Voltage‐sensitive outward currents in cat motoneurones.

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Abstract
The soma membrane of cat motoneurons was voltage-clamped in vivo using intracellular current and voltage electrodes whose tips were separated by at least 5 .mu.m. Depolarization activates 2 separate, non-interacting K conductance systems whose rates of activation and decay differ by a factor of about 10. These conductances have a similar reversal potential, in the range of -6 to -21 mV (these and all subsequent voltages are expressed relative to the resting potential). Both conductances show linear instantaneous current-voltage relationships. The steady-state magnitudes of both conductances increase with increasing depolarization. Neither conductance inactivates substantially during prolonged depolarizations. The faster K conductance is similar to that described for squid axons and frog node. Activation begins at about +30 mV and is more than 90% complete within 5 ms of a depolarizing voltage step to +50 mV. Activation kinetics appear to be nonlinear. This fast K conductance contributes to the fast falling phase of the action potential. Following repolarization, this conductance decays with a time constant of 2-4 ms. The slower K conductance activates during depolarizations of 10 mV or greater. The activation and decay of this conductance can be described by 1st-order exponential functions with time constants ranging from 20 to 50 ms. The slow K conductance underlies the prolonged hyperpolarization that follows motoneuron action potentials. This slow K conductance may be regulated by intracellular Ca ions. Motoneurons exhibit another distinct conductance system that is activated by hyperpolarization. This 3rd system has a reversal potential near the resting potential. Activation of this conductance during a hyperpolarizing voltage step can be fitted by a single exponential function with a time constant of 50-60 ms over the range -20 to -50 mV. This hyperpolarization-activated conductance accounts for some aspects of the anomalous rectification reported in cat motoneurons. When the clamp circuit was turned off and the motoneurons were stimulated to discharge repetitively by depolarizing current steps, the apparent soma threshold voltage increased as the applied current (and discharge frequency) increased. The basic features of the motoneuron action potential were reconstructed by simulations based on voltage clamp measurements of the voltage dependent conductance systems and previous measurements of passive membrane properties. These simulations assumed that the kinetics of the fast Na and K conductance systems in motoneurons can be described by equations of the same form as the Hodgkin-Huxley equations. These action potential reconstructions indicated that a major portion of the delayed depolarization following the action potential is attributable to capacitative currents from the dendrites. The interspike voltage trajectories measured during low-frequency (primary range) repetitive discharge could be simulated reasonably well by summing the fast and slow K conductances; during high-frequency (secondary range) discharge simulated trajectories were less depolarized than recorded trajectories.