The Synapse (Cambridge (CIE) O Level Biology): Revision Note

Structure of a Synapse

• The junction between two neurones is known as a synapse

• A synapse consists of:

  • The synaptic cleft, also known as the synaptic gap

  • The presynaptic knob; here vesicles are found which contain neurotransmitter molecules

  • The postsynaptic membrane, which contains receptor proteins

The junctions between neurones are known as synapses

Synapses & Neurotransmitters

• Neurones never touch each other

• The junctions (gaps) in between neurones are called synapses

• The electrical impulse travels along the first axon

• This triggers the nerve-ending of the presynaptic neurone to release chemical messengers called neurotransmitters from vesicles which fuse with the presynaptic membrane

• The neurotransmitters diffuse across the synaptic gap (or cleft) and bind with receptor molecules on the membrane of the second neurone (known as the postsynaptic membrane)

  • The vesicles stay in the presynaptic membrane

• This stimulates the second neurone to generate an electrical impulse that travels down the second axon

• The neurotransmitters are then destroyed to prevent continued stimulation of the second neurone which would cause repeated impulses to be sent

• Synapses ensure that impulses only travel in one direction, avoiding confusion within the nervous system if impulses were travelling in both directions

• As this is the only part of the nervous system where messages are chemical as opposed to electrical, it is the only place where drugs can act to affect the nervous system - e.g. this is where heroin works

Synapses and Neurotransmitters Diagram

How an impulse is passed on at a synapse