Final answer:
Action potentials involve a positive feedback cycle where depolarization leads to more depolarization until an action potential is triggered. Graded potentials, such as generator potentials or receptor potentials, do not require a positive feedback cycle and are proportional to the strength of the stimulus.
Step-by-step explanation:
The question addresses whether action potentials or graded potentials require a positive feedback cycle. Action potentials indeed involve a positive feedback mechanism, where an initial depolarizing stimulus that is strong enough to reach the threshold potential leads to the opening of voltage-gated Na+ channels, causing further depolarization and more opening of Na+ channels. This positive feedback cycle rapidly depolarizes the membrane and generates the spike characteristic of an action potential. On the other hand, graded potentials do not require such a positive feedback cycle; instead, they are caused by stimuli that induce a change in membrane potential directly proportional to the stimulus strength, leading to either a depolarization or hyperpolarization depending on the type of stimulus and channels involved.
Graded potentials include generator potentials in dendrites of sensory neurons, which influence the generation of an action potential, as well as receptor potentials in other sensory receptor cells that directly cause neurotransmitter release without necessarily reaching a threshold. Graded potentials can summate spatially or temporally to increase the likelihood of bringing a neuron to threshold to fire an action potential, but this summation is not considered a positive feedback loop.