Distribution of facial motor neurons in the pond turtle Pseudemys scripta elegans

A tract tracing study was performed to examine the localization of the facial nucleus in the brain stem of the pond turtle, Pseudemys scripta elegans. Neurobiotin and the fluorescent tracers alexa fluor 488 and 594 were used to retrogradely label neurons of the abducens or facial nerves. The results showed that the facial nucleus has two subnuclei, a medial group and a lateral group. Measurements of cell size revealed no significant differences between these populations. Double labeling studies showed that the medial cell group of the facial nucleus lies between the principal and accessory abducens nuclei in the pons, whereas the lateral group lies adjacent to the accessory abducens nucleus. The facial nucleus of pond turtles largely overlaps the rostrocaudal extent of the accessory abducens nucleus, but extends well beyond it into the medulla. These data elucidate the position and distribution of the facial nucleus in the brain stem of pond turtles and contribute to the body of comparative neuroanatomical literature on the distribution of the cranial nerve nuclei of reptiles.