Corticifugal influences on the activity of reticular formation neurons in cats

Abstract

Corticifugal influences descending through the subthalamus have been shown to regulate the excitability of the reticular formation (RF) and maintain attentive behavior. Previous work demonstrated that cooling the secondary somatosensory cortex (SSII) modified RF sensory-evoked potentials. The present experiments indicate that the primary sensorimotor cortex (SSI) also projects to single neurons at those levels of the RF which have been shown to receive subthalamic influences. In acutely prepared, unanesthetized, immobilized cats, bilateral cooling of SSI caused a reduction in the response of midbrain and medullary RF neurons to sciatic stimulation. In some RF cells which responded to both sciatic and forepaw stimulation, cortical cooling decreased responses to both inputs. The generalized, tonic nature of these corticifugal influences was also illustrated by showing that cooling reduced spontaneous activity. Rewarming led to a recovery of reticular responsiveness and background activity. If care was taken to prevent cortical deterioration, cooling could be repeated with subsequent response recovery back to control levels upon rewarming. In some reticular neurons which had both early and late responses it was possible to show that the cortex could also exert differential effects on these responses. Thus, corticifugal projections from both SSI and SSII are important in determining the excitability of neurons along the length of the RF to somatosensory input. Although these influences were usually tonic and generalized in nature, in some cases they could be shown to exert more specific effects.