Characterization of some morphological parameters of orbicularis oculi motor neurons in the monkey

The primate facial nucleus is a prominent brainstem structure that is composed of cell bodies giving rise to axons forming the facial nerve. It is musculotopically organized, but we know little about the morphological features of its motor neurons. Using the Lucifer Yellow intracellular filling method, we examined 11 morphological parameters of motor neurons innervating the monkey orbicularis oculi (OO) muscle, which plays an important role in eyelid closure and voluntary and emotional facial expressions. All somata were multipolar and remained confined to the intermediate subnucleus, as did the majority of its aspiny dendritic branches. We found a mean maximal cell diameter of 54 μm in the transverse dimension, cell diameter of 60 μm in the rostrocaudal dimension, somal surface area of 17,500 μm2 and somal volume of 55,643 μm3. Eight neurons were used in the analysis of dendritic parameters based upon complete filling of the distal segments of the dendritic tree. We found a mean number of 16 dendritic segments, an average dendritic length of 1036 μm, diameter of 7 μm, surface area of 12,757 μm2 and total volume of 16,923 μm3. Quantitative analysis of the dendritic branch segments demonstrated that the average number, diameter and volume gradually diminished from proximal to distal segments. A Sholl analysis revealed that the highest number of dendritic intersections occurred 60 μm distal to the somal center with a gradual reduction of intersections occurring distally. These observations advance our understanding of the morphological organization of the primate facial nucleus and provide structural features for comparative studies, interpreting afferent influence on OO function and for designing studies pinpointing structural alterations in OO motor neurons that may accompany disorders affecting facial movement.