Describe the process that allows an action potential to be passed across a synapse in the parasympathetic nervous system.

The action potential arrives at the presynaptic knob and causes the depolarisation of the plasma membrane. This causes voltage gated calcium channels to open allowing calcium to diffuse into the presynaptic knob. The influx of calcium causes vesicles containing acetylcholine (Ach) to move to and fuse with the plasma membrane of the presynaptic knob and Ach is released into the synaptic cleft by exocytosis. The Ach diffuses across the synaptic cleft and binds to complementary receptors on sodium channels in the postsynaptic membrane. This causes the sodium channels to open and sodium to diffuse into the postsynaptic knob causing the depolarisation of the postsynaptic membrane. The depolarisation causes a generator potential in the postsynaptic knob and if this potential exceeds the +40mV threshold then a new action potential will be propagated. In order to prevent the continuous firing of action potentials, the Ach in the synaptic cleft will be broken down by acetylcholinesterase into choline and acetate.