To wrap up the muscle series we are looking at neuromuscular junctions – the synapses between motor neurones and muscle fibre cells. This article will relate closely to the synapse article, but there are some differences between cholinergic synapses and neuromuscular junctions.
Neuromuscular junction structure
There are three main differences between neuromuscular junctions and a cholinergic synapses:
- The presynaptic neurone is always a motor neurone, whereas in a cholinergic synapse it could also be a sensory neurone, relay neurone or bipolar neurone.
- The postsynaptic membrane in a neuromuscular junction is the sarcolemma (the cell-surface membrane of a muscle fibre cell) rather than another neurone. The sarcolemma has many receptors to bind to the neurotransmitter (acetylcholine).
- The sarcolemma has clefts containing the acetylcholinesterase enzyme. This enzyme breaks down acetylcholine. Inhibiting this enzyme with a drug could cause problems with muscle control due to too much acetylcholine binding to receptors.
Acetylcholine always has an excitatory effect at neuromuscular junctions, whereas in cholinergic synapses it can be excitatory or inhibitory. When acetylcholine binds to its receptors in a neuromuscular junction, Na+ channels open and Na+ diffuse into the muscle cell. If threshold is reached, the sarcolemma depolarises and the depolarisation spreads into the muscle fibre cells via the T-tubules, triggering the release of calcium ions from the sarcoplasmic reticulum. Muscle contraction begins.
Summary
- Neuromuscular junctions work in a similar way to cholinergic synapses with a few differences.
- They are found between motor neurones and muscle fibre cells, and use acetylcholine as an excitatory neurotransmitter.
- Acetylcholinesterase is present to break down acetylcholine.
