Final answer:
If two equal but opposite graded potentials summate, they cancel each other out, leading to no net change in the membrane potential and decreasing the likelihood of reaching the threshold for an action potential. This summation can impact whether a neuron fires an action potential and is influenced by both spatial and temporal factors.
Step-by-step explanation:
When two equal but opposite graded potentials summate, such as a depolarizing graded potential with a hyperpolarizing graded potential, they effectively cancel each other out. This is because graded potentials result in temporary changes in the membrane voltage due to various stimuli. Depolarizing graded potentials, which often occur due to Na⁺ or Ca²⁺ entering the cell, make the inside of the cell less negative relative to the outside. In contrast, hyperpolarizing graded potentials, which can be caused by K⁺ leaving the cell or Cl⁻ entering, make it more negative. If these two graded potentials are of the same magnitude but opposite in effect, they neutralize one another, leading to no net change in the membrane potential. This makes it less likely for the neuron to reach the threshold necessary to generate an action potential.
Summation, which includes both spatial and temporal summation, refers to the process where multiple graded potentials can add together over space and time to affect the overall membrane potential. If excitatory and inhibitory postsynaptic potentials mix, the net effect on the membrane potential depends on the dominance of either summative effect.