5.3.8 The Role of Synapses

• The junction where two neurones meet is known as a synapse
• A synapse consists of a presynaptic knob, synaptic cleft and postsynaptic membrane 
• Synapses that use acetylcholine (ACh) as a neurotransmitter are described as cholinergic synapses

The structure of a cholinergic synapse

Transmission at a synapse

• Electrical impulses cannot ‘jump’ across synapses
• When an electrical impulse arrives at the end of the axon on the presynaptic neurone (also called the synaptic knob), chemical messengers called neurotransmitters are released from vesicles at the presynaptic membrane 
• The neurotransmitters diffuse across the synaptic cleft and temporarily bind with receptor molecules on the postsynaptic membrane
• This stimulates the postsynaptic neurone to generate an electrical impulse that then travels down the axon of the postsynaptic neurone
• The neurotransmitters are then destroyed or recycled to prevent continued stimulation of the second neurone, which could cause repeated impulses to be sent

The events that occur when an action potential reaches a cholinergic synapse