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  1. 1-biological-molecules

    1-1-biological-molecules-carbohydrates
    11 主题
  2. 1-2-biological-molecules-lipids
    3 主题
  3. 1-3-biological-molecules-proteins
    5 主题
  4. 1-4-proteins-enzymes
    12 主题
  5. 1-5-nucleic-acids-structure-and-dna-replication
    8 主题
  6. 1-6-atp-water-and-inorganic-ions
    4 主题
  7. 2-cell-structure
    2-1-cell-structure
    7 主题
  8. 2-2-the-microscope-in-cell-studies
    4 主题
  9. 2-3-cell-division-in-eukaryotic-and-prokaryotic-cells
    8 主题
  10. 2-4-cell-membranes-and-transport
    7 主题
  11. 2-5-cell-recognition-and-the-immune-system
    7 主题
  12. 2-6-vaccines-disease-and-monoclonal-antibodies
    6 主题
  13. 3-exchange-and-transport
    3-1-adaptations-for-gas-exchange
    6 主题
  14. 3-2-human-gas-exchange
    10 主题
  15. 3-3-digestion-and-absorption
    5 主题
  16. 3-4-mass-transport-in-animals
    6 主题
  17. 3-5-the-circulatory-system-in-animals
    8 主题
  18. 3-6-mass-transport-in-plants
    6 主题
  19. 4-genetics-variation-and-interdependence
    4-1-dna-genes-and-chromosomes
    7 主题
  20. 4-2-dna-and-protein-synthesis
    6 主题
  21. 4-3-genetic-diversity-mutations-and-meiosis
    7 主题
  22. 4-4-genetic-diversity-and-adaptation
    6 主题
  23. 4-5-species-and-taxonomy
    4 主题
  24. 4-6-biodiversity
    9 主题
  25. 5-energy-transfers-in-and-between-organisms-a-level-only
    5-1-photosynthesis-a-level-only
    5 主题
  26. 5-2-respiration-a-level-only
    7 主题
  27. 5-3-energy-and-ecosystems-a-level-only
    9 主题
  28. 5-4-nutrient-cycles-a-level-only
    4 主题
  29. 6-organisms-respond-to-changes-in-their-environments-a-level-only
    6-1-response-to-stimuli-a-level-only
    12 主题
  30. 6-2-nervous-coordination-a-level-only
    10 主题
  31. 6-3-skeletal-muscles-a-level-only
    6 主题
  32. 6-4-homeostasis-a-level-only
    11 主题
  33. 7-genetics-populations-evolution-and-ecosystems-a-level-only
    7-1-inheritance-a-level-only
    6 主题
  34. 7-2-populations-a-level-only
    3 主题
  35. 7-3-evolution-a-level-only
    5 主题
  36. 7-4-populations-in-ecosystems-a-level-only
    7 主题
  37. 8-the-control-of-gene-expression-a-level-only
    8-1-genetic-mutations-a-level-only
    2 主题
  38. 8-2-regulation-of-gene-expression-a-level-only
    11 主题
  39. 8-3-using-genome-projects-a-level-only
    4 主题
  40. 8-4-gene-technologies-a-level-only
    13 主题
  41. exam-guidance-and-skills
    essay-guidance
    3 主题
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Neuromuscular junctions

  • Neuromuscular junctions are specialised synapses located at the junctions between neurones and muscles

    • The presynaptic cell is a motor neurone

    • The postsynaptic cell is a muscle cell

Diagram of a neuromuscular junction: synaptic knob, vesicles with acetylcholine, motor neurone axon, synaptic cleft, sarcolemma, mitochondria, nucleus.
Neuromuscular junctions are specialised synapses located between motor neurones and muscle cells

Transmission at a neuromuscular junction

  • When an impulse travelling along the axon of a motor neurone arrives at a neuromuscular junction, the signal is transmitted at the synapse as follows:

    1. calcium ion channels open and calcium ions diffuse into the neurone

    2. vesicles containing the neurotransmitter acetylcholine (ACh) fuse with the presynaptic membrane

    3. ACh is released and diffuses across the cleft where it binds to receptor proteins on the sarcolemma

    4. sodium ion channels in the sarcolemma open, allowing sodium ions to diffuse into the muscle cell

    5. the sarcolemma is depolarised, and if a threshold is reached then an action potential is generated

    6. the action potential is transmitted into the interior of the muscle cell via deeply infolded regions of the sarcolemma known as T-tubules

    7. voltage-gated calcium ion channels in the membranes of the sarcoplasmic reticulum open

    8. calcium ions diffuse out of the sarcoplasmic reticulum and into the sarcoplasm surrounding the myofibrils

    9. the protein filaments inside muscle cells move, and muscle contraction occurs

    10. When stimulation ends the enzyme acetylcholinesterase (AChE) breaks down ACh and the products are reabsorbed by the presynaptic cell

Diagram illustrating neuromuscular junction, with motor neurone, action potentials, ion movements and muscle fibre. Key for ACh, Na+, and Ca2+ pathways.
Sequence of transmission at a neuromuscular junction, numbered 1 to 9, detailing action potential, ion diffusion, and ACh binding leading to muscle contraction.

Comparing cholinergic synapses and neuromuscular junctions

  • As neuromuscular junctions are a types of synapse, they share many features with cholinergic synapses, e.g. in both:

    • calcium ions trigger the release of neurotransmitter

    • acetylcholine is the neurotransmitter

    • acetylcholine moves across the cleft by diffusion

    • acetylcholine binds to receptors on the postsynaptic membrane to open sodium ion channels

    • acetylcholine is broken down by AChE

  • There are also many differences between the two types of synapse:

Cholinergic synapses

Neuromuscular junctions

Between two neurones

Between a motor neurone and a muscle cell

Postsynaptic membrane has no folds

Postsynaptic membrane has folds in which AChE can be stored

No T-tubules

T-tubules are present

Can be excitatory or inhibitory

Always excitatory

ACh binds to receptors on postsynaptic cell

ACh binds to receptors on muscle cell

Action potential may be initiated and transmitted onwards

The end of a nerve pathway