Nervous and endocrine systems sample question 1

Inactivation of voltage-gated Na⁺ channels

Nervous and endocrine systems sample question 2

Acetylcholine persists longer in the cleft and activates the remaining receptors more.

Nervous and endocrine systems sample question 3

A pituitary tumour that secretes TSH independently of feedback by thyroid hormone

MCAT · Biological and Biochemical Foundations · Biology

Nervous and endocrine systems

Nervous and endocrine systems is part of Biology in the MCAT Biological and Biochemical Foundations section. The questions below are written in the style of the real test, each with the answer and a worked explanation.

Question types you will see here include action potential and channel effects, myelin and conduction velocity, synaptic drug and toxin effects and summation of postsynaptic potentials.

Common traps: mixing which toxin blocks Na+ (rising phase) and K+ (falling phase); reversing the effect of high extracellular K+; confusing absolute and relative refractory periods.

Sample questions

Question 1
What is the main cause of the absolute refractory period in a neuron?
- A.
  Reversal of the Na⁺ concentration gradient
- B.
  Depletion of intracellular K⁺
- C.
  Closing of voltage-gated K⁺ channels
- D.
  Inactivation of voltage-gated Na⁺ channels
Answer: D. Inactivation of voltage-gated Na⁺ channels
After opening, voltage-gated Na⁺ channels enter an inactivated state and cannot reopen until the membrane repolarizes, so no second action potential can fire. The ion gradients barely change during a single action potential, so neither ion depletion nor gradient reversal is the cause.
Question 2
Why do acetylcholinesterase inhibitors improve muscle strength in patients with myasthenia gravis, a disease in which antibodies reduce the number of functional nicotinic acetylcholine receptors?
- A.
  The drugs directly depolarize the muscle cell membrane.
- B.
  The drugs increase synthesis of new nicotinic receptors.
- C.
  Acetylcholine persists longer in the cleft and activates the remaining receptors more.
- D.
  The drugs block the autoantibodies from binding to the acetylcholine receptors on muscle.
Answer: C. Acetylcholine persists longer in the cleft and activates the remaining receptors more.
Blocking acetylcholinesterase slows the breakdown of acetylcholine, so each nerve impulse produces a larger and longer-lasting activation of the receptors that remain. These drugs do not affect the autoimmune process or the number of receptors.
Question 3
A patient has a high plasma concentration of free thyroxine (T₄) and a high plasma TSH. Which explanation is most consistent with both results?
- A.
  Graves disease, in which antibodies stimulate the TSH receptor on thyroid cells
- B.
  Primary hypothyroidism caused by autoimmune destruction of the thyroid gland
- C.
  A pituitary tumour that secretes TSH independently of feedback by thyroid hormone
- D.
  Taking excess thyroxine tablets without medical supervision, which raises the T₄ in the blood
Answer: C. A pituitary tumour that secretes TSH independently of feedback by thyroid hormone
Normally high T₄ suppresses TSH by negative feedback, so finding both high means the TSH source is not responding to feedback: a TSH-secreting pituitary tumour drives the thyroid to overproduce. In Graves disease and with exogenous thyroxine, T₄ is high but TSH is suppressed; in primary hypothyroidism TSH is high because T₄ is low.

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