Corticifugal influences on reticular formation evoked activity in cats

Abstract

Corticifugal influences descending through the subthalamus have been postulated to be responsible for its role in determining the excitability of the reticular formation (RF) and maintaining the attentive, affective and motor behavior of the animal. Support for this concept comes from acute experiments involving cooling of the secondary somatosensory cortex (SSII) in unanesthetized, immobilized cats. Bilateral cooling of SSII reduced the amplitude of sciatic evoked potentials (EP) along the length of the brain stem RF, though reductions in EP of midbrain RF were more consistent than those in the medullary RF. Sciatic EP in sensory and motor areas at comparable levels of the neuraxis, including ventralis posterior lateralis, showed little or no change. These results suggest that the cortex can exert tonic facilitatory influences on the RF. Cooling of SSII also decreased the amount of inhibitory interaction between paired sciatic EP in the RF which implies an interference with phasic, inhibitory influences. Extirpation of SSII had much the same effect as cooling in reducing sciatic EP in the RF. Unilateral removal of SSII produced a significant decrease in sciatic EP; subsequent removal of contralateral SSII resulted in still more reduction implying that these corticifugal influences originate bilaterally, though the ipsilateral projection may be more pronounced. Persistence of sciatic EP in the RF after bilateral extirpation or pro-longed cooling of SSII suggests that other areas of the cortex are also involved in determining the excitability of the RF to entering sensory input.