Synapse or Neuro-junction

Synapse is the junction between dendrites of two adjacent neurones.

Function: allow the transmission of nerve impulses from neurone to neurone.

In the pre-synaptic knob, there are vesicles which contain a transmitter substance which is usually acetylcholine.

When an impulse reaches the synaptic knob, it stimulates the vesicles to move towards the pre-synaptic membrane releasing the acetylcholine.

This transmitter substance then diffuses across the synaptic cleft to the post-synaptic membrane which then becomes depolarized.

Sodium ions from the cleft then flow through the post-synaptic membrane into the post-synaptic knob causing an action potential at this point. This action potential is then transmitted as a nerve impulse along the neurone.

Immediately afterwards the acetylcholine liberated in the synaptic cleft is destroyed by an enzyme called cholinesterase into inactive end 1-1 products namely; choline and acetic acid (ethanoic acid).

These end-products are reabsorbed by the axon terminals and reconstituted into acetylcholine using energy in form of ATP provided by mitochondria.

This rapid breakdown of acetylcholine is necessary to re-polarise the pre-synaptic knob membrane for the next nerve impulse propagation so that there is no merging of the successive nerve impulse from neurone to neurone.