Question

Which combination of the following statements about presynaptic inhibition is TRUE?

1. Axoaxonal synapse mediated
2. Prolongs IPSP
3. Prolonged by anaesthesia
4. Not affected by pharmacological agents
5. It affects motor endplate potential
6. Decreased by strychnine
7. Decreased by picrotoxin

• A. a, d, f
• B. b, e, f
• C. a, c, f
• D. a, c, g

Hint:

Presynaptic inhibition is axo-axonal and GABA mediated: blocked by picrotoxin (not strychnine), prolonged by anaesthesia.

Answer 1

The Correct Option is D

Step 1: Recall what presynaptic inhibition is.
Presynaptic inhibition happens when one neuron's terminal reduces how much neurotransmitter a second neuron's terminal releases, instead of directly hyperpolarising the postsynaptic cell body.

Step 2: Check the anatomy (statement a).
This kind of inhibition is set up by an axo-axonal synapse: an inhibitory axon terminal synapses directly onto another axon terminal (not onto a dendrite or soma). Statement (a) is TRUE.

Step 3: Check statement b (prolongs IPSP).
Presynaptic inhibition does not work by hyperpolarising the postsynaptic membrane, so it does not generate or prolong an IPSP in the postsynaptic cell. That kind of hyperpolarising IPSP belongs to postsynaptic (direct) inhibition instead. Statement (b) is FALSE.

Step 4: Check statement c (anaesthesia).
General anaesthetics, and drugs like barbiturates, enhance GABA-A receptor activity at the axo-axonal synapse. This strengthens and prolongs presynaptic inhibition. Statement (c) is TRUE.

Step 5: Check statement d (unaffected by drugs).
Presynaptic inhibition is very much affected by drugs acting on GABA-A receptors, so calling it drug-resistant is wrong. Statement (d) is FALSE.

Step 6: Check statement e (motor endplate potential).
The motor endplate potential is the depolarisation at the neuromuscular junction, a separate synapse outside the central nervous system. Presynaptic inhibition, as classically described, acts within the spinal cord and brain, not at the endplate. Statement (e) is FALSE.

Step 7: Check statements f and g (strychnine vs picrotoxin).
Strychnine blocks glycine receptors, and glycine is the transmitter of postsynaptic (direct) inhibition, so strychnine knocks out postsynaptic inhibition, not presynaptic inhibition. Picrotoxin blocks GABA-A receptors, and GABA is the transmitter released at the axo-axonal synapse that produces presynaptic inhibition, so picrotoxin is the drug that actually decreases presynaptic inhibition. Statement (f) is FALSE and statement (g) is TRUE.

Step 8: Put together the true statements.
The true statements are (a), (c), and (g).

Final Answer:
\[ \boxed{a,\ c,\ g} \]
Note: the printed source listed two of the four answer choices with identical, garbled text ("a c f" appearing twice). Based on the pharmacology above, GABA-A blockade by picrotoxin (not glycine blockade by strychnine) is what reduces presynaptic inhibition, so the option set here has been corrected to a, c, g as the true combination.