Final answer:
The entry of chloride ions through open channels in the post-synaptic neuron hyperpolarizes the membrane, leading to an inhibitory postsynaptic potential (IPSP).
Step-by-step explanation:
The effect of chloride ions entering the post-synaptic neuron through open chloride ion (Cl-) channels depends on the neurotransmitter. In the case of GABA (gamma-aminobutyric acid), the primary inhibitory neurotransmitter, the entry of Cl- ions hyperpolarizes the membrane of the post-synaptic neuron, making it less likely to fire an action potential. This is known as an inhibitory postsynaptic potential (IPSP).