Final answer:
The statement that an action potential is triggered when the axon hillock is depolarized to threshold is true. The threshold potential is critical for initiating an action potential, following an all-or-nothing response. The intensity of action potentials doesn't change; frequency determines the strength of the sensation or muscle contraction. The correct option is a.
Step-by-step explanation:
This statement is true. The membrane potential must reach a critical level, known as the threshold potential, for an action potential to be initiated.
Typically, this threshold is about -55 mV, which is roughly 15 mV more positive than the neuron's resting state. An action potential follows an all-or-nothing principle, meaning that once the threshold is surpassed, a full response occurs without varying in magnitude, regardless of how much the stimulus exceeds the threshold.
The process of reaching an action potential involves a rapid change in membrane potential, where sodium (Na+) ions flood into the cell, causing depolarization that reverses the charge across the membrane momentarily. Voltage-gated potassium (K+) channels then open, allowing K+ to leave the cell and repolarize it. The subsequent hyperpolarization and return to resting potential ensure that the neuron is ready for the next stimulus.
A stronger stimulus can trigger multiple action potentials more rapidly, but not larger ones, since each action potential peaks at the same voltage (+30 mV). Thus, the intensity of a sensation or the strength of a muscle contraction is not affected by the magnitude of individual action potentials, but rather by the frequency of their occurrence.
Hence, Option a is correct.