Gustatory pathways in the bullhead catfish. II. Facial lobe connections.

The second order projections of the gustatory system in catfish were examined using both retrograde (HRP) and anterograde (degeneration and autoradiographic) hodological methods. Golgi-stained material was used to demonstrate the different cell types in the primary gustatory sensory area, the facial lobe. Efferents from the facial lobe gather into ascending and descending secondary gustatory tracts. The descending tract terminates largely in the medial funicular nucleus and the commissural nucleus of Cajal in the region of the obex. A small portion of the descending tract continues caudally to terminate in the dorsal horn of the spinal cord. The ascending secondary gustatory tract terminates mostly in the ipsilateral superior secondary gustatory nucleus in the isthmic region. A small portion of the ascending tract continues rostrally to terminate in the posterior thalamic nucleus and in the region of the nucleus lobo-bulbaris. Sparser contralateral projections are also seen in the posterior thalamus and isthmic gustatory regions. Three cell types can be discerned in the facial lobe: small, medium and large. The small cells are intrinsic neurons, the medium cells project to the isthmic gustatory nucleus, and the large cells send fibers to the other terminal areas described above, as summarized in figure 19.