The currents that flow in the somatosensory cortex during the direct cortical response.

A current-flow and current-source-density analysis of the sensory evoked response (SER) and the direct cortical response (DCR) in the somatosensory cortex of rats was performed to determine the origin of these potentials. The SER was found to originate in layers II and III, as in cats, with a single excitatory neuronal circuit component. The DCR, on the other hand, has five components, three inhibitory and two excitatory. The activation and magnitude of these components vary with stimulus strength and frequency. During the second and fourth ms of the response, two inhibitory currents flow in layers V and VI; 2 ms later, excitatory current flows in layers II and III. This excitatory current appears to be the same one involved in the SER. Five ms later, the superficial excitatory current is replaced by an inhibitory one in the neighborhood of the DCR's negative peak. At strong stimulus strengths, this is followed by an excitatory current in layer V. The early inhibitory and excitatory components step up through layers upper-VI, V and III over time, implying that inhibition followed by excitation moves upward through cortex. The currents associated with the DCR in somatosensory cortex are compared with those for the DCR in motor and association cortex.