Unit activity of primary somatosensory cortex during primary responses

Abstract

1. In three series of experiments on cats responses of 490 neurons in a circumscribed area of the somatosensory cortex were recorded and at the same time primary responses (PR) were recorded to electrical stimulation of a nerve. 2. The following type of unit responses were distinguished: A) nonbackground spikes during the PR, B) nonbackground spikes during the PR followed by an afterdischarge, C) temporary cessation of activity, and D) nonbackground spikes during the PR followed by temporary cessation of activity. The number of neurons with responses of type A is greatest in all series of experiments. The response of the type B neurons is most marked when general anesthesia subsides, while the type C response is most marked under moderately deep anesthesia and the type D response during strychnine poisoning. 3. Latent periods of single unit responses in all series of experiments lie approximately between 6 and 50 msec, and two-thirds of all neurons responding to nerve stimulation with nonbackground spikes begin to discharge in the positive phase of the PR. Several groups of neurons, presumably located successively in the chain of neurons processing a stimulus arriving from the periphery, can be distinguished on the basis of latent periods of their discharges, their variability, and depth of recording. 4. The positive phase of the PR is due to depolarization of neurons lying at a depth of 800–1400 μ, and the negative phase to hyperpolarization of the same neurons at deep levels and also, perhaps, to depolarization of apical dendrites of neurons in an inactive state.