The origin of cell excitation

The mechanisms we have discussed above account for the propagation and for the termination of the action potential. However, so far we have relied on external electrodes for its initiation. Under physiological conditions,action potentials can be evoked in various ways. 

The first, very important means of action potential generation consists in synaptic transmission. A synapse coimects a presynaptic cell (always a neuron) to a postS5Uiaptic cell (a neuron or muscle cell). In brief, a synapse works as follows  

Excitation of the pres5uiaptic cell leads to the release of a neurotransmitter substance. 

The neurotransmitter binds to a receptor on the postsy-naptic cell, very commonly a ligand-gated charmel. 

The receptor charmel opens and locally depolarizes the membrane. 

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