Ascending fibers from the gigantocellular nucleus to the centromedian nucleus of the thalamus in cats.

Centrum medianum (CM) afferent pathways, which are involved in pain sensation, were analyzed using physiological techniques. Thirty-four neurons in the gigantocellular nucleus (nucleus gigantocellularis 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 7.2 mV (n = 101) and a duration of 6.8 ms. 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 10.9 ms (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 1.6 ms. Another 33 responded with a long latency of 6.7, and 11 of them were able to follow GC stimulation of over 200 Hz with a fixed long latency; 22 responded with varying long latencies but were not able to follow stimuli over 15 Hz. 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.