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Muscular tissue

 

Wat is muscle?

  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.

  • Coetzee et al p131-149
  • Stevens and Lowe p65-76, 227-234
  • Wheater p97-115

Skeletal muscle

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.


Cardiac muscle

Cardiac muscle is a form of striated muscle, but differs from skeletal muscle in the following:
  1. Cardiac muscle fibres contain a single elongated nucleus centrally positioned.
  2. The fibres may branch longitudinally.
  3. Individual cardiac muscle cells are connected end to end by specialized junctions called intercalated disks.
  4. The T-tubules are located at the Z-line and not at the A-I junction.

Smooth muscle

  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.


Diseases of muscle

 
  • Duchenne muscular dystrophy is caused by defecting gene coding for dystrophin which links the muscle actin cytoskeleton to the external lamina.

  • Myasthenia gravis is an autoimmune disease caused by the formation of antibodies to the acetyleholine receptor of the motor end plate.

  • Over-active smooth muscle cells in the walls of small airways cause asthma.
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© augustus 2002 - julie 2006 marius loots