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muscle

 [mus´'l]
a bundle of long slender cells (muscle fibers) that have the power to contract and hence to produce movement. Muscles are responsible for locomotion and play an important part in performing vital body functions. They also protect the contents of the abdomen against injury and help support the body. See appendix 3-4 and see color plates.

Muscle fibers range in length from a few hundred thousandths of a centimeter to several centimeters. They also vary in shape, and in color from white to deep red. Each fiber receives its own nerve impulses, so that fine and varied motions are possible. Each has its small stored supply of glycogen, which it uses as fuel for energy. Muscles, especially the heart, also use free fatty acids as fuel. 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.

Some muscles are attached to bones by tendons. Others are attached to other muscles, or to skin (producing the smile, the wink, and other facial expressions, for example). All or part of the walls of hollow internal organs, such as the heart, stomach, intestines, and blood vessels, are composed of muscles. The last stages of swallowing and of peristalsis are actually series of contractions by the muscles in the walls of the organs involved.
Types of Muscle. There are three types of muscle: involuntary, voluntary, and cardiac, composed respectively of smooth, striated, and mixed smooth and striated tissue.
Types and structure of muscle. From Dorland's, 2000.


Involuntary muscles are those not under the control of the conscious part of the brain; they respond to the nerve impulses of the autonomic nervous system. They include the countless short-fibered, or smooth, muscles of the internal organs and power the digestive tract, the pupils of the eyes, and all other involuntary mechanisms.

Voluntary muscles are those controlled by the conscious part of the brain, and are striated. These are the skeletal muscles that enable the body to move, and there are more than 600 of them in the human body. Their fibers are grouped together in sheaths of muscle cells. Groups of fibers are bundled together into fascicles, surrounded by a tough sheet of connective tissue to form a muscle group such as the biceps. Unlike the involuntary muscles, which can remain in a state of contraction for long periods without tiring and are capable of sustained rhythmic contractions, the voluntary muscles are readily subject to fatigue.

Cardiac muscles (the muscles of the heart) are the third kind; they are involuntary and consist of striated fibers different from those of voluntary muscle. The contraction and relaxation of cardiac muscle continues at a rhythmic pace until death unless the muscle is injured in some way.
Voluntary muscles extend from one bone to another, cause movements by contraction, and work on the principle of leverage. For every direct action made by a muscle, an antagonistic muscle can cause an opposite movement. To flex the arm, the biceps contracts and the triceps relaxes; to extend the arm, the triceps contracts and the biceps relaxes.
(See also heart.)
Physiology of Muscles. No muscle stays completely relaxed, and as long as a person is conscious, it remains slightly contracted. This condition is called tonus, or tone. It keeps the bones in place and enables a posture to be maintained. It allows a person to remain standing, sitting up straight, kneeling, or in any other natural position. Muscles also have elasticity. They are capable of being stretched and of performing reflex actions. This is made possible by the motor and sensory nerves which serve the muscles.

Muscles enable the body to perform different types of movement. Those that bend a limb at a joint, raising a thigh or bending an elbow, are called flexors. Those that straighten a limb are called extensors. Others, the abductors, make possible movement away from the midline of the body, whereas the adductors permit movement toward the midline. Muscles always act in opposing groups. In bending an elbow or flexing a muscle, for example, the biceps (flexor) contracts and the triceps (extensor) relaxes. The reverse happens in straightening the elbow.

A muscle that has contracted many times, and has exhausted its stores of glycogen and other substances, and accumulated too much lactic acid, becomes unable to contract further and suffers from fatigue. In prolonged exhausting work, fat in the muscles can also be used for energy, and as a consequence the muscles become leaner.
agonistic muscle one opposed in action by another muscle, the antagonistic muscle. Called also agonist.
antagonistic muscle one that counteracts the action of another (the agonistic muscle). Called also antagonist.
appendicular muscle one of the muscles of a limb.
articular muscle one that has one end attached to the capsule of a joint.
auricular m's
1. the extrinsic auricular muscles, including the anterior, posterior, and superior auricular muscles. See appendix 3-4.
2. the intrinsic auricular muscles that extend from one part of the auricle to another, including the helicis major, helicis minor, tragicus, antitragicus, transverse auricular, and oblique auricular muscles. See appendix 3-4.
cruciate muscle a muscle in which the fiber bundles are arranged in the shape of an X.
cutaneous muscle striated muscle that inserts into the skin.
deltoid muscle the muscular cap of the shoulder, often used as a site for an intramuscular injection. See appendix 3-4.
extraocular m's the six voluntary muscles that move the eyeball: superior, inferior, middle, and lateral recti, and superior and inferior oblique muscles. See appendix 3-4.
extrinsic muscle one that originates in another part than that of its insertion, as those originating outside the eye, which move the eyeball.
fixation m's (fixator m's) accessory muscles that serve to steady a part.
gluteal m's three muscles, the greatest, middle, and least, that extend, abduct, and rotate the thigh. See appendix 3-4.
hamstring m's the muscles of the back of the thigh, including the biceps femoris, semitendinosus, and semimembranosus. See appendix 3-4.
intraocular m's the intrinsic muscles of the eyeball. See appendix 3-4.
intrinsic muscle one whose origin and insertion are both in the same part or organ, as those entirely within the eye.
multipennate muscle a muscle in which the fiber bundles converge to several tendons.
palatine m's the intrinsic and extrinsic muscles that act upon the soft palate.
pectoral m's four muscles of the chest; See appendix 3-4.
quadrate muscle a square-shaped muscle; see appendix 3-4.
quadriceps muscle a name applied collectively to four muscles of the thigh; see anatomic Table of Muscles in the Appendices.
scalene m's four muscles of the upper thorax that raise the first two ribs, aiding in respiration. See appendix 3-4.
skeletal m's striated muscles that are attached to bones and typically cross at least one joint.
sphincter muscle a ringlike muscle that closes a natural orifice; called also sphincter.
synergic m's (synergistic m's) those that assist one another in action.
thenar m's the abductor and flexor muscles of the thumb. See appendix 3-4.
triangular muscle a muscle that is triangular in shape.
yoked m's those that normally act simultaneously and equally, as in moving the eyes.

mus·cle

(mŭs'ĕl), [TA]
A primary tissue, consisting predominantly of highly specialized contractile cells, which may be classified as skeletal muscle, cardiac muscle, or smooth muscle; microscopically, the latter is lacking in transverse striations characteristic of the other two types; one of the contractile organs of the body by which movements of the various organs and parts are effected; typical muscle is a mass of musculus fibers (venter or belly), attached at each extremity, by means of a tendon, to a bone or other structure; the more proximal or more fixed attachment is called the origin (q.v.), the more distal or more movable attachment is the insertion (q.v.); the narrowing part of the belly that is attached to the tendon of origin is called the caput or head. For gross anatomic description, see musculus
Synonym(s): musculus [TA]
[L. musculus]

muscle

(mŭs′əl)
n.
1. A tissue composed of fibers capable of contracting to effect bodily movement.
2. A contractile organ consisting of a special bundle of muscle tissue, which moves a particular bone, part, or substance of the body: the heart muscle; the muscles of the arm.
3. Muscular strength: enough muscle to be a high jumper.

mus′cly adj.

MUSCLE

Abbreviation for:
Multi Station Clinical Examination (OSCE)

mus·cle

(mŭs'ĕl) [TA]
A primary tissue, consisting predominantly of highly specialized contractile cells, which may be classified as skeletal muscle, cardiac muscle, or smooth muscle; microscopically, the latter is lacking in transverse striations characteristic of the other two types; one of the contractile organs of the body by which movements of the various organs and parts are effected; typical muscle is a mass of muscle fibers (venter or belly), attached at each extremity, by means of a tendon, to a bone or other structure; the more proximal or more fixed attachment is called the origin, the more distal or more movable attachment is the insertion; the narrowing part of the belly that is attached to the tendon of origin is called the caput or head.
Synonym(s): musculus.
[L. musculus]

muscle

(mus'el ) [L. musculus, diminutive of mus, mouse]
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MORPHOLOGICAL FORMS OF MUSCLE
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MORPHOLOGICAL FORMS OF MUSCLE
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MORPHOLOGICAL FORMS OF MUSCLE
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MORPHOLOGICAL FORMS OF MUSCLE
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MORPHOLOGICAL FORMS OF MUSCLE
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MORPHOLOGICAL FORMS OF MUSCLE
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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MUSCLES OF THE TRUNK

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.
illustration

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.
illustration

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.
Enlarge picture
MORPHOLOGICAL FORMS OF MUSCLE
Enlarge picture
MORPHOLOGICAL FORMS OF MUSCLE
Enlarge picture
MORPHOLOGICAL FORMS OF MUSCLE

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

soleus

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.
SmoothCardiacStriated
SynonymsInvoluntaryMyocardialVoluntary
NonstriatedSkeletal
Visceral
Fibers
Length (in/m)50–20025,000
Thickness (in/m) 4–8 75
ShapeSpindlesCylinders
MarkingsNo striationStriationMarked striation
NucleiSingleSingleMultiple
Effects of cutting related nerveSlightRegulation of heart rate is lostComplete paralysis
illustration

muscle

A tissue consisting of large numbers of parallel elongated cells with the power of shortening and thickening so as to approximate their ends and effect movement. Up to 50% of the body weight consists of muscle, most being attached to bone in such a way that muscle contraction causes joints to bend (flex) or straighten (extend). Muscle fibres convert chemical energy into mechanical energy. There are three kinds of muscle—striped (striated) or voluntary muscle; smooth or involuntary muscle occurring in the walls of arteries, the intestines and the urinary tract; and heart muscle (MYOCARDIUM), a network (syncytium) of muscle fibres that contract regularly and automatically without external stimulus.
Muscleclick for a larger image
Fig. 225 Muscle . A sarcomere.

muscle

the fleshy part of any animal that consists of tissue made up of highly contractile cells which serve to move parts of the body relative to each other.

A muscle is composed of many fibres or muscle cells. In STRIATED MUSCLE, each cell contains a bundle of MYOFIBRILS each exhibiting a banding pattern and being made up of a number of SARCOMERES arranged end to end. The sarcomere is the unit of contraction and the banding visible over its surface results from the longitudal filaments which make up the myofibril being of two types, thick (dark) and thin (light). These filaments overlap as shown in Fig. 225. The thick filaments are composed of the protein MYOSIN and the thin filaments of ACTIN. H.E. Huxley and K. Harrison found that on contraction, the light zones (I-BANDS) were comparatively narrow; on relaxation of the muscle the I-bands were broad. Where very strong contraction takes place the H-zone disappears and the thin filaments overlap.

Huxley and Harrison proposed the sliding filament hypothesis, to account for their observations. Bridges occur between thick and thin filaments and in contraction the bridges pull thin filaments past the thick ones using a ratchet mechanism. Some filaments are retained in this ‘pulled past’ position whilst others detach then reattach and repeat the ‘pulling past’ action. ACTOMYSIN is formed at the point of contact of bridge and thin filament. For each bridge to go through its cycle of attachment, contraction and reattachment, the splitting of one molecule of ATP is required, the cycles occurring between 50 and 100 times per second. The supply of ATP comes from MITOCHONDRIA between the fibrils. Calcium ions are released from vesicles in the sarcoplasmic reticulum, by the ACTION POTENTIAL passing along the surface of the fibre and these split the ATP. Troponin activated by the calcium displaces tropomyosin, which prevents myosin bridges from binding with actin fibrils. Once binding takes place this activates ATPase and on hydrolysis of ATP the bridge goes through its cycle of movement.

muscle

A contractile organ of the body which produces movements of the various parts or organs. Typically it is a mass of fleshy tissue, attached at each extremity by means of a tendon to a bone or other structure. Muscles are classified according to structure as non-striated (or unstriated or unstriped or smooth) or striated (or striped), by control as voluntary or involuntary, or by location as cardiac, skeletal or visceral.
abducens muscle See lateral rectus muscle.
adducens muscle See medial rectus muscle.
agonistic muscle A muscle that performs the desired movement, or does the opposite to an antagonistic muscle. Example: the left lateral rectus is the agonistic muscle when the left eye turns to the left. See antagonistic muscle.
antagonistic muscle A muscle that opposes the action of another. Example: the right superior rectus muscle is the contralateral antagonist of the left superior oblique. See agonistic muscle; synergistic muscles.
Brücke's muscle See ciliary muscle.
ciliary muscle The smooth (unstriated and involuntary) muscle of the ciliary body. In a meridional section of the eye it has the form of a right-angled triangle, the right angle being internal and facing the ciliary processes. The posterior angle is acute and points to the choroid, the hypotenuse runs parallel with the sclera. Some of its fibres have their origin in the scleral spur at the angle of the anterior chamber, while other fibres take origin in the trabecular meshwork. The fibres radiate backward in three directions: (1) Fibres coursing meridionally or longitudinally more or less parallel to the sclera and can be traced posteriorly into the suprachoroid to the equator or even beyond. They end usually in branched stellate figures known as muscle stars with three or more rays to each. These fibres represent Brücke's muscle. (2) Other fibres course radially. These fibres lie deep in the longitudinal fibres from which they are distinguished by the reticular character of their stroma but are often very difficult to separate from the circular fibres. (3) The circular fibres (Müller's muscle) occupy the anterior and inner portion of the ciliary body and run parallel to the limbus. As a whole, these fibres form a ring.Innervation to the ciliary muscle (mainly parasympathetic fibres derived from the oculomotor nerve) is provided through the short ciliary nerves and stimulation causes a contraction of the muscle. However, a small amount of sympathetic supply is also believed to act and relax the muscle. Blood supply to the ciliary muscle is provided by the anterior and long posterior ciliary arteries. Contraction of the ciliary muscle causes a reduction in its length thus causing the whole muscle to move forward and inward. Consequently the zonule of Zinn, which suspends the lens, relaxes. This leads to a decrease in the tension in the capsule of the lens allowing it to become more convex and thereby providing accommodation. Syn. Bowman's muscle. See mechanism of accommodation; adrenergic receptors; ciliary body; scleral spur; Helmholtz's of accommodation theory; zonule of Zinn.
muscle cone A structure formed by the sheath of the four recti muscles as they pass forward from their common origin at the apex of the orbit in the fibrous ring called the annulus of Zinn (and around the optic nerve) to be inserted into the sclera around the eyeball. Some authors consider the muscle cone to include the superior oblique muscle. See annulus of Zinn.
dilator pupillae muscle Smooth (unstriated and involuntary) muscle whose fibres constitute the posterior membrane of the iris. This muscle extends from the ciliary body close to the margin of the iris where it fuses with the sphincter pupillae muscle. Contraction of the dilator pupillae muscle draws the pupillary margin towards the ciliary body and therefore dilates the pupil. This muscle is supplied by the sympathetic fibres in the long ciliary nerves and by a few parasympathetic fibres. See adrenergic receptors; sphincter pupillae muscle; mydriatic.
elevator muscle's See inferior oblique muscle; superior rectus muscle.
external rectus muscle See lateral rectus muscle.
extraocular muscle's The striated (voluntary) muscles that control the movements of the eyes. There are six such muscles: four recti muscles (lateral rectus, medial rectus, superior rectus and inferior rectus) which move the eye more or less around the transverse and vertical axes, and two oblique muscles (inferior oblique and superior oblique) which move the eyes obliquely. The muscles are composed of striated fibres of varying length, mostly running parallel to the direction of the muscle and united by fibrous connective tissue. They have a greater ratio of nerve fibres to muscle fibres than other striated muscles of the body. The fibre thickness varies from 3 to 50 μm, although functionally there seem to be two main types of fibres, the fast and the slow fibres. The former are the thickest and probably responsible for the fast movements of the eyes (saccades) and the latter consist of thin fibres. The tendons (bands of connective tissue) at one end of each extraocular muscle are attached to bones. This is the origin of the muscle. At the other end of the muscle the tendon is attached to the eye and this area is called the insertion. The substance proper of the muscle is called the belly. Contraction of a muscle occurs in the direction of its constituent fibres and causes a shortening of the muscle. Consequently the eye turns in a given direction depending upon which extraocular muscle is contracting. Contraction results from nervous impulses arriving at the motor end-plate (the junction between an axon and a striated muscle fibre) of the muscle through one of the ocular motor nerves. This causes a neurotransmitter substance to be discharged in the microscopic gap between the end-plate and a muscle fibre. These muscles also possess specialized receptors called muscle spindles, which are small groups of muscle fibres that are provided with both a sensory and a motor nerve supply. There are between 12 and 50 in each muscle. The muscle spindles provide a constant and continuous monitoring of the degree of tension of the muscle itself. Syn. extrinsic muscles; oculorotary muscles. See cholinergic; felderstruktur fibres; motor unit; strabismus surgery; motility test; three-step test.
extrinsic muscle's See extraocular muscles.
eyelid retractor muscle's See levator palpebrae superioris muscle; Müller's palpebral muscles.
Horner's muscle A thin layer of fibres that originates behind the lacrimal sac from the upper part of the posterior lacrimal crest (a ridge on the lacrimal bone which borders the fossa for the lacrimal sac). The muscle passes outward and forward and divides into two slips surrounding the canaliculi. It then becomes continuous with the pretarsal portions of the orbicularis muscle of the upper and lower lids and with the muscle of Riolan. Horner's muscle may be involved in tear drainage through action on the lacrimal sac. Syn. pars lacrimalis muscle; tensor tarsi muscle. See muscle of Riolan.
inferior oblique muscle (IO) One of the extraocular muscles, it takes its origin at the antero-medial corner of the floor of the orbit. It passes underneath the inferior rectus in a backward direction (making an angle of about 50º with the sagittal plane of the eye), then under the lateral rectus to be inserted by the shortest tendon of all extraocular muscles on the posterior, temporal portion of the eyeball, for the most part below the horizontal meridian, some 5 mm away from the optic nerve. It is innervated by the oculomotor nerve and it extorts (main action), elevates and abducts the eyeball when the eye is in the primary position. Combined with the action of the superior rectus muscle, it directs the eye upward (Fig. M15). See Bielschowsky's head tilt test; three-step test.
inferior rectus muscle (IR) This is the shortest of the four recti muscles. It arises from the lower part of the annulus of Zinn, runs forward, downward and outward (making an angle of about 23º with the sagittal plane) and inserts into the inferior portion of the sclera about 6.5 mm from the corneal limbus. It is innervated by the inferior division of the oculomotor nerve and it depresses (main action), adducts and extorts the eyeball when the eye is in the primary position (Fig. M15). See annulus of Zinn; Müller's palpebral muscles; Bielschowsky's head tilt test.
inferior tarsal muscle See Müller's palpebral muscles.
internal rectus muscle See medial rectus muscle.
intraocular muscle's The smooth (unstriated and involuntary) muscles found within the eye. They are the ciliary, the dilator pupillae and the sphincter pupillae muscles. Syn. intrinsic muscles. See cholinergic.
lateral rectus muscle (LR) One of the extraocular muscles, it arises from both the lower and upper parts of the annulus of Zinn which bridge the superior orbital fissure. The muscle passes forward along the lateral wall of the orbit, crosses the tendon of the inferior oblique muscle and inserts into the sclera about 6.9 mm from the corneal limbus. It is innervated by the abducens nerve and it abducts the eyeball when the eye is in the primary position (Fig. M15). Syn. external rectus muscle; abducens muscle. See annulus of Zinn; check ligament.
levator palpebrae superioris muscle Striated muscle that arises from the under surface of the lesser wing of the sphenoid bone above and in front of the optic canal. It passes forward below the roof of the orbit and above the superior rectus muscle and terminates in a tendinous expansion or aponeurosis (also called levator aponeurosis), which spreads out in a fan-shaped manner so as to occupy the whole breadth of the orbit and thus gives the whole muscle the form of an isosceles triangle. From the inferior surface of the aponeurosis arises a thin sheet of smooth muscle fibres called Müller's palpebral muscle (or superior tarsal muscle) which inserts into the posterior margin of the superior tarsal plate and into the superior fornix of the conjunctiva. These smooth muscle fibres are innervated by sympathetic nerves from the superior cervical sympathetic ganglion and assist in elevating the upper eyelid. The fibres of the aponeurosis are attached to the anterior margin of the superior tarsal plate while some fuse with bundles of the orbicularis oculi muscle to attach to the skin. These latter sets of fibres produce the horizontal skin crease of the upper eyelid. The striated levator aponeurosis is innervated by the superior division of the oculomotor nerve and elevates the upper eyelid. Its antagonist is the orbicularis muscle. See orbicularis muscle.
medial rectus muscle (MR) One of the extraocular muscles, it arises from the medial part of the annulus of Zinn. It passes forward along the medial wall of the orbit and is inserted into the sclera about 5.5 mm from the corneal limbus. It is innervated by the inferior division of the oculomotor nerve and it adducts the eyeball when the eye is in the primary position (Fig. M15). Syn. internal rectus muscle; adducens muscle. See annulus of Zinn; check ligament.
Müller's muscle See ciliary muscle.
Müller's palpebral muscle's Smooth muscles of the eyelids. The superior one (also called superior tarsal muscle) originates from the under surface of the levator palpebrae superioris muscle and passes below to insert into the upper margin of the tarsal plate of the upper eyelid. The inferior one (also called inferior tarsal muscle) originates from the muscular fascia covering the inferior rectus muscle. It extends upward and inserts into the bulbar conjunctiva and the lower margin of the tarsal plate of the lower eyelid. Müller's palpebral muscles are innervated by sympathetic fibres and help in lifting the upper eyelid and depressing the lower eyelid. They are sometimes referred to as the eyelid retractors.
oculorotary muscle's See extraocular muscles.
orbicularis muscle A thin oval sheet of striated muscle that surrounds the palpebral fissure, covers the eyelids and spreads out for some distance onto the temple, forehead and cheek. It consists of three portions: (1) The marginal or ciliary portion (muscle of Riolan). (2) The palpebral portion (also called the pars palpebralis muscle) which is the essential part of the muscle and is confined to the lids and may itself be divided into pretarsal portion whose fibres lie in front of the tarsal plates, and the preseptal portion whose fibres extend from the tarsal plates to the orbital margin. The palpebral portion is used in closing the eye without effort or in reflex blinking. (3) The orbital portion (also called the pars orbitalis muscle) which is found in the eyebrow, the temple, the forehead and the cheek. This portion of the muscle is used to close the eye tightly and the skin of the forehead, temple and cheek is drawn towards the inner side of the orbit. The orbicularis muscle is innervated by the facial nerve. Syn. sphincter oculi muscle. See ectropion; Horner's muscle; levator palpebrae superiotis muscle; muscle of Riolan; myokymia.
pars ciliaris muscle See muscle of Riolan.
pars lacrimalis muscle See Horner's muscle.
pars orbitalis muscle See orbicularis muscle.
pars palpebralis muscle See orbicularis muscle.
pupillary muscle's The dilator pupillae and the sphincter pupillae muscles.
muscle of Riolan The ciliary portion of the orbic-ularis muscle, it consists of very fine striated muscle fibres which lie in the dense tissue of the eyelids near their margin. It is continuous with Horner's muscle and encircles the eyelid margins mainly between the tarsal glands and the eyelash follicles. Its action is to bring the eyelid margins together when the eyes are closed. Syn. pars ciliaris muscle. See Horner's muscle; orbicularis muscle.
sphincter oculi muscle See orbicularis muscle.
sphincter pupillae muscle Smooth, circular muscle about 1 mm broad, forming a ring all round the pupillary margin near the posterior surface of the iris. It is innervated by parasympathetic fibres of the oculomotor nerve that synapse in the ciliary ganglion and by a few sympathetic fibres. Its contraction produces a reduction in the diameter of the pupil. See miotics; dilator pupillae muscle; pupil light reflex.
muscle spindle See extraocular muscles.
superior oblique muscle (SO) This is the longest and thinnest of the extraocular muscles. It arises above and medial to the optic foramen on the small wing of the sphenoid bone. It passes forward between the roof and medial wall of the orbit to the trochlea (which is in the form of a pulley made of fibrocartilage) located at the front of the orbit where it loops over and turns sharply backward, downward and outward (making an angle of about 55º with the sagittal plane), passes under the superior rectus and inserts into the sclera just behind the equator on the superior temporal portion of the eyeball. It is innervated by the trochlear nerve and it intorts (main action), depresses, and also abducts the eyeball when the eye is in the primary position (Fig. M15). See trochlear fossa; Bielschowsky's head tilt test; three-step test.
superior rectus muscle (SR) One of the extraocular muscles, it arises from the upper part of the annulus of Zinn. It passes forward and outward (making an angle of about 23º with the sagittal plane) and inserts into the sclera about 7.7 mm from the corneal limbus. It is innervated by the superior division of the oculomotor nerve and elevates (main action), adducts, and also intorts the eyeball when the eye is in the primary position (Fig. M15). See annulus of Zinn; Bielschowsky's head tilt test.
synergistic muscle's Muscles having a similar and mutually helpful action as, for example, the inferior rectus and superior oblique muscles in depressing the eyeball. See Table M5.
superior tarsal muscle See Müller's palpebral muscles.
tarsal muscle's See Müller's palpebral muscles.
tensor tarsi muscle See Horner's muscle.
yoke muscle's Muscles of the two eyes which simultaneously contract to turn the eyes in a given direction. Example: the medial rectus of the right eye and the lateral rectus of the left eye when turning the eyes to the left. See Hering's law of equal innervation; motility test; version.
Fig. M15 Extraocular muscles of the eye (the left superior rectus muscle is not shown to allow a clearer view of the muscles underneath)enlarge picture
Fig. M15 Extraocular muscles of the eye (the left superior rectus muscle is not shown to allow a clearer view of the muscles underneath)

Table M5 Agonistic, antagonistic and synergistic extraocular muscles
agonistipsilateral antagonistipsilateral synergist(s)contralateral synergist
lateral rectusmedial rectussuperior obliquemedial rectus
inferior oblique
medial rectuslateral rectussuperior rectuslateral rectus
inferior rectus
superior rectusinferior rectusinferior obliqueinferior oblique
inferior rectussuperior rectussuperior obliquesuperior oblique
superior obliqueinferior obliquesuperior rectusinferior rectus
inferior obliquesuperior obliqueinferior rectussuperior rectus

Table M6 Innervation and action of the six extraocular muscles
muscleinnervationaction in the primary position
medial rectusoculomotor (III)adduction
lateral rectusabducens (VI)abduction
inferior rectusoculomotor (III)depression*
adduction
extorsion
superior rectusoculomotor (III)elevation
adduction
intorsion
inferior obliqueoculomotor (III)extorsion
elevation
abduction
superior obliquetrochlear (IV)intorsion
depression
abduction
*Bold characters indicate main action.

Table M7 Intraocular muscles of the eyeball (unstriated muscles)
name of musclenerve supplyaction
sphincter pupillaeparasympathetic via oculomotor nerveconstricts pupil
dilator pupillaesympathetic via trigeminal nervedilates pupil
ciliaryparasympathetic via oculomotor nervecontrols shape of lens in accommodation

Table M8 Yoke muscles
right eyeleft eyeversion*
lateral rectusmedial rectusto the right
medial rectuslateral rectusto the left
superior rectusinferior obliqueup and to the right
inferior rectussuperior obliquedown and to the right
superior obliqueinferior rectusdown and to the left
inferior obliquesuperior rectusup and to the left
*The directions refer to those of the patient.

Table M9 Dimensions of the four recti muscles
muscle length (mm)insertion distance from limbus (mm)tendon length (mm)
lateral rectus486.98.8
medial rectus405.53.7
inferior rectus406.55.5
superior rectus427.75.8

mus·cle

(mŭs'ĕl) [TA]
Primary tissue, consisting predominantly of highly specialized contractile cells, which may be classified as skeletal muscle, cardiac muscle, or smooth muscle.
Synonym(s): musculus [TA] .
[L. musculus]

Patient discussion about muscle

Q. What are muscle cramps caused from? I am a 30 year old woman and am pregnant. I keep on getting a muscle cramps on the back on my lower leg. It really hurts! What is causing it and how can I prevent it?

A. You can get muscle cramps almost anywhere in your body during pregnancy, but the most common site is your calves. Although the spasms may only last a short time, they can be very severe.
No one knows for certain what causes leg cramps in pregnancy, though there are some theories: Deficiencies in salt, calcium, magnesium and vitamin C or changes in blood circulation.
To prevent it make sure to stretch your muscles before bed and if you do get a cramp, immediately stretch your calf muscles: Straighten your leg, heel first, and gently flex your toes back toward your shins. It might hurt at first, but it will ease the spasm and the pain will gradually go away.

Q. Why do my muscles sometimes burn when I'm exercising? I do exercise twice a day. Why do my muscles sometimes burn when I'm exercising?

A. The idea that lactic acid is what causes muscle burn during exercise is outdated and not supported by the most recent research. Lactic acid is actually a primary source of fuel during anaerobic exercise.

Muscle cells take up glucose (muscle glycogen) and convert them into lactic acid, which the mitrochondria in the cells then use for energy. The old theory was that lactic acid was a waste product that hindered performance. New scholarship on this actually shows that lactic acid is a SOURCE of fuel, not a "dead end as far as energy production is concerned."

Much of this new thinking has come from research performed by Dr. George Brooks at the University of California - Berkely. You can read more here: http://berkeley.edu/news/media/releases/2006/04/19_lactate.shtml

Researchers also now believe that muscle acidosis (that burning sensation during exercise)is not caused by increases in lactate within the muscle, but rather by a completely separate reaction when ATP is h

Q. What can I do to build muscle and develop immunity? I'm Mickey, 21. My height is 5’5” and I weigh 176 lbs. I love out door games especially soccer. I have poor immunity that I get sick very often. What can I do to build muscle and develop immunity?

A. You must keep your GI tract healthy. Eat plenty of soluble and insoluble fiber every day minimum of 25 grams, but gradually shoot up to 35 grams. Include yogurt or encapsulated probiotics in your daily diet. The more robust your GI tract, the more available nutrients such as glutamine is for anabolic muscle metabolism. Another nutrient is ImmunoLin, a purified source of immunoglobin G (IgG), which fights off viruses that may enter the body through the GI tract. Research has shown that IgG not only improves get immune health, which helps you to stay healthy, but also helps people who suffer from various allergies. Do exercise regularly. If you follow the above tips, I am sure you will get the desired results.

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