The current-to-frequency conversion of extraocular motoneurons

A description of the current to frequency conversion of extraocular neurons is presented. The discharge frequency of extraocular motoneurons of the cat to intracellular stimulation is rate sensitive to both depolarizing and hyperpolarizing current waveforms. From a time domain analysis of the relationship between the stimulus input current and the mathematically described measured neuronal discharge rate, the system impulse response is obtained by deconvolution. The description includes two non-linearities 1) a threshold element and 2) a current saturation element. The output of the derived process description is compared to experimental observations, evoked by a variety of current waveforms. The rate sensitive behaviour of extraocular motoneurons is demonstrated by a graph of gain versus the modulation frequency of stimulus current. It is suggested that the high frequency gain characteristic of the system partially compensates for the low pass transfer property of the extraocular muscles and orbital tissues. Thus the adaptive characteristics of these neurons could generate the neural signal necessary for the occurrence of saccadic eye movements.