MORPHOLOGY OF PHYSIOLOGICALLY IDENTIFIED RETINAL X-AXON AND Y-AXON INTHE CATS THALAMUS AND MIDBRAIN AS REVEALED BY INTRAAXONAL INJECTION OF BIOCYTIN

Prior morphological studies of individual retinal X and Y axon arbors based on intraaxonal labeling with horseradish peroxidase have been li mited by restricted diffusion or transport of the label. We used biocy tin instead as the intraaxonal label, and this completely delineated e ach of our six X and 14 Y axons, including both thalamic and midbrain arbors. Arbors in the lateral geniculate nucleus appeared generally as has been well documented previously. Interestingly, all of the labele d axons projected a branch beyond thalamus to the midbrain. Each X axo n formed a terminal arbor in the pretectum, but none continued to the superior colliculus. In contrast, 11 of 14 Y axons innervated both the pretectum and the superior colliculus, one innervated only the pretec tum, and two innervated only the superior colliculus. Two of the Y axo ns were quite unusual in that their receptive fields were located well into the hemifield ipsilateral with respect to the hemisphere into wh ich they were injected. These axons exhibited remarkable arbors in the lateral geniculate nucleus, diffusely innervating the C-laminae and m edial interlaminar nucleus, but, unlike all other X and Y arbors, they did not innervate the A-laminae at all. In addition to these qualitat ive observations, we analyzed a number of quantitative features of the se axons in terms of numbers and distributions of terminal boutons. We found that Y arbors contained more boutons than did X arbors in both thalamus and midbrain. Also, for axons with receptive fields in the co ntralateral hemifield (all X and all but two Y axons), 90-95% of their boutons terminated in the lateral geniculate nucleus; the other two Y axons had more of their arbors located in midbrain. (C) 1995 Wiley-Li ss, Inc.