Impulses are transmitted from neuron to neuron by the release of a transmitter chemical across synaptic clefts which separate neurons. This transmitter chemical is stored in synaptic vesicles of the presynaptic ending. One of the commonly referred to transmitter chemicals is acetylcholine..

When an impulse arrives at a presynaptic ending, calcium ions cause the vesicles to fuse with the presynaptic membrane and the transmitter chemical in these fused vesicles is ejected into the synaptic cleft.

The transmitter chemical diffuses across the cleft and combines with the postsynaptic receptors. Depending on the postsynaptic receptor, the transmitter chemical will produce either an excitatory postsynaptic potential (EPSP) or an inhibitory postsynaptic potential (IPSP). An EPSP has the effect of depolarizing the neuron and the IPSP has the effect of hyperpolarizing it.

What do you suppose happens if both an EPSP and an IPSP occur at a given neuron? The concept of neural summation provides the answer.

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