Time Structure of Impulsation of Spinal Motoneurons of Cats under Conditions of Realization of the Stretch Reflex

We analyzed the characteristics of impulse activity recorded intracellularly from motoneurons of the mm. gastrocnemius + soleus in decerebrated cats; this activity was induced by controlled stretching of the above muscle group. Long-lasting (30 min long or more) sufficiently qualitative records (membrane potential, MP, greater than −50 mA throughout the entire recording period) were obtained from 17 motoneurons. Muscle stretching (sinusoidal half-waves of different amplitudes and combinations of such waves) allowed us to obtain well-reproducible successions of impulses with no significant changes in the resting MP of the studied motoneurons. In 14 of such cells, we found significant correlation between normalized variations of the instantaneous frequency (in a sequence of preceding and subsequent action potentials, APs), indicative of a considerable probability of alternation of high and low values of this parameter. The same trend (toward alternation of high and low values) was observed in successive magnitudes of afterhyperpolarization (AH) that followed APs. Between the values of the extrema of AH trajectories and values of the instantaneous frequency of impulsation, we observed significant negative correlation. The mechanisms underlying the formation of AH in spinal motoneurons and also the effects of this hyperpolarization on the characteristics of impulse activity generated by these cells are discussed.