The central projections of lateral line and cutaneous sensory fibres (VII and X) in Xenopus laevis

Abstract

The central terminal fields of anterior (VII) and posterior (X) lateral line nerves, as well as the somatosensory fibres of VII and X, have been studied in Xenopus laevis by CoCl2 backfilling of appropriate nerves followed by Timms intensification of whole brains. Lateral line fibres terminate in a dorsomedial region of the medulla that extends from the auricular lobe of the cerebellum to the obex. There is great overlap in the terminal fields of lateral line fibres from nerves innervating different groups of stitches. The technique also reveals second order cells within the lateral line nucleus, just ventral and medial to the zone of fibre termination. Lateral line efferent cell bodies lie within the medullary motor areas and their ventrally descending dendrites ramify profusely within the reticular formation. Somatosensory fibres of VII and X project into a totally separate medullary-spinal pathway, the tractus descendens trigemini. Collaterals of these fibres innervate an area of neuropil considered to be the most posterior portion of the trigeminal sensory nucleus. At the region of the calamus criptus further collaterals ramify within distinct transversely arranged plexi and may decussate to innervate similar plexi on the contralateral side. The functional significance of these anatomical findings is discussed.