If the nerve supply to a muscle is disrupted, the muscle cell undergoes regressive changes known as tissue atrophy. The most conspicuous indication of this atrophy is thinning of the muscle and its cells. If innervation is reestablished surgically or by the slower process of natural regeneration of the nerve, the muscle can regain normal shape and strength.
The events leading to contraction of skeletal muscle can be summarized as a series of steps.
The events involved in contraction can be summarized as follows (the numbers refer to the numbers in Fig. 11.11): 1.
The contraction of a skeletal muscle fiber is initiated when a nerve impulse traveling along the axon of a motor neuron arrives at the neuromuscular junction.
2.
The nerve impulse prompts the release of acetylcholine into the synaptic cleft that binds into ACh-gated Na channels causing local depolarization of sarcolemma.
3.
Voltage-gated Na channels open, and Na enters the cell.
indicated by the numerals. ACh, acetylcholine.
4.
General depolarization spreads over the plasma membrane of the muscle cell and continues via membranes of the T tubules.
5.
Voltage sensor proteins in the plasma membrane of T tubules change their conformation.
6.
At the muscle cell triads, the T tubules are in close contact with the lateral enlargements of the sarcoplasmic reticulum, where gated Ca2-release channels are activated by conformational changes of voltage-sensor proteins.
7.
Ca2 is rapidly released from the sarcoplasmic reticulum into the sarcoplasm.
8.
Ca2 binds to the TnC portion of the troponin  complex.
9.
The contraction cycle is initiated, and Ca2 is returned to the terminal cisternae of the sarcoplasmic reticulum.
Encapsulated sensory receptors in muscles and tendons are examples of proprioreceptors. These receptors are part of  FOLDER 11.4
Clinical Correlation: Myasthenia Gravis