Answer:
local depolarization of the sarcolemma
Explanation:
The neuromuscular junction is basically the set of an axon and a muscle fiber. The axon or nerve end adopts, at the end of the contact area with the muscle, an oval shape about 32 microns wide. In this final area of the axon are mitochondria and other elements that participate in the formation and storage of the neurotransmitter of muscle stimulation: acetylcholine.
The binding of acetylcholine to the motor plate receptor can be in two places, which are different in affinity and which will also define the affinity of certain acetylcholine agonists and antagonists. It is necessary that two molecules of acetylcholine be attached to the two sites of the receptor so that the receptor acquires a channel form inside and allows the transfer of ions that will allow the modification of the action potential. The ion traffic will consist of a passage of sodium and calcium cations inside and potassium out. The most striking flow from the quantitative point of view corresponds to sodium and potassium. Potassium follows a concentration gradient, while sodium takes a concentration and electrical gradient. The result is the increase in the positive charge on the inner side of the muscle cell membrane.
The opening of the receiver follows a phenomenon called "all or nothing". It means that when a sufficient number of receptors is open simultaneously, the depolarization threshold of the motor plate is exceeded and the action potential that propagates to the rest of the muscular membrane is triggered. Once the motor plate depolarizes the action potential runs through the entire sarcolemma.
