Neural encoding of input transients investigated by intracellular injection of ramp currents in cat α‐motoneurones

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Abstract
1. Input—output relations were analysed in spinal α‐motoneurones during current transients reaching a steady level after a linear growth of different slopes. The motoneurone output considered in the analysis was the instantaneous frequency of the cell discharge.2. In all motoneurones firing frequency during the ramp exceeded that of the final steady level and it was related to the velocity of rise of the current. In the majority of motoneurones the instantaneous frequency grew during the ramp stimulus, as if it were dependent on current intensity as well as on its rate of rise. Only in a few cells was firing frequency constant over the first two interspike intervals during the ramp, as would be expected if this response depended solely on the rate of rise.3. Frequency—velocity (f/v) plots for different rates of rise of the injected current showed a linear relation for each interspike interval. Presence or absence of an intensity component was revealed in these plots by divergence or, respectively, overlapping of the f/v relations for the first and second intervals. Divergence was eliminated by subtraction of the estimated intensity component. The slope of the f/v relation for the first interval did not change significantly after subtraction of the intensity component and was taken as an index of the dynamic sensitivity of the motoneurones. The slope of the f/v relation varied greatly (from 47 to 330 impulses s−1. (nA ms−1)−1) in the population examined and was higher in motoneurones with a long‐lasting afterhyperpolarization (a.h.p.) than in those where it was short‐lasting.4. It is proposed that the ability of the motoneurones to encode both the steady level and the rate of change of input signals depends on the conductance changes responsible for the a.h.p. and their accumulation. A positive correlation was found between the size of the a.h.p. potassium current, estimated as a.h.p. peak voltage/cell input resistance, and the slope of the f/v relation for the first interval.