Cortical innervation of the facial nucleus in the non-human primate: a new interpretation of the effects of stroke and related subtotal brain trauma on the muscles of facial expression.

The corticobulbar projection to musculotopically defined subsectors of the facial nucleus was studied from the face representation of the primary (M1), supplementary (M2), rostral cingulate (M3), caudal cingulate (M4) and ventral lateral pre- (LPMCv) motor cortices in the rhesus monkey. We also investigated the corticofacial projection from the face/arm transitional region of the dorsal lateral premotor cortex (LPMCd). The corticobulbar projection was defined by injecting anterograde tracers into the face representation of each motor cortex. In the same animals, the musculotopic organization of the facial nucleus was defined by injecting fluorescent retrograde tracers into individual muscles of the upper and lower face. The facial nucleus received input from all face representations. M1 and LPMCv gave rise to the heaviest projection with progressively diminished intensity occurring in the M2, M3, M4 and LPMCd projections, respectively. Injections in all cortical face representations labelled terminals in all nuclear subdivisions (dorsal, intermediate, medial and lateral). However, significant differences occurred in the proportion of labelled boutons found within each functionally characterized subdivision. M1, LPMCv, LPMCd and M4 projected primarily to the contralateral lateral subnucleus, which innervated the perioral musculature. M2 projected bilaterally to the medial subnucleus, which supplied the auricular musculature. M3 projected bilaterally to the dorsal and intermediate subnuclei, which innervated the frontalis and orbicularis oculi muscles, respectively. Our results indicate that the various cortical face representations may mediate different elements of facial expression. Corticofacial afferents from M1, M4, LPMCv and LPMCd innervate primarily the contralateral lower facial muscles. Bilateral innervation of the upper face is supplied by M2 and M3. The widespread origin of these projections indicates selective vulnerability of corticofacial control following subtotal brain injury. The finding that all face representations innervate all nuclear subdivisions, to some degree, suggests that each motor area may participate in motor recovery in the event that one or more of these motor areas are spared following subtotal brain injury. Finally, the fact that a component of the corticofacial projection innervating both upper and lower facial musculature arises from the limbic proisocortices (M3 and M4) and frontal isocortices (M1, M2, LPMCv and LPMCd) suggests a potential anatomical substrate that may contribute to the clinical dissociation of emotional and volitional facial movement.

Segregated parallel inputs to the brachial spinal cord from the cingulate motor cortex in the monkey.

The corticospinal projection from the cingulate motor cortex to the lower cervical enlargement (C5-T1) was investigated in four rhesus monkeys. Each received an injection of biotinylated dextran amine involving the arm representation of M3 (area 24c) or M4 (area 23c). In M3 cases, contralateral terminal label occurred in the lateral part of laminae V and VI of the intermediate zone including the reticulated marginal border. Lighter labeling was found in laminae IV, VII and the dorsolateral part of the anterior horn (lamina IX). In marked contrast, M4 cases demonstrated contralateral terminal labeling in the medial part of the dorsal and intermediate zones (laminae III, IV, V and VI). Lighter labeling involved the medial part of laminae VII, X and the dorsolateral anterior horn (lamina IX). Our experiments demonstrate that the corticospinal projection from the arm representations of M3 and M4 innervate distinct and separate parts of the spinal gray. Along with the noted differences in the cortical inputs to M3 and M4, these data suggest that the two cingulospinal systems may mediate independent and specialized forms of information effecting upper limb movement.