An impulse is an electric message transmitted along the nerve fibres. It moves very fast as ions. The nerve impulse is initiated by stimulation of receptors by a given stimulus e.g. light, sound etc. the stimulus causes enough stimulation to a point that triggers off an impulse called the threshold level. If the stimulation does not reach the threshold, no impulse is formed and that stimulus is not detected.

When a stimulus reaches a receptor cell, it generates an impulse which is passed to the cell body of a neuron. The impulse is then transmitted from one the dendrites of another neuron via a gap called the synapse.
The arrival of an impulse at the end of an axon triggers the release of the transmitter substance into the synapse. This diffuses across the gap and stimulates the dendrites of an adjacent neuron to form an impulse hence the impulse being passed on.
After the passage of an impulse across the synapse, the transmitter substance is destroyed and a new one is made within the axon.
The mechanism ensures that an impulse travels only in one direction across a synapse.

A synapse is a junction or space between the terminals of two adjacent neurons. This junction links the dendrites of one neuron to the dendrites of another adjacent neuron. Movement of impulses across the synapse occurs by secretion
of a chemical called a transmitter chemical in the space known as the synaptic cleft.

Structure of a synapse

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A single impulse may fail to get across the synapse and it may require two or more impulses arriving quickly in succession (one after the other); temporal or
simultaneously (at the same time) from two or more neurons. This is termed as summation.
Sometimes inhibitory chemicals are secreted into the synapse and when an impulse comes, it gets blocked. This is referred to as inhibition.

Functions of the synapse
1) It enables propagation (movement) of an impulse from one neuron to another.
2) It ensures that an impulse moves in one direction by having vesicles on one side of the synapse.
3) The synapse acts as a junction in the nervous system that can diverge, or converge information.
4) It prevents continuous stimulation of body organs.