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

Neuromuscular-junction

The structure of the neuromuscular junction

 

 

Stimulation-of-muscle-contraction-1Stimulation-of-muscle-contraction-2

The steps involved in muscle contraction via the neuromuscular junction

 

Comparison of Cholinergic Synapses and Neuromuscular Junctions

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.

 

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