According to this theory when a depolarization wave or nerve impulse, after transmitting down the presynaptic neuron, reaches at the synapse it brings about the release of a chemical compound (neurohumor) which is known as a transmitter ; this substance in the gap of synapse is responsible for the conduction of nerve impulse through synapse.
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This transmitter is a acetylcholine. This substance goes on accumulating in the cleft of synapse to an extent that it causes depolarization of the dendrites of the postsynaptic neuron and now the wave of impulse is triggered in the second neuron.
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Acetylcholine is secreted only by the axon terminations—this is the reason why the axon is not excited anywhere and the nerve impulse travels in it both direction, it will be continued further only on the synaptic end of the axon and not on the dendritic end.
This is the answer to the question why the nerve impulses always travel in a single direction.
For a proper continuous working of the synapse it is necessary that chemical excitation must come to an end after it has acted, so that the synapse may become ready for the passage of the next impulse.
So, the act of removing acetylcholine is performed by an enzyme called cholinesterase. The time taken for the impulse to travel from one neuron to another through the “synapse is 0-6 milli second.
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It is also believed that single impulse dies out on reaching synapse but several impulses reaching a synapse within a short period “fire” the impulses into the next neuron.
The reason for it may be that a single impulse is unable to produce the adequate quantity of the transmitting substance.
On the contrary, this also has been found that a certain synapse gets ‘fatigued’ by working repeatedly, and its capacity to transmit subsequent impulses is reduced.
The possible reason for it may be that the secretory reserve of that particular transmitting substance is depleted.