the threshold value controls whether or not the incoming stimuli are sufficient to generate an action potential. it relies on a balance of incoming inhibitory and excitatory stimuli. the potentials generated by the stimuli are additive, and they may reach threshold depending on their frequency and amplitude. normal functioning of the central nervous system entails a summation of synaptic inputs made largely onto a neuron's dendritic tree. these local graded potentials, which are primarily associated with external stimuli, reach the axon initial segment and build until they manage to reach the threshold value. the larger the stimulus, the greater the depolarization, or attempt to reach threshold. the task of depolarization requires several key steps that rely on anatomical factors of the cell. the ion conductances involved depend on the membrane potential and also the time after the membrane potential changes.