Final answer:
The intensity of a stimulus is encoded in the rate of action potentials and activation of more receptors, not the size of individual action potentials, due to the all-or-none nature of action potentials consisting of threshold, log-linear, and saturation phases.
Step-by-step explanation:
Intensity Coding and Action Potential Phases
Action potentials in neurons follow an "all-or-none" principle, with intensity of a stimulus not being directly proportional to the size of the action potential. Rather, it is coded in distinct phases:
Threshold: A minimum level of depolarization (-55 mV) is required to trigger an action potential. Below this threshold, no action potential occurs.
Log-linear Phase: Once the threshold is surpassed, voltage-gated Na+ channels open and the neuron rapidly depolarizes, resulting in a uniform action potential that peaks at +30 mV regardless of stimulus strength.
Saturation: The maximum frequency of action potentials is limited by the refractory period, during which a new action potential cannot be initiated. Thus, even with a strong stimulus, there is a cap on how rapidly action potentials can fire.
Stimulus intensity is encoded by the rate of action potentials and the number of receptors stimulated, not by the magnitude of individual action potentials. Each action potential is consistent in magnitude, so a stronger stimulus does not create a "bigger" action potential but can cause a higher frequency of action potentials or recruit more neurons.