Cat spinoreticular neurons: locations, responses and changes in responses during repetitive stimulation.

Neurons in the lumbosacral spinal cord which project to the medial pontomedulary reticular formation were studied in the chloralose-anesthetized cat. Such neurons, identified by antidromic activation, were found predominantly in lamina VIII and medial lamina VII, and most were found to project to the contralateral reticular formation. Receptive fields for natural stimuli were generally complex, having various combinations of excitatory and inhibitory areas ipsi- and/or contralaterally. Adequate stimuli ranged from innocuous to noxious, with the stimuli required for decreasing a neuron's activity usually more intense than the stimulus required for increasing it. Electrical stimulation of hindlimb nerves indicated the presence of extensive convergence. Responses of spinoreticular neurons were found to decline during periods of repetitive stimulation. The response decrements were found to have many of the parametric features of behavioral habituation and were similar to response decrements previously observed in the medial pontomedullary reticular formation.