Neuromuscular Junction
Biology: Cambridge International A Level, topic 15.1.9
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.
Test yourself on Neuromuscular Junction
What is a neuromuscular junction between?
Transmission Across a Neuromuscular Junction
What happens after the arrival of the action potential at the synaptic knob?
Which of the following statements about acetylcholine and acetylcholinesterase are true?