Synapses (Edexcel GCSE Biology: Combined Science)

• Neurones do not actually come into direct contact with each other
• Where the dendrites of two neurones meet (to make a connection between the neurones) a junction known as a synapse is formed
• At a synapse, there is a very small gap between neurones
  • This very small gap is known as the synaptic cleft or synaptic gap

• Electrical impulses cannot travel directly from one neurone to the next due to the synaptic cleft (electricity cannot 'jump' the gap)
• Instead, the electrical signal is briefly converted to a chemical signal that can cross the synaptic cleft
  • The chemical signalling molecules used to transfer the signal between neurones at a synapse are known as neurotransmitters

• Once these neurotransmitters cross the synaptic cleft and meet the neurone on the opposite side, the signal is converted back into an electrical impulse, which can then pass along the neurone

A synapse

How an impulse is passed across a synapse

• The electrical impulse travels along the first axon (of the first neurone, known as the presynaptic neurone)
• This triggers the end of the presynaptic neurone to release chemical messengers called neurotransmitters from vesicles
  • These vesicles fuse with the presynaptic membrane, releasing their contents into the synaptic cleft

• The neurotransmitters diffuse across the synaptic cleft and bind with receptor molecules on the membrane of the second neurone (known as the postsynaptic membrane)
• This stimulates the second neurone to generate an electrical impulse (which then travels down the second axon)
• The neurotransmitters are then destroyed to prevent continued stimulation of the second neurone (otherwise the neurotransmitters would cause repeated impulses to be sent)
• Synapses ensure that impulses only travel in one direction, avoiding the confusion that would be caused within the nervous system if impulses were able to travel in both directions

How an impulse is passed on at a synapse