Describe the synaptic transmission of nerve impulses.

The nerve impulse arrives at the pre-synaptic knob. Gated voltage-sensitive calcium ion channels open and calcium ions rapidly diffuse into the pre-synaptic knob. The influx of calcium ions stimulates synaptic vesicles full of the neurotransmitter acetylcholine to fuse with the pre-synaptic membrane. Acetylcholine is released into the synaptic cleft by exocytosis. Acetylcholine diffuses across the synaptic cleft. It bonds with receptor sites associated sodium ion channels, causing them to open. Sodium ions rapidly diffuse in and depolarise the post-synaptic membrane. If there is adequate depolarisation then an action impulse will be initiated in the post-synaptic neurone. Acetylcholine remaining in the synaptic cleft with be rapidly broken down by the enzyme acetylcholinesterase to prevent repeated stimulation of the post-synaptic neurone. The products (choline and ethanoic acid) diffuse back across the synaptic cleft into the pre-synaptic knob. ATP is then required to reform acetylcholine and ‘package’ it in the synaptic vesicles ready for the arrival of the next nerve impulse.