COMPARISON OF THE CONNECTIONAL PROPERTIES OF THE 2 FORELIMB AREAS OF THE RAT SENSORIMOTOR CORTEX - SUPPORT FOR THE PRESENCE OF A PREMOTOR OR SUPPLEMENTARY MOTOR CORTICAL AREA

The existence of multiple motor cortical are as that differ in some of their ProPerties is well known in primates, but is less clear in the rat. The present study addressed this question from the point of view of connectional properties by comparing the afferent and efferent proj ections of the caudal forelimb area (CFA), considered to be the equiva lent of the forelimb area of the primary motor cortex (MI), and a seco nd forelimb motor representation, the rostral forelimb area (RFA). As a result of various tracing experiments (including double labeling), i t was observed that CFA and RFA had reciprocal corticocortical connect ions characterized by preferential, asymmetrical, laminar distribution , indicating that RFA may occupy a different hierarchical level than C FA, according to criteria previously discussed in the visual cortex of primates. Furthermore, it was found that RFA, but not CFA, exhibited dense reciprocal connections with the insular cortex. With respect to their efferent projection to the basal ganglia, it was observed that C FA projected very densely to the lateral portion of the ipsilateral ca udate putamen, whereas the contralateral projection was sparse and mor e restricted. The ipsilateral projection originating from RFA was slig htly less dense than that from CFA, but it covered a larger portion of the caudate putamen (in the medial direction); the contralateral proj ection from RFA to the caudate putamen was of the same density and ext ent as the ipsilateral projection. The reciprocal thalamocortical and corticothalamic connections of RFA and CFA differed from each other in the sense that CFA was mainly interconnected with the ventrolateral t halamic nucleus, while RFA was mainly connected with the ventromedial thalamic nucleus. Altogether, these connectional differences, compared with the pattern of organization of the motor cortical areas in prima tes, suggest that RFA in the rat may well be an equivalent of the prem otor or supplementary motor area. In contrast to the corticocortical, corticostriatal, and thalamocortical connections, RFA and CFA showed s imilar efferent projections to the subthalamic nucleus, substantia nig ra, red nucleus, tectum, pontine nuclei, inferior olive, and spinal co rd.