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Related to skeletal muscle: smooth muscle
, cardiac muscle
grossly, a collection of striated voluntary muscle fibers connected at either or both extremities with the bony framework of the body; may be appendicular or axial; histologically, a muscle consisting of elongated, multinucleated, transversely striated skeletal muscle fibers together with connective tissues, blood vessels, and nerves; individual muscle fibers are surrounded by fine reticular and collagen fibers (endomysium); bundles (fascicles) of muscle fibers are surrounded by irregular connective tissue (perimysium); the entire muscle is surrounded, except at the muscle tendon junction, by a dense connective tissue (epimysium).
A usually voluntary muscle that is made up of elongated, multinucleate, transversely striated muscle fibers and is typically attached to a bone.
skel·e·tal mus·cle (skel'ĕ-tăl mŭs'ĕl)
Grossly, a collection of striated muscle fibers connected at either or both extremities with the bony framework of the body; it may be an appendicular or an axial muscle; histologically, a muscle consisting of elongated, multinucleated, transversely striated skeletal muscle fibers together with connective tissues, blood vessels, and nerves; individual muscle fibers are surrounded by fine reticular and collagen fibers (endomysium); bundles (fascicles) of muscle fibers are surrounded by irregular connective tissue (perimysium); the entire muscle is surrounded, except at the muscle-tendon junction, by a dense connective tissue (epimysium).
MORPHOLOGICAL FORMS OF MUSCLE
MORPHOLOGICAL FORMS OF MUSCLE
MORPHOLOGICAL FORMS OF 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.
Muscle connected to, and necessary for the movement of, bones.
Axon terminal at its junction with a muscle fibre, from an electronmicrograph.
neuromuscular junction the site where a motor nerve axon terminal makes close contact with the skeletal muscle fibre which it supplies. An action potential arriving at the terminal causes release of the neurotransmitter acetylcholine (ACh), which crosses the very narrow synaptic cleft to binding sites on the muscle membrane and initiates its depolarization; this triggers an action potential in the adjacent muscle fibre membrane and so sets in train the process of excitation-contraction coupling. In almost all mammalian/human extrafusal muscle fibres, the junction takes the form of a motor endplate but less extensive structures occur in some other locations.
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.
muscle 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.
Types of skeletal muscle contraction.
muscle contraction the process of force-generation in the fibres of any class of muscle, by the interaction of myosin head-groups in the thick filaments with actin molecules in one of the immediately neighbouring thin filaments. This is set in train ('activated') by a rise in the concentration of calcium ions [Ca2+] in the muscle fibre cytoplasm in all types of muscle, but the mechanism for this rise differs in important respects between them. With reference to skeletal muscle, 'contraction', though literally implying shortening, is used to describe force-generation, whether it actually results in shortening (concentric action), tension without movement (isometric action) or even lengthening against the muscle's own resistance (eccentric action); the last is sometimes called an 'eccentric contraction' or, even worse, a 'lengthening contraction' - paradoxical usages better avoided. See also excitation-contraction coupling, force-velocity relationship, myofibrils.