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Muscular tissue |
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Specialised cells whose primary function is contraction characterize muscular tissue. These cells, called muscle fibres, are elongated and arranged in parallel arrays. Their coordinated contraction results in movement.
Muscular tissue may be classified based on appearance and location. Striated muscle exhibits cross-striations visible under the microscope, while smooth muscle does not. Striated muscle may be further divided into skeletal muscle and cardiac muscle. Skeletal muscle is, for obvious reasons, under voluntary nervous control and is often referred to as voluntary muscle. Smooth muscle is under autonomic nervous control and is largely restricted to the viscera and blood vessels. |
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Skeletal muscle fibres are multinucleated cells, with the nuclei situated immediately beneath the sarcolemma (cell membrane). An individual fibre may be up to 35cm long. A mature skeletal muscle fibre is composed of a large number parallel running myofibrils. The myofibrils in turns consist of overlapping parallel-arranged thin actin filaments and thick myosin filaments. The filaments are anchored to specific accessory proteins. The functional unit of the fibril is the sarcomere, which is situated between two Z-lines.
A thin layer of connective tissue, the endomysium, surrounds each muscle fibre. Several muscle fibres are bundled together as a fasciculus (Pl fasciculi). The connective tissue layer surrounding the fasciculi is known as the perimysium. An individual muscle is a number of fasciculi enclosed by the epimysium, a thick dense connective tissue capsule. The connective tissue framework is continuous with that of the tendons and muscle attachments to direct the muscle forces to the bone, skin, etc. Muscles are highly vascular with the blood vessels running in the various connective tissue layers. The different fibre types may histologically visualized by the use of histochemical techniques. (See Wheater Fig 6.13, Stevens and Lowe Fig 13.4). Skeletal muscle contraction is regulated by intracellular calcium stored in the T-tubule system. |
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Cardiac muscle is a form of striated muscle, but differs from skeletal muscle in the following:
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Smooth muscle fibres are spindle-shaped with a single centrally located nucleus. The actin and myosin filaments are arranged in a meshwork throughout the cell except in the nuclear area, therefore smooth muscle cells do not display the striations characteristic of striated muscle.
The filaments are anchored to each other and to the cell membrane by dense bodies. These cells lack a T-tubule system and rely on extracellular calcium for contraction. |
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