MONROY, J.A.; ANDERSON, C.W.; NISHIKAWA, K.C.: Neural control of feeding in microhylid frogs

In this study, we investigated the neural pathways involved in the control of feeding behavior in microhylid frogs. Microhylid frogs use a muscular hydrostatic mechanism to protract their tongues and have the unusual ability to aim their tongues independently of their heads during protraction. The tongue protractor muscle (m. genioglossus) of microhylids has a unique morphology in which the muscle fibers are oriented in two directions. Results from retrograde labeling revealed a larger number of motor neurons in the ventrolateral nucleus compared to other anurans, probably due to the evolution of this novel tongue protractor morphology and aiming ability. Data also suggest that in one microhylid species sensory afferents, originating from the tongue and traveling in the hypoglossal nerve, enter the brainstem in the third spinal nerve and synapse onto neurons in the dorsal horn. It appears that the second order neurons then contact cells in the reticular formation and Purkinje cell layer of the cerebellar cortex. Sensory feedback from the hypoglossal nerve may coordinate head turning, mouth opening, and tongue protraction. Kinematic analysis suggests that the feeding behavior of microhylid frogs differs depending on the location of prey around the frog. Transection of the hypoglossal nerve disrupts sensory feedback and the coordination of mouth opening with tongue protraction. These data suggest a neuronal mechanism for modulating motor output during feeding in microhylid frogs.