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For the question below,

electric stimulation of the musculocut. nerve causes a rapid twitch of the biceps. EM shows numerous vesicles within musculocutaneous nerve terminals. in order for these vesicles to release their contents into the synaptic cleft which of the following substances is required?

I put sodium, thinking you would need depolarization first in order for the calcium to be released, but the answer is calcium. Could anyone explain this concept?? I don't seem to understand what really goes on at the neuromuscular junction
 

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Na+ is important for activating voltage-gated calcium channels. Ca2+ binds to receptors on neurotransmitter vesicles, inducing their fusion with the presynap membrane and subsequent release.

So, calcium influx is directly related to vesicle release. Sodium is important since its influx (depolarization) activates the calcium channels.

Hope that helps!
 

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Na+ is important for activating voltage-gated calcium channels. Ca2+ binds to receptors on neurotransmitter vesicles, inducing their fusion with the presynap membrane and subsequent release.

So, calcium influx is directly related to vesicle release. Sodium is important since its influx (depolarization) activates the calcium channels.

Hope that helps!
Actually this is not entirely accurate.

When the vesicles reach the synapse they are loaded with the neurotransmitter via an active process that involves a proton pump ATPase which provides an electrochemical gradient.

After the action potential, calcium comes into to the presynaptic membrane. Calcium then binds to specific proteins in the cytoplasm, one of which is synaptotagmin, which in turn triggers the complete fusion of the synaptic vesicle with the cellular membrane.

Hope its more clear.
 
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