5.5.15 Transmission Across a Neuromuscular Junction (learning)

Question 1

What is a neuromuscular junction?

Answer 1

• located between neurone and muscle cell
• impulse travels from axon of motor neurone to presynaptic membrane
• action potential causes calcium ions to diffuse into neurone
• Stimulates vesicles containing ACh to fuse with presynaptic membrane
• ACh released and diffuses across neuromuscular junction binding to receptor proteins on sarcolemma
• Ion channels in sarcolemma open and sodium ions diffuse in
• Depolarises sarcolemma
• Action potential passes down T-tubules to centre of muscle fibre
• Voltage gated calcium ion channel protein in membranes of sarcoplasmic reticulum open
• Calcium ions diffuse out of SR and into sarcoplasm surrounding myofibrils
• Calcium ions bind to troponin molecules (initiating muscle contraction)
• Troponin and tropomyosin proteins change position on thin actin filaments
• Myosin-binding sites exposed to actin molecules
• Muscle contraction occurs

Question 2

How to stop muscle contraction

Answer 2

• Acetylcholinesterase enzyme present in synaptic cleft
• Breaks down ACh
• Calcium ions pumped back into SR once sarcolemma, T tubules and SR are no longer polarised
• Movement of Calcium ions back into SR terminates muscle contraction