CIE A Level Biology (9700) 2019-2021

Revision Notes

15.1.9 Stimulating Contraction in Striated Muscle

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.

Author:

Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.
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