Cerebral cortical control of orbicularis oculi motoneurons

Cerebral cortical neural networks associated with eyelid movement play a critical role in facial animation, contribute to the regulation of blink frequency, and help prevent ocular injury. Eyelid closure depends, in part, on motoneurons that innervate the orbicularis oculi (OO) muscles. In this study, OO motoneuron cortical afferents were identified in rhesus monkeys with rabies virus, a retrograde transneuronal tracer. Virus was injected into the right OO muscle and immunohistochemically localized after 4-6 day transport intervals. Labeled motoneurons were limited to dorsal portions of the ipsilateral facial motor nucleus. After 4- and 4.5-day transport intervals, most labeled cortical neurons were localized to ventrolateral premotor (LPMCv), dorsolateral premotor (LPMCd), and motor (M1) cortices. Labeled neurons were more sparsely distributed in supplementary (M2), caudal (M4), and rostral (M3) cingulate motor cortices; the frontal eye fields (FEF); pre-supplementary motor cortex (pre-SMA); somatosensory cortices (areas 3a, 3b, and 1); and prefrontal cortex. At longer transport intervals (5-6 days), labeled neurons increased substantially in LPMCv, LPMCd, M2, M3, M4, pre-SMA, and FEF. Concentrations of labeled neurons also appeared in cortices along the lateral fissure and intraparietal sulcus. Overall, the densest collection of labeled neurons was localized to the caudal junction of LPMCd and LPMCv with M1. Rostral M3 was another focus of OO premotor neurons. Labeled neurons were distributed bilaterally in all motor cortical areas with a modest contralateral predominance for M2, LPMC, and M1. Thus, the cortical control of OO motor activity is distributed bilaterally among multiple motor areas.