Impulse coding of ramp currents intracellularly injected into pyramidal tract neurones.

Relationships between the repetitive discharge and dynamic aspects of the input were analyzed in pyramidal tract neurones of the cat. Inputs were intracellularly injected currents reaching a steady level after ramps of different slopes (from 0.02 to 1.5 nA ms-1). Output was the instantaneous frequency of the discharge. During the transient phase, instantaneous frequency appeared to be related both to the velocity of rise and to the intensity of the stimulating current. The dynamic component of the cell response was estimated by subtraction of the intensity-bound component (derived from the steady-state response to current steps). After subtraction, the instantaneous frequency of interspike intervals following the first one became proportional to the ramp slope. The instantaneous frequency of the first interval also increased with the current slope, but at a lower rate than the frequency of other intervals. Moreover its dynamic component virtually stopped growing when the ramp slope exceeded 0.3-0.5 nA ms-1.