Surely a direct connection (i.e. an electrical synapse) between motor neurone and the sarcolemma would allow for much faster neuromuscular transmission? It is my understanding that chemical synapses are only useful in facilitating spatial summation of action potentials from multiple presynaptic terminals... muscles have no need for this as they are innervated by a single motor neurone only?
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$\begingroup$ The motor neurons develop from one germ layer, the muscles from another. They are different cells. Are you asking why evolution didn't happen in another way? $\endgroup$– kmmNov 15, 2017 at 1:49
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$\begingroup$ @kmm An electrical synapse can be between any cells. $\endgroup$– Bryan Krause ♦Nov 15, 2017 at 4:42
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$\begingroup$ @kmm I am essentially asking whether there is any advantage in having a chemical synapse at a neuromuscular interface over having a electrical one (and from my understanding I can't see any). It seems strange to me that evolution would result in such a seemingly convoluted mechanism for neuromuscular transmission; I feel like there must be a good reason for having a chemical transmission $\endgroup$– ogu01Nov 15, 2017 at 15:29
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$\begingroup$ @BryanKrause Can be, but generally aren't: hence my question for the OP about why evolution didn't happen another way. $\endgroup$– kmmNov 15, 2017 at 22:20
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$\begingroup$ My point was just that them being different cell types doesn't exclude electrical synapses; maybe you weren't trying to imply that but your comment seemed to. Both chemical and electrical synapses are highly complex structures, chemical synapses arguable more so, so there's no a priori reason they couldn't form between dissimilar cells $\endgroup$– Bryan Krause ♦Nov 16, 2017 at 0:47
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After much searching on the internet I've found an answer to this question: the neuromuscular junction facilitates amplification of the motor neurone potential to generate a large enough current to depolarise a muscle fibre as muscle fibres have a much larger area of membrane (ie a larger capacitance) than neurons.
Source: Encyclopedia of Neuroscience: Volume One
The reason is the tremendous amplification of the neuromuscular junction. The amplification occurs since a small current is generated in a nerve can rekease large amounts of molecules of the transmitter acetyl-choline ($\require{mhchem} \ce{ACh}$), each of which can open channels in the much larger muscle membrane and can create much larger currents. In addition, a motor nerve fiber often branches profusely to innervate hundreds of muscle fibers, thus providing further amplification.
Finally, the capacitance of the muscular membrane has long been known to be much greater than the capacitance of the nerve membrane, because of the additional membranous systems, such as the transverse tubules in muscular fibers. Thus the capacity of the muscle to store charge is much greater than that of a nerve membrane and much more current must be supplied to depolarize muscle cells directly.
