Nervous and endocrine systems sample question 1

They block the presynaptic serotonin transporter, so serotonin stays longer in the cleft and keeps stimulating receptors.

Nervous and endocrine systems sample question 2

Inactivation of voltage-gated Na⁺ channels

Nervous and endocrine systems sample question 4

recall childhood events and perform learned motor skills but be unable to form new declarative memories.

Nervous and endocrine systems sample question 5

Extracellular K+ accumulates, which makes the K+ equilibrium potential less negative, depolarizes neurons and raises their excitability.

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
Selective serotonin reuptake inhibitors are used to treat depression. What is their direct effect at the synapse?
- A.
  They block postsynaptic serotonin receptors, so serotonin accumulates in the cleft.
- B.
  They block the presynaptic serotonin transporter, so serotonin stays longer in the cleft and keeps stimulating receptors.
- C.
  They inhibit serotonin synthesis in the presynaptic terminal, so less serotonin is released.
- D.
  They open voltage-gated Ca²⁺ channels in the terminal, so more vesicles fuse with the terminal membrane with each impulse.
Answer: B. They block the presynaptic serotonin transporter, so serotonin stays longer in the cleft and keeps stimulating receptors.
Reuptake by the presynaptic transporter ends the signal; blocking it raises synaptic serotonin and prolongs receptor activation. A would block the signal rather than strengthen it; C would lower serotonin levels; D describes a different mechanism that SSRIs do not have.
Question 2
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 3
A patient has a tremor that is absent at rest but grows worse as she reaches for a cup, along with a wide-based, unsteady walk. Which lesion best explains these findings?
- A.
  A lesion of the substantia nigra, with a resting tremor that eases on movement
- B.
  A lesion of the cerebellum
- C.
  A lesion of the primary somatosensory cortex on one side
- D.
  A lesion of Broca's area in the frontal lobe
Answer: B. A lesion of the cerebellum
B is correct: the cerebellum coordinates movement, and its damage gives an intention tremor and ataxia. A describes the pattern of Parkinson disease, a resting tremor that eases on movement; C would cause sensory loss; D would cause nonfluent speech.
Question 4
After bilateral damage to the hippocampi, a patient is most likely to
- A.
  form new memories normally but be unable to recall events from before the damage.
- B.
  form new declarative memories but lose previously learned motor skills such as riding a bicycle.
- C.
  recall childhood events and perform learned motor skills but be unable to form new declarative memories.
- D.
  show fluent speech with poor comprehension, with normal memory for events.
Answer: C. recall childhood events and perform learned motor skills but be unable to form new declarative memories.
The hippocampus is needed to consolidate new declarative (episodic and semantic) memories; damage causes anterograde amnesia while remote memories and procedural learning (basal ganglia, cerebellum) are largely spared. Loss of old memories with intact new learning (A) is the reverse pattern, loss of motor skills (B) is not hippocampal, and fluent poor-comprehension speech (D) is the Wernicke pattern.
Question 5
A drug blocks the uptake of extracellular K+ by astrocytes. During intense neuronal firing, which result is most likely?
- A.
  Extracellular K+ falls, which hyperpolarizes neurons and decreases their excitability.
- B.
  Extracellular K+ accumulates, which makes the K+ equilibrium potential less negative, depolarizes neurons and raises their excitability.
- C.
  Intracellular K+ rises in neurons, which hyperpolarizes them because K+ is positive.
- D.
  The Na+/K+ ATPase of neurons is activated directly by the drug, which increases the K+ gradient and thus keeps the resting potential more negative.
Answer: B. Extracellular K+ accumulates, which makes the K+ equilibrium potential less negative, depolarizes neurons and raises their excitability.
Astrocytes buffer K+ released during action potentials; without uptake, extracellular K+ rises, the K+ equilibrium potential and resting potential become less negative, and neurons are more excitable. A has the wrong direction of change, C confuses the effect of intracellular K+ with the resting potential, and D invents a drug action.

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