Response properties and functional organization of neurons in midline region of medullary reticular formation of cats

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Abstract
Extracellular single-unit recordings were made in the anesthetized cat from neurons within the medullary raphe nuclei and nearby reticular formation. The descending axons from some of these neurons were characterized in terms of length, conduction velocity and location within the white matter of the spinal cord. The sensory properties were characterized following somatic, baroreceptor, visual and auditory stimuli. The mean conduction velocities of the descending axons from neurons in the medullary raphe nuclei and in the magnocellular tegmental field (26 m/s) were significantly slower than the mean conduction velocities of units in the regions immediately dorsal to them (50 m/s). Action potentials in neurons in the medullary raphe nuclei and in the magnocellular tegmental field were evoked by antidromic stimulation from the dorsolateral portion of the spinal cord (30 or 43, 70%), whereas neurons located in more dorsal regions along the midline and in the reticular formation projected into the ventral columns (18 of 25, 72%). Neurons were most easily activated by a tap stimulus to the body surface. This stimulus activated 84% of the neurons tested. The receptive fields were large, often including the four limbs, back, and head. Tap-sensitive neurons were found throughout the regions investigated. stimulus (pinch), 35% responded to baroreceptor stimuli, 32% responded to visual stimuli, and 33 responded to auditory stimuli, and 33% responded to auditory stimuli. Neurons responsive to pinch were likely to respond to baroreceptor stimuli and unlikely to respond to visual stimuli. Neurons responsive to visual stimuli were likely to respond to auditory stimuli. Neurons responsive to pinch or baroreceptor stimuli were located predominantly in the medullary raphe nuclei and magnocellular tegemental field. The axons of medullary-spinal neurons responsive to these stimuli had a low average conduction velocity (28 .+-. 4 m/s) and the axons were located predominantly in the dorsolateral funiculus. These neurons may participate in endogenous analgesia-producing pathways and the baroreceptor-mediated reflex control of blood pressure. Neurons responsive to visual or auditory stimuli were located predominantly in the dorsal region along the midline and in the dorsal portion of the reticular formation. The axons of medullary-spinal neurons responsive to these stimuli had a high average conduction velocity (52 .+-. 5 m/s) and the axons were located predominantly in the ventral columns. These neurons may participate in the reflex control of body position.