Comparison of the Connectional Properties of the Two Forelimb Areas of the Rat Sensorimotor Cortex: Support for the Presence of a Premotor or Supplementary Motor Cortical Area

Abstract

The existence of multiple motor cortical areas 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 projections of the caudal forelimb area (CFA), considered to be the equivalent of the forelimb area of the primary motor cortex (MI), and a second forelimb motor representation, the rostral forelimb area (RFA). As a result of various tracing experiments (including double labeling), it was observed that CFA and RFA had reciprocal corticocortical connections characterized by preferential, asymmetrical, laminar distribution, indicating that RFA may occupy a different hierarchical level than CFA, 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 CFA projected very densely to the lateral portion of the ipsilateral caudate putamen, whereas the contralateral projection was sparse and more restricted. The ipsilateral projection originating from RFA was slightly less dense than that from CFA, but it covered a larger portion of the caudate putamen (in the medial direction); the contralateral projection from RFA to the caudate putamen was of the same density and extent 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 thalamic 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 primates, suggest that RFA in the rat may well be an equivalent of the premotor or supplementary motor area. In contrast to the corticocortical, corticostriatal, and thalamocortical connections, RFA and CFA showed similar efferent projections to the subthalamic nucleus, substantia nigra, red nucleus, tectum, pontine nuclei, inferior olive, and spinal cord.

Functional and Anatomical Reorganization of the Sensory-Motor Cortex after Incomplete Spinal Cord Injury in Adult Rats

“…This suggests their origin in the motor rather than the sensory cortex (Kuang and Kalil, 1990;Bareyre et al, 2002). A motor bias of the midlinecrossing CST is also demonstrated by the high proportion of neurons that originated from the RFA, which is suggested to be primarily motor (Neafsey and Sievert, 1982;Sanderson et al, 1984;Rouiller et al, 1993).…”

Progressive Motor Cortex Functional Reorganization Following 6-Hydroxydopamine Lesioning in Rats

“…Hindlimb representation delimited the posterior boundary of the vibrissa and forelimb representation, and ineffective sites formed the basis for delineating the rostral M1 border. Based on physiological and anatomical evidence (Neafsey and Sievert, 1982;Neafsey et al, 1986;Rouiller et al, 1993), we distinguished two subregions within the forelimb area, namely the caudal and rostral areas (posterior and anterior to AP, 3.25 mm, respectively). Forelimb movements were classified as either distal (wrist/digit) or proximal (elbow/shoulder).…”

Restoration of function after brain damage using a neural prosthesis

“…intact animals, they appear to be relatively weak compared with M1's connections with the somatosensory cortex (9,11,12).…”

“…The rostral forelimb area (RFA) is a premotor area in the rodent's frontal cortex that shares many properties with PM of primates and is thought to participate in recovery of function after injury to M1 (5,(7)(8)(9). PM areas are so-named because the principal target of their output fibers is M1 (10).…”