Biopsychology: Synaptic Transmission

Once the action potential reaches the end of the axon it needs to be transferred to another neuron or tissue. It must cross over a gap between the pre-synaptic neuron and post-synaptic neuron – which is known as the synaptic gap. At the end of the neuron (in the axon terminal) are the synaptic vesicles which contains chemical messengers, known as neurotransmitters. When the electrical impulse (action potential) reaches these synaptic vesicles, they release their contents of neurotransmitters.

Neurotransmitters then carry the signal across the synaptic gap. They bind to receptor sites on the post-synaptic cell that then become activated. Once the receptors have been activated, they either produce excitatory or inhibitory effects on the post-synaptic cell.

Some neurotransmitters are excitatory and some are inhibitory. Excitatory neurotransmitters (e.g. noradrenaline) make the post-synaptic cell more likely to fire, whereas inhibitory neurotransmitters (e.g. GABA) make them less likely to fire. For example, if an excitatory neurotransmitter like noradrenaline binds to the post-synaptic receptors it will cause an electrical charge in the cell membrane which results in an excitatory post-synaptic potential (EPSP), which makes the post-synaptic cell more likely to fire. Whereas, if an inhibitory neurotransmitter like GABA binds to the post-synaptic receptors it will result in an inhibitory post-synaptic potential (IPSP), which makes the post-synaptic cell less likely to fire.