Final answer:
Inhibitory synapses increase permeability primarily to chloride (Cl-) and potassium (K+) ions, leading to hyperpolarization of the neuron and inhibiting action potential formation.
Step-by-step explanation:
Inhibitory synapses become more permeable to ions such as chloride (Cl-) and potassium (K+). When an inhibitory neurotransmitter like GABA (gamma-aminobutyric acid) is released, it can cause Cl- channels to open, leading to an influx of Cl- ions. This influx results in hyperpolarization of the neuron's membrane, making it less likely to fire an action potential; this is known as an inhibitory postsynaptic potential (IPSP). Potassium channels may also open, allowing K+ to exit the neuron, which can also contribute to the hyperpolarization effect.