Suppose a particular neurotransmitter causes an IPSP in postsynaptic cell X and an EPSP in postsynaptic cell Y. A likely explanation is that a. the threshold value in the postsynaptic membrane is different for cell X and cell Y. b. the axon of cell X is myelinated, but that of cell Y is not. c. only cell Y produces an enzyme that terminates the activity of the neurotransmitter. d. cells X and Y express different receptor molecules for this particular neurotransmitter.

Verified step by step guidance

1. Step 1: Understand the problem. The problem is asking why a particular neurotransmitter causes an inhibitory postsynaptic potential (IPSP) in one cell (cell X) and an excitatory postsynaptic potential (EPSP) in another cell (cell Y).
2. Step 2: Review the options. Option a suggests that the threshold value in the postsynaptic membrane is different for cell X and cell Y. However, the threshold value is not related to whether a neurotransmitter causes an IPSP or EPSP. Option b suggests that the axon of cell X is myelinated, but that of cell Y is not. Myelination of the axon affects the speed of signal transmission, not the type of postsynaptic potential. Option c suggests that only cell Y produces an enzyme that terminates the activity of the neurotransmitter. This would affect the duration of the neurotransmitter's effect, not whether it causes an IPSP or EPSP.
3. Step 3: Evaluate the correct option. Option d suggests that cells X and Y express different receptor molecules for this particular neurotransmitter. This is the most likely explanation. Different receptors can respond differently to the same neurotransmitter. Some receptors can cause an IPSP when activated, while others can cause an EPSP.
4. Step 4: Understand why this is the correct answer. The type of postsynaptic potential that a neurotransmitter induces depends on the type of receptor it binds to. Therefore, if a neurotransmitter causes an IPSP in one cell and an EPSP in another, it is likely because the two cells have different types of receptors for that neurotransmitter.
5. Step 5: Finalize the answer. Therefore, the correct answer is option d: cells X and Y express different receptor molecules for this particular neurotransmitter.