Abstract
Abstract: :
Purpose: Multiple electrode (tungsten) penetrations into the brain stem during eye movement behavioral studies is a commonly employed technique. Eye movements generally remain robust and accurate during these long term chronic studies. However, neither the extent of damage to the central nervous system nor the possible effects on the extraocular muscles have been assessed. Methods: Two adult rhesus monkeys that had undergone two years of electrode penetrations into their abducens and vestibular nuclei were examined histologically. An analysis of their VIth nucleus neurons and glia as well as lateral rectus muscle fibers was carried out. Results: Twenty–two percent of the large neurons (≈ 30µm in diameter), on average, were missing and extensive neuropil disruption and gliosis was evident in the experimental side abducens nuclei as compared to the control side in each animal. The lateral rectus muscles showed small, but inconsistent, changes in total fiber number. However, the muscle fiber diameters were altered leading to a more homogenous muscle as the typical orbital and global subdivisions of the muscle became less distinct. In spite of these morphological findings, eye movement records from before and after the electrophysiological studies were comparable. Conclusions: The complex architecture of the extraocular muscles, including longitudinal and serial connections among muscle fibers, plus the possibility of polyneuronal innervation of single muscle fibers could explain why saccadic and pursuit eye movements as well as fixations remained largely unaffected in the face of this damage to the brain stem.
Keywords: anatomy • eye movements • extraocular muscles: structure