THE SYNAPSE

The synapse is where the nerve impulse passes from one cell to the next. In most cases the electrical signal (the action potential) stops and a chemical signal takes over to cross the gap between the cells.

The chemical messenger is called a neurotransmitter. It crosses the gap by diffusion, which creates a small delay, up to a millisecond, in the nerve signal transmission.

The advantage of using neurotransmitter is that the nerve impulse can be given some more specificity (all action potentials are the same, they only vary in their frequency and in the neurone they are travelling along). Neurotransmitters can also control the operation of the nervous system by inhibition or excitation. Many drugs that try to cure problems in the nervous system operate at synapses.

In many ways neurotransmitters are hormones working over a very short distance (about 20nm). Indeed some of them (e.g. noradrenaline) work both at synapses and in the circulatory system.

An action potential travels down an axon to the terminal buttons or synaptic knobs at the end.

The action potential depolarises the membrane of a terminal button causing voltage-gated Ca²⁺ channels to open.

Ca²⁺ ions flood into the terminal button. This stimulates hundreds of synaptic vesicles packed with neurotransmitter to fuse with the membrane of the terminal button. The Ca²⁺ ions are then pumped out again.

Exocytosis releases the neurotransmitter into the synaptic cleft.

The neurotransmitter rapidly diffuses across the cleft to the postsynaptic membrane.

The neurotransmitter molecules bind with specific receptor sites on the postsynaptic membrane.

The receptor sites are part of a ligand-gated ion channel. These channels let Na⁺ ions in or K⁺ ions out causing localised depolarisation of the membrane.

If the localised depolarisations build up to the nerve cell threshold, a full action potential will be produced. This will travel away, down the postsynaptic neurone.

The action of the neurotransmitters stops as they dilute by diffusion in the synaptic cleft and by hydrolysis through the action of enzymes there. This is most important. The signal must not be perpetuated indefinitely.

The motor end plate is the terminal button of a motor neurone which makes contact with a muscle cell. The motor end plate releases the neurotransmitter acetylcholine that ultimately causes the muscle cell to contract.

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