Answer
Excitation of a nerve fiber leads to excitation of its targeted muscle fiber by the following process;
When the signal (AP) arrives at he synaptic knob of a nerve fiber, it opens voltage-gated ca++ ion channels. and Ca++ ions pour out into the sarcoplasm of the synaptic knob. . Calcium ions stimulate the synaptic vesicles to release acetylcholine(ACH) by exocytocis into the synaptic cleft. The ACH molecules cross the synaptic cleft and bind to the acetylcholine receptors (nicotinic) in the sarcolemma of the motor and plate facing the presynaptic neuron. The diffusion of Na+ ions down their electrochemical gradient into the sarcoplasm depolarises the myocyte locally.
Work Step by Step
The binding of ACH molecules to ACHRs causes chemical-gated ion channels in the sarcolemma to open. As a consequence, Na+ ions pour (diffuse) into the sarcoplasm down their electrochemical gradient depolarize local areas of the myocyte. This means the potential may go from RMP of -90 mV to zero -and may overshoot zero to positive +30 mV. The K+ ion gate are slow to open , and they open after the Na+ ion channels. When the K+ ion channels open K+ ions diffuse out of the cell being repelled by the Na+ ions and moved by the propensity to flow down their chemical gradient. As a consequence the potential in the cell goes from negative (-90 mV) to zero to peak positive voltage difference. As the Na+ ions become less active and the Na+ inflow and the K+ out flow equalize, the RMP is re-established and the cell my even hyperpolarize. This generates MEPPs which summate to form an end plate potential (EPP). The rapid fluctuation of cell potential from RMP to peak and back again is the EPP. hen EPP reach threshold they trigger action potentials which are travelling waves of potential that spread across muscle fibers.
The EPP triggers the opening of voltage-gated ion channels ( Na+ and K+). If the EPP reches threshold it will trigger an action potential at first locally. But the action potential will spread from point to point along the sarcolemma . Tus the muscle fiber is excitedand ready for excitation- contraction coupling preparatory to contraction.
The muscle cell becomes excited as the AP is propagated over the entire cell
