An excitatory pathway through dorsal columns to rubrospinal cells in the cat.

A somaesthetic excitatory pathway to rubrospinal cells, via other than cortico- or cerebello-rubral relays, has been demonstrated in cats, using intracellular recording techniques. After lesions of the rostral part of the cerebral cortex and the cerebellum, electrical stimulation of the contralateral forepaw and low intensity stimulation of the dorsal columns produced excitatory post-synaptic potentials (e.p.s.p.s) in rubrospinal cells, which often reached the threshold for firing bursts of spikes. These somaesthetic e.p.s.p.s are composed of a burst of small elementary e.p.s.p.s which progressively create a depolarization with a long-lasting time course. Evidence has been obtained that the pathway runs through the dorsal columns to reach the red nucleus after a relay. Stimulation applied to the dorsal columns gives rise to the response with very low threshold current, close to 20 microA. Also, the firing pattern of lemniscal fibres to single-shock dorsal column stimulation has been correlated with the time course of excitation of the rubrospinal cells. As calculated from latencies of e.p.s.p.s the conduction velocity in the investigated pathway is over 55 m/s in the peripheral nerve fibres, indicating that thick, myelinated fibres are involved. It is suggested that the fast somaesthetic loop might, via the rubrospinal tract, participate in exploratory motor behaviour and in the adaptation of gait to the environment.