The terminal branches of motor axons lie in troughs in the surface of the muscle cell, where the plasma membrane is highly folded. The ACh interacts with receptors on the muscle cell plasma membrane and results in depolarization of the muscle cell surface. Excess ACh is hydrolyzed by acetylcholine esterase localized in the basement membrane between the muscle cell and the nerve ending. The axon of one motor neuron may branch and innervate more than one muscle cell. The group of muscle cells so innervated is called a motor unit .
Motor units vary in size in different muscles.
The muscle spindles serve as stretch sensors. Muscle disease Myasthenia gravis is an autoimmune disease. In some forms neuromuscular transmission is compromised by antibodies to ACh receptors. It is coded by a very large gene on the X chromosome. Dystrophin lines the cytoplasmic surface of the plasma membrane of muscle cells.