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Synapse - excitation transfer


Excitation transmission between nerve cells


If an action potential reaches a synaptic end-point, then the calcium channels and Ca are opened by the voltage change+Ions enter the synaptic endknob. As a result, the neurotransmitter-filled vesicles are pushed toward the synaptic cleft. There, the vesicles merge with the presynaptic membrane and release the neurotransmitters into the synaptic cleft. These migrate through the synaptic cleft and bind to the receptors of the postsynaptic membrane.
Receptors and ion channels are so interconnected that the ion channels open when a receptor is occupied by a neurotransmitter. So it comes to an influx of Na+ Ions into the postsynaptic membrane. This in turn ensures a positive depolarization in the dendrite and thus a transmission of electrical excitation in the episode neuron. If the threshold at the axon hill is exceeded, there is another action potential and the process repeats itself.
In the meantime certain enzymes provide for the breakdown of released neurotransmitters in the synaptic cleft. The split neurotransmitters diffuse back into the end-caps and are "recycled" by the cell. The longer the degradation of released neurotransmitters lasts, the longer the transmitted excitement will continue. Therefore, the degradation of the neurotransmitters by the enzymes is so important, otherwise the arousal would permanently stop and cramp the body.
It can therefore be stated that an electrical excitation in the synaptic cleft becomes a chemical signal (neurotransmitter), which in turn causes electrical excitation.
The figure above shows an example of the excitation transmission of two neurons. In the case of a nerve cell adjacent to a muscle cell, instead of a postsynaptic membrane, there is a so-called motor end plate (right figure, T = synaptic end point, M = muscle fiber). The corresponding neurotransmitter for such processes between nerve and muscle cells is called acetylcholine. The 'final' action potential then causes no forwarding of the stimulus anymore, but a muscle contraction.