The axon initial segment plays important roles in spike initiation and invasion of axonal spikes into the soma. Among primary sensory neurons, those in the mesencephalic trigeminal nucleus (MTN) are exceptional in their ability to initiate soma spikes (S-spikes) in response to synaptic inputs, consequently displaying two kinds of S-spikes, one caused by invasion of an axonal spike arising from the sensory receptor and the other initiated by somatic inputs. We investigated where spikes are initiated in such MTN neurons and whether there are any differences between the two kinds of S-spikes. Simultaneous patch-clamp recordings from the soma and axon hillock revealed a spike-backpropagation from the spike-initiation site in the stem axon to the soma in response to 1-ms somatic current pulse, which disclosed the delayed emergence of S-spikes after the current-pulse offset. These initiated S-spikes were smaller in amplitude than S-spikes generated by stimulation of the stem axon; however, 4-AP (< or =0.5 mM) eliminated the amplitude difference. Furthermore, 4-AP dramatically shortened the delay in spike initiation without affecting the spike-backpropagation time in the stem axon, whereas it substantially prolonged the refractory period of S-spikes arising from axonal-spike invasion without significantly affecting that of presumed axonal spikes. These observations suggest that 4-AP-sensitive K(+) currents exert two opposing effects on S-spikes depending on their origins: suppression of spike initiation and facilitation of axonal-spike invasion at higher frequencies. Consistent with these findings, strong immunoreactivities for Kv1.1 and Kv1.6, among 4-AP-sensitive and low-voltage-activated Kv1 family examined, were detected in the soma but not in the stem axon of MTN neurons.