A-Level Biology October/November 2024 Q8(c): Lambert-Eaton myasthenic syndrome (LEMS) is a rare disorder of the neuromuscular juncti…

1 answer

• ▲0
  accepted ✓

  In LEMS, antibodies bind to and block the voltage-gated calcium channels on the presynaptic knob.

  1. When an action potential arrives at the presynaptic knob, fewer voltage-gated calcium channels can open. This results in a reduced influx of calcium ions ($Ca^{2+}$) into the presynaptic knob.
  2. As $Ca^{2+}$ ions are required to trigger the movement and fusion of synaptic vesicles with the presynaptic membrane, fewer vesicles fuse.
  3. Consequently, a smaller quantity of the neurotransmitter acetylcholine (ACh) is released into the synaptic cleft by exocytosis.
  4. Less ACh is available to bind to the receptors on the sarcolemma. This leads to fewer sodium ion channels opening, causing insufficient depolarisation of the sarcolemma. The threshold potential is less likely to be reached, generating fewer action potentials in the muscle fibre, which results in a weaker contraction.

  ---

  How the marks are awarded

  • B1 — Stating that fewer calcium ions enter the presynaptic knob.
  • B1 — Explaining that fewer vesicles move to and fuse with the presynaptic membrane.
  • B1 — Concluding that less acetylcholine (ACh) is released into the synaptic cleft.
  • B1 — Linking the reduced ACh to less depolarisation of the sarcolemma or the threshold potential not being reached.

  Common mistakes

  • Confusing LEMS with Myasthenia Gravis by stating that antibodies block the ACh receptors on the postsynaptic membrane.
  • Using vague terms like 'the signal is blocked' without specifying the ions, neurotransmitters, and potentials involved.
  • Incorrectly identifying the ions, for example stating that sodium ions enter the presynaptic knob to trigger vesicle release.
  • Failing to link the chain of events to the final outcome, e.g., stopping the explanation after 'less ACh is released' without explaining why this causes a weaker contraction.

  Examiner tip: For any physiological process, learn the precise sequence of events and the specific role of each molecule (e.g., ion, neurotransmitter, enzyme) at each stage.

  ---

  AI-generated model answer, grounded in the official Cambridge mark scheme and reviewed by the MarkScheme team. Mark your own answer to this question →

  @markscheme_answers

Your answer

Sign in to answer this question.