Final answer:
An action potential originates at the trigger zone, which is the initial segment of the axon or axon hillock. Graded potentials such as generator potentials and PSPs can lead to the generation of an action potential if they collectively bring the membrane potential to the threshold required.
Step-by-step explanation:
Graded Potentials and Action Potentials
The question relates to what type of potential originates at the trigger zone of neurons. The answer is an action potential. Graded potentials, such as generator potentials and postsynaptic potentials (PSPs), can summate and lead to the generation of an action potential if they reach a threshold level. This threshold level is a critical point at which there is a high density of voltage-gated Na+ channels that initiate the depolarizing phase of the action potential. This occurs at the initial segment of the axon in sensory neurons, also known as the axon hillock in other neurons. Unlike receptor potentials, which directly cause neurotransmitter release without needing to reach a threshold, the generator and postsynaptic potentials can trigger the action potential at the trigger zone if they collectively bring the membrane potential to the threshold.
Generator potentials are graded potentials from the dendrites of a unipolar cell which generate the action potential in the initial segment of that cell's axon. These graded potentials can be caused by various stimuli, like temperature changes in thermoreceptors, and are key to neuronal communication.
Spatial summation is the combination of graded potentials that result from different locations on the neuron, while temporal summation is the combination of potentials that occur at the same place but separated in time. Both these forms of summation can contribute to reaching the threshold necessary to initiate an action potential.