5.5.15 Transmission Across A Neuromuscular Junction Flashcards
How does transmission across a neuromuscular junction occur?
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
How can muscle contraction be stopped?
In order to prevent the muscle from being continually stimulated by a single impulse the acetylcholinesterase enzyme present in the synaptic cleft breaks down the acetylcholine molecules
Calcium ions are also pumped back into the SR once the sarcolemma, T tubules and SR are no longer polarised
The movement of calcium ions terminates muscle contraction
