Question

Which of the following is the most direct result of depolarizing the presynaptic membrane of an axon terminal?

(A) Voltage-gated calcium channels in the membrane open.

(B) Synaptic vesicles fuse with the membrane.

(C) Ligand-gated channels open, allowing neurotransmitters to enter the synaptic cleft.

(D) An EPSP or IPSP is generated in the postsynaptic cell.

Short answer

(A) The statement“Voltage-gated calcium channels in the membrane open” is true.

(B) The statement “Synaptic vesicles fuse with the membrane” is false.

(C) The statement “Ligand-gated channels open, allowing neurotransmitters to enter the synaptic cleft” is false.

(D) The statement “An EPSP or IPSP is generated in the postsynaptic cell” is false.

Step-by-step solution

Presynaptic end

A synapse is a structure between two neurons (or between a neuron and a target cell). In this structure, the neuron that propagates the signal (action potential) in the forward direction is denoted by the presynaptic end. Typically, the axon terminals serve as the presynaptic end.

Explanation of option ‘(A)’

When a signal depolarizes the neuronal membrane of the presynaptic terminal, the ion channels for calcium are stimulated and opened. This opening causes a profuse inflow of calcium ions within the axonal terminal, which regulates further propagation of the signal to the postsynaptic terminal.

Thus, the immediate effect of axon terminal depolarization is the activation of calcium ion channels.

Therefore, the given statement is true.

Explanation of option ‘(B)’

The synaptic vesicles are the components that contain neurotransmitters. The inflow of calcium in the presynaptic end causes the fusion of the synaptic vesicle’s membrane with the presynaptic terminal’s membrane.

Thus, the fusion of the synaptic vesicle with the presynaptic membrane is not a direct effect of depolarization.

Therefore, the given statement is false.

Explanation of option ‘(C)’

The neurotransmitters are transferred from the presynaptic terminal to the synaptic cleft through a process of exocytosis. In this mechanism, the fused membrane of the synaptic vesicle and presynaptic end opens up, and the neurotransmitters are emitted out.

Thus, ligand-gated ion channels are not involved in the transfer of neurotransmitters to the synaptic cleft.

Therefore, the given statement is false.

Explanation of option ‘(D)’

A postsynaptic potential, either IPSP (inhibitory postsynaptic potential) or EPSP (excitatory postsynaptic potential), is generated when the molecules of neurotransmitters link with their respective receptors on the postsynaptic end.

Thus, the generation of IPSP or EPSP occurs at a much later stage after depolarization.

Therefore, the given statement is false.