VIBRISSAL MOTOR CORTEX IN THE RAT - CONNECTIONS WITH THE BARREL FIELD

The flow of information in the sensorimotor cortex may determine how s omatic information modulates motor cortex neuronal activity during vol untary movement. Electrophysiological recordings and neuroanatomical t racing techniques were used to study the connections between the prima ry somatosensory cortex (SI) and the vibrissal representation of the p rimary motor cortex (MI) in rodents, Intracortical microstimulation (I CMS) was applied to the vibrissal region of the motor cortex to identi fy a site from which stimulation evoked movements of the vibrissae. Mo vements of only a single whisker were evoked by applying low-intensity stimulating current to particular locations within MI. A single injec tion of either horseradish peroxidase (HRP) or biocytin was made at th e stimulus site in each animal, to retrogradely label cells in the som atosensory cortex. Receptive field (RF) responses were recorded from n eurons in the barrel cortex to identify the sensory cortex representat ion of the same whisker that responded to ICMS. The site at which neur ons responded predominately to manual stimulation of this particular v ibrissa was marked by a small electrolytic lesion. The projection from the somatosensory cortex to the identified whisker representation in the motor cortex was determined by mapping the location of labeled neu rons in tissue sections processed for either HRP or biocytin. The rela tionship of the labeled cells in SI to the barrel structures was deter mined from adjacent sections that were stained for cytochrome oxidase. In all cases, the barrel column associated with the relevant whisker contained labeled cells. Surrounding barrels also contained labeled ce lls, although fewer in number. Very few labeled cells were found in no n-contiguous barrels. These results show that the SI to MI projection is somatotopically arranged, such that the sensory cortex representati on of a whisker is morphologically connected to the motor cortex repre sentation of the same whisker. Thus, sensory information is relayed to MI from the relevant whisker region in SI. Adjacent whisker regions a lso appear to relay somatic input, but presumably to a lesser degree. A second group of animals received single small injections of the ante rograde tracer, Phaseolus vulgaris leucoagglutinin, to an electrophysi ologically identified whisker representation in the sensory cortex. A single narrow column of labeled fibers was found in the motor cortex f ollowing such injections. Thus, the sensory cortex appears to relay so matic information from the vibrissae to restricted regions of the moto r cortex in a somatotopically organized manner. Furthermore, the stimu lus-evoked whisker movements suggest that certain features of the outp ut map of the motor cortex are discretely organized. These input/outpu t relationships suggest that complex information processing within the vibrissal sensorimotor cortex is highly organized.