Graded potentials: (a) do not occur until the membrane reaches threshold (b) undergo spatial and temporal summation (c) are self-propagating (d) have refractory periods
The Correct Answer and Explanation is:
Correct answer: (b) undergo spatial and temporal summation
Graded potentials are localized changes in membrane potential that occur in response to stimuli, usually in the dendrites or cell body of a neuron. These changes can vary in magnitude and duration depending on the strength and duration of the stimulus. Unlike action potentials, which are all-or-nothing and self-propagating, graded potentials are not transmitted over long distances and diminish in strength as they spread from the site of origin.
One of the key features of graded potentials is their ability to undergo spatial and temporal summation. Spatial summation refers to the additive effect of multiple simultaneous stimuli occurring at different locations on the neuron. When these stimuli are close enough in space, their combined effect can bring the membrane closer to the threshold for firing an action potential. Temporal summation, on the other hand, occurs when multiple stimuli are received at the same location in rapid succession. If the timing is close enough, their effects can accumulate, increasing the likelihood of reaching the threshold.
Graded potentials do not require the membrane to reach a specific threshold to occur, making choice (a) incorrect. These potentials can occur in response to subthreshold stimuli. Choice (c) is incorrect because graded potentials are not self-propagating; they spread passively and decay with distance. Choice (d) is also incorrect because graded potentials do not have refractory periods, which are characteristic of action potentials due to the inactivation and reset of voltage-gated ion channels.
Graded potentials play a crucial role in neural communication by integrating incoming signals through summation. If the resulting depolarization is strong enough to reach threshold at the axon hillock, an action potential will be initiated and propagate along the axon. This process enables complex signal processing in the nervous system.
