Stimulating Contraction in Striated Muscle
- Striated muscle contracts when it receives an impulse from a motor neurone via the neuromuscular junction
- 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 on the actin molecules
- The process of muscle contraction (known as the sliding filament model) can now begin
Exam Tip
You may have noticed that there are a lot of similarities between the events at the neuromuscular junction and those that occur at cholinergic synapses. A cholinergic synapse is between two neurones, a neuromuscular junction is between a neurone and muscle. Make sure you understand the similarities and differences and don’t get confused between the two.
