What happens immediately after ACh binds to postsynaptic receptor?

• A. ACh is reabsorbed into presynaptic neuron
• B. Receptor returns to resting state
• C. ACh is hydrolyzed by AChE
• D. Induced fit triggers secondary message

Answer: (D)

Binding of acetylcholine to its postsynaptic receptor first causes the receptor to change shape—an induced fit. This conformational shift is what converts the chemical signal into a physical change that starts the downstream response: for ionotropic receptors, it opens the channel to allow ion flow; for metabotropic receptors, it activates G proteins and second messenger cascades. The other options describe later steps: reuptake doesn’t happen immediately after binding; the receptor returning to resting state happens after signaling ends; and acetylcholinesterase hydrolyzes ACh in the cleft to terminate the signal, not as the immediate consequence of binding.