We know that nerve impulses are carried along neurones in a wave of depolarisation. But how does a nerve impulse get transmitted from one neurone to the next? Well, that is where synapses come in.
Structure of synapses
Synapses are the microscopic gaps between neurones. The neurone before the synapse is called the presynaptic neurone, and has a rounded end called the synaptic knob. This contains a number of synaptic vesicles filled with neurotransmitters. These are chemicals which carry the message across the synaptic cleft (the gap). The neurone after the synapse is called the postsynaptic neurone. Receptors specific to the neurotransmitter are embedded in the cell membrane of the postsynaptic neurone.
Transmission of nerve impulses across cholinergic synapses
A cholinergic synapse is a type of synapse that uses acetylcholine as the neurotransmitter. The nerve impulse is transmitted across it as follows:
- An action potential arrives at the synaptic knob which stimulates voltage-gated calcium (Ca2+) ion channels to open in the presynaptic neurone membrane.
- Ca2+ ions diffuse into the synaptic knob through the open ion channels which triggers the synaptic vesicles to move towards the presynaptic membrane.
- The vesicles fuse with the membrane and release acetylcholine (the neurotransmitter) into the synaptic cleft. This is an example of exocytosis.
- Acetylcholine diffuses across the synaptic cleft and binds to acetylcholine receptors on the postsynaptic membrane.
- Sodium (Na+) ion channels open in the postsynaptic neurone membrane and Na+ ions diffuse into the neurone. The membrane is depolarised and the action potential continues if the threshold is reached.
- Acetylcholine is broken down by acetylcholinesterase in the synaptic cleft, and the products of the reaction are taken back up into the presynaptic neurone to be recycled. The Ca2+ ions are removed from the synaptic knob by active transport.
- Synapses transmit nerve impulses between neurones.
- Movement of calcium ions triggers synaptic vesicles to release the neurotransmitter into the synaptic cleft.
- Neurotransmitters bind to specific receptors on the postsynaptic membrane.