Nervous System Organization and Function (3A) Practice Test

A neuron receives two simultaneous synaptic inputs: an excitatory synapse on a distal dendrite and an inhibitory synapse located on the soma near the axon hillock. Both synapses are activated at the same time with equal transmitter release probability. The inhibitory receptor is a ligand-gated Cl$^-$ channel whose reversal potential is near the resting membrane potential. Based on the scenario, which function is most consistent with the somatic inhibitory synapse?

Assume: action potentials initiate at the axon hillock when local depolarization reaches threshold.

A cultured excitatory synapse was voltage-clamped at the postsynaptic neuron. The presynaptic terminal was stimulated with a single action potential. Under control conditions, a fast postsynaptic current (PSC) is recorded. When extracellular Ca$^{2+}$ is reduced from 2 mM to 0.2 mM (Mg$^{2+}$ unchanged), the PSC amplitude decreases markedly, while presynaptic action potential shape is unchanged.

Which mechanism best explains the reduced PSC amplitude?

(Assume the same number of postsynaptic receptors are present and the neurotransmitter is cleared normally.)