|
Neuromuscular junction disease is a medical condition where the normal conduction through the neuromuscular junction fails to function correctly. ==Neuromuscular junction diseases== The neuromuscular junction is a specialized synapse between a neuron and the muscle it innervates. It allows efferent signals from the nervous system to contact muscle fibers causing them to contract. In vertebrates, the neuromuscular junction is always excitatory, therefore to stop contraction of the muscle, inhibition must occur at the level of the efferent motor neuron. In other words, the inhibition must occur at the level of the spinal cord. Release of acetylcholine vesicles from the presynaptic terminal occurs only after adequate depolarization of the efferent nerve. Once a motor nerve action potential reaches the presynaptic nerve terminal it causes an increase in intracellular calcium concentration by causing an increase in ion conductance through voltage gated calcium channels. This increase in calcium concentration allows the acetylcholine vesicles to fuse with the plasma membrane at the presynaptic membrane, in a process calledexocytosis, thus releasing acetylcholine into the synapse. Once acetylcholine is present in the synapse it is able to bind to nicotinic acetylcholine receptors increasing conductance of certain cations, sodium and potassium in the postsynaptic membrane and producing an excitatory end plate-current. As cations flow into the postsynaptic cell, this causes a depolarization, as the membrane voltage increases above normal resting potential. If the signal is of sufficient magnitude, than an action potential will be generated post synaptically. The action potential will propagate through the sarcolemma to the interior of the muscle fibers eventually leading to an increase in intracellular calcium levels and subsequently initiating the process of Excitation–contraction coupling. Once coupling begins it allows the sarcomeres of the muscles to shorten, thus leading to the contraction of the muscle. Neuromuscular junction diseases are a result of a malfunction in one or more steps of the above pathway. As a result, normal functioning can be completely or partially inhibited, with the symptoms largely presenting themselves as problems in mobility and muscle contraction as expected from disorders in motor end plates. Neuromuscular junction diseases can also be referred to as end plate diseases or disorders. Among neuromuscular diseases some can be autoimmune disease, or hereditary disorders. They can affect either presynaptic mechanisms or postsynaptic mechanisms, preventing the junction from functioning normally. The most studied diseases affecting the human acetylcholine receptor are Myasthenia Gravis and some forms of congenital myasthenic syndrome. Other diseases include the Lambert-Eaton syndrome and botulism. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Neuromuscular junction disease」の詳細全文を読む スポンサード リンク
|