Final answer:
Two depolarizing graded potentials that summate can lead to the neuron reaching threshold, potentially triggering an action potential through spatial or temporal summation.
Step-by-step explanation:
When two depolarizing graded potentials summate, they combine to create an overall change in the membrane potential that can lead to the neuron reaching its threshold. This process is known as summation. If the total change in voltage reaches a positive 15 mV, for example, from -70 mV up to -55 mV, these potentials can cause the membrane to reach the threshold required for an action potential to occur. Summation can occur spatially, with graded potentials coming from different locations on the neuron, or temporally, with potentials occurring at the same location but at different times. Ultimately, spatial and temporal summation work together to facilitate the initiation of action potentials.