Recurrent inhibition and afterhyperpolarization following motoneuronal discharge in the cat.

1. The relation between the size of a monosynaptic reflex (varying from the smallest values to the maximal motor response) and the output from Renshaw cells was investigated. This relation was extremely variable from one Renshaw cell to another. However, a linear relation between the reflex size and the early discharge emerged when the responses of all the Renshaw cells were averaged or when the summed activity of a pool of Renshaw cells was estimated by recording the recurrent inhibition in their target motoneurones. It was concluded that the lowest threshold motoneurones were efficient in producing recurrent inhibition. 2. In motoneurones, recorded intracellularly, the size of the depression caused by the afterhyperpolarization was compared to the maximum autogenetic recurrent inhibition. Under the particular experimental conditions used to mimic human experiments (Hultborn & Pierrot-Deseilligny, 1979a), it was found that recurrent inhibition had the same order of magnitude as the depression caused by afterhyperpolarization. 3. The additional depression caused by the summation of afterhyperpolarizations of two consecutive spikes was measured. It was shown that a summation of importance equal to the maximum autogenic recurrent inhibition required a mean interspike interval of 25 msec.