EFFECTS OF STIMULATION OF PONTINE AND BULBAR RETICULAR FORMATION UPON SPINAL MOTONEURONS OF THE CAT

Abstract

Pontine and bulbar reticular formation (RF) and Deiters1 nucleus were stimulated with brief electric stimuli (2-3 msec.) and their effects on the intracellular potential of lumbar alpha motoneurons as well as on the size of monosynaptic reflex were studied in the cat. Secondary effects through the gamma efferents-muscle spindle loop were eliminated. Stimulation of pontine RF, especially its rostral part, produced a marked inhibition of extensor monosynaptic reflex at a latency of 15-20 msec. and a slight facilitation of flexor monosynaptic reflex at a latency of about 10 msec. Stimulation of Dieters'' nucleus was facilitatory to the extensor monosynaptic reflex. The facilitation appeared in two separate phases, the one set up at a latency of less than 10 msec. with relatively short and simple time course and the other at a latency of 15-20 msec. with a longer duration. The effect of stimulation of the bulbar RF varied extensively with a slight change in the electrode position. The most prevalent effect was facilitatory to the extensor monosynaptic reflex, which in some cases resembled that of Deiters'' nucleus in its time course. Generally speaking, changes by stimulation of intracellular potential of spinal motoneurons reflected the influence selicitable from the same site upon the size of mono-synaptic reflex. Facilitatory or inhibitory influences upon the reflex size could mostly be accounted for by the potential change of the motoneuron membrane which was most likely due to excitatory post-synaptic potentials and inhibitory post-synaptic potentials. Single spinal motoneurons fired from different levels of depolarization, when the rate of rise of depolarizing post-synaptic potentials was different according to the stimulated site in the brain stem. The effect of repetitive stimulation of the RF on the membrane potential of single spinal motoneurons could be illustrated in general as a summated effect of single stimulation at the same site. Tonic suprasegmental influences and their modification by reticular stimulation were visualized as maintained discharges of descending tract fibers and spinal interneurons. A hypothetical explanation of decerebrate rigidity was put forward based on the present findings.