Nerve and muscle: synaptic integration: practice question

A Physiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Physiology · Nerve and muscle: synaptic integration

Sample item

A motor neuron at -70 mV has a threshold of -55 mV. Four excitatory synapses fire together, each producing a 5 mV EPSP, while one inhibitory synapse fires and produces a 3 mV IPSP. Assuming linear summation, what happens?

AIt reaches -43 mV because inhibitory potentials do not summateBIt reaches -53 mV, which is above threshold, so the neuron firesCIt reaches -58 mV, which is below threshold, so the neuron does not fireDIt reaches -73 mV because the IPSP overrides all excitation

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

B. It reaches -53 mV, which is above threshold, so the neuron fires

Postsynaptic potentials summate spatially. Four EPSPs give +20 mV and the IPSP, produced by chloride influx, gives -3 mV, so the net change is +17 mV and the membrane reaches -53 mV. This is more positive than the -55 mV threshold at the axon hillock, so an action potential is triggered.

Option by option

A. It reaches -43 mV because inhibitory potentials do not summate
Inhibitory potentials do summate algebraically with excitatory ones, giving -53 mV, not -43 mV.
B. It reaches -53 mV, which is above threshold, so the neuron fires (correct)
Correct. Net depolarisation of 17 mV crosses the threshold.
C. It reaches -58 mV, which is below threshold, so the neuron does not fire
This ignores the size of the combined excitatory input; net depolarisation is 17 mV, not 12 mV.
D. It reaches -73 mV because the IPSP overrides all excitation
An IPSP of 3 mV cannot cancel 20 mV of excitation.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is B, It reaches -53 mV, which is above threshold, so the neuron fires. Postsynaptic potentials summate spatially.

Why is "It reaches -43 mV because inhibitory potentials do not summate" not the answer?

Inhibitory potentials do summate algebraically with excitatory ones, giving -53 mV, not -43 mV.

Why is "It reaches -58 mV, which is below threshold, so the neuron does not f…" not the answer?

This ignores the size of the combined excitatory input; net depolarisation is 17 mV, not 12 mV.

Why is "It reaches -73 mV because the IPSP overrides all excitation" not the answer?

An IPSP of 3 mV cannot cancel 20 mV of excitation.

What concept does this question test?

It tests Nerve and muscle: synaptic integration in Physiology. Four EPSPs give +20 mV and the IPSP, produced by chloride influx, gives -3 mV, so the net change is +17 mV and the membrane reaches -53 mV. This is more positive than the -55 mV threshold at the axon hillock, so an action potential is triggered.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Nerve and muscle: synaptic integration?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.