Rhombomeric organization of vestibular pathways in larval frogs

Rhombencephalic subnuclei and projection pathways related to vestibular fun ction were mapped in larval ranid frogs. The retention of overt postembryon ic rhombomeres (r) allowed direct visualization of the locations of neurons retrogradely labeled with fluorescent dextran amines from the midbrain ocu lomotor complex, cerebellum, vestibular nuclei, and spinal cord. Oculomotor projecting vestibular neurons were mainly located in bilateral r1/2, ipsil ateral r3, and contralateral r5-8, and spinal projecting vestibular neurons mainly in ipsilateral r4 and contralateral r5. Vestibular commissural neur ons were located in r1-3 and r5-7 and were largely excluded from r4. Cerebe llar projecting neurons included contralateral inferior olivary neurons in r8 and vestibular neurons in bilateral r6/7 and contralateral r1/2. Mapping these results onto adult anuran vestibular organization indicates that the superior vestibular nucleus derives from larval r1/2, the lateral vestibul ar nucleus from r3/4, and the major portions of the medial and descending v estibular nuclei from r5-8. The lateral vestibulospinal tract projects from an origin in r4, whereas a possible ascending tract of Deiters arises in r 3. Rhombomere 5 contains a nuclear group that appears homologous to the tan gential nucleus of fish, reptiles, and birds and thus likely serves gravist atic and linear vestibulomotor reflexes. Comparisons between frogs and othe r vertebrates suggest that vestibular neurons performing similar computatio nal roles during head movements originate from the same segmental locations in different species. (C) 2001 Wiley-Liss, Inc.