Explain the process that occurs from when the action potential reaches the pre-synaptic knob to transmission in the next neurone. (6 Marks)

Once an action potential reaches the synaptic cleft of a pre-synaptic neuron, voltage-gated calcium channels open. It is the depolarisation of the membrane that causes the opening. Calcium ions subsequently enter the cell down its electrochemical gradient through these channels, leading to an increase in intracellular calcium ion concentration. This triggers the exocytosis of neurotransmitters which involves vesicle fusion with the membrane and the release of the transmitters into the synaptic cleft.
The neurotransmitters then diffuse across the synaptic cleft and bind to ligand-gated sodium channels associated with the membrane of the post-synaptic neurone. This leads channel opening, and in increase in permeability to sodium ion movement across the cell membrane; depolarisation follows and if the threshold is reached, an action potential is generated in the post-synaptic neuron. Neurotransmitters are eventually broken down by enzymes or recycled back into the pre-synaptic neurone, terminating transmission. One example of a typical neurotransmitter is acetylcholine which binds to nicotinic acetylcholine receptors. These receptors are usually found post-synaptically. The opening of these channels leads to a net influx of positive ions into the cell, which can result in depolarisation of the post-synaptic neuron. Acetylcholine is broken down by acetylcholinesterase.