Responses of single units in laminae 2 and 3 of cat spinal cord.

333 units were recorded in laminae 2 and 3 of lumbar cord in decerebrate cats. Recording of small-amplitude spikes was made possible by the use of platinum-surfaced tungsten microelectrodes, continuously variable filters and an analogue delay line display. Stimulation of the lateral Lissauer tract showed that a sample of the units sent axons into this tract. Using iron-plated electrodes, recording sites were marked and shown to be within laminae 2 and 3. Axons of peripheral afferent axons were excluded from the sample, as were long-range descending axons. By using one electrode placed close to a cell body in lamina 4 and a roving electrode in the dendritic region dorsal to the cell body, it was possible to show that the recorded units were not field spread of deeper cells. Of the units 94% had peripheral receptive fields, RF; 30% had small RFs less than 2 sq.cm, intermediate in size between RFs of peripheral axons and RFs of large cells in dorsal horn. These small RF cells occurred in clusters and their RFs constituted a fraction of the larger RF of nearby large cells. Of the units 56% responded to brush and touch, 19% to brush, touch and pressure, while 19% required pressure on skin to excite them. Latency of response to electrical stimulation showed that all cells were excited by myelinated afferents. While no cells were detected exclusively by C afferents, many may have been excited by both A and C afferents. Eighteen per cent of the cells showed a prolonged discharge lasting more than 5 sec after a single stimulus. Some of these long discharge cells continued firing for minutes. Another unusual class, 14% of all cells, habituated very powerfully to intermittant natural or electrical stimuli, and remained unresponsive for many seconds after responding to the first stimulus.