Musculotopic organization of the hypoglossal nucleus in the grass frog, Rana pipiens.

Recent neural tracer studies in several mammalian species have demonstrated a similar musculotopic organization of the hypoglossal motoneurons which innervate individual tongue muscles. The distribution of this musculotopic organization in nonmammalian tetrapods, however, has not received detailed investigation. As part of an ongoing study on the comparative organization of the vertebrate hypoglossal nucleus, the musculotopic organization of the hypoglossal nucleus of Rana pipiens was studied by injection of lectin-conjugated horseradish peroxidase into four distinct tongue muscles and the geniohyoid muscle. Injections into the hyoglossus muscle label neurons in dorsal regions of the hypoglossal nucleus in middle and rostral nucleus levels. Injections into the genioglossus basalis muscle label neurons in ventral and lateral regions of the hypoglossal nucleus in caudal nucleus levels. Injections into the genioglossus medialis muscle label neurons in dorsal regions in caudal levels, throughout the nucleus in middle levels, and in ventral regions in more rostral levels. Injections into the geniohyoid muscle label neurons in the ventral tip of the hypoglossal nucleus and in the ventromedial corner of the medullary gray matter in middle and rostral nucleus levels. These results demonstrate that the organization of the hypoglossal nucleus in Rana pipiens is more complex than previous tracer studies indicated. Similarities in the musculotopic organization of the amphibian and mammalian hypoglossal nuclei suggest an evolutionary conservatism of the motor system controlling tongue movement.