The functional anatomy and evolution of hypoglossal afferents in the leopard frog, Ranapipiens

Abstract

Previously, we suggested that afferents are present in the hypoglossal nerve of the leopard frog, Rana pipiens. The basis for this was behavioral data obtained after transection of the hypoglossal nerve. These afferents coordinate the timing of tongue protraction with mouth opening during feeding. The goal of the present study was to define anatomically these hypoglossal afferents in Rana pipiens. Retrograde tracing was performed using horseradish peroxidase, fluorescent dextran amines and neurobiotin. Data show that the cell bodies of hypoglossal afferents are located in the dorsal root ganglion of the third spinal nerve and enter the brainstem through its dorsal root. The afferents ascend in the dorsomedial funiculus and move laterally after they pass the obex. They project in the granular layer of the cerebellum and the medial reticular formation. The cervical afferents that travel in this pathway are known to carry proprioceptive and cutaneous sensory information. We hypothesize that the presence of afferents in the hypoglossal nerve is a derived characteristic of anurans, which has resulted from the re-routing of afferent fibers from the third spinal nerve into the hypoglossal nerve. The appearance of hypoglossal afferents coincides with the morphological acquisition of a highly protrusible tongue.