DISTRIBUTION OF HYPOGLOSSAL MOTOR-NEURONS INNERVATING THE PREHENSILE TONGUE OF THE AFRICAN PIG-NOSED FROG, HEMISUS-MARMORATUM

Using retrograde neuronal tracers, a study of the distribution of hypo glossal motor neurons innervating the tongue musculature was performed in the African pig-nosed frog, Hemisus marmoratum. This species is a radically divergent anuran amphibian with a prehensile tongue that can be aimed in three dimensions relative to the head. The results illust rate a unique rostrocaudal distribution of the ventrolateral hypogloss al nucleus and an unusually large number of motor neurons within this cell group. During the evolution of the long, prehensile tongue of Hem isus, the motor neurons innervating the tongue have greatly increased in number and have become more caudally distributed in the brainstem a nd spinal cord compared to other anurans. These observations have impl ications for understanding neuronal reconfiguring of motoneurons for n ovel morphologies requiring new muscle activation patterns. (C) 1998 E lsevier Science Ireland Ltd.