Abstract
Centrum medianum (CM) afferent pathways, which are involved in pain sensation, were analyzed using physiological techniques. Thirty-four neurons in the gigantocellular nucleus (nucleus gigantocellu- laris medullae oblongatae, GC) in cats were recorded intracellularly. Of these, 5 (15%) did not respond to electrical stimulation applied to any of the 4 limbs. Twenty-nine (85%) showed spike potentials that were superimposed on excitatory post-synaptic potentials (EPSPs) with an amplitude of 7.2mV (n=101) and a duration of 6.8ms. The latencies from contra- and ipsilateral forelimb, contra- and ipsilateral hindlimb to the GC were 9.3 (n=25), 7.2 (n=23), 12.9 (n=28), and 10.9ms (n=25), respectively. Of these responding neurons, 19 (66%) responded to stimuli to all 4 limbs, 7 (24%) to 3 limbs, 1 (3%) to 2 limbs, and 2 (7%) to 1 limb. These afferent neurons in the GC showed spike potentials without EPSPS after stimulation of the CM in the thalamus. Extracellular activities of 37 CM neurons were also tested. Of these, 4 neurons responded to GC stimulation with a short latency of 1.6ms. Another 33 responded with a long latency of 6.7, and 11 of them were able to follow GC stimulation of over 200Hz with a fixed long latency; 22 responded with varying long latencies but were not able to follow stimuli over 15Hz. Three-quarters of the CM neurons received ipsilateral inputs from the GC, and the other contralateral inputs. Three CM neurons responded to stimulation of the GC in quasi-intracellular records. These findings suggest that some ascending fibers from the GC terminate on CM neurons and play a role in pain sensation.
