Some properties of spinal neurons projecting to the medial brain-stem reticular formation

Experiments were done to locate and characterize neurons projecting from the spinal cord to the brain-stem reticular formation. In barbiturate-anesthetized or midcollicular-decerebrate cats, concentric bipolar electrodes were placed in the medial brain-stem reticular formation of the caudal pons and rostral medulla. Neurons were identified as projecting to brain-stem reticular formation using the antidromic collision criterion. Of 30 cells, 20 projected to the ipsilateral and ten to the contralateral brain-stem reticular formation. The mean conduction velocity was 49.6 m/sec. Cell somata were located deep to lamina V of the dorsal horn. Neurons in this sample fell into two categories based on their receptive field. One type of cell was excited by firm pressure over such deep structures as muscles, ligaments and periosteum. The second type had a large cutaneous receptive field. Neurons of this second type responded maximally to noxious cutaneous stimuli. Inhibitory receptive field components were common in neurons with both types of receptive field. In addition to antidromic activation, stimulation of medial brain-stem reticular formation produced both synaptic excitation and marked inhibition of some cells projecting to the brain-stem reticular formation. It is concluded that spinoreticular neurons may relay information concerning noxious stimuli.