Neuromuscular junctions

A neuromuscular junction (or synapse) is the site of communication between a motor neurone and muscle fibres. there is a specific sequence of events, which results in the conduction of an action potential. This sequence is as follows:

• The action potential arrives at the presynaptic neurone and stimulates the opening of voltage-gated calcium channels.

• Calcium ions rush into the presynaptic neurone down their concentration gradient. This causes vesicles containing neurotransmitter to fuse with the presynaptic membrane.

• Neurotransmitter is release into the synaptic cleft via exocytosis.

• Neurotransmitter molecules diffuse across the synaptic cleft and bind to ligand-gated sodium channels on the postsynaptic membrane.

• These ligand-gated sodium channels open upon binding of the neurotransmitter and sodium ions rush into the postsynaptic cell down their concentration gradient. These channels also permit the flow of potassium ions out of the postsynaptic cell.

• The postsynaptic membrane is depolarised and an action potential is triggered in the postsynaptic cell. 

However, this does not always occur. There are two types of synapse, which are excitatory and inhibitory. At an excitatory synapse, the above events cause the depolarisation of the postsynaptic membrane resulting in an action potential. However, at an inhibitory synapse, the neurotransmitter binds to an inhibitory receptor on the postsynaptic membrane, causing the opening of potassium or chloride ion channels,hence leading to an influx of potassium or chloride ions from the cytosol. This inhibits the generation of an action potential as the membrane becomes hyper-polarised. A single neurone can be affected by multiple signals received at both inhibitory and excitatory synapses. An action potential is only generated if the sum of these depolarisation's reaches threshold potential .