Recurrent facilitation of spinal reflexes

Antidromic volleys in muscle nerves may facilitate monosynaptic reflexes originating from neighboring motoneurons. This facilitation has been studied in spinal cats. It is at its peak with a conditioning-test interval of 20 to 30 msec., and can last 50 to 100 msec. The threshold of facilitation is about the same as that of recurrent inhibition. Both phenomena appear to be activated by stimulation of the large motor axons. The latency of facilitation seems to be longer than that of recurrent inhibition by approximately 1 msec., suggesting the presence of at least one more synaptic delay. Facilitation often follows an inhibition of variable depth and duration. Frequently, however, the facilitation is not preceded by inhibition, and therefore it cannot be a rebound effect. The pharmacological properties of facilitation resemble those of recurrent inhibition. Dihydro-beta-erythroidine(1) partially blocks facilitation; the peak is decreased and occurs earlier, and the duration is shortened. Eserine increases the duration of facilitation and inhibition and sometimes enhances their magnitude. It is concluded that recurrent facilitation is mediated by the cholinergic axon collaterals, and that at least two interneurons are located between collateral and motoneuron. Possible mechanisms of facilitation are discussed.