AP Psychology: The Neuron and Neural Firing (Drill 3)

Question 1. A neuroscience student is reviewing a diagram of a neuron. She identifies a long, thin projection that carries electrical signals away from the cell body toward the terminal buttons. Which structure is she describing?

• A) A dendrite, which receives incoming signals from other neurons
• B) The myelin sheath, which insulates the neuron and speeds signal transmission
• C) The axon, which transmits the electrical impulse away from the cell body ✓
• D) The synapse, which is the gap between two communicating neurons

Explanation: The axon is the elongated fiber that conducts the action potential away from the cell body toward the axon terminals, where neurotransmitters are released. (A) is a common misconception based on confusing the two main projections of a neuron: dendrites receive incoming signals toward the cell body, while the axon sends signals away. (B) is a true-but-irrelevant distractor: the myelin sheath does surround the axon and speed transmission, but the student is identifying the projection itself, not its insulating covering. (D) is wrong because a synapse is not a structure of the neuron; it is the junction between neurons. [Practice 1]

Question 2. A researcher applies a weak electrical current to a neuron in a lab dish. The current is below the level needed to produce a response. She gradually increases the intensity. Suddenly, at threshold, the neuron fires a full-strength action potential. Which principle does this demonstrate?

• A) Summation, in which multiple synaptic inputs combine to bring a neuron closer to threshold
• B) Reuptake, in which neurotransmitters are reabsorbed after the signal is transmitted
• C) The refractory period, in which a neuron cannot fire again immediately after activation
• D) The all-or-none principle, in which a neuron either fires completely or not at all ✓

Explanation: The all-or-none principle states that once the threshold for an action potential is reached, the neuron fires at full strength; there is no partial firing. The researcher's observation (no response below threshold, then a complete action potential at threshold) is a direct demonstration of this principle. The AP Psychology CED uses the term "all-or-none principle." (A) describes summation, which involves combining inputs from multiple synapses or repeated stimulation to reach threshold, a different process from the single-stimulus threshold response demonstrated here. (C) is a true-but-irrelevant distractor: the refractory period does follow an action potential and limits refiring, but the scenario describes what happens leading up to the first firing, not what occurs immediately after. [Practice 1]

Question 3. A pharmacologist develops a drug that binds to postsynaptic receptor sites but does not activate them, effectively blocking natural neurotransmitters from binding. In a clinical trial, participants who take the drug report reduced feelings of pleasure and motivation. Which of the following is the most accurate description of how this drug works and what the findings suggest?

• A) The drug acts as an antagonist, blocking the neurotransmitter's effect; the findings suggest the targeted neurotransmitter normally contributes to pleasure and motivation. ✓
• B) The drug acts as an agonist, increasing the neurotransmitter's effect; the findings suggest the targeted neurotransmitter normally suppresses pleasure.
• C) The drug prevents reuptake of the neurotransmitter; the findings suggest that excess neurotransmitter activity reduces motivation.
• D) The drug triggers an action potential in postsynaptic neurons; the findings suggest overstimulation of reward pathways reduces pleasure.

Explanation: A drug that occupies receptor sites without activating them is an antagonist; it blocks the natural neurotransmitter from binding. Because participants report reduced pleasure and motivation after the blockade, the findings suggest the targeted neurotransmitter normally facilitates these states. (B) contains a definitional error: an agonist mimics or enhances the neurotransmitter's effect, which is the opposite of what is described. (C) is a true-but-irrelevant distractor: preventing reuptake is a real drug mechanism that does affect neurotransmitter availability, but the stem clearly describes receptor blocking, not reuptake inhibition, students who conflate these two distinct mechanisms may be drawn to (C). (D) is wrong because a drug that occupies but does not activate receptors would not trigger an action potential. [Practice 2]

Question 4. A researcher records the electrical activity of a single neuron at rest and during stimulation. She reports the following: Resting membrane potential: -70 mV Threshold of excitation: -55 mV Peak of action potential: +40 mV Duration of absolute refractory period: approximately 1-2 milliseconds A student reviews the data and concludes: "If the researcher applies two stimuli in rapid succession, the second arriving 0.5 milliseconds after the first fires, the neuron will produce two separate action potentials of equal strength." Which of the following best evaluates this conclusion?

• A) The conclusion is correct; neurons can always fire again immediately after an action potential if the stimulus is strong enough.
• B) The conclusion is correct; the all-or-none principle guarantees equal-strength firing whenever the threshold is reached.
• C) The conclusion is incorrect; during the absolute refractory period, the neuron cannot fire again, so the second stimulus will produce no action potential. ✓
• D) The conclusion is incorrect; the second action potential would be weaker because the neuron has not fully repolarized.

Explanation: Immediately after an action potential, the neuron enters the absolute refractory period, a window during which it cannot fire again regardless of stimulus strength. At 0.5 ms, the second stimulus falls well within this period (approximately 1-2 ms per the data), so no second action potential is produced. (A) is false: even a very strong stimulus cannot cause firing during the absolute refractory period. (B) is a true-but-irrelevant distractor: the all-or-none principle is accurate, neurons do fire at full strength when they fire, but the principle describes the character of a firing that occurs, not whether firing is possible at all. A student who recalls the all-or-none principle without thinking carefully about the absolute refractory period will be tempted by (B). (D) is incorrect: action potentials that do occur are always full-strength by the all-or-none principle; the neuron does not produce weakened potentials during recovery; it simply cannot fire during the absolute refractory period. [Practice 3]

Question 5. During synaptic transmission, vesicles in the terminal buttons release neurotransmitter molecules into the synaptic gap. Most of these molecules bind to receptors on the postsynaptic membrane. Some molecules are then broken down by enzymes in the synapse, and others are taken back up into the presynaptic neuron. A student learns that a certain antidepressant medication works by interfering with the step in this process that returns neurotransmitters to the presynaptic neuron. What mechanism does the medication target, and what is the most direct effect on synaptic transmission?

• A) The medication inhibits enzymatic breakdown, increasing the amount of neurotransmitter available in the synapse.
• B) The medication inhibits reuptake, keeping more neurotransmitter molecules in the synapse and prolonging their effect on the postsynaptic neuron. ✓
• C) The medication blocks postsynaptic receptors, preventing the neurotransmitter from binding and reducing its effect.
• D) The medication triggers additional vesicle release, increasing the total amount of neurotransmitter initially secreted.

Explanation: The step that "returns neurotransmitters to the presynaptic neuron" is reuptake. Medications that inhibit reuptake prevent neurotransmitter molecules from being pulled back into the presynaptic terminal, leaving more molecules in the synapse to continue stimulating postsynaptic receptors. (A) is a true-but-irrelevant distractor: enzymatic breakdown is a real process that also reduces available neurotransmitter, and inhibiting it would similarly increase synaptic availability, but the question specifically identifies reuptake as the targeted step, and these are two distinct mechanisms. A student reading quickly may select (A) because "increasing available neurotransmitter" sounds correct without noticing the mechanism described differs from what the stem identifies. (C) describes an antagonist effect that would reduce postsynaptic activation, the opposite of what a reuptake-inhibiting antidepressant achieves. (D) is wrong because the medication targets reuptake, not the initial release of vesicles. [Practice 1]