The most important factor determining whether a receptor-operated ion channel is inhibitory or excitatory is

Question

BlackTom AI · Accepted Answer

Whether a receptor-operated ion channel is inhibitory or excitatory depends on the relationship between its reversal potential (Erev) and the neuronal threshold for action potential firing.

1) Relevant concepts:
- Reversal potential (Erev): the membrane potential at which net ionic current through the open channel is zero; opening the channel drives the membrane potential toward Erev.  
- Resting membrane potential and threshold: typical values are about -65 to -70 mV (rest) and ≈ -50 to -55 mV (threshold).  
- Excitatory effect: opening the channel moves Vm toward a value at or above threshold, increasing probability of action potential. Inhibitory effect: opening the channel moves Vm away from threshold (hyperpolarizes) or clamps it below threshold, decreasing firing probability.  
- Ion charge or ligand binding per se does not determine sign; the electrochemical gradient (Erev relative to threshold) does.

2) Step-by-step reasoning:
- Start with an example resting Vm = -70 mV and threshold = -55 mV.  
- If a channel has Erev = 0 mV (e.g., nonselective cation channel permeable to Na+), opening it will depolarize Vm toward 0 mV, passing threshold (from -70 toward 0), so it is excitatory.  
- If a channel has Erev = -80 mV (e.g., Cl− under many conditions), opening it will drive Vm more negative (from -70 toward -80), moving away from threshold, so it is inhibitory.  
- If Erev lies between resting and threshold (for example -60 mV, still below threshold), opening the channel may depolarize but not reach threshold and often exerts a shunting (inhibitory) effect; if Erev is more positive than threshold (e.g., -50 mV), opening will be excitatory.

3) Intermediate calculation examples:
- Rest -70 mV → Erev 0 mV: change toward +70 mV relative to rest, crosses threshold (-55), excitatory.  
- Rest -70 mV → Erev -80 mV: change of -10 mV, hyperpolarizing, inhibitory.

4) Why the correct answer is correct:
Because the direction and functional effect of current flow through the channel are set by Erev relative to threshold; channels drive Vm toward Erev, so whether that movement brings Vm toward or away from threshold determines excitation vs inhibition.

5) Why other options are incorrect:
- Charge of the permeant ion: an ion’s sign alone does not guarantee excitatory vs inhibitory effects (Cl− can be excitatory if ECl is above threshold).  
- Ligand-binding properties: these determine gating and kinetics, not the sign of the voltage effect.  
- Number of different ions that can pass: permeability to multiple ions affects Erev value but it is the resulting Erev relative to threshold that matters, not the count itself.  
- "None of the above" is wrong because one option (Erev relative to threshold) is correct.

Conclusion: The correct choice is “whether the permeant ion's reversal potential is positive or negative to threshold,” which matches the provided answer.

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