6.2.10 Neuromuscular Junctions

Key Features of Neuromuscular Junctions

1. Structure:

• Similar to synapses between neurones, but occur between a motor neurone and a muscle fibre.
• The presynaptic membrane contains vesicles filled with acetylcholine (ACh).
• The postsynaptic membrane is the sarcolemma of the muscle fibre, containing nicotinic acetylcholine receptors.

2. Neurotransmitter:

• The neurotransmitter used is acetylcholine (ACh).

3. Action Potential Transmission:

• An action potential arrives at the presynaptic neurone, triggering the opening of voltage-gated calcium ion channels.
• Calcium ions enter the presynaptic terminal, causing vesicles filled with acetylcholine to fuse with the presynaptic membrane.
• Acetylcholine is released into the synaptic cleft by exocytosis.

4. Activation of the Muscle Fibre:

• Acetylcholine binds to receptors on the sarcolemma, causing sodium ion channels to open.
• Sodium ions enter the muscle fibre, causing depolarisation of the sarcolemma.
• This generates an action potential in the muscle, leading to muscle contraction.

5. Acetylcholine Breakdown:

• The enzyme acetylcholinesterase breaks down acetylcholine in the synaptic cleft into acetate and choline, stopping continuous muscle contraction.
• Choline is reabsorbed by the presynaptic neurone and recycled to form more acetylcholine.

Differences Between Neuromuscular Junctions and Synapses

Feature	Neuromuscular Junction	Synapse (Between Neurones)
Postsynaptic structure	Muscle fibre (sarcolemma)	Another neurone
Neurotransmitter	Always acetylcholine	Varies (e.g., acetylcholine, dopamine)
Action potential effect	Always excitatory, leading to muscle contraction	Can be excitatory or inhibitory
Response	Muscle contraction	Nerve impulse or inhibitory signal