INFORMATION-PROCESSING WITHIN THE MOTOR CORTEX .1. RESPONSES OF MORPHOLOGICALLY IDENTIFIED MOTOR CORTICAL-CELLS TO STIMULATION OF THE SOMATOSENSORY CORTEX

Inputs from the somatosensory cortex to the motor cortex have been pro posed to function in learning of motor skills. In an attempt to analyz e how these somatosensory inputs were processed in the motor cortex, n eurons in the superficial layer of the cat motor cortex were classifie d into three groups on the basis of synaptic responses elicited by int racortical microstimulation (ICMS) of area 2. ICMS was delivered throu gh seven electrodes implanted in area 2. When ICMS through one of the seven sites produced a response that was greater than 50% of the respo nse produced by stimulating the seven sites at a time, the site was ca lled 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 s ite but displayed a variable latency. Type III cells had no dominant s ite 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 typ e III responses. Seventy-nine neurons in layers II/III of the motor co rtex, which responded to ICMS in area 2, were stained by intracellular injection of biocytin. From the presence of an apical dendrite and ri ch spines on the dendrites, 23 type I, 21 type II, and 15 type III cel ls were classified as pyramidal cells. Type II pyramidal cells were lo cated 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 nonpyram idal cells. These cells consisted of five small multipolar cells in la yer II and a large multipolar cell and a small bitufted cell in layer III. The remaining 11 cells were not classified because of insufficien t staining. Since type I and type II cells are considered to represent monosynaptic and polysynaptic responses to stimulation of area 2, res pectively, information flow from type I cells to more superficially lo cated type II cells is presumed in layers II/III of the motor cortex. Type III responses suggest the presence of a convergent flow of impuls es inside of and/or between areas 2 and 4. (C) 1994 Wiley-Liss, Inc.