Ionic Basis And Nonlinear Dynamical Mechanism Of The Rhythm Transitions In The Spontaneous Discharges Of Injured Nerve

Experimental ectopic pacemakers, which generate spontaneous discharges in various rhythms, arc formed at the injured site of the rat's sciatic nerve subjected to chronic injury. Such spontaneous discharges arc input into the central nervous system and arc recongnized as afferent message from the receptive field of the injured nerve, causing abnormal sensations. Rcccnt study suggests that sodium channels, potassium channels and calcium channels arc involved in the generation of the spontaneous discharges. In the present study, surgical operation was performed as described by Bennett & Xie to produce the pacemaker using adult male SD rats(200-300g). After 8-12 days survival time, the animals were rcancsthesized, the previously injured nerve was exposed and perfused with warm Kreb's solution. About 20-30mm proximal to the injured site, the sciatic nerve was exposed and covered with warm paraffin oil. The spike trains of individual fibres ending at the pacemakers in the injured site were recorded in this oil pool by separating a fine bundle of fibers which were disconnected from the central nervous system but whose connection to the injured site was left intact. If the observed bundle contains only one active fiber, the spike trains were recorded into a computer via an A/D converter. The time intervals between the maximum values of the successive spikes were recorded seriatim as ISI series.The main results are as follows:1. According to the nonlinear dynamical features of the ISI series, the rhythms of the spontaneous discharges can be described as periodic rhythm, periodic rhythm with intermittency and chaotic rhythm. About 57.63% of the 59 ISI series of A-fibers(conduction velocity > 10m/s) recorded while the pacemaker is in normal Kreb's solution are periodic, including period one, period two, and period three.2. With alterations of [Ca++] in the Kreb's solution perfusing the pacemaker, the rhythm of the observed spike trains changes in a regular way of period transition. The transitions from period one to period two, to chaos, to period three, and even to period four in ISI series are observed as [Ca++]o is decreased by perfusing the pacemaker with Ca~++-free Kreb's solution. This period-adding transition in ISI series represents the changes in the rhythm of the spontaneous discharges. During the increase of [Ca++]o from 1.2mM to 5.0mM, the period two ISI series turns to period one, while the period three ISI series turns into chaos and then into period two. When [Ca++]o is maintained at about 3.5mmoI/L, the rhythm of the original period three(in 1.2 [Ca++]o) discharges presents a steady irregularity, but the return map obtained by plotting the ISI series of this irregular rhythm shows that the irregulariy is not stochastic. The...