Impulse frequency encoding of the dynamic aspects of excitation

Like receptors, neurones of the mammalian CNS react to both the static and the dynamic features of the input signals. The quantitative aspect of the current-to-frequency transduction during input transients were analyzed in spinal motoneurones and in corticospinal neurones by the technique of injecting intracellularly either ramp of sine-wave currents. It was found that, in both types of neurones, the instantaneous firing frequency is linearly correlated, within definite ranges, both to the intensity and to the velocity of change of the input currents. By recording the isometric mechanogram of the muscle units innervated by the impaled motoneurones, it was disclosed that the motoneurones discharges produced by the ramp currents develop tension changes, whose average velocity is proportional to the ramp slope. For both types of neurones, results are consistent with the hypothesis that the major determinants of the double sensitivity, to the intensity and to the velocity, are the kinetics of the potassium conductance system responsible for the spike afterhyperpolarization.