Types of Synapse

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Neuromuscular Junction

A neuromuscular junction is a synapse between a motor neurone and a muscle cell. An action potential is transmitted across the synapse using the neurotransmitter acetylcholine. The steps involved are:

Arrival at the synaptic knob

An action potential arrives at the synaptic knob at the end of the motor neurone.
The action potential depolarises the membrane of the synaptic knob. This causes voltage-gated calcium (Ca²⁺) ion channels to open.
Ca²⁺ ions diffuse into the synaptic knob.

Release of acetylcholine (ACh)

The Ca²⁺ ion concentration inside the synaptic knob begins to increase.
This causes the synaptic vesicles to move and fuse with the presynaptic membrane.
Acetylcholine, the neurotransmitter inside the vesicles, is released into the synaptic cleft.
- This process is called exocytosis.

Binding to receptors

Acetylcholine binds to specific receptors on the postsynaptic membrane called nicotinic cholinergic receptors.
Binding of the neurotransmitter opens sodium ion channels in the postsynaptic muscle cell.
As Na⁺ ions diffuse into the cell, the membrane becomes depolarised.
If the potential difference reaches the threshold value, an action potential is generated and flows along the motor cell.

Removal of acetylcholine

An enzyme called acetylcholinesterase (AChE) breaks down acetylcholine in the synaptic cleft.
The products of the break down are reabsorbed by the presynaptic neurone and reused to synthesise more acetylcholine.
It is important that the acetylcholine is removed from the receptors. This stops action potentials from being continuously generated in the postsynaptic cell.

Cholinergic Synapses

Synapses that use acetylcholine as a neurotransmitter are called cholinergic synapses. Although acetylcholine is also used at a neuromuscular junction, there are some key differences:

Type of postsynaptic cell

Cholinergic synapses are between two neurones.
Neuromuscular junctions are between a motor neurone and a muscle cell.

Number of receptors

There are less receptors in the postsynaptic membrane at a cholinergic synapse than at a neuromuscular junction.

Type of response

A cholinergic synapse can trigger an inhibitory or excitatory response in the postsynaptic membrane.
An action potential at a neuromuscular junction always triggers an excitatory response in the muscle cell.

Result of depolarisation

In a cholinergic synapse, depolarisation of the postsynaptic membrane results in an action potential.
At a neuromuscular junction, depolarisation of the postsynaptic membrane results in muscle contraction.

Acetylcholinesterase

Acetylcholinesterase is the enzyme that breaks down acetylcholine after it has bound to the receptors on the postsynaptic membrane.
- In cholinergic synapses, the enzyme is located in the synaptic cleft.
- At a neuromuscular junction, the enzyme is stored in clefts in the postsynaptic membrane.