Striated muscle physiology

Myofibrils are responsible for the contraction of the muscles.   They are the contractile elements.   They are also responsible for the striations seen in the muscle cell.   Under the electron microscope the myofibrils have been resolved to have myofilaments of two nature- myosin and actin. Myosin is the thick and short myofilament.   It has also been demonstrated that it has ATPase (adenosine triphosphatase) activities.  

Actin is a long myofilament and does not possess ATPase activity. In skeletal muscle it possesses troponin.  It has been demonstrated that actin and myosin form actomyosin complexes which break down periodically in relation to contraction and relaxation of muscle fibres.

Under the light microscope two bands of the striated muscle can be seen, the dark bands known as the A (anisotropic) band and the light band known as the I (isotropic) band.   There is also a light intermediate zone of the A band known as the Hensen (H) zone or line.   Further resolution will show an M line within the H zone.   Ultrastructural studies demonstrates that the A band is formed mainly by myosin and some actin filaments.   The light longitudinal lines described as the Z disc.   A sarcomere (muscle cell) extends from one Z disc top another.   The above description is for a relaxed muscle.

In a contracted muscle, the A and I bands completely disappear giving room for the contraction band made up of the actin and myosin filaments.   Also in the contracted muscle, the sarcomeres shortens leading to the shortening of the distance between the 2 discs.   The actin filaments become almost completely approximated.   This leads to the shortening of the length of the muscle fiber and the whole of the muscle itself.   The mechanism is explained by the sliding filament theory of muscle contraction.

Some functions of muscles- histophysiology

Muscles perform minimal regeneration. When damaged they are usually replaced by fibrous tissue. When they do regenerate the capacity for regeneration of new muscle fibers is very minimal, although the so called satellite cells  which are actually myoblasts or muscle stem cells present in adult muscles can produce a modicum of muscle fibers.

Exercise causes hypertrophy of skeletal muscles cells.

Nerve which supplies muscles enters the muscle substance to the neuromuscular hilus which is usually closer to the origin of the muscle than to its insertion. The neurons which reach the muscle fibers are alpha efferents. Gamma efferents supply intrafusal fibers of the muscle spindles. The actual microscopic junction between neuron and muscle fiber is called the motor end plate.

Apart from actin and myosin, other proteins have been described in skeletal muscles and they include desmin, titin, myomesin and actinin.

Neuromuscular junction is just another synaptic junction but this one is between the neuron and the muscle cell, where the postsynaptic membrane is the sarcolemma and the presynaptic membrane is the neurilemma. Synaptosomes contain depolarizing agents which are neurotransmitters that pass across the synaptic cleft to reach the postsynaptic membrane that is depolarized.

Red and white muscle

Red muscles are dark, slow and have a long latency. They are found in muscles of the back. White muscles on the other hand are lighter and have short twitch duration adapted for fine movement. They are found in extra-ocular muscles.

 


Striated muscle explanatory video


Muscle physiology


Actin and myosin filaments

Muscular contraction

 

 

 


 

Actin (green) and  myosin microfilaments and other arrangements on the sarcomere. One myosin is surrounded by six actin filaments.

 

Depolarization

 

 

 

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