Visual system of the channel catfish (Ictalurus punctatus): III. Fiber order in the optic nerve and optic tract.
In channel catfish the ganglion cell axons leave the retina via a ring of approximately 13 separate optic papillae. Each papilla serves an area of retina extending from the central zone of the retina to the periphery. Papillae located at a dorsal position in the ring serve exclusively dorsal retina. Ventrally located papillae, however, have an exaggerated peripheral retinal representation, so that they serve mostly ventral retina but also some areas of peripheral retina dorsal to the nasal and temporal poles. The ganglion cell axon bundles departing from the retina via individual papillae were labelled with horseradish peroxidase, and sections of the optic pathway were examined to reveal the topographic organization of the fibers. The topographic order of the optic nerve was dissimilar to that of cichlids and goldfish. Fibers from individual papillae remained together throughout the optic nerve. Close to the optic nerve head, the papillae were arranged as a continuum around the U-shaped optic nerve, without the discontinuity in the representation of the ventral retina seen in other fish. Fibers associated with the dorsal papillae were located at the tip of the caudolateral arm of the U, and fibers from ventral papillae were on the rostromedial arm. Fibers from nasally and temporally located papillae were found on the base of the U. By the level of the optic chiasm the U shape had flattened out but retained the relative ordering of the papillae. Rotation of the nerve as it became the optic tract brought the representation of the ventral papillae to the dorsal pole of the tract, and the dorsal papillae to the ventral tract. It was only in the optic tract that rearrangement of fibers became apparent. As described above, the axons of some ganglion cells in dorsal, peripheral retina left the retina and travelled through the optic nerve with axons from extreme ventral retina. In the optic tract, these dorsal fibers joined the main body of fibers from the dorsal retina. The significance of these observations for theories of fiber rearrangement is discussed.
Dunn-Meynell
A A
AA
Department of Ophthalmology, New York Medical College, Valhalla 10595.
Sharma
S C
SC
D000818:Animals; D002397:Catfishes / Q000033:anatomy & histology*; D007059:Ictaluridae / Q000033:anatomy & histology*; D009412:Nerve Fibers / Q000166:cytology*; D009897:Optic Chiasm / Q000033:anatomy & histology; D009900:Optic Nerve / Q000033:anatomy & histology*; D012160:Retina / Q000166:cytology*; D012165:Retinal Ganglion Cells / Q000166:cytology*; D014795:Visual Pathways / Q000033:anatomy & histology*