AQA A Level Biology复习笔记6.2.10 Neuromuscular Junctions

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


Comparison of Cholinergic Synapses and Neuromuscular Junctions


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. The main difference is that a cholinergic synapse is located between two neurones whereas a neuromuscular junction is between a neurone and muscle.