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F. Schroedl, A. Brehmer, W.L. Neuhuber, D. Nickla; Neuronal Tracing in the Autonomic Facial Nerve in Chickens . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3322.
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In the avian eye, choroidal blood flow is under parasympathetic control of the ciliary and pterygopalatine ganglion, the latter one consisting of a chain of interconnected microganglia within the orbit. One or both of these ganglia might be involved in regulating choroidal thickness, a mechanism that comprises one of the compensatory responses in the visual regulation of refractive state. Because of the anatomical peculiarity of the pterygopalatine ganglion, functional experiments on the autonomic part of the facial nerve (aVII) have not been done. The aim of this study was the search for an extra–orbital access to the preganglionic facial nerve in preparation for studies on its possible role in the visual regulation of ocular growth.
aVII in chickens was accessed within the tympanic cavity and marked with Texas–red labelled dextran (TxR). Structures of the orbit and brainstem were removed followed by immunohistochemistry against nNOS, ChAT, VIP, CGRP, GAL, and SOM. Lesions of the preganglionic part were performed, followed by IOP (TonoPen)– and ocular dimensions measurements (high–frequency A–scan ultrasonography) before and at various intervals (for at least 8 days) after the lesion.
TxR+ anterograde–traced nerve fibres were found within the microganglionic chain within the orbit, forming TxR+ boutons on nNOS+ neurons. Retrograde TxR+ labelled neurons were found within the metencephalon (cell diameters ∼20 µm). These neurons were ChAT+, but negative for nNOS/VIP/SOM/GAL/CGRP. Most likely, these represent preganglionic neurons of the superior salivatory nucleus. Close to the TxR+ neurons another neuronal population (TxR–) was found, ChAT+/CGRP+ but GAL– (cell diameters ∼40µm). Most likely, they represent motor neurons of the facial nerve. In preliminary transection experiments of aVII (n= 7), no changes in IOP were detectable. Further, no significant effect on axial elongation (experimental vs control, change over 8 days: 600 vs 533 µm; ns) or choroidal thickness (change over 8 days: 23 vs 21 µm; ns) has been observed, however, there was a trend toward an increase in retinal thickness over this time (22 vs 4.8 µm; p=0.09).
Accessing aVII via the tympanic cavity enables excellent possibilities for functional experiments with intact orbital structures. While there were no significant effects on choroidal thickness or on eye growth in normal birds, this does not preclude potential effects on eyes responding to the imposition of blur. Future experiments will explore the influence of this pathway on the visual regulation of eye growth.
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