Abstract
The axon initial segment plays important roles in spike-initiation and invasion of axonal spikes into the soma. Among PSNs, those in the MTN can exceptionally initiate spikes in response to synaptic inputs, consequently displaying two kinds of spikes, one caused by invasion of an axonal spike arising from the sensory receptor and the other initiated by somatic inputs. We addressed where spikes are initiated in MTN neurons and whether there are any differences between initiated and invaded soma spikes (S-spikes). Simultaneous patch-clamp recordings from the soma and axon hillock revealed a spike-backpropagation from the initiation site in the stem axon to the soma in response to somatic current pulse, which brought about the delayed emergence of S-spikes after the offset of the current pulse. These initiated S-spikes were smaller in amplitude than invaded ones generated by stimulation of the stem axon; however, 4AP (<=0.5mM) eliminated the amplitude difference. Furthermore, 4AP markedly shortened the delay in spike-initiation without affecting the latency to spike-invasion, whereas it prolonged the refractory period of invaded S-spikes without affecting that of presumed axonal spikes markedly. These observations suggest that 4AP-sensitive K+ currents exert two opposing effects on S-spikes depending on their origins; suppression of spike-initiation and facilitation of spike-invasion at higher frequencies. [J Physiol Sci. 2006;56 Suppl:S152]
