Two neurons, A and B, synapse onto a third neuron, C.If neurotransmitter from A opens ligand-gated channels permeable to `Na^+` and `K^+` and neurotransmitter from B opens ligand-gated `Cl^-` channels, which of the following statements is true ?

Options

A. An action potential in neuron A causes a depolarisation in neuron B.
B. An action potential in neuron B causes a depolarisation in neuron C.
C. Simultaneous action potentials in A and B will cause hyperpolarisation of neuron C.
D. Simultaneous action potentials in A and B will cause less depolarisation of neuron C than if only neuron A fired an action potential.

Answer

D

Explanation

Once neurotransmitters are released from the pre-synaptic axon terminal, they diffuse across the cleft.This results in binding of neurotransmitter to receptor and opening or closing of specific ion channels in the post-synaptic plasma membrane.Usually at an excitatory snapse, channels in the post-synaptic cell that are permeable to `Na^+, K^+` and other small positive ions open, but `Na^+` flux dominates, because it has the largest electrochemical gradient.At inhibitory synapses, channels to `Cl^-` or `K^+` open.In the given situation, the neuron B sends inhibitory pre-synaptic signals onto post-synaptic neuron C which would summate (subtract from ) with the amplitude of excitatory pre-synaptic signal caused by neuron A.Therefore depolarisation will be of lower amplitude.