Neurophysiology – Cell Types, Signals, and Sensory Pathways Practice Test

GABA is the main inhibitory neurotransmitter in the CNS. When it binds to GABA_A receptors, it opens Cl− channels, allowing chloride to enter the neuron and hyperpolarize the membrane, which lowers the chances of firing an action potential. GABA_B receptors, a slower, metabotropic type, activate potassium channels and suppress calcium entry, producing longer-lasting inhibition. This inhibitory signaling is essential for balancing excitation, shaping network activity, and preventing runaway neuronal firing. In contrast, glutamate is the primary excitatory transmitter in the CNS, dopamine has modulatory roles with both excitatory and inhibitory effects depending on receptor type, and acetylcholine can be excitatory in many CNS circuits and at the neuromuscular junction, so none of them serve as the main inhibitory transmitter. (Note: glycine is also inhibitory, mainly in the spinal cord and brainstem, but GABA is the principal CNS inhibitor.)