Which receptors mediate fast synaptic transmission?

• A. Nuclear receptors
• B. Ionotropic receptors
• C. Metabotropic receptors
• D. Tyrosine kinase receptors

Answer: (B)

Fast synaptic transmission relies on receptors that directly gate ion channels. These ionotropic receptors respond to a neurotransmitter by opening an intrinsic pore in the receptor, letting ions flow across the membrane in an instant and producing immediate postsynaptic potentials within milliseconds. Examples include AMPA and NMDA receptors for glutamate, GABA_A receptors for GABA, and nicotinic acetylcholine receptors. This direct coupling to ion flow explains the rapid, point-to-point signaling at fast synapses.

In contrast, nuclear receptors are intracellular and regulate gene expression, taking minutes to hours to manifest effects—far too slow to mediate fast transmission. Metabotropic receptors (GPCRs) work through second messenger pathways, producing slower and more prolonged responses. Tyrosine kinase receptors initiate phosphorylation cascades that modulate cellular signaling over a slower timescale than direct ion flow.