Synapses
- Where two neurones meet, they do not actually come into physical contact with each other
- Instead, a very small gap, known as the synaptic cleft, separates them
- The ends of the two neurones, along with the synaptic cleft, form a structure known as a synapse
- Synapses act as the junctions between any cells in the nervous system, e.g.
- In the sense organs, there are synapses between sensory receptor cells and sensory neurones
- In muscles, there are synapses between motor neurones and muscle fibres
A synapse
Synaptic transmission
- Electrical impulses cannot ‘jump’ across the synaptic cleft
- When an electrical impulse arrives at the end of the axon on the presynaptic neurone, the membrane of the presynaptic neurone becomes depolarised, triggering an influx of calcium ions into the presynaptic cell via calcium ion channels in the membrane
- The calcium ions cause vesicles in the presynaptic neurone to move towards the presynaptic membrane where they fuse with it and release chemical messengers called neurotransmitters into the synaptic cleft
- A common neurotransmitter is acetylcholine, or ACh
- The neurotransmitters diffuse across the synaptic cleft and bind with receptor molecules on the postsynaptic membrane; this causes associated sodium ion channels on the postsynaptic membrane to open, allowing sodium ions to diffuse into the postsynaptic cell
- If enough neurotransmitter molecules bind with receptors on the postsynaptic membrane then an action potential is generated, which then travels down the axon of the postsynaptic neurone
- The neurotransmitters are then broken down to prevent continued stimulation of the postsynaptic neurone
- The enzyme that breaks down acetylcholine is acetylcholinesterase
Synaptic transmission using the neurotransmitter acetylcholine
Unidirectionality
- Synapses ensure the one-way transmission of impulses
- Impulses can only pass in one direction at synapses because neurotransmitter is released on one side and its receptors are on the other – chemical transmission cannot occur in the opposite direction
- This prevents impulses from travelling the wrong way