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The neuromuscular junction. Acetylcholine receptors

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The neuromuscular junction. Acetylcholine receptors

Acetylcholine receptors are ion channels that span the postsynaptic membrane, and they have extracellular, intramembranous, and cytoplasmic portions. They are located principally over thepeaks of the postsynaptic folds, where they are present at high density. They consist of five subunits arranged around the central ion channel. One of the subunits is represented twice. The different subunits are products of separate genes. This implies that the acetylcholine receptor has been highly conserved in evolution and suggests that the genes coding for the different subunits may have evolved from a single primeval acetylcholine receptor gene.

Models of acetylcholine receptors indicate that its structure contains an extracellular N-terminal portion, several intramembranous portions (of which one provides part of the wall of the ion channel) separated by short intracellular and extracellular portions, and a cytoplasmic C-terminal part.

The supply of junctional acetylcholine receptors is continuously renewed. Receptors are internalized by the muscle cell and degraded in lysosomes (specialized cytoplasmic organelles), while new receptors are synthesized and inserted into the muscle membrane.

In normally innervated muscle, receptors are confined to the neuromuscular junction. In non-innervated fetal muscle and in denervated adult muscle, however, acetylcholine receptors are found elsewhere as well. These receptors have slightly different properties from junctional receptors, notably a much higher rate of turnover.

Acetylcholine/acetylcholine receptor interaction
The resting membrane potential of the muscle cell is held at about -80 millivolt. Binding of acetylcholine to each of the two alpha subunits causes the receptor molecule to alter its configuration so that the ion channel is opened for about one millisecond (0.001 second). This permits the entry of small positive ions, mainly sodium. The resulting local depolarization (the end plate potential) causes voltage-gated sodium channels located around the end plate to open. At a critical point (the firing threshold for the muscle cell) a self-generating action potential is triggered, causing the membrane potential to reverse and become briefly positive. The action potential propagates over the muscle fibre membrane to activate the contractile process.

The amplitude of the end plate potential is normally sufficient to bring the membrane potential of the muscle cell well above the critical firing threshold. The extent to which it does so represents a “safety factor” for neuromuscular transmission. The safety factor will be reduced by any event that, by interfering with presynaptic or postsynaptic function, reduces the size of the end plate potential.

John M. Newsom-Davis

 




 







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