1. A total of 14.4% of medial reticular formation (RF) cells discharged maximally in relation to movements of the facial musculature. Cells related to movement of the ipsilateral eyelids, ipsilateral lip, ipsilateral pinna, and cells related to movement of several facial regions were seen.
2. Cells related to ipsilateral eyelid closure constituted 2.2% of all RF cells recorded. Responses to stroboscopic visual stimuli causing eyelid closure were seen at a 30- to 40 ms latency.
3. Cells related to ipsilateral lip movement constituted 2.2% of all RF cells recorded.
4. Cells related to ipsilateral pinna movement constituted 7.1% of all RF cells recorded.
5. Ipsilateral eyelid, lip, and pinna movement cells were completely silent during movements restricted to the contralateral facial musculature. They discharged only during movement of the affected ipsilateral region in a single direction. They were significantly more likely than adjacent nonfacial cells to respond during the reflex head shake and significantly less likely to respond to auditory stimuli. They had little or no tonic activity during waking and sleep and no response to electric pulse stimulation of the skin.
6. Cells related to several facial movements constituted 2.9% of all RF cells recorded. Maximal discharge rates occurred during movement of the ipsilateral pinna, but smaller rate increases were also seen during movement of the ipsilateral lip and eyelid and during contralateral facial movements. In contrast to other facial cells, these cells also: a) responded during movements of ipsilateral facial regions in any direction, b) responded to somatosensory stimulation and passive movements of facial skin, c) responded to auditory and electric pulse stimulation of the skin, d) were inactive during the reflex head shake, and e) had high levels of tonic activity in both waking and sleep.
7. Each facial movement-related cell type had a different anatomical distribution within the brain stem. Cells related to eyelid, lip, and multiple facial movements formed localized clusters ventral to the abducens nucleus. Cells related to pinna movement were scattered throughout the gigantocellular tegmental field. Facial movement-related reticular cells may have premotor and proprioceptive roles in the regulation of facial movements.