Final answer:
An action potential is followed by a refractory period, during which the neuron regains its negative charge, returning to its resting state before it can be depolarized again.
Step-by-step explanation:
An action potential is followed by a brief refractory period, when the neuron regains its negative charge.
Direct answer: A) Refractory; negative
After an action potential, a neuron enters a refractory period during which it cannot generate another action potential. This period is crucial for the neuron to reset its ion channels to their resting state. During an action potential, sodium (Na+) channels open, allowing Na+ ions to flood into the cell, leading to depolarization. As the peak is reached, potassium (K+) channels open, and Na+ channels close, resulting in K+ moving out of the cell and initiating repolarization.
Eventually, the membrane becomes slightly more negative than the resting potential, a state termed hyperpolarization. At this point, the Na+/K+ pump, which actively transports Na+ out of the cell and K+ into the cell, helps to restore the resting membrane potential, making the inside of the cell negative again compared to the outside.