striated muscle

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stri·at·ed mus·cle

skeletal or voluntary muscle in which cross-striations occur in the fibers as a result of regular overlapping of thick and thin myofilaments; contrast with smooth muscle. Although cardiac muscle (which is not voluntary muscle) also looks striated, the term striated muscle is incorrectly used as a synonym for voluntary skeletal muscle.

striated muscle

Muscle, including skeletal and cardiac muscle in vertebrates, that is distinguished from smooth muscle by having transverse striations across the fibers.

striated muscle

Etymology: L, striatus, striped, musculus, muscle
any muscle, including all of the skeletal muscles, in which the fibers are divided by bands of cross-striations (stripes) as a result of overlapping of thick and thin myofilaments. Contractions in such muscles are voluntary. The heart, a striated involuntary muscle, is an exception. Cardiac muscle is often placed in its own category because it has microanatomical and contraction physiological characteristics very different from those of most striated muscle. Also called skeletal muscle, voluntary muscle. Compare cardiac muscle, smooth muscle.
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Striated muscle

stri·at·ed mus·cle

(strī'āt-ĕd mŭs'ĕl)
Skeletal or voluntary muscle in which cross-striations occur in the fibers as a result of regular overlapping of thick and thin myofilaments; contrast with smooth muscle. Although cardiac muscle is also striated in appearance, the term "striated muscle" is commonly used as a synonym for voluntary skeletal muscle.


(sel) [L. cella, a chamber]
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The basic unit of life. A cell is a group of self-sustaining biochemical reactions that are isolated from the environment by a selectively permeable lipid membrane. Among the key reactions are those that maintain a stable intracellular concentration of ions; for mammalian cells, typical internal concentrations include 140 mM K+, 5-15 mM Na+, 5-15 mM Cl-, and a pH of 7.2, which can be significantly different from their concentrations outside the cell. Other key reactions move molecules and molecular complexes within the cell, sometimes changing the cell's shape. These reactions, along with many others, require energy, and the generation of energy by breaking apart preexisting hydrocarbon molecules ("food") is the job of glycolysis and other characteristic intracellular metabolic reactions. See: glycolysis; metabolism; mitochondrion.


Intracellular chemical reactions are controlled by enzymes that are organized in stable molecular complexes called organelles. The polymer-based organelles include centrioles and the cytoskeleton; nucleic acid-based organelles include ribosomes; and membrane-enclosed organelles include the nucleus, endoplasmic reticula, Golgi complexes, lysosomes, peroxisomes, mitochondria, and storage and transport vesicles. See: illustrationIndividual mammalian cells are usually microscopic, typically ranging from 5 µm to 50 µm in diameter. In humans, lymphocytes are small cells (~6 µm in diameter), columnar epithelial cells (10 µm x 20 µm) are medium-size cells, and mature ova (120-150 µm) are some of the largest cells.

Cell Division

In mammals, all new cells arise from existing cells through cell division, and an animal's growth results largely from increases in the number of its cells, most of which differentiate into specialized cell types to form the body's various tissues. Cell division involves two major processes: karyokinesis, the division of the nucleus, and cytokinesis, the division of the remainder of the cell. When generating somatic daughter cells, karyokinesis uses a process called mitosis, which produces daughter cells with a full complement of chromosomes. When generating germ cells, karyokinesis includes a process called meiosis, which produces daughter cells with half the normal number of chromosomes. See: meiosis and mitosis for illus.

A cell

Alpha cell of the pancreas.

accessory cell

A monocyte or macrophage that participates in the immune response.
See: antigen-presenting cell; macrophage

acidophilic cell


acinar cell

A cell present in the acinus of an acinous gland, e.g., of the pancreas.

adipose cell


adult stem cell

A precursor cell that can also give rise to identical precursor cells: daughters of a stem cell can develop into a terminally-differentiated cell type or they can remain a stem cell. Adult stem cells are found in many tissues, such as bone marrow, brain, retina, skin, intestines, liver, testis, and pancreas.
Synonym: somatic stem cell See: embryonic stem cell

adventitial cell

A macrophage along a blood vessel, together with perivascular undifferentiated cells associated with it.

air cell

An air-filled sinus cavity in a bone.

alpha cell

1. An enteroendocrine cell that produces glucagon and is found in the pancreatic islets. Synonym: A cell
2. An acidophil of the adenohypophysis (anterior pituitary gland).

alveolar cell

1. In the lung, either of two types of epithelial cells lining the alveoli. Type I cells are simple thin squamous epithelial cells. Type II cells secrete pulmonary surfactant. Type II cells are smaller and more numerous than Type I cells.
2. In the mammary glands, the milk-secreting epithelial cells, which are activated during lactation.

amacrine cell

A modified nerve cell in the retina that has dendrites but no axon.
See: neuron

ameloblast cell


anterior horn cell

A somatic motor neuron that has its cell body in the ventral (anterior) horn of the gray matter of the spinal cord; its axon passes out through a ventral root and innervates skeletal muscle.

antigen-presenting cell

Abbreviation: APC
A cell that breaks down antigens and displays their fragments on surface receptors next to major histocompatibility complex molecules. This presentation is necessary for some T lymphocytes that are unable to recognize soluble antigens. Macrophages are the primary antigen-presenting cells, but B cells and dendritic cells also can act as APCs.
See: T cell; macrophage processing cell


The abbreviated name for an 'amine precursor uptake and decarboxylation cell'. These cells are the constituents of a diffuse neuroendocrine system and all have metabolic pathways that make and utilize serotonin (5-HT). APUD cells include chromaffin cells, enterochromaffin cells, and SIF cells as well as certain cells found in the parathyroid gland, thyroid gland, pituitary gland, hypothalamus, and placenta.

argentaffin cell

A cell in the epithelium of the stomach, intestines, and appendix that secretes serotonin.

astroglial cell


atypical glandular cells

Abbreviation: AGC
An abnormal finding on a Pap test. This classification is divided into “favor neoplasia” or “not otherwise specified (NOS).” NOS is subdivided into endocervical or endometrial origin. Atypical endocervical cells are important because of their risk for significant disease.
Synonym: atypical glandular cells of undetermined significance

atypical glandular cells of undetermined significance

Abbreviation: AGUS
Atypical glandular cells.

B cell

1. A lymphocyte that synthesizes and secretes antibodies. B lymphocytes originate and differentiate in the bone marrow and then populate the spleen, lymph nodes, and other lymphoid tissues. When exposed to an antigen, a B cell divides to form (a) plasma cells, which produce antigen-specific antibodies, and (b) a lesser number of memory B cells, which can quickly differentiate into plasma cells upon a second exposure to the original antigen. Antibody production is a key part of the humoral immune response of adaptive immunity. The humoral immune response is effective against bacteria, viruses, and other pathogens, and provides the rationale for vaccination. Synonym: B lymphocyte See: ; T cell;
2. Pancreatic beta cell.

band cell

The developing leukocyte at a stage at which the nucleus is not segmented.

basal cell

1. A rounded or cuboidal epithelial stem cell found in the bottom layer of pseudostratified epithelia, such as the epidermis and the lining of the airways of the lung.
2. Either of two types of cell found in the bottom layer of the olfactory epithelium; one type is a flattened "basal cell proper", and the other is a rounded stem cell called a globose cell.
3. A rounded stem cell found in the taste buds and a progenitor of the specialized taste receptor cells.

basket cell

1. Myoepithelial cell.
2. One of the nonspiny granule cells found in the cerebral cortex.
3. One of the small interneurons found in the outermost layer of the cerebellar cortex along with stellate cells.

basophilic cell


beta cell

1. Any of the insulin-secreting cells of the pancreas that constitute the bulk of the islets of Langerhans. Synonym: B cell
2. A basophil cell of the adenohypophysis (anterior pituitary gland).

Betz cell

See: Betz cells

bipolar cell

Bipolar neuron.

blast cell

1. A precursor cell for a specific cell type.
2. An immature cell of a specific type.

blood cell

Any cell normally found circulating in the blood stream. Blood cells include red cells and white cells; red cells generally remain inside blood vessels, while white cells can also more into the tissues outside the blood vessel walls.
See: blood for illus.

bone cell

An osteoblast, osteoclast, or osteocyte.

bone marrow cell

Marrow cell.

brush cell

An epithelial cell found sparsely in the lining of the bronchial tree. The cell's surface has long stiff microvilli, and the cell has the appearance of an absorptive cell.

burr cell

An erythrocyte with 10 to 30 spicules distributed over the surface of the cell, as seen in heart disease, stomach cancer, kidney disease, and dehydration. Synonym: echinocyte

cancer cell

A cell present in a neoplasm and differentiated from normal tissue cells because of its degree of anaplasia, irregularity of shape, nuclear size, changes in the structure of the nucleus and cytoplasm, increased number of mitoses, and ability to metastasize.

capsule cell

Satellite cell.

cartilage cell


castration cell

An enlarged and vacuolated basophil cell seen in the pituitary in gonadal insufficiency or following castration.

CD3 cell

T cell.

CD4 cell

Helper T cell.

CD8 cell

A suppressor T cell, e.g., a cytotoxic T cell.

CD 34+ cell

A cell with the CD34 protein on its surface membrane. Some CD34 cells that are hemopoietic stem cells can be separated out from peripheral blood.

cementoblast cell


cementocyte cell

Any of the cells trapped within cementum that maintain cementum as a living calcified tissue by their metabolic activity.

centroacinar cell

A duct cell of the pancreas more or less invaginated into the lumen of an acinus.

chalice cell

Goblet cell.

chemoreceptor cell


chief cell

1. Any of the cells of the parathyroid gland that secretes the parathyroid hormone.
2. Any of the cells of the gastric glands that secretes pepsinogen.

chromaffin cell

A cell that produces, stores, and secretes catecholamines (dopamine and norepinephrine). Chromaffin cells are found in the medulla of the adrenal glands and in small clusters in the sympathetic ganglia.

chromophobe cell


Clara cell

A cuboidal epithelial cell found in the lining of the terminal and the respiratory bronchioles of the lungs. Clara cells are nonciliated, and they secrete surfactant, like the type II alveolar epithelial cells found deeper in the bronchial tree.

cleavage cell


clue cell

A vaginal epithelial cell, thickly coated with coccobacillary organisms. Clue cells are a hallmark of bacterial vaginosis.

columnar cell

An epithelial cell with height greater than its width.

columnar epithelial cell

Columnar cell.

cone cell

A cell in the retina whose scleral end forms a cone that serves as a light receptor. Vision in bright light, color vision, and acute vision depend on the function of the cones. See: rod cell

contrasuppressor cell

A T cell that inhibits the activity of suppressor T cells. Although a contrasuppressor cell shares this functional capability with T helper cells, it is distinguished from other CD4+ cells by its other cell surface markers and the unique group of cytokines it produces.

cortical cell

A cell in the cortex of an organ, e.g., a neuron in the cerebral cortex.

corticotroph cell


cuboid cell

A cell – usually epithelial – with a height about equal to its width and depth.

cytotoxic cell

Cytotoxic T cell.

cytotoxic T cell

A CD8+ T lymphocyte that can destroy microorganisms directly through the release of perforin and proteolytic enzymes. These cells are particularly important in the defense against viruses, rejection of allografts, and, possibly, new malignant cells. Synonym: CD8 cell; cytotoxic cel; killer T cell

D cell

An enteroendocrine cell that produces somatostatin and is found in the pancreatic islets, stomach, and small intestine. Synonym: delta cell; somatostatin cell

daughter cell

A cell formed by cell division.

decoy cell

A cell found in the urine with inclusion bodies in its nucleus. It indicates infection with BK virus in renal transplant recipients.

delta cell

Pancreatic D cell.

dendritic cell

One type of antigen-presenting cell that helps T cells respond to foreign antigens. Dendritic cells are found in epithelial tissues and include the Langerhans' cells of the skin and the interdigitating cells in lymph nodes; they also circulate in the blood.

Downey cell

See: Downey cell

dust cell

A macrophage that migrates into the lumen of lung aveoli and ingests debris, particles of air pollution, and pathogens to keep the airspaces clear.

EC cell

1. An embryonal carcinoma cell, which is a cultured cell line.
2. An enterochromaffin cell that secretes substance P and is found in the stomach and small intestine.

effector cell

A cell that carries out the final response or function of a particular process. The main effector cells of the immune system, for example, are activated lymphocytes and phagocytes—the cells involved in destroying pathogens and removing them from the body.
See: leukocyte

embryonic stem cell

Abbreviation: ES cell
A cell from the inner cell mass of the blastocyst (the 3-5 day old mammalian embryo) that can give rise to all the somatic cells of the body. Embryonic stem cells can be maintained as pure stem cell cultures.
See: adult stem cell

endothelial cell

The type of epithelial cell that lines blood vessels and lymph vessels; these cells are usually squamous (flattened) and form sheets one layer thick. Endothelial cells are derived from mesenchyme cells of the embryo. A sheet of endothelial cells is called an endothelium.
See: endothelium

enterochromaffin cell

Abbreviation: EC cell
An enteroendocrine cell that produces serotonin and is found in the small intestine. Enterochromaffin cells are very similar to the cells, found throughout the peripheral sympathetic nervous system, that are called simply 'chromaffin cells'.

enteroendocrine cell

One of the scattered hormone-producing cells found in the pancreatic islets and throughout the gastrointestinal (mainly, small intestinal) mucosa.

ependymal cell

Any of the epithelial cells that form a one-cell-thick layer lining the ventricles and the central canal of the central nervous system. The ventricular (apical) surfaces of many ependymal cells are covered with cilia or microvilli. In most places, the ependymal layer does not have a basal lamina. Specialized regions of ependymal cells include the covering of the blood vessels and loose connective tissue of the choroid plexuses; here, the ependyma is specialized to secrete cerebrospinal fluid.

epithelial cell

Any of the cells forming the cellular sheets that cover surfaces, both inside and outside the body. Epithelial cells are closely packed and take on polyhedral shapes, from tall (columnar) through squat (cuboidal) to flat (squamous). Epithelial cells adhere strongly to one another, and one of their surfaces -- the basal surface -- sticks firmly to a thin extracellular film of fibrils called a basal lamina. A sheet of epithelial cells derived from embryonic epithelia (the ectoderm or the endoderm) is called an epithelium. See: epithelium


Ethmoid sinus.

ethmoid air cell

Ethmoid sinus.

eukaryotic cell

The type of cell composing multicellular, as well as a number of unicellular, organisms. Unlike prokaryotic cells, eukaryotic cells have many of their intracellular functions organized within structures called organelles. Some organelles -- notably, the nucleus, which contains the DNA -- are enclosed by intracellular membranes.

F cell

An enteroendocrine cell that produces pancreatic polypeptide and is found in the pancreatic islets.

fat cell


flame cell

A bone marrow cell with a bright red cytoplasm, occasionally found in the marrow of patients with multiple myeloma.

flow cell

An optical cell used in photometers and cell counters, through which the sample and any standards are passed for detection and measured or counted by optical or electrometric means.
See: cytometry

foam cell

A cell that contains vacuoles; a lipid-filled macrophage.

follicle cell

Follicular cell.

follicular cell

1. The secretory cell of the thyroid gland; it produces the thyroid hormones, T3 (triiodothyronine) and T4 (tetraiodothyronine or thyroxine).
2. Any of the flattened somatic cells that form a monolayer around each primary oocyte in the ovary. After puberty, when an oocyte matures, during a monthly cycle, its follicular cells divide, become cuboidal, and form a multilayered coating for the oocyte; at this stage, the follicular cells are called granulosa cells.

folliculostellate cell

A supporting cell in the adenohypophysis (anterior pituitary gland); it produces bioactive peptides, including growth factors and cytokines.

foreign body giant cell

Giant cell.

G cell

An enteroendocrine cell found in the stomach that produces the hormone gastrin.

ganglion cell

1. Any neuron whose cell body is located within a ganglion.
2. A neuron of the retina of the eye whose cell body lies in the ganglion cell layer. The axons of ganglion cells form the optic nerve.

germ cell

A cell whose function is to reproduce the organism. Early in development, primordial germ cells are found in the genital ridges of the embryo. Later, in the testis, the primordial germ cells are called spermatogonia, and in the ovary, they are called oogonia. When they mature, the germ cells (i.e., spermatogonia and oogonia) differentiate into haploid gametes (i.e., spermatozoa and ova).
Synonym: primordial germ cell

giant cell

1. A multinucleated phagocyte created by several individual macrophages that have merged around a large pathogen or a substance resistant to destruction, such as a splinter or surgical suture. See: granuloma; tuberculosis
2. A large multinucleated (40-60 nuclei) tumor cell characteristic of certain bone and tendon tumors.
3. A large multinucleated cell that invades the walls of the aorta and its major branches in giant cell arteritis.

glial cell

One of three types of nonneuronal cell in the central nervous system: astrocytes, oligodendrocytes, and microglial cells. SYN neuroglial cell
Synonym: neuroglial cell

gitter cell

A macrophage present at sites of brain injury. The cells are packed with lipoid granules from phagocytosis of damaged brain cells.
See: microglia

globus cell

One of the two varieties of basal cell found in the olfactory epithelium. It is a rounded neuroblast or neural stem cell for the olfactory receptor cells.

goblet cell

A mucous cell sitting between nonsecretory cells, such as is found in the intestinal epithelium.

Golgi cell

See: Golgi, Camillo

gonadotroph cell


graft facilitating cell

Any of a group of CD8 positive, t-cell receptor negative cells that help donated bone marrow engraft in the recipient.

granule cell

1. Any of the small neurons that pack the granular cell layer of the cerebellar cortex, immediately below the Purkinje cell layer. Granule cells receive inputs (mossy fibers) from the spinal cord and brainstem (except the inferior olive). Axons of granule cells run perpendicular to the Purkinje cell dendrites, on which they synapse.
2. Any of the neurons of the cerebral cortex that are not pyramidal cells. Cortical granule cells are categorized as spiny or nonspiny. Synonym: stellate cell
3. A small axon-less neuron found in the olfactory bulb.

granulosa cell

One of the many cuboidal cells that surround and nurture the maturing oocyte.
See: follicular cell (2)

gustatory cell

Taste cell.

hair cell

An epithelial cell possessing stereocilia in the maculae, cristae ampullaris, and the organ of Corti. These cells are receptors for the senses of position and hearing.

heart failure cell

A red-colored (from ingested red cells) lung macrophage often found in the sputum of patients with congestive heart failure.

HeLa cell

A line of human epithelial cells that grows well in culture. It is an immortal cancer cell that has been maintained in continuous tissue cultures for decades from a patient with carcinoma of the cervix. It is named for the first two letters of the patient's first and last names, Henrietta Lacks. HeLa cells have been used in thousands of experiments on cell growth, differentiation, and cancer, and in virology, pharmacology, and other fields.
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HELMET CELLS: (Orig. mag. ×640)

helmet cell

A schistocyte or fragmented blood cell, seen in hemolytic anemias.
See: illustration

helper cell

A type of T lymphocyte whose surface is marked by CD4 receptors; it is involved in both cell-mediated and antibody-mediated immune responses. It secretes cytokines that stimulate the activity of B cells and other T cells and binds with class II histocompatibility antigens, which are processed by macrophages and other antigen-presenting cells. Synonym: helper T cell See: antigen processing; T cell; cell-mediated immunity

helper T cell

Helper cell.

hematopoietic stem cell

A progenitor cell in the bone marrow that can replicate itself as well as produce precursor cells of the various blood cell lineages.

hilus cell

An androgen-producing cell found in the ovarian hilum. It is analogous to the male Leydig cell.

holly leaf cell

A cell found in blood smears of persons with sickle cell anemia.

horizontal cell

A neuron of the inner nuclear layer of the retina. The axons of these cells run horizontally and connect various parts of the retina.

Hürthle cell

See: Hürthle cell

hybridoma cell

See: hybridoma

hyperchromatic cell

A cell that contains more than the normal number of chromosomes and hence stains more densely.

I cell

An enteroendocrine cell that produces the enzyme cholecystokinin-pancreozymin (pancreaticozymin) and is found in the small intestine.

interdigitating cell

A type of antigen-presenting cell found in lymph nodes and lymphoid tissue.

interstitial cell

Any of the many cells found in connective tissue of the ovary, in the seminiferous tubules of the testes, and in the medulla and cortex of the kidney. The cells in the testes and ovaries produce hormones such as testosterone and estrogen.

intestinal absorptive cell

In the small intestine, any of the tall columnar cells topped with a brush border made of thousands of microvilli.

islet cell

A cell of the islets of Langerhans of the pancreas.

juvenile cell

The early developmental form of a leukocyte.

juxtaglomerular cell

A modified smooth muscle cell in the wall of the afferent arteriole leading to a glomerulus of the kidney. This type of cell secretes renin when blood pressure decreases to activate the renin-angiotensin mechanism, which increases sodium retention, thus elevating the blood pressure.

K cell

An enteroendocrine cell that produces gastric inhibitory peptide (glucose-dependent insulinotropic peptide) and is found in the small intestine. This peptide stimulates the beta cells of the pancreas to secrete insulin.

killer cell

Natural killer cell.

killer T cell

Cytotoxic T cell.

Kulchitsky cell

An APUD cell found in the lung.

Kupffer cell

See: Kupffer cell

L cell

An enteroendocrine cell that produces glucagon-like peptide-1 and is found in the small intestine. This peptide signals the pancreas to secrete insulin after a meal.

labile cell

A cell that is always mitotically active, such as the epithelial cells lining the stomach and the stem cells in the red bone marrow.

lactotroph cell


LAK cell

Abbreviation for lymphokine-activated killer cell. These natural killer cells, obtained from the patient's blood, have been activated in culture with interleukin-2.LAK cells; the cells can then be used to treat patients with solid malignant tumors.


A type of dendritic antigen-presenting cell that typically resides in the skin.
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L.E. CELL (center): (Orig. mag. ×1000)

L.E. cell

Historically, an abbreviation for lupus erythematosus cell, a polymorphonuclear leukocyte that contains the phagocytized nucleus of another cell. It is characteristic but not diagnostic of lupus erythematosus.

This distinctive cell may form when the blood of patients with systemic lupus erythematosus is incubated and further processed according to a specified protocol. The plasma of some patients contains an antibody to the nucleoprotein of leukocytes. These altered nuclei, which are swollen, pink, and homogeneous, are ingested by phagocytes. These are the L.E. cells. The ingested material, when stained properly, is lavender and displaces the nucleus of the phagocyte to the inner surface of the cell membrane. The L.E. cell phenomenon can be demonstrated in most patients with systemic lupus erythematosus but is not essential for diagnosis.


Leydig cell

See: Leydig's cell

littoral cell

A macrophage found in the sinuses of lymphatic tissue.

liver cell


lutein cell

A cell of the corpus luteum of the ovary that contains fatty yellowish granules. Granulose lutein cells are hypertrophied follicle cells; these lutein (paralutein) cells develop from the theca interna.

lymphoid cell

An obsolete term for lymphocyte.

lymphokine-activated killer cell

LAK cell.

M cell

1. A microfold cell, which is a cell in the gastrointestinal epithelium covering patches of lymphoid tissue. M cells transport antigens from the intestinal lumen to the underlying lymphoid tissues for recognition and processing.
2. An APUD cell that produces melanotropin and is found in the pituitary gland.

macroglial cell

An astrocyte or an oligodendrocyte.

marrow cell

Bone marrow cell.
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mast cell

A large tissue cell resembling a basophil, which is essential for inflammatory reactions mediated by immunoglobulin E (IgE) but does not circulate in the blood. Mast cells are present throughout the body in connective tissue, but are concentrated beneath the skin and the mucous membranes of the respiratory and digestive tracts. Mast cells are covered with IgE molecules, which bind with foreign antigens and stimulate degranulation, releasing such mediators as histamine, prostaglandins, leukotrienes, and proteinases from densely packed granules within the cytoplasm. These mediators produce type I (immediate) hypersensitivity reactions (e.g., urticaria, allergic rhinitis, asthma, angioedema, and systemic anaphylaxis). See: illustration

mastoid cell

Mastoid air cell.

mastoid air cell

Any of the variable-sized, air-filled sinuses inside the mastoid antrum. About 20% of adult skulls have no mastoid air cells.

matrix stem cell

A stem cell derived from Wharton's jelly.

memory cell

A cell derived from B or T lymphocytes that can quickly recognize a foreign antigen to which the body has been previously exposed. Memory T cells stimulate T helper lymphocytes and cytotoxic T cells; memory B cells stimulate the production of antigen-specific antibodies by B plasma cells. Both types of memory cells survive for years, providing a durable adaptive immune response against foreign antigens.

mesenchyme cell

One of the two basic somatic cell lineages -- the other being epithelial cells. In contrast to epithelial cells, mesenchyme cells are not polarized and are frequently motile. In the early embryo, mesenchyme cells fill many of the spaces enclosed by epithelia. Later, mesenchyme cells will secrete the space-filling extracellular matrix molecules, such as collagen and glycoproteins, that characterize connective tissue.

mesenchymal stem cell

A stem cell found in connective tissue and capable of producing cells of the connective tissue lineages, such as cartilage, bone, muscle, and fat cells.

mesothelial cell

The type of epithelial cell that lines serous (pleural, peritoneal, and pericardial) cavities, blood vessels, and lymph vessels; these cells are usually squamous (flattened) and form sheets one layer thick. Mesothelial cells are derived from mesenchyme cells of the embryo. A sheet of mesothelial cells is called a mesothelium.
See: mesothelium

microglial cell

A small glial cell of the central nervous system and retina. Microglia have spiky branched processes and are arranged homogeneously throughout the brain and spinal cord. They are activated by disease and injury, after which they become phagocytic and sometimes resume their embryonic motility like a macrophage.

mitral cell

One of the two principal neurons of the olfactory bulb -- the other being the tufted cell. In a complex synaptic formation called a glomerulus, each of the mitral and tufted cells receives synaptic inputs from axons of the olfactory nerve. The axons of the mitral and tufted cells form the olfactory tract and synapse in the olfactory cortex.

mossy cell

An astrocyte or other glial cell with many branching processes. See: neuroglia

mother cell

A cell that gives rise to similar cells through fission or budding. Synonym: parent cell

mucosal cell

Any cell in a mucosal epithelium.

mucous cell

An epithelial cell that secretes mucus and IgA antibodies. Mucous cells and serous cells are the two varieties of secretory cells found in exocrine glands.
Synonym: mucus cell

mucus cell

Mucous cell.

multinucleated giant cell

Giant cell.

multipolar cell

Multipolar neuron.

muscle cell

See: muscle

myeloid cell

Any white blood cell other than lymphocytes.

myeloma cell

A cell present in the bone marrow of patients with multiple myeloma.

myoepithelial cell

A smooth muscle cell found in some epithelia; it lies between glandular cells and the basal lamina of sweat, mammary, lacrimal, and salivary glands.
Synonym: basket cell

natural killer cell

Abbreviation: NK cell
A large granular lymphocyte – a defensive cell of innate immunity – that bonds to cells and lyses them by releasing cytotoxins. Natural killer cells are null cells, lymphocytes that do not have B cell or T cell surface markers, and they can be activated without previous antigen exposure. NK cells destroy cells infected with viruses and some types of tumor cells in cultures. They also secrete gamma interferon (INF?), tumor necrosis factor alpha (TNFa), and granulocyte-macrophage colony-stimulating factor (GMCSF), enhancing the effect of T lymphocytes.
Synonym: killer cell

nerve cell


neural crest cell

Embryonic cells of the neuron-glia lineage that form along the ridges (neural folds) of the neural plate and that migrate into the developing organism to produce a variety of tissues. The migratory ability of these embryonic epithelial cells is similar to the motility of mesenchyme cells; this has led neural crest cells to be called mesectodermal cells. In the neural lineage, neural crest cells give rise to the dorsal root ganglia, the placodes that will develop into the olfactory and auditory sensory organs, the pituitary gland, the peripheral autonomic nervous system, and the neurenteric and APUD cells. In the glial lineage, neural crest cells give rise to Schwann cells and other peripheral satellite cells. In addition, neural crest cells of the cranial region give rise to certain facial connective tissue, including the bones of the nasal cavities, the roof of the mouth, and the sella turcica.

neuroglial cell

Glial cell.

Niemann-Pick cell

See: Niemann-Pick cell

NK cell

Natural killer cell.

nonstem cell

Any cell found in the bone marrow that cannot reconstitute the marrow or give rise to more differentiated blood cells.

null cell

A large lymphocyte without the cell markers of either a T cell or a B cell. Natural killer cells are examples of null cells.

odontoblast cell


oligodendroglial cell


olfactory cell

Olfactory receptor cell.

olfactory receptor cell

A cell of the olfactory mucosa that has receptors for the sense of smell. Olfactory cells are continuously replaced from stem cells throughout adult life.
Synonym: olfactory cell

osteoprogenitor cell

Any of the mesenchyme precursor cells committed to the bone lineage and capable of producing osteoblasts and osteocytes. Osteoprogenitor cells are found in bone, bone marrow, and other connective tissue.

oxyntic cell

A parietal cell of the gastric glands; it produces hydrochloric acid and the intrinsic factor.

parabasal cell

An abnormal but not malignant cell seen in some cytologic specimens obtained during Papanicolaou tests (Pap tests). It is found in women with vaginal atrophy, in some postpartum women, some women suffering from anorexia or starvation, and some who have used progesterone for contraception.

parent cell

Mother cell.

phalangeal cell

One of the cells supporting the hair cells of the organ of Corti. These cells form several rows of outer phalangeal cells (Deiters' cells) and a single row of inner phalangeal cells.

phantom cell

See: red cell ghost

pigment cell

Any cell that normally contains pigment granules.

plasma cell

A cell derived from a B lymphocyte that has been sensitized to a specific foreign antigen and produces antibodies to that particular antigen. It may be found in the blood or in tissue fluid. Synonym: plasmacyte

postganglionic cell

Postganglionic neuron.

PP cell

An enteroendocrine endocrine cell found in the pancreatic islets that produces pancreatic polypeptide.

pre-B cell

The immediate precursor of a lymphocytic B cell.

preganglionic cell

Preganglionic neuron.

prickle cell

A cell with spiny processes that connect with similar processes of adjoining cells. These are found in the stratum spinosum of the keratinized epithelium of the epidermis.

primary cell

In physical therapy, a device consisting of a container, two solid conducting elements, and an electrolyte for the production of electric current by chemical energy.

primordial cell

Primordial germ cell.

primordial germ cell

A germ cell before it begins its maturation into a haploid gamete.
Synonym: primordial cell

progenitor cell

A cell (sometimes a stem cell) that produces cells of a particular lineage, e.g., a neuroblast.

prokaryotic cell

The form of cell composing many primitive unicellular organisms, such as bacteria. Prokaryotic cells do not have nuclei, which are partitioned by an intracellular membrane; instead the DNA forms one main coil in the cell cytoplasm.

Purkinje cell

See: Purkinje, Johannes E. von

pus cell

A leukocyte present in pus. Pus cells are often degenerated or necrotic.

pyramidal cell

A large, common neuron found in the cerebral cortex. Pyramidal cells are flask-shaped or triangular, and, in the parts of the cortex with six layers, they occupy the fifth layer. Pyramidal cell dendrites project up into the most superficial layer of the cortex, while pyramidal cell axons run in the opposite direction, i.e., downward and out of the cortex.
Synonym: pyramidal neuron

radial glial cell

A structural macroglial cell that is a key component of the developing nervous system. Radial glial cells first appear in the neural tube, where their cell bodies are suspended between two thin cell processes; the apical process attaches to the inner (ventricular) surface of the neural tube, and the basal process attaches to the outer (pial) surface. Early in development, neuroblasts migrate radially along the scaffolding formed by the radial glial cell processes, and growing axons may follow the scaffolding longitudinally. Later, many radial glial cells retract their processes and differentiate into astrocytes.

red cell

A small cell that is filled with hemoglobin, has no nucleus, and is shaped like a biconcave disc. Red cells transport oxygen to tissues and carbon dioxide to the lungs. Individual red cells have a life span of 3-4 months, and new red cells are continually being produced in the bone marrow. In a healthy person, 99% of the cells circulating in the blood are red cells.
Synonym: erythrocyte; red blood cell; red blood corpuscle See: erythrocyte

red blood cell

Abbreviation: RBC
Red cell.

Renshaw cell

See: Renshaw cell

resting cell

1. A cell that is not dividing. See: interphase
2. A cell not performing its normal function (i.e., a nerve cell that is not conducting an impulse or a muscle cell that is not contracting).

reticular cell

1. An undifferentiated cell of the spleen, bone marrow, or lymphatic tissue that can develop into one of several types of connective tissue cells or into a macrophage.
2. A cell of reticular connective tissue. See: reticular tissue

reticuloendothelial cell

An out-of-date term for a cell of the mononuclear phagocytic system.

Rieder cell

See: Rieder cell

rod cell

A cell in the retina of the eye whose scleral end is long and narrow, forming a rod-shaped sensory receptor. Rods are stimulated by light and are essential for vision in dim light. See: cone cell

rosette cell

A rose-shaped cluster of phagocytes surrounding lysed nuclear material or red blood cells. Rosette cells occur frequently in blood in which L.E. cells are present. Rosette cells are not diagnostic of lupus erythematosus. See: L.E. cell

Rouget cell

See: Rouget cell

S cell

An enteroendocrine cell that produces secretin and is found in the small intestine.

satellite cell

1. A stem cell associated with skeletal muscle that may form a limited number of new muscle cells after injury.
2. One of the neuroglia cells enclosing the cell bodies of sensory neurons in spinal ganglia. Synonym: capsule cell

scavenger cell

A phagocyte that cleans up disintegrating tissues or cells.

Schwann cell

See: Schwann cell

segmented cell

A segmented neutrophil (i.e., one with a nucleus of two or more lobes connected by slender filaments).

selenoid cell

See: red cell ghost

sensory cell

A cell that when stimulated gives rise to nerve impulses that are conveyed to the central nervous system.

septal cell

A type II alveolar cell that secretes pulmonary surfactant; it is adjacent to a septum of the alveoli.

serous cell

An epithelial cell that secretes a watery fluid containing proteins, glycoproteins, and often antibodies (IgA, IgG, and IgM). Serous cells and mucous cells are the two varieties of secretory cells found in exocrine glands.

Sertoli cell

See: Sertoli cell

sex cell

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sickle cell

An abnormal erythrocyte shaped like a sickle. Synonym: drepanocyte See: anemia, sickle cell

signet-ring cell

A vacuolated cell with the nucleus off center. Mucus-secreting adenocarcinomas usually contain these cells.

skeletal muscle cell

See: muscle

smooth muscle cell

See: muscle

somatic cell

Any cell that is not a germ cell.

somatic stem cell

Adult stem cell.

somatostatin cell

D cell.

spiculed red cell

Crenated red blood cells with surface projections. In most instances, this is a normal variation in red cell equilibrium and is reversible.
See: acanthocyte

spider cell


squamous cell

A flat epithelial cell.
Synonym: squamous epithelial cell

squamous epithelial cell

Squamous cell.

stellate cell

1. Granule cell of the cerebral cortex.
2. One of the small interneurons found in the outer layer of the cerebellar cortex along with basket cells.

stellate reticuloendothelial cell

A Kupffer cell, one of the macrophages that line the sinusoids of the liver.

stem cell

An embryonic stem cell or an adult stem cell.

Sternberg-Reed cell

See: Reed-Sternberg cell

stipple cell

A red blood cell that contains small basophilic-staining dots. It is seen in lead poisoning, malaria, severe anemia, and leukemia.

striated muscle

See: muscle

suppressor T cell

A subpopulation of regulatory T lymphocytes that develop in the thymus gland, that slows or stops a specific immune response.

sustentacular cell

A supporting cell, as in the acoustic macula, organ of Corti, olfactory epithelium, taste buds, or testes.

sympathicotrophic cell

One of the large epithelial cells that occur in groups in the hilus of the ovary. They are thought to be chromaffin cells.

sympathochromaffin cell

A chromaffin cell of ectodermal origin present in the fetal adrenal gland. Sympathetic and medullary cells originate from these cells.

syncytial giant cell

Giant cell.

T cell

A lymphocyte that responds to specific antigens, with the assistance of antigen-presenting cells (APCs). T cells arise in the bone marrow and migrate to the thymus gland, where they mature; then they circulate between blood and lymph, serving as one of the primary cells of the adaptive immune response. Immature T cells are called thymocytes. Mature T cells are antigen specific. Their surface receptors (T cell receptors, abbrev. TCRs) respond only to a single antigen. T cells are further categorized using another family of surface protein markers called clusters of differentiation (CDs). All T cells have a CD3 marker. Additional markers differentiate the subclasses of T cells. CD4 T helper cells serve primarily as regulators, secreting cytokines that stimulate the activities of other white blood cells. CD8 T cells (cytotoxic T cells) directly lyse (kill) organisms, an important defense against viruses; most CD8 T cells also produce gamma interferon (INF?), one of the strongest stimulators of macrophage activity. Synonym: T lymphocyte See: immune response; lymphocyte; immunological surveillance; T-cell receptor

A T cell can only recognize the "foreignness" of antigens after they have been modified by macrophages and other antigen-presenting cells (APCs). After this, T cells dominate the adaptive immune response by mobilizing B cells and other T cells of the cell-mediated immune pathways. T cells are responsible for type IV hypersensitivity reactions, such as graft rejection, and for tumor cell recognition and destruction.

See: cytokine; cell-mediated immunity
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TARGET CELLS: In hemoglobin C disease (×600)

target cell

1. . An erythrocyte with a dark rounded central area surrounded by a lightly stained clear ring, which in turn is surrounded by a dense ring of peripheral cytoplasm. It is present in certain blood disorders, such as thalassemia, and in patients who have no spleen. See: hemoglobin C disease for illus Synonym: codocyte; leptocyte
2. . The cell at which a signal (e.g., hormone or nerve impulse) is aimed.

tart cell

A phagocyte that has ingested the unaltered nuclei of cells. These nuclei can be observed unchanged within the phagocytes.

taste cell

Any of the neuroepithelial cells within a taste bud that are receptors for the sense of taste. Each possesses on the free surface a short gustatory hair that projects through the inner taste pore.
Synonym: taste receptor cell

taste receptor cell

taste cell.

tear-drop cell

An abnormally shaped blood cell, sometimes found on blood smears of patients with bone marrow fibrosis, iron deficiency, or thalassemias.
Synonym: dacrocyte

tendon cell

Any of the fibroblasts of white fibrous connective tissue of tendons arranged in parallel rows.

terminally differentiated cell

A cell sufficiently committed to a particular function that it can no longer divide, e.g. a red cell.

thymic epithelial cell

The epithelial cells that form the internal scaffolding of the thymus. These cells vary in shape and size but generally align in sheets and cords, partitioning the thymus into islands of close-packed lymphocytes in the organ's cortex. Thymic epithelial cells are not simply structural and they interact actively with adjacent lymphocytes.

thymus cell

Any cell characteristic of the thymus, including thymic epithelial cells and thymocytes (thymic lymphocytes).

thyroid cell

Any cell characteristic of the thyroid gland, but usually referring to a thyroid follicular cell.

totipotent cell

An undifferentiated embryonic cell that has the potential to develop into any type of cell.

Touton giant cell

See: Touton cell

transitional cell

The stretchable epithelial cells that compose the transitional epithelium (uroepithelium), which lines most of the urinary tract. Transitional cells are strongly interconnected. They are cuboidal when not under pressure, and they become flattened and squamous when stretched. Transitional epithelia are 4-6 cells thick, and the top transitional cells -- those on the lumenal surface -- fuse to become larger and polyploid.

trophoblast cell

One of the epithelial cells forming the surface of the spherical blastocyst stage embryo. Trophoblast cells are destined to give rise to many of the extraembryonic tissues.


See: mitral cell

Türk irritation cell

See: Türk irritation cell

Tzanck cell

See: Tzanck cell

undifferentiated cell

A cell resembling an embryonic cell in that it does not have the specific morphologic or functional characteristics of any particular adult cell type.

unipolar cell

A cell with a single cell process.

Vero cell

A lineage of cells used in cell cultures and isolated from kidney epithelial cells of the African green monkey (Cercopithecus aethiops).

visual cell

A rod cell or cone cell of the retina.

wandering cell

A rarely used term for a cell (such as a macrophage) that moves like an ameba.

white cell


white blood cell

Abbreviation: WBC


(mus'el ) [L. musculus, diminutive of mus, mouse]
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A type of tissue composed of contractile cells. Each muscle cell is filled with parallel actin and myosin filaments. When activated by an internal release of calcium, the filaments use the energy in ATP to crawl along each other in opposite directions. This movement shortens the length of the cell, which then contracts.

The three general classes of muscle cells (myocytes) are skeletal (striated), cardiac (striated), and smooth; most of the muscle in humans is skeletal. A typical muscle has a central portion called the belly and two or more attachment ends with tendons; the more stationary of the attachments is called the muscle's origin, while the more movable attachment is called the muscle's insertion. See: illustration

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abdominal muscles

The abdominal muscles are made up of the cremaster, external abdominal oblique, iliacus, psoas major, pyramidalis, quadratus lumborum, rectus abdominis, and transversus abdominis muscles.

abducens muscle

, abducens oculi
Lateral rectus muscle, one of the extraocular muscles. Nerve: cranial nerve (CN VI). In clinical practice, referred to as the lateral rectus muscle.

abductor muscle

A muscle that on contraction draws a part away from the median plane of the body or the axial line of an extremity.
See: adductor muscle

abductor digiti minimi muscle

Hand muscle. Origin: pisiform bone of wrist. Insertion: base of proximal phalanx of digit 5. Nerve: ulnar (C8-T1). Action: abducts digit 5.

abductor pollicis brevis muscle

Hand muscle. Origin: flexor retinaculum of wrist, scaphoid and trapezium bones. Insertion: lateral base of proximal phalanx of thumb. Nerve: median (C8-T1). Action: abducts thumb, aides in opposition with digit 5.
See: armfor illus. (Muscles of the Arm)

adductor muscle

A muscle that draws toward the midline.
See: abductor muscle

adductor brevis muscle

A muscle of the medial thigh originating on the ramus of the pubis and inserted in the linea aspera of the femur. It adducts, flexes, and medially rotates the thigh and is controlled by the obturator nerve.

adductor longus muscle

Hip and thigh muscle. Origin: front of pubis (below crest). Insertion: linea aspera of femur. Nerve: obturator (L2-L4). Action: adducts, flexes, and rotates thigh medially.
See: leg for illus. (Muscles of the leg)

adductor magnus muscle

Hip and thigh muscle. Origin: inferior ramus of pubis, ramus of ischium, ischial tuberosity. Insertion: linea aspera and adductor tubercle of femur. Nerve: obturator and sciatic (L2-L4). Action: adducts, flexes, and rotates thigh medially.
See: leg for illus. (Muscles of the leg)

adductor pollicis muscle

Hand muscle. Origin: capitate bone of wrist and metacarpals 2-3. Insertion: proximal phalanx of thumb and medial sesamoid bone. Nerve: ulnar (C8-T1. Action: adducts thumb, aides in opposition with digit 5.

agonist muscle

Controlled movements involve two opposing muscles: the agonist muscle produces the main action, while the antagonist muscle produces the opposite action to a lesser degree. The balance between agonist and antagonist muscles allows precise control of the final action.
Synonym: antagonist muscle See: PNF Stretching Techniques

anconeus muscle

A short muscle along the back of and outside the elbow. It originates from the lateral epicondyle of the humerus, crosses the back of the elbow joint on the same side, attaches to the lateral surface of the olecranon process and the adjacent surface of the ulna. It extends the forearm and abducts the elbow as the forearm pronates. It is innervated by the radial nerve (C7, C8, T1).

antagonist muscle

Agonist muscle.

antigravity muscles

Muscles that pull against gravity to maintain normal posture.
Synonym: postural muscles

appendicular muscle

One of the skeletal muscles of the limbs.

arrector pili muscle

Arrector pili.

arm muscle

Arm: biceps brachii, brachialis, coracobrachialis, and triceps muscles. Forearm, anterior: flexor carpi radialis, flexor carpi ulnaris, flexor digitorum profundus, flexor digitorum superficialis, flexor pollicis longus, and pronator quadratus muscles. Forearm, posterior: abductor pollicis longus, anconeus, brachioradialis, extensor carpi radialis brevis, extensor carpi radialis longus, extensor carpi ulnaris, extensor digitorum, extensor digitorum minimi, extensor indicis, extensor pollicis brevis, extensor pollicis longus, and supinator muscles.
See: arm for illus. (Muscles of the Arm)

articular muscle

A muscle attached to the capsule of a joint.

arytenoid muscle

The oblique or the transverse arytenoid -- laryngeal muscles. Origins: arytenoid cartilage. Insertions: contralateral arytenoid cartilage. Nerve: recurrent laryngeal and superior laryngeal of the vagus (CN X). Action: closes laryngeal inlet by bringing arytenoid cartilages toward each other.

auditory muscles

The tensor tympani and stapedius muscles.

axial muscle

A skeletal muscle that moves or stabilizes the head or the trunk.

back muscle

Superficial: latissimus dorsi and trapezius muscles. Middle layer: levator scapulae, rhomboid major, and rhomboid minor muscles. Deep layer: erector spinae and splenius. Deepest layer: interspinalis, intertransverse, multifidus, rotatores, semispinalis, and spinalis capitis.

biceps brachii muscle

Arm muscle. Origin: supraglenoid tubercle, coracoid process of scapula. Insertion: tuberosity of radius, posterior border of ulna (via bicipital aponeurosis). Nerve: musculocutaneous (C5-C6). Action: flexes forearm, supinates hand.
See: arm for illus. (Muscles of the Arm)

biceps femoris muscle

Leg muscle. Origin: ischial tuberosity, linea aspera and second supracondylar ridge of femur. Insertion: lateral condyle of tibia, head of fibula. Nerve: sciatic (L5-S2). Action: flexes leg, rotates leg laterally, extends thigh.
See: leg for illus. (Muscles of the leg)

bipennate muscle

A muscle in which the fibers converge from both sides to a central tendon.

brachialis muscle

Arm muscle. Origin: anterior surface of lower (distal) humerus. Insertion: coronoid process of ulna. Nerve: musculocutaneous and radial (C5-C7). Action: flexes forearm.
See: arm for illus. (Muscles of the Arm)

brachioradialis muscle

Arm muscle. Origin: lateral supracondylar ridge of distal humerus. Insertion: distal end of radius. Nerve: radial (C5-C7). Action: flexes forearm.
See: arm for illus. (Muscles of the Arm)

buccinator muscle

Facial muscle. Origin: pterygomandibular raphe and alveolar processes of jaws. Insertion: orbicularis oris muscle at angle of mouth. Nerve: facial (CN VII). Action: compresses check against teeth, retracts angle of mouth.

cardiac muscle

A tissue composed of mitochondrion-filled muscle cells that also contain neatly packed actin and myosin filaments; the filaments are arranged in cylindrical bundles called myofibrils. In each cell, the myofibrils are all aligned in the same direction and are parceled into longitudinal blocks (called sarcomeres) of similar lengths. Under the microscope, the ends of the blocks appear as lines, making cardiac muscle cells appear to have regularly arranged striations. In the muscle tissue, the cardiac muscle cells are connected in branching networks.

Cardiac muscle is innervated by both sympathetic and parasympathetic autonomic motor axons. In addition, cardiac muscle: is stimulated by blood—borne molecules, can conduct electrical impulses from cell to cell, and can independently generate rhythmical contractions. Cardiac muscle, which is found only in the heart, cannot be controlled consciously.

See: table

chest wall muscle

Pectoralis major, pectoralis minor, serratus anterior, subclavius, subscapularis, or teres major muscle.

chewing muscle

Mastication muscle.

ciliary muscle

Internal eye muscle. Origin: edges of sclera. Insertion: ciliary process of lens. Nerve: oculomotor (CN III). Action: allows lens to become more curved to focus on near objects.

constrictor muscle of pharynx

A muscle that constricts the pharynx; it is important for swallowing.

core muscle

One of the major muscles that stabilizes and controls the pressure inside the trunk; these are the pelvic floor, abdominal wall, back, and diaphragm muscles.

corrugator muscle

Facial muscle. Origin: medial part of supraorbital margin. Insertion: skin above middle of eyebrow. Nerve: facial (CN VII). Action: pulls eyebrows toward midline and downward.
Synonym: Corrugator supercilii

cremaster muscle

Spermatic cord muscle. Origin: inguinal ligament and pubic tubercle. Insertion: cremasteric fascia covering spermatic cord. Nerve: genitofemoral (L1-L2). Action: elevates testis in males.
See: penis for illus.

cricoarytenoid muscle

The lateral or the posterior cricoarytenoid -- laryngeal muscles. Origin: cricoid cartilage. Insertion: muscular process of arytenoid cartilage. Nerve: recurrent laryngeal of the vagus (CN X). Action: rotates arytenoid cartilages for vocalizations.

cricothyroid muscle

Laryngeal muscle. Origin: cricoid cartilage. Insertion: lower edges of thyroid cartilage. Nerve: superior laryngeal of the vagus (CN X). Action: tenses (stretches) vocal cords
See: thyroidfor illus.

deep neck muscle

One of the various neck muscles that surround the vertebral column and base of the skull and which are contained in the prevertebral cylinder of deep cervical fascia. All these muscles are innervated by cervical spinal nerves, and most of these muscles act primarily to move and stabilize the head.

deltoid muscle

Shoulder muscle. Origin: a bony ellipse from the lateral third of the clavicle over the acromial process and along the spine of the scapula. Insertion: deltoid tuberosity on the lateral shaft of the humerus. Nerve: axillary (C5-C6). Action: abducts arm.
See: arm for illus. (Muscles of the Arm)

detrusor muscle

The three-layered muscular wall of the urinary bladder. Nerve: primarily parasympathetic (S2-S4), secondarily sympathetic (T11-L2). Action: empties bladder.

diaphragm muscle

Origin: internal surfaces of lower six ribs, xiphoid process, vertebral bodies L1-L3. Insertion: central tendon (of diaphragm). Nerve: phrenic, lower six intercostals. Action: inflates lungs

digastric muscle

Neck muscle with two bellies. Origin: anterior belly attaches to the digastric fossa in mandible at base of anterior midline, posterior belly attaches to mastoid process. Insertion: tendon connecting both bellies in a loop of fascia that is attached to hyoid bone. Nerve: anterior belly -- trigeminal (CN V), posterior belly -- facial (CN VII). Action: lowers mandible and raises hyoid bone.
See: neck for illus.

erector spinae muscle

Three adjacent vertical bands of deep back muscles -- the iliocostalis, longissimus, and spinalis muscles. Origins: a wide tendon running along the iliac crest to the sacrum, the lower lumbar and sacral spinous processes. Insertions: along the back in the angles of the lower ribs, transverse processes of the thoracic and cervical vertebrae. Nerves: dorsal rami of the spinal nerves. Actions: extends (bends backward) the vertebral column and neck, twists the back.

extensor carpi ulnaris muscle

Forearm muscle. Origin: lateral epicondyle of humerus, proximal edge of ulna. Insertion: proximal end of fifth metacarpal. Nerve: radial (C7-C8). Action: adducts hand, extends wrist.
See: arm for illus. (Muscles of the Arm)

extensor digitorum muscle

Forearm muscle. Origin: lateral epicondyle of humerus. Insertion: common extensor tendon of fingers. Nerve: radial (C7-C8). Action: extends fingers and wrist.
See: arm for illus. (Muscles of the Arm)

extensor digitorum brevis muscle

Foot muscle. Origin: dorsolateral surface of calcaneus. Insertion: extensor tendons of toes. Nerve: deep peroneal (S1-S2). Action: extends toes.
See: leg for illus. (Muscles of the leg)

extensor digitorum longus muscle

Foot muscle. Origin: lateral condyle of tibia, upper three-fourths of fibula. Insertion: extensor tendons of toes 2-5. Nerve: deep peroneal (L5-S1). Action: extends toes, dorsiflexes foot.
See: leg for illus. (Muscles of the leg)

extensor hallucis longus muscle

Foot muscle. Origin: middle of fibula. Insertion: base of proximal phalanx of big toe. Nerve: deep peroneal (S1-S2). Action: dorsiflexes big toe.
See: leg for illus. (Muscles of the leg)

external intercostal muscles

The outer layer of muscles between the ribs, originating on the lower margin of each rib and inserted on the upper margin of the next rib. During inspiration, they draw adjacent ribs together, pulling them upward and outward, and increasing the volume of the chest cavity. They are controlled by the intercostal nerves.

external oblique muscle

Abdominal wall muscle. Origin: lower costal margin. Insertion: anterior half of iliac crest, rectus sheath, inguinal ligament. Nerve: intercostals 8-12, iliohypogastric, ilioinguinal (L1). Action: tenses and compresses abdomen, flexes and laterally rotates spine, lowers rib cage.

external pterygoid muscle

Lateral pterygoid muscle.

extraocular muscle

Abbreviation: EOM
Six muscles that attach outside the eyeball and that move the eye in its socket. The EOM are: the inferior and superior oblique muscles, and the lateral, medial, inferior, and superior rectus muscles.
See: extraocular for illus.

extrinsic muscle

Abbreviation: EM
The muscles outside an organ that control its position, such as the EM of the eye or tongue.

muscles of facial expression

Thin muscles that insert into the skin of the face; all are innervated by the facial nerve (CN VII). Scalp: frontalis and occipitalis muscles. Ear: anterior, posterior, and superior auricular muscles. Eye: orbicularis oculi. Nose: depressor septi, nasalis, and procerus muscles. Mouth: buccinator, depressor anguli oris, depressor labii inferioris, levator anguli oris, levator labii superioris, mentalis, orbicularis oris, risorius, and zygomaticus muscle. Neck: platysma.
See: face and headfor illus.

muscles of facial expression

Facial muscles.

fibularis muscles

The newer name for the peroneus muscles.

fibularis longus muscle

Peroneus longus muscle.

fixation muscle

A muscle that steadies a part so that more precise movements in a related structure may be accomplished.

flexor carpi radialis muscle

Forearm muscle. Origin: medial epicondyle of humerus. Insertion: bases of second and third metacarpals. Nerve: median (C6-C7). Action: abducts hand, flexes wrist.
See: Arm, muscles of the arm (illus.)

flexor carpi ulnaris muscle

Forearm muscle. Origin: medial epicondyle of humerus, medial side of olecranon, proximal posterior edge of ulna. Insertion: pisiform, hamate, and base of fifth metacarpal. Nerve: ulnar (C7-C8). Action: adducts hand, flexes wrist.

flexor digitorum longus muscle

Foot muscle. Origin: posterior surface of middle tibia. Insertion: distal phalanges of toes 2-5. Nerve: tibial (S2-S3). Action: flexes toes 2-5, plantarflexes foot.
See: leg for illus. (Muscles of the leg)

flexor digitorum profundus muscle

Forearm muscle. Origin: proximal three-fourths of ulna. Insertion: distal phalanges of fingers (digits 2-5). Nerve: ulnar, median (C8-T1). Action: flexes distal finger joints, aids in wrist flexion.

flexor digitorum superficialis muscle

Forearm muscle. Origin: medial epicondyle of humerus, coronoid process of ulna. Insertion: middle phalanges of fingers (digits 2-5). Nerve: median (C7-T1). Action: flexes fingers and wrist.
See: arm for illus. (Muscles of the Arm)

flexor hallucis longus muscle

Foot muscle. Origin: distal two-thirds of posterior tibia. Insertion: plantar side of distal phalanx of big toe. Nerve: tibial (S2-S3). Action: flexes big toe, plantarflexes foot.

flexor pollicis brevis muscle

A muscle of the hand originating on the flexor retinaculum and trapezium, trapezoid, and capitate and inserted on the lateral side of the base of the first phalanx of the thumb. It flexes the thumb at both the carpometacarpal joint and the metacarpophalangeal joint and is controlled by the median and the ulnar nerves.

flexor pollicis longus muscle

Forearm muscle. Origin: coronoid process of ulna, anterior surface of radius. Insertion: distal phalanx of thumb. Nerve: median (C8-T1). Action: flexes thumb.
See: arm for illus. (Muscles of the Arm)

foot muscles

Dorsal: dorsal interosseous, extensor digitorum brevis, extensor digitorum longus, extensor hallucis longus, and tibialis anterior muscles. Plantar: abductor digiti minimi, abductor hallucis, adductor hallucis, flexor digitorum brevis, flexor digiti minimi brevis, flexor hallucis brevis, lumbrical, plantar interosseous, and quadratus plantae muscles.
See: leg for illus. (Muscles of the leg)

frontalis muscle

Front half of occipitofrontalis muscle – a facial muscle. Origin: epicranial (scalp) aponeurosis. Insertion: skin of eyebrows, root of nose. Nerve: facial (CN VII). Action: elevates eyebrows, wrinkles forehead.
See: face and head for illus.

fusiform muscle

A muscle resembling a spindle. See: bipennate muscle for illus.

gastrocnemius muscle

Leg muscle. Origin: medial condyle of femur, lateral condyle of femur. Insertion: calcaneus (via Achilles tendon). Nerve: tibial (S1-S2). Action: plantarflexes foot, flexes knee.
See: leg for illus. (Muscles of the leg)

gemellus muscle

Either of the two muscles that attach to the medial surface of the greater trochanter of the femur (the trochanteric fossa) where they mesh with the tendon of the obturator internus muscle. The superior gemellus muscle arises from the ischial spine and is innervated by the nerve to the obturator internus; the inferior arises from the ischial tuberosity and is innervated by the femoral nerve. Both muscles hold the head of the femur in the acetabulum, rotate (laterally) the thigh in extension, and abduct the thigh when it is flexed.

genioglossus muscle

Tongue muscle. Origin: genial tubercle on inside of mandibular symphysis. Insertion: ventral tongue, hyoid bone. Nerve: hypoglossal (CN XII). Action: protrudes and depresses tongue.

gluteus maximus muscle

Thigh muscle. Origin: upper outer edge of ilium and sacrum. Insertion: iliotibial tract of fascia lata, gluteal tuberosity of femur. Nerve: inferior gluteal (L5-S2). Action: extends, abducts, and laterally rotates thigh.

gluteus medius muscle

Thigh muscle. Origin: lower half of ilium. Insertion: proximal medial tibia. Nerve: obturator (L2-L3). Action: adducts, flexes, and medially rotates thigh.

gracilis muscle

Thigh muscle. Origin: lower half of pubis. Insertion: proximal medial tibia. Nerve: obturator (L2-L3). Action: adducts, flexes, and medially rotates thigh.
See: leg for illus. (Muscles of the leg)

hamstring muscles

Posterior thigh muscles that originate on the ischial tuberosity and act across both the hip and knee joints; they are the biceps femoris, gracilis, sartorius, semitendinosus, and semimembranosus muscles.

hand muscles

Abductor digiti minimi, abductor pollicis brevis, adductor pollicis, dorsal interosseous, flexor digiti minimi, flexor pollicis brevis, lumbrical, opponens digiti minimi, opponens pollicis, palmaris brevis, and palmar interosseous muscles.

Hilton muscle

See: Hilton, John

hyoglossus muscle

A sheet of muscle extending up from the hyoid bone to the ipsilateral base and sides of the tongue. It depresses the sides of the tongue and is innervated by cranial nerve XII (hypoglossal nerve).

iliacus muscle

Thigh muscle. Origin: iliac fossa. Insertion: lesser trochanter of femur, psoas major tendon. Nerve: femoral (L2-L3). Action: flexes thigh.

iliopsoas muscle

The iliacus and psoas major muscles considered together.
See: leg for illus. (Muscles of the leg)

inferior oblique muscle

Extraocular muscle. Origin: inside front lower margin of maxillary part of orbit. Insertion: lateral surface of eyeball behind its equator. Nerve: oculomotor (CN III). Action: turns eye up and outward with lateral rotation.
See: extraocular for illus.

inferior rectus muscle

Extraocular muscle. Origin: tendinous ring around optic nerve at rear of orbit. Insertion: lower edge of eyeball in front of its equator. Nerve: oculomotor (CN III). Action: turns eye down and medially.
See: extraocular for illus.

infraspinatus muscle

Shoulder muscle. Origin: medial two-thirds of infraspinatus fossa of scapula. Insertion: posterior side of greater tubercle of humerus. Nerve: suprascapular (C4-C6). Action: rotates arm laterally.

internal intercostal muscles

The muscles between the ribs, lying beneath the external intercostals. During expiration, they pull the ribs downward and inward, decreasing the volume of the chest cavity and contributing to a forced exhalation.

internal pterygoid muscle

Medial pterygoid muscle.

intrinsic muscle

A muscle that has both its origin and insertion within a structure, as intrinsic muscles of the tongue, eye, hand, or foot.

involuntary muscle

A muscle not under conscious control: smooth, cardiac, and some skeletal muscles.

laryngeal muscle

Any of six short muscles inside the larynx that move the vocal apparatus and (except for the cricothyroid muscle) are innervated by the recurrent laryngeal branch of the vagus nerve (CN X).

lateral pterygoid muscle

One of the mastication muscles. Origin: greater wing of sphenoid bone, lateral pterygoid plate. Insertion: pterygoid fovea of condyle of mandible. Nerve: trigeminal (CN V). Action: opens mouth, protrudes mandible.
Synonym: external pterygoid muscle See: arm for illus.

lateral rectus muscle

Extraocular muscle. Origin: tendinous ring around optic nerve at rear of orbit. Insertion: temporal edge of eyeball in front of its equator. Nerve: abducens (CN VI). Action: turns eye laterally.
See: extraocular for illus.

latissimus dorsi muscle

Back muscle. Origin: spinous processes of vertebrae T7-S3, thoracolumbar fascia, iliac crest. Insertion: bicipital groove of humerus. Nerve: thoracodorsal (C6-C8). Action: adducts, extends, and medially rotates arm.

leg muscles

Anterior and lateral: extensor digitorum longus, extensor hallucis longus, peroneus, peroneus longus, peroneus tertius, and tibialis anterior muscles. Posterior: flexor digitorum longus, flexor hallucis longus, gastrocnemius, plantaris, popliteus, soleus, and tibialis posterior muscles.
See: leg for illus. (Muscles of the leg)

levator ani muscle

The set of pelvic floor muscles, which include the iliococcygeus, levator prostatae or vaginal sphincter, pubococcygeus, and puborectalis muscles. Origins: insides of pelvic bones (pubis, arcus tendinaeus, ischial spine, and sacrospinous ligament). Insertions: perineal body, coccyx, anococcygeal ligament, lower sacrum. Nerve: perineal of spinal S4, pudendal. Action: supports pelvic viscera, contributes to urethral, vaginal, and anal sphincter actions.

levator palpebrae muscle

Eyelid muscle. Origin: inner roof of orbit. Insertion: skin and tarsal plate of upper eyelid. Nerve: oculomotor (CN III). Action: raises upper eyelid.
See: extraocular for illus.

lumbrical muscle

Hand and foot muscles. Origins: tendons of flexor digitorum profundus or flexor digitorum longus. Insertions: extensor tendons of digits 2-5. Nerve, hand: median (C8-T1), ulnar (C8-T1). Nerve, foot: medial plantar (S2-S3), lateral plantar (S2-S3). Action: flex the straightened digits (specifically, flex the metacarpophalangeal or metatarsophalangeal joints while extending the interphalangeal joints).

masseter muscle

Muscle of mastication. Origin: zygomatic process of maxilla, zygomatic arch. Insertion: coronoid process, lower half of ramus, and angle of mandible. Nerve: trigeminal (CN V). Action: elevates mandible to close jaw.
See: headfor illus.

mastication muscle

The chewing muscle, which is innervated by the mandibular division of the trigeminal nerve (CN V). These muscles include the masseter, temporalis, and medial and lateral pterygoid muscles. Synonym: chewing muscle

medial pterygoid muscle

Muscle of mastication. Origin: lateral pterygoid plate. Insertion: medial surface of ramus and angle of mandible. Nerve: trigeminal (CN V). Action: closes mouth, protrudes mouth, moves jaw sideways.
Synonym: internal pterygoid muscle

medial rectus muscle

Extraocular muscle. Origin: tendinous ring around optic nerve at rear of orbit. Insertion: nasal edge of eyeball in front of its equator. Nerve: oculomotor (CN III). Action: turns eye medially.

mentalis muscle

Facial muscle. Origin: incisive fossa at front of mandible. Insertion: skin of chin. Nerve: facial (CN VII). Action: raises and protrudes lower lip.
See: face and headfor illus.

mimetic muscles

Facial muscles. Synonym: muscles of facial expression

multipennate muscle

A muscle with several tendons of origin and several tendons of insertion, in which fibers pass obliquely from a tendon of origin to a tendon of insertion on each side. See: bipennate muscle for illus.

mylohyoid muscle

Neck muscle. Origin: mylohyoid line of mandible. Insertion: hyoid bone, mylohyoid raphe. Nerve: trigeminal (CN V). Action: elevates hyoid and larynx, lowers jaw.

nasalis muscle.

The major nose muscle and a muscle of facial expression.

neck muscles

Anterior and lateral: digastric, geniohyoid, mylohyoid, omohyoid, platysma, sternocleidomastoid, sternohyoid, sternothyroid, stylohyoid, and thyrohyoid muscles. Posterior: levator scapulae, scalene muscles, and trapezius. Suboccipital: obliquus capitis and rectus capitis muscles.
See: headfor illus.

nonstriated muscle

Smooth muscle.

obturator muscle

Either of the two muscles on each side of the pelvic region that rotate the thighs outward.

opponens pollicis muscle

A muscle of the hand originating on the trapezium and flexor retinaculum and inserted in the first metacarpal. It flexes and adducts the thumb (brings it across the palm) and is controlled by the median nerve.

orbicular muscle

A muscle encircling an opening.

orbicularis oculi muscle

Facial muscle. Origin: completely surrounds eye, attaches to medial palpebral ligament (and adjacent bones) and lacrimal crest (and adjacent bones). Insertion: medial palpebral raphe (after encircling orbit), lateral palpebral raphe, tarsi of eyelids. Nerve: facial (CN VII) Action: closes eyelids, lifts cheeks, compresses lacrimal sac.
See: face and headfor illus.

orbicularis oris muscle

Facial muscle. Origin: adjacent facial muscles that surround mouth. Insertion: into itself and skin of lips while encircling mouth. Nerve: facial (CN VII). Action: closes and purses lips.
See: face and headfor illus.

muscles of the palate

Levator veli palatini, musculus uvulae, palatoglossus, palatopharyngeus, pharyngeal constrictor, salpingopharyngeus, and tensor veli palatine muscles.

palmaris longus muscle

Forearm muscle. Origin: medial epicondyle of humerus. Insertion: palmar surface of flexor retinaculum, palmar aponeurosis. Nerve: median (C7-C8). Action: flexes hand.
See: arm for illus. (Muscles of the Arm)

papillary muscle

Internal conical heart muscles. Origin: ventricular wall. Insertion: tricuspid and mitral valve leaflets via chordae tendinae. Action: anchor leaflets of valves during heart contractions.

pectinate muscle

A ridge of myocardium on the inner wall of either atrium of the heart.

pectoralis major muscle

Chest wall muscle. Origin: medial half of clavicle, sternum, costal cartilages 4-6. Insertion: lateral edge of bicipital groove of humerus. Nerve: lateral and medial pectoral (C5-T1). Action: adducts and medially rotates arm.

pectoralis minor muscle

Chest wall muscle. Origin: Anterior medial surface of ribs 3-5. Insertion: coracoid process of scapula. Nerve: lateral and medial pectoral (C6-C8). Action: pulls shoulder forward and down, elevates rib cage.

peroneus longus muscle

Leg muscle. Origin: lateral two-thirds of fibula. Insertion: medial cuneiform bone, base of first metatarsal. Nerve: superficial peroneal (L5-S1). Action: everts and plantar flexes foot.
Synonym: fibularis longus muscle See: leg for illus. (Muscles of the leg)

pharynx and tongue muscles

Cricothyroid, genioglossus, geniohyoid, hyoglossus, palatoglossus, pharyngeal constrictor, styloglossus, stylopharyngeus, salpingopharyngeus, and thyrohyoid muscles.

piriformis muscle

Thigh muscle. Origin: anterior surface of sacrum. Insertion: upper part of greater trochanter of femur. Nerve: spinal L5-S2. Action: laterally rotates thigh.

platysma muscle

Neck and facial muscle. Origin: superficial fascia of upper chest. Insertion: skin of lower face. Nerve: facial (CN VII). Action: lowers jaw, widens neck.
See: face and headfor illus.

postaxial muscle

A muscle on the posterior or dorsal aspect of a limb.

postural muscles

Antigravity muscles.

preaxial muscle

A muscle on the anterior or ventral aspect of a limb.

procerus muscle

A muscle that arises in the skin over the nose and is connected to the forehead. It acts to draw the eyebrows down.

pronator teres muscle

Arm muscle. Origin: medial epicondyle of humerus, coronoid process of ulna. Insertion: lateral side of middle of radius. Nerve: median (C6-C7). Action: pronates forearm.

psoas major muscle

Thigh muscle. Origin: bodies of vertebrae T12-L1. Insertion: lesser trochanter of femur. Nerve: lumbar L1-L3. Action: flexes thigh.

pterygoid muscle

The lateral or the medial pterygoid muscle.

puborectalis muscle

Pelvic muscle, part of levator ani. Origin: back surface of pubis. Insertion: joins other levator ani muscles forming a bowl shaped diaphragm, encircles anal canal, and attaches to sacrum and coccyx. Nerve: inferior rectal and sacral (S4). Action: supports pelvis, holds anal canal at right angle to rectum.

quadriceps muscle

The rectus femoris, vastus intermedius, vastus lateralis, and vastus medius muscles together.

rectus abdominis muscle

Abdominal wall muscle. Origin: crest and symphysis of pubis. Insertion: xiphoid process, costal cartilages 5-7. Nerve: spinal T7-T12. Action: tenses abdomen, flexes vertebral column.

rectus femoris muscle

Thigh muscle. Origin: anterior inferior iliac spine, upper edge of acetabulum. Insertion: tibial tuberosity (via the patellar ligament). Nerve: femoral (L2-L4). Action: extends leg, flexes thigh.
See: leg for illus. (Muscles of the leg)

red muscle

Twitch skeletal muscle cells containing myoglobin and many mitochondria. These cells largely generate energy via aerobic oxidation and are suited for maintaining contractions for an extended time.

muscle of respiration

Any of the muscles used in breathing, including the diaphragm, the muscles of the rib cage, and the abdominal muscles. See: diaphragm; expiration; inspiration

rhomboid muscle

The major or the minor rhomboid muscle -- shoulder muscles. Origins: nuchal ligament, spinous processes of vertebrae C7-T5. Insertion: vertebral edge of scapula. Nerve: dorsal scapular (C4-C5). Action: pulls scapulae toward each other.
See: illus. (Muscles of the Trunk)

rotator cuff muscles

Shoulder muscles -- the infraspinatus, subscapularis, supraspinatus, and teres minor muscles -- which hold the head of the humerus in the glenoid fossa of the scapula.

sartorius muscle

Thigh muscle. Origin: anterior superior iliac spine. Insertion: medial side of proximal tibia. Nerve: femoral (L2-L3). Action: flexes thigh and leg, laterally rotates thigh.
See: leg for illus. (Muscles of the leg)

scalene muscle

The anterior, the middle, or the posterior scalene muscle -- neck muscles. Origins: transverse processes of vertebrae C1-C7. Insertions: upper surfaces of ribs 1-2. Nerves: cervical spinal C4-C8. Actions: raises ribs 1-2, bends neck ipsilaterally.

semimembranosus muscle

Thigh muscle. Origin: ischial tuberosity. Insertion: medial condyle of tibia. Nerve: sciatic (L5-S2). Action: extends thigh, flexes and medially rotates leg.
See: leg for illus. (Muscles of the leg)

semitendinosus muscle

Thigh muscle. Origin: ischial tuberosity. Insertion: upper medial tibia near tuberosity. Nerve: sciatic L5-S2). Action: extends thigh, flexes and medially rotates leg.
See: leg for illus. (Muscles of the leg)

serratus muscle

Any of several muscles arising from the ribs or vertebrae by separate slips.

serratus anterior muscle

Chest muscle. Origin: outer surface of ribs 1-8. Insertion: anterior side of vertebral edge of scapula. Nerve: long thoracic (C5-C7). Action: pulls scapula forward (anterior) and laterally (abduction), rotates scapula upward.

shoulder muscles

Deltoid, infraspinatus, subscapularis, supraspinatus, teres major and teres minor muscles.
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skeletal muscle

A tissue composed of muscle cells (often multinucleated) that contain neatly packed actin and myosin filaments; these filaments are arranged in cylindrical bundles called myofibrils. In each cell, the myofibrils are all aligned in the same direction and are parceled into longitudinal blocks (called sarcomeres) of similar lengths. Under the microscope, the ends of the blocks look like lines, making skeletal muscle cells appear to have regularly arranged striations. See: illustration

Skeletal muscle is innervated by somatic (as opposed to autonomic) motor axons at a synaptic structure called a motor endplate, where acetylcholine is the neurotransmitter. Most skeletal muscles can be controlled consciously, and skeletal muscle is sometimes referred to as voluntary muscle. Skeletal muscle cells contract more forcefully than smooth or cardiac muscle cells.

Skeletal muscle got its name because it usually attaches at one end to bone. Skeletal muscle is by far the most common type of muscle in the body and it plays a major role in normal metabolism, e.g., after a meal, excess glucose is removed from the blood stream primarily by skeletal muscle.

smooth muscle

A tissue composed of muscle cells that contain loosely-organized actin and myosin filaments. The lack of tight organization means that smooth muscle cells do not appear striated when examined under a microscope. Smooth muscle tissue tends to occur as sheets and is typically found in the walls of tubes, e.g., arteries, and sacs, e.g., the gastrointestinal system.

Smooth muscles are innervated by both sympathetic and parasympathetic autonomic motor axons; they are also stimulated by blood-borne molecules. Smooth muscles cannot be consciously controlled, and this form of muscle tissue is called involuntary muscle. Smooth muscle cells contract more slowly than skeletal or cardiac muscle cells.

Synonym: nonstriated muscle.; unstriated muscle. See: table


Leg muscle. Origin: proximal ends of tibia and fibula. Insertion: calcaneus via Achilles tendon. Nerve: tibial (S1-S2). Action: plantarflexes foot.
See: leg for illus. (Muscles of the leg)

somatic muscle

Muscle derived from mesodermal somites, including most skeletal muscle.

sphincter muscle

A muscle that encircles a duct, tube, or orifice, thus controlling its opening.

sphincter muscle of urinary bladder

The smooth muscle fibers around the origin of the urethra. Contraction of this muscle prevents urination; relaxation permits it.

stabilizer muscle

A muscle that supports a body segment so muscles attached to it can function.

stapedius muscle

Middle ear muscle. Origin: posterior wall of middle ear. Insertion: neck of stapes. Nerve: facial (CN VII). Action: tilts stapes, dampens excessive vibrations.

sternocleidomastoid muscle

Neck muscle. Origin: upper edge of manubrium, middle of upper clavicle. Insertion: mastoid process. Nerve: accessory (CN XI), spinal C2. Action: contralaterally rotates head.
See: face and headfor illus.

striated muscle

See: table

subscapularis muscle

Shoulder muscle. Origin: medial subscapular fossa. Insertion: lesser tubercle of humerus. Nerve: upper and lower subscapular (C5-C7). Action: medially rotates arm.

superior oblique muscle

Extraocular muscle. Origin: sphenoid bone deep in medial side of orbit. Insertion: lateral surface of eyeball behind its equator. Nerve: trochlear (CN IV). Action: turns eye down and outward with medial rotation.

superior rectus muscle

Extraocular muscle. Origin: tendinous ring around optic nerve at rear of orbit. Insertion: upper edge of eyeball in front of its equator. Nerve: oculomotor (CN III). Action: turns eye up and medially.
See: extraocular for illus.

supraspinatus muscle

Shoulder muscle. Origin: medial supraspinous fossa of scapula. Insertion: greater tubercle of humerus. Nerve: suprascapular (C4-C6). Action: abducts arm.

synergistic muscles

Muscles aiding one another in function.

temporalis muscle

Muscle of mastication. Origin: temporal fossa of skull. Insertion: coronoid process of mandible. Nerve: trigeminal (CN V). Action: closes mouth, clenches teeth, retracts jaw.
See: headfor illus.

tensor fascia lata muscle

Thigh muscle. Origin: iliac crest, anterior superior iliac spine. Insertion: iliotibial tract of fascia lata. Nerve: superior gluteal (L4-L5). Action: stabilizes (abducts) thigh, extends and laterally rotates leg.

tensor tympani muscle

Middle ear muscle. Origin: wall of auditory tube. Insertion: handle of malleus. Nerve: trigeminal (CN V). Action: tenses tympanic membrane, dampens excessive vibrations.

teres major muscle

Shoulder muscle. Origin: lower lateral edge of scapula. Insertion: bicipital groove of humerus. Nerve: lower scapular (C6-C7). Action: adducts and medially rotates arm.

teres minor muscle

Shoulder muscle. Origin: upper lateral edge of scapula. Insertion: greater tubercle of humerus. Nerve: axillary (C4-C6). Action: laterally rotates arm.

thenar muscle

The abductor or flexor muscle of the thumb.

thigh muscles

Anterior: iliopsoas, quadriceps (rectus femoris, vastus intermedius, vastus lateralis, and vastus medius), and sartorius muscles. Medial: adductor brevis, adductor longus, adductor magnus, gracilis, and pectineus muscles. Gluteal region: gemelli, gluteus maximus, gluteus medius, gluteus minimus, obturator externus, obturator, internus, piriformis, quadratus femoris, and tensor fasciae lata muscles. Posterior: biceps femoris, semimembranosus, and semitendinosus muscles.
See: leg for illus. (Muscles of the leg)

thyroepiglottic muscle

A muscle arising on the inner surface of the thyroid cartilage. It extends upward and backward and is inserted on the epiglottis. It depresses the epiglottis.

tibialis anterior muscle

Leg muscle. Origin: lateral side of proximal tibia. Insertion: medial side of cuneiform bone, base of metatarsal 1. Nerve: deep peroneal (L4-L5). Action: inverts and dorsiflexes foot.

tibialis posterior muscle

Leg muscle. Origin: anterior tibia and fibula. Insertion: navicular, cuneiform, and cuboid bones; metatarsals 2-4. Nerve: tibial (L4-L5). Action: inverts and plantarflexes foot.

tonic muscle

Skeletal muscle fibers that contract slowly and that cannot propagate an action potential along their cell membranes. Tonic muscles are uncommon in humans and are found only in the extraocular muscles, stapedius muscle, and intrafusal fibers of the muscle spindles. The remainder of human skeletal muscle contains only twitch fibers.

trapezius muscle

Neck and back muscle. Origin: occipital bone (superior nuchal line), nuchal ligament, spinous processes of vertebrae C7-T12. Insertion: posterior edge of lateral clavicle, acromion, posterior edge of spine of scapula. Nerve: accessory (CN XI), spinal C3-C4. Action: elevates, retracts, and rotates scapula.
See: face and headfor illus.

triangular muscle

A flat muscle with a broad origin and narrow insertion.

triceps muscle

Arm muscle. Origin: infraglenoid tubercle of scapula, posterior of proximal humerus, posterior of distal humerus. Insertion: olecranon process. Nerve: radial (C6-C8). Action: extends forearm.
Synonym: triceps brachii muscle See: arm for illus. (Muscles of the Arm)

triceps brachii muscle

Triceps muscle.

tricipital muscle

A muscle with three tendons of origin and a single, common insertion.

twitch muscle

Muscle fibers that can conduct axon potentials along their cell membranes. Almost all skeletal muscle in humans is twitch muscle. A very small number of muscles in humans are tonic muscles. Twitch muscles cells can be categorized into a number of types on the basis of the biochemical cycle that they use to produce their energy: red (oxidative), white (glycolytic), or intermediate (oxidative/glycolytic). Most human muscles are composed of a mix of twitch muscle cell types.

unipennate muscle

A muscle whose fibers converge on only one side of a tendon. See: bipennate muscle for illus.

unstriated muscle

Smooth muscle.

uterine muscle

See: myometrium

vastus intermedius muscle

Thigh muscle. Origin: anterior and lateral sides of proximal femur. Insertion: common tendon of quadratus muscles, tibial tuberosity via patellar ligament. Nerve: femoral (L2-L4). Action: extends leg.

vastus lateralis muscle

Thigh muscle. Origin: lateral side of proximal femur. Insertion: common tendon of quadratus muscles, tibial tuberosity via patellar ligament. Nerve: femoral (L2-L4). Action: extends leg.
See: leg for illus. (Muscles of the leg)

vastus medialis muscle

Thigh muscle. Origin: medial side of femur Insertion: common tendon of quadratus muscles, tibial tuberosity via patellar ligament. Nerve: femoral (L2-L4). Action: extends leg.

vocalis muscle

Laryngeal muscle. Origin: midline of inner surface of thyroid cartilage. Insertion: arytenoid cartilage. Nerve: recurrent laryngeal of vagus (CN X). Action: changes tension of vocal cords.

voluntary muscle

A muscle that can be controlled voluntarily; most skeletal muscles are voluntary.
Length (in/m)50–20025,000
Thickness (in/m) 4–8 75
MarkingsNo striationStriationMarked striation
Effects of cutting related nerveSlightRegulation of heart rate is lostComplete paralysis

striated muscle

Voluntary, skeletal muscle and heart (cardiac) muscle, characterized by microscopic transverse stripes or striation. Compare SMOOTH MUSCLE.

Striated muscle

Also known as striped muscle; it includes muscles of the skeletal system and of the heart.
Mentioned in: Trichinosis
Figure 1: Muscles of the back.
Figure 2: Muscles of the abdominal wall. Superficial layer shown on the right side of the body, deeper layer on the left.
Figure 3: Muscles of the shoulder girdle and upper limb.
Figure 4: Muscles of the shoulder girdle and upper limb.
Figure 5: Muscles of the lower limb. Left leg from the front.
Figure 6: Muscles of the lower limb. Left leg from the back.
Figure 7: Structure of skeletal muscle at progressively higher magnification, from whole muscle to contractile proteins (A-D, F). E represents the 'sliding filaments' diagrammatically.


contractile soft tissue, responsible for all significant active movements and force-generations in an animal body. Divisible into three classes: (1) skeletal or voluntary muscle the class of muscle acting, in almost all body locations, to move one bone relative to another, the more superficial skeletal muscles being visible under the skin in all but the most obese subjects; (2) cardiac muscle the type unique to the heart; (3) smooth muscle composing the actively adjustable components of the walls of blood vessels and of the gastrointestinal, respiratory, urinary and reproductive tracts. Skeletal and cardiac are the striated muscles; cardiac and smooth share the property of being involuntary. See also muscle fibres, muscle fibre types, myofibrils; Figure 1, Figure 2, Figure 3, Figure 4, Figure 5, Figure 6, Figure 7.
Figure 1: The nervous system.


longitudinally oriented cytoplasmic components of skeletal and cardiac muscle fibres, which are the loci of force-generation when the muscle is activated. Each myofibril extends the whole length of the fibre though it is typically only ∼1 μm diameter; thus a fibre of 100 μm diameter has several thousand fibrils in its cross-section. In turn, each fibril is composed of numerous parallel filaments of myofibrillar proteins, chiefly myosin in thick filaments and chiefly actin in thin filaments, alternating and partially overlapping along the length of each myofibril, and so giving rise to the cross-banding pattern of the lengthwise repeating sarcomeres; these are aligned side by side across the fibre, giving it (under appropriate histological stains or optical imaging techniques) the striated appearance characteristic of these two classes of muscle. Interaction between the two proteins, myosin and actin, at the cross-bridges results in the generation of active force during muscle contraction. See also sarcoplasmic reticulum; Figure 1.

striated muscle

voluntary/skeletal muscle tissue (see muscle, striated)

stri·at·ed mus·cle

(strī'āt-ĕd mŭs'ĕl)
Skeletal or voluntary muscle in which cross-striations occur in the fibers as a result of regular overlapping of thick and thin myofilaments.


an organ composed of bundles of fibers that has the power to contract and hence to produce movement. Muscles are responsible for locomotion and help support the body, generate heat and perform a number of other functions. They are of two varieties: striated (or striped, voluntary or skeletal), which makes up most of the meat of a carcass, and smooth (unstriated), which includes all the involuntary muscle of the viscera, heart and blood vessels.
Skeletal muscle fibers range in length from a few millimeters to many centimeters. They also vary in color from white to deep red. Each muscle fiber receives its own nerve impulses, which trigger fine and varied motions. At the signal of an impulse traveling down the nerve, the muscle fiber changes chemical energy into mechanical energy, and the result is muscle contraction. At least two major types of muscle fiber have been identified by histochemical techniques: type I (red) fibers, which have a slow contraction; and type II (white) fibers, which have a fast contraction.
Some muscles are attached to bones by tendons. Others are attached to other muscles, and to skin, producing, for example, the skin twitch, the eye blink and hair erection. Parts of the walls of hollow internal organs, such as the heart, stomach and intestines and also blood vessels, are composed of muscles. See also muscular. For a complete list of named muscles see Table 13.

agonistic muscle
prime mover; a muscle opposed in action by another muscle, called the antagonist.
antagonistic muscle
one that counteracts the action of another muscle (the agonist).
appendicular muscle
one of the muscles of a limb.
arrector pili muscle
small, smooth muscle attached to the bulb of the hair which causes erection of the hair and compression of the attending sebaceous gland when it contracts.
arterial muscle
part of the tunica media; smooth muscle fibers arranged in a circular pattern around the lumen.
articular muscle
one that has one end attached to the capsule of a joint.
axial muscle
1. muscles derived from the somites in the embryo.
2. the muscles around the vertebral column.
muscle biopsy
sample of living muscle obtained by excision or punch.
cardiac muscle
striated involuntary muscle with branched fibers and containing modified fibers which act as cardiac conducting cells.
congenital muscle defects
may be environmental, e.g. nutritional muscular dystrophy, or inherited, e.g. splayleg of piglets.
congenital type II muscle fiber hypertrophy
occurs in the hip joint musculature in German shepherd dogs but there is no detectable abnormality of gait.
cutaneous muscle
striated muscle that inserts into the skin.
double muscle
see myofiber hyperplasia.
esophageal muscle
the tunica muscularis of the esophagus in most domestic animals is mostly striated; in pigs, horses and cats there are small segments of smooth muscle; in birds the entire tunic is smooth muscle.
extraocular m's
the six or seven voluntary muscles that move the eyeball: dorsal, ventral, medial and lateral recti, dorsal and ventral oblique, and retractor bulbi muscles.
extrinsic muscle
one that originates in another part than that of its insertion, e.g. those originating outside the eye, which move the eyeball.
fast-twitch skeletal muscle
two of the three types of skeletal muscle are pale in color and fast-twitch—type IIa (fast-twitch oxidative-glycolytic), type IIb (fast-twitch glycolytic). Type IIa fibers are fatigue-resistant, type IIb fatigue easily.
muscle fiber
see muscle (above).
fixation m's, fixator m's
accessory muscles that serve to steady a part.
hamstring m's
the biceps, semimembranosus and semitendinosus muscles. See also hamstring.
intraocular m's
the intrinsic muscles of the eyeball.
intrinsic muscle
one whose origin and insertion are both in the same part or organ, such as those entirely within the eye.
involuntary muscle
see smooth muscle (below).
iridial muscle
layers of circular (sphincter) and radial (dilator) muscles. See also iris.
jaw muscle
see Table 13.1H muscles of mastication.
laryngeal muscle
see Table 13.1E muscles of the larynx.
limb muscle
see Table 13.3, 13.4 muscles of the fore- and hindlimbs.
masseter muscle
the principal muscle of mastication. See also Table 13.1H.
mylohyoid muscle
see Table 13.1D muscles of the hyoid apparatus.
muscle neoplasms
of striated muscle—rhabdomyoma, rhabdomyosarcoma; of plain muscle—leiomyoma, leiomyosarcoma.
muscle nonstriated
see smooth muscle (below).
orbicular muscle
one that encircles a body opening, e.g. the eye or mouth.
muscle-paralyzing drugs
drugs which produce neuromuscular blockade, used as muscle relaxants during surgical procedures. Include d-tubocurarine, alcuronium chloride, pancuronium, vecuronium, atracurium besylate, succinylcholine.
red muscle
type 1 fibers predominate with slow contraction cycles and aerobic metabolism.
muscle rupture
the muscle may have torn away from its insertion, in which case the tendon will be slack, or it may be a complete or partial separation of the belly of the muscle, when the muscle will be swollen and hard. Structural and conformational changes may result, e.g. in rupture of the gastrocnemius muscle, and the hernias caused by rupture of the ventral abdominal muscles or the diaphragm.
skeletal m's
striated muscles that are attached to bones and typically cross at least one joint. Called also voluntary or striated muscles.
slow-twitch skeletal muscle
type 1 skeletal muscle fibers are bright red and contain large amounts of myoglobin; not easily fatigued.
smooth muscle
plain or involuntary muscle which powers the internal organs and is controlled by the autonomic nervous system; slow contracting cycles and fatigue resistant. Two types listed, visceral and vascular.
sphincter muscle
a ringlike muscle that closes a natural orifice; called also sphincter.
muscle spindle
sensory end-organ attached to the perimysial connective tissue of the muscle.
muscle strain
soreness and stiffness in a muscle due to overexertion or contusion, especially in muscles that have not been conditioned for hard use; some of the muscle fibers may actually tear.
striated muscle
see skeletal muscles (above).
synergic m's
those that assist one another in action.
temporal muscle
a significant muscle of mastication. See also Table 13.1H.
muscle-tendon junction
the union between connective tissue investing muscles and anchoring connective tissue.
type I muscle fiber
see slow-twitch skeletal muscle (above).
type II muscle fiber
see fast-twitch skeletal muscle (above).
type II muscle fiber deficiency
a relative deficiency of type II muscle fibers, with a predominance of type I fibers. An inherited defect in Labrador retrievers. Clinical signs include stunted growth, and muscle weakness and abnormal gait, which subside with rest, from an early age.
voluntary muscle
see skeletal muscle (above).
white muscle
consist of type II fibers; fast contraction fibers and aerobic metabolism are characteristic.
yoked m's
those that normally act simultaneously and equally, as in moving the eyes.