An investigation of pyramidal tract cells in the somatosensory cortex of the rat.

1. An electrophysiological study has been made of pyramidal tract (PT) cells in the somatosensory cortex of the rat; the axons of the cells were shown to project either to the dorsal column nuclei or to the corticospinal tract.2. The corticospinal axons had conduction velocities of 7.6-10.8 m/sec. and therefore must have belonged to the large population of PT axons with relatively small diameters.3. PT cells constituted almost a quarter of the units encountered; they were distributed throughout the forepaw area within the deeper layers of the cortex.4. The sizes of the receptive fields of PT cells varied widely and did not differ significantly from those of non-PT cells.5. The latencies of PT cell discharges following peripheral stimulation showed less variation than those of non-PT cells though the mean latencies were identical.6. Most PT cells could be driven from the periphery and exhibited spontaneous activity in addition; they therefore function to maintain a continuous control over the somatosensory input to the brain which can be increased whenever the skin is stimulated.