Intracellular recording and stimulation techniques were used in a comparison of electrical properties of triceps surae and plantaris motoneurons between unlesioned and 6-wk chronic spinal (L(1-)L(2)) cats. The primary analysis was restricted to 195 moteneurons with action potential heights greater than or equal to 80 mV. Voltage transients resulting from short-duration current pulses (0.5 ms) were used to estimate membrane time constant (tau(m)) and equivalent cylinder electrotonic length (L). Although L was unchanged, tau m and the equalization phase time constant were significantly lower (17%) in motoneurons from chronic spinal preparations. Estimated total cell surface area was also reduced by 11%. The incidence of sag conductances, as judged from observations of the decay of voltage transients, increased from 3% to 29% after chronic spinalization. Input resistance, as measured from either the amplitude of voltage responses to long-duration (50 ms) hyperpolarizing pulses (R(inL)) or from the area of the short-duration current pulse-induced voltage transients, was unchanged in the chronic spinal preparation. Rheobase current was unchanged but threshold voltage (V-Th) was increased in chronic spinal motoneurons. Increased V-Th was not a result of membrane hyperpolarization because both mean action potential height (88 mV) and resting membrane potential (70 mV) were identical in both preparations. The threshold current for action potential activation by short-duration (0.5 ms) current pulses increased 28% in chronic spinal preparations. This is consistent with the increase in V-Th in the same motoneurons. Measured V-Th was identical to that calculated from the product of R(inL) and rheobase in the unlesioned preparation but was significantly larger than calculated V-Th in chronic spinal preparations. This may indicate an increased incidence or magnitude of subthreshold rectification processes in motoneurons from chronic spinal preparations. These results in barbiturate-anesthetized preparations suggest that ankle extensor motoneurons are less excitable in the chronic spinal state. Mean afterhyperpolarization duration was 10% shorter in motoneurons from chronic spinal preparations, whereas amplitude was unchanged. Electrical properties were also compared in chronic spinal and unlesioned preparations using motoneurons with action potential heights of 60-79 mV. In these motoneurons with presumably poorer impalements there were no significant differences between unlesioned and chronic spinal preparations. Ia monosynaptic excitatory postsynaptic potentials (EPSPs) recorded in the same motoneurons have decreased half-widths and rise times and increased amplitudes. The present results suggest that changes in the passive electrical properties of motoneurons in 6-wk chronic spinal cats may contribute to, but are not wholly responsible for, changes in EPSP shape. Because the increased EPSP amplitude in chronic spinal preparations cannot be accounted for by membrane property changes, alterations in the synaptic mechanisms underlying Ia EPSPS are likely in chronic spinal preparations.
