Transmission Across a Neuromuscular Junction
- Striated muscle contracts when it receives an impulse from a motor neurone via the neuromuscular junction
- Neuromuscular junctions work in a very similar way to synapses
- They are located between a neurone and a muscle cell
- When an impulse travelling along the axon of a motor neurone arrives at the presynaptic membrane, the action potential causes calcium ions to diffuse into the neurone
- This stimulates vesicles containing the neurotransmitter acetylcholine (ACh) to fuse with the presynaptic membrane
- The ACh that is released diffuses across the neuromuscular junction and binds to receptor proteins on the sarcolemma (surface membrane of the muscle fibre cell)
- This stimulates ion channels in the sarcolemma to open, allowing sodium ions to diffuse in
- This depolarises the sarcolemma, generating an action potential that passes down the T-tubules towards the centre of the muscle fibre
- These action potentials cause voltage-gated calcium ion channel proteins in the membranes of the sarcoplasmic reticulum (which lie very close to the T-tubules) to open
- Calcium ions diffuse out of the sarcoplasmic reticulum (SR) and into the sarcoplasm surrounding the myofibrils
- Calcium ions bind to troponin molecules, stimulating them to change shape
- This causes the troponin and tropomyosin proteins to change position on the thin (actin) filaments
- The myosin-binding sites are exposed to the actin molecules
- The process of muscle contraction (known as the sliding filament model) can now begin
- There are multiple neuromuscular junctions spread across several muscle fibres within the muscle
The structure of the neuromuscular junction
The steps involved in muscle contraction via the neuromuscular junction
Stopping muscle contraction
- In order to prevent the muscle from being continually stimulated by a single impulse the acetylcholinesterase enzyme present in the synaptic cleft breaks down the acetylcholine molecules
- Calcium ions are also pumped back into the SR once the sarcolemma, T tubules and SR are no longer polarised
- The movement of calcium ions terminates muscle contraction
Comparison of Cholinergic Synapses and Neuromuscular Junctions Table
Exam Tip
The images above describe the events that occur in a single muscle fibre (not the whole muscle). An impulse arriving at a motor endplate will always cause a contraction in that specific muscle fibre. However, if only a few impulses are sent via a small number of motor neurones then the total number of muscle fibres that contract will be minimal. If an insufficient number of muscle fibres contract, the whole muscle won't look like it has contracted at all!
