Explain the process of synaptic transmission.

The arrival of an action potential at the presynaptic neurone causes voltage gated Ca2+ channels to open, allowing Ca2+ to diffuse into the synaptic knob. The increased intracellular Ca2+ concentration in the presynaptic neurone causes vesicles containing the neurotransmitter to move to the presynaptic membrane and fuse with it. It releases the neurotransmitters into the synaptic cleft by a process called exocytosis. These neurotransmitters bind to ligand gated receptors on the postsynaptic neurone membrane. Activation of these receptors by the ligand opens the Na+ channel proteins so that Na+ enters the presynaptic neurone and creates a new action potential in the neurone.
Using the cholinergic synapse as an example, acteylcholinesterase hydrolyses the neurotransmitter acetylcholine. This allows the neurotransmitter to be recycled and reformed in the presynaptic neurone. The absence of acetylcholine in the synaptic cleft deactivates the postsynaptic receptors so there is no longer an influx of Na+ into the post synaptic neurone.