Information processing within the motor cortex. I. Responses of morphologically identified motor cortical cells to stimulation of the somatosensory cortex.

Inputs from the somatosensory cortex to the motor cortex have been proposed to function in learning of motor skills. In an attempt to analyze how these somatosensory inputs were processed in the motor cortex, neurons in the superficial layer of the cat motor cortex were classified into three groups on the basis of synaptic responses elicited by intracortical microstimulation (ICMS) of area 2. ICMS was delivered through seven electrodes implanted in area 2. When ICMS through one of the seven sites produced a response that was greater than 50% of the response produced by stimulating the seven sites at a time, the site was called a "dominant" site. Type I cells were those that had a dominant stimulation site and showed a constant response latency when examined by a double shock test. Type II cells were those that had a dominant site but displayed a variable latency. Type III cells had no dominant site and showed a variable latency. Latency of type I responses was 1.2-2.6 milliseconds, which was much shorter than that of type II and type III responses. Seventy-nine neurons in layers II/III of the motor cortex, which responded to ICMS in area 2, were stained by intracellular injection of biocytin. From the presence of an apical dendrite and rich spines on the dendrites, 23 type I, 21 type II, and 15 type III cells were classified as pyramidal cells. Type II pyramidal cells were located more superficially than type I and type III pyramidal cells. On the basis of the absence or sparseness of dendritic spines, three type I and four type II cells in layers II/III were classified as nonpyramidal cells. These cells consisted of five small multipolar cells in layer II and a large multipolar cell and a small bitufted cell in layer III. The remaining 11 cells were not classified because of insufficient staining. Since type I and type II cells are considered to represent monosynaptic and polysynaptic responses to stimulation of area 2, respectively, information flow from type I cells to more superficially located type II cells is presumed in layers II/III of the motor cortex. Type III responses suggest the presence of a convergent flow of impulses inside of and/or between areas 2 and 4.