Cortical innervation of the facial nucleus in the non-human primate

Abstract

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.

Judgments of social awkwardness from brief exposure to children with and without high-functioning autism

“…However, this type of dysmorphology appears to define a subgroup of individuals with autism who have more complex medical involvement, lower IQ, and poorer outcomes (Corbett et al, 2012; Corbett et al, 2013), which does not match the population with high-functioning autism in our stimuli. Recent work using high-definition 3-D imaging has identified several facial features that are significantly more prevalent in children with ASD and their unaffected siblings, compared to TD peers (Morecraft et al, 2001). However, these features are most prevalent in unaffected siblings, making it doubtful that they would result in easily perceived social awkwardness in children with ASD.…”

Face to face with the social brain

“…This is important to note because frontal lobe size is argued to be the main constraint on social network size in anthropoids [57]. Perhaps the best argument for an evolutionary link between facial expressivity and neocortical expansion is the fact that at least five different areas of the motor cortex connect directly to the facial motor nucleus via descending axons in the corticobulbar tract [58]. However, this point has received little attention with regard to anthropoid brain evolution until now.…”

From facial mimicry to emotional empathy: A role for norepinephrine ?

“…Tract tracing studies that show connectivity between lateral-basal amygdala nucleus, cingulate motor cortex (M3, M4) and brainstem facial motor nucleus suggest a potential anatomical basis for this mimicry (Morecraft et al 2007; Morecraft, Lousie, Herrick, & Stilwell-Morecraft, 2001). For example, the lateral-basal amygdala nucleus, which together with the lateral amygdala nucleus is believed to play a key role in extracting emotionally salient sensory information across modalities (LeDoux, 2000), shows dense bidirectional connectivity to the rostral cingulate motor cortex (M3) which codes a representation predominantly of the upper face.…”