THE MODE OF ACTIVATION OF A BARREL COLUMN - RESPONSE PROPERTIES OF SINGLE UNITS IN THE SOMATOSENSORY CORTEX OF THE MOUSE UPON WHISKER DEFLECTION

Response properties of single units in the mouse barrel cortex were st udied to determine the sequence in which the neurons that form a corti cal column become activated by a single 'natural' stimulus. Mice (n = 11) were anaesthetized with urethane. For a total of 153 cells, groupe d by cortical layer, responses to a standardized deflection of a singl e whisker were characterized using poststimulus time and latency histo grams. Usually, for each unit, data were collected for stimulation of its principal whisker (PW; the whiskers corresponding to the barrel co lumn in which the cell was located) and of the four whiskers surroundi ng the PW. In all layers, PW stimulation evoked responses at shorter l atency than surround whisker stimulation. In layers II - III and IV a bimodal distribution of cells according to latency to PW stimulation w as found. Statistical analysis indicated the presence of two classes o f cells in each of these layers: 'fast' units (latency <15 ms) and 'sl ow' units (latency greater-than-or-equal-to 15 ms). The great majority of cells in layers I, V and VI fired at latencies of >20 ms to PW sti mulation. In general, stimulation of surround whiskers evoked a smalle r response than PW stimulation, The fast cells of layer IV showed the greatest response to PW stimulation (mean = 1.78 spikes/100 ms poststi mulus). Their firing was maximal during the 10 - 20 ms poststimulus ep och, while the slow layer IV cells fired maximally during the 20 - 30 ms poststimulus epoch. Surround inhibition occurred in all layers with in the first 10 ms after stimulus onset, during which period the fast cells are the most active ones, and are thus likely to be responsible for the surround inhibition. This notion is supported by an analysis o f spike duration that showed that eight of the ten cells with a thin s pike (supposed to be GABAergic; McCormick et al., J. Neurophysiol., 54 , 782 - 806, 1985), had PW latencies of < 15 ms. We conclude that the activation of a barrel column is initially inhibitory in nature.