Inhibitory synapses

Excitatory synapses cause an impulse to be transmitted to another neurone by stimulating an action potential to occur in the postsynaptic neurone. An example of an excitatory synapse is a cholinergic synapse, that uses the neurotransmitter acetylcholine. Inhibitory synapses prevent an action potential from occurring in the postsynaptic neurone by entering the neurone into a state of hyperpolarisation. An example of an inhibitory synapse is some synapses in the brain, where GABA is the neurotransmitter used.

1. There is an influx of sodium ions into the synaptic knob of the presynaptic neurone, increasing the positive charge in the axon.

2. The increase in positive charge stimulates voltage-gated calcium ion channels to open, and calcium ions diffuse into the synaptic knob. The calcium ions induce vesicles containing GABA to move down the synaptic knob and fuse with the pre-synaptic membrane, releasing the neurotransmitter into the synaptic cleft.

3. GABA diffuses across the synaptic cleft and binds to receptor sites on chloride ion channels on the post-synaptic neurone membrane. This causes the chloride ion channels to open, and chloride ions to diffuse into the postsynaptic neurone. Potassium ion channels are also triggered to open, so potassium ions diffuse out of the postsynaptic neurone. This means that the neurone enters a state of hyperpolarisation and an action potential cannot be generated.

This is important during summation, for example, during spatial summation, as there may be a mix of inhibitory and excitatory neurones connected to another neurone to control whether threshold potential is reached. The excitatory neurones have to summate to overcome the hyperpolarisation caused by the inhibitory neurone to trigger an action potential in the postsynaptic neurone.