Carbamazepine prevents neuron excitation by blocking which channels from recovering from which state?

• A. Potassium channels from resting state
• B. Sodium channels from resting state
• C. Sodium channels from the inactivated state
• D. Calcium channels from the inactivated state

Answer: (C)

Carbamazepine lowers neuronal excitability by a state-dependent block of voltage-gated sodium channels. These channels cycle through resting, open, and inactivated states. After an action potential, channels enter the inactivated state and must recover to the resting state before they can reopen. Carbamazepine binds preferentially to sodium channels while they are in the inactivated state and slows their return to the resting state. This reduces the pool of channels available to fire again, especially during rapid firing, which dampens repetitive neuronal activity. So the drug prevents excitation by blocking sodium channels from recovering from the inactivated state.