Antidromic activation of sensory afferent fibres in sympathetic nerves of the cat by stimulation of collaterals within the dorsal medulla oblongata.

Action potentials were evoked in the white ramus of the third thoracic segment by electrical stimulation in the dorsal medulla oblongata. The following findings indicate that these potentials are due to antidromic activation of collaterals of afferent fibres in sympathetic nerves rather than to orthodromic synaptic activation of preganglionic sympathetic neurones via bulbospinal sympatho-excitatory pathways: (i) they had short latencies yielding intraspinal conduction velocities of 13-43 m/sec; (ii) they followed short trains of stimuli at frequencies up to 600 Hz; and (iii) they were abolished by cutting the dorsal roots of the same spinal segment.