Repetitive impulse firing: comparisons between neurone models based on 'voltage clamp equations' and spinal motoneurones.

Abstract

AbstractRepetitive impulse firing was elicited in neurone models by steady stimulating currents of abrupt onset. The neurone models were based on the Frankenhaeuser—Huxley equations (1964) for voltage clamp data from the amphibian peripheral nerve. A frequency‐current curve (‘f‐I curve’), initial adaptation, and minimum firing rate similar to those of cat spinal motoneurones were obtained in the Frankenhaeuser—Huxley model if it were provided with (i) a long‐lasting after‐hyperpolarization due to potassium permeability changes, and (ii) a decreased subthreshold sodium inactivation. For detailed comparisons to the repetitive impulse firing of spinal motoneurones, model versions were used in which the subthreshold sodium inactivation was very slight, and the passive membrane properties as well as the afterpotentials resembled those of spinal motoneurones. In their repetitive behaviour, these models were quantitatively similar to spinal motoneurones. In the motoneurone‐like model versions, initial adaptation was due to a kind of ‘summation’ of the potassium permeability changes underlying the after‐hyperpolarizations of consecutive spikes. The slope of the f‐I curve was markedly affected by modifications of the size or time course of the potassium permeability changes responsible for the after‐hyperpolarization.