The gap between two nerve cells is called a synapse. Synapses have several roles in nervous communication. Primarily they allow action potentials to pass from one neurone to the next. The way the impulse is transmitted over synapses is by transmitter chemicals (called neurotransmitters). This means the transmission across synapses is significantly slower than the transmission along the axons of neurones.
The structure of a single synapse is shown in figure 10, below. It is important to realise that individual neurones may have synapses with many other neurones. In the CNS each interneurone may have thousands of synapses.
|Figure 10 : The structure of a single synapse
The sequence of events as information is transmitted across a synapse is as follows:
- When an action potential reaches the end of the presynaptic neurone it causes voltage gated calcium channels to open.
- Ca2+ enter the end of the presynaptic neurone (called the synaptic knob) and cause synaptic vesicles to fuse with the presynaptic membrane.
- Neurotransmitter molecules are released into the synaptic cleft and diffuse across to the postsynaptic membrane.
- The neurotransmitter molecules bind to specific receptor proteins in the post-synaptic membrane.
- This causes the sodium ion channels to open and sodium ions flood in.
- This causes a new action potential in the postsynaptic neurone.
- Specific enzymes in the synaptic cleft break down the neurotransmitter molecules.
- The breakdown products diffuse back into the pre-synaptic neurone and are used to re-synthesise more neurotransmitter molecules.
- This biosynthesis requires energy in the form of ATP, which is why many mitochondria are present.
There are several different molecules which act as neurotransmitters. The most common neurotransmitter is called acetylcholine ( ACh) it is released from neurones in the CNS and the PNS. It is the main neurotransmitter used in the somatic and the autonomic branches of the PNS.
Neuromuscular junctions are a specific type of synapse. They are the synapses formed between motor neurones and muscle cells. They use the neurotransmitter acetylcholine.