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.

Key Concepts

Neurotransmitter Receptors

Neurotransmitter receptors are specialized proteins located on the postsynaptic membrane that bind to neurotransmitters. Different types of receptors can lead to varying cellular responses; for instance, some receptors may generate excitatory postsynaptic potentials (EPSPs), while others may produce inhibitory postsynaptic potentials (IPSPs). The specific receptor type expressed by a neuron determines how it responds to a given neurotransmitter.

Postsynaptic Potentials

Postsynaptic potentials are changes in the membrane potential of a postsynaptic neuron following the binding of neurotransmitters to its receptors. An EPSP depolarizes the membrane, making it more likely to fire an action potential, while an IPSP hyperpolarizes the membrane, making it less likely to fire. The balance between these potentials is crucial for neuronal communication and overall brain function.

Myelination and Conduction Velocity

Myelination refers to the presence of a myelin sheath around the axon of a neuron, which increases the speed of electrical signal transmission. Myelinated axons conduct action potentials more rapidly than unmyelinated axons due to saltatory conduction, where the action potential jumps between nodes of Ranvier. This difference in conduction velocity can influence how quickly a postsynaptic cell responds to neurotransmitter release.