CUTANEOUS PROJECTION TO SECOND-ORDER NEURONS OF THE DORSAL COLUMN SYSTEM

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Abstract
By use of microelectrodes, extracellular unitary potentials were recorded from gracilis neurons of decerebrate cats. Most of the 282 units studied in detail fell into one of three groups. One type responded to hair deflection with a rapidly adapting discharge and the evoked responses could be inhibited by stimulation of nerves or receptors from skin surrounding the excitatory receptive field. A second type of unit responded to light skin pressure (touch), with a relatively slowly adapting discharge and exhibited a conspicuous background discharge which could be altered by varying the temperature of the receptive field. A third group of units responded to tap or vibratory stimuli. For a period such vibration-sensitive units discharged at least once for each cycle of sinusoidal vibration from 70 to 350 cycles/sec. The cutaneous receptive field size for hair and touch units varied with location on the body surface. The smallest receptive fields were those located distally on the hind limb while the largest involved the proximal limb and trunk. Receptive fields for touch units regularly were much larger than for hair units. There was a tendency for units of each type to be found at certain positions of the rostral-caudal dimension of the nucleus. The responses of each of the three frequently encountered gracilis units were similar to known characteristics of a receptor. For this reason it is concluded that the projection to gracilis neurons is separate and specific from each cutaneous receptor. The major change in sensory signals after synaptic transfer in this nucleus seems to be a consequence of excitatory convergence of several to many receptors of one type upon a gracilis neuron. For the hair receptor projection the excitatory effects are modified by inhibition initiated from regions outside the receptive field. The findings and conclusions are discussed in relation to the functional organization of the dorsal column sensory projection.