inhibitory

Meaning: An inhibitory input pushes the postsynaptic membrane potential away from threshold. The transmitter, such as GABA or glycine, typically binds a ligand-gated channel that lets Cl⁻ enter or K⁺ leave. The result is an inhibitory postsynaptic potential (IPSP), a graded hyperpolarization. IPSPs and excitatory postsynaptic potentials (EPSPs) sum in space and time at the axon hillock. The neuron fires only if the net depolarization reaches threshold. In biochemistry, an inhibitory molecule instead lowers enzyme activity.

On the MCAT: Passages describe a toxin, drug, or mutation that changes a channel or transmitter, and you predict whether the target neuron fires more or less. Expect questions on Cl⁻ or K⁺ gradients and the direction of ion flow, on summation of EPSPs and IPSPs, and on reflex arcs where an inhibitory interneuron relaxes the antagonist muscle. Blocking an inhibitory input raises firing, so activity is disinhibited.

Common confusion: Students label a transmitter as inherently excitatory or inhibitory. The receptor decides. Acetylcholine excites skeletal muscle through nicotinic receptors but slows the heart through muscarinic receptors. Also, an inhibitory neuron still fires action potentials. The inhibition is the effect on its target cell.