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J. R. Dearworth, Jr., J. F. Blaum, D. T. Bednarz, M. S. Jones, M. Ariel; Abduction of the Eye in the Turtle Evoked by Stimulation of the Trochlear Nerve. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4125.
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© ARVO (1962-2015); The Authors (2016-present)
Since trochlear motoneurons respond robustly to horizontal rotation in turtle (Jones and Ariel, 2006), we hypothesized that stimulation of CN IV produces not only torsion but also abduction. We compared eye movement responses during stimulation of CN IV with that of the oculomotor nerve (CN III) and the abducens nerve (CN VI).
Using an isolated head preparation, glass suction electrodes were used to stimulate CN III, IV, and VI. Two red-eared slider turtles, Trachemys scripta elegans, were euthanized to remove the brains leaving long nerves on the cranial floor. During trains of constant current pulses, eye movements were measured by an automated eye tracking system (ViewPoint EyeTracker®, Arrington Research, Inc.). To calibrate movements, isolated heads were fixed horizontally to a gimble. Mean responses evoked using various sets of current parameters were compared.
In absence of stimulations, the iris lines of turtle eyes were not aligned parallel with the horizontal axis of the fixed heads, but instead were offset with rotation toward the nostrils. In the first preparation, the left eye started at a rotation ~33° relative to the horizon. Stimulations (n=5) using 100ms 30Hz trains of pulses (100µA; 2ms) of the left CN VI translated the eye on a trajectory parallel with the iris line, corresponding with an upward (1.5° ± 0.2, SEM) and horizontal movement (2.1° ± 0.5), away from the midline of the fixed head. In the same preparation, stimulations (n=5) with 500ms 100Hz trains of pulses (70µA; 2ms) of the left CN IV produced similar direction of translation, upward (4.1° ± 0.2) and horizontal (5.9° ± 0.3), but occurred together with rotation (12.3° ± 1.4) toward the nostril. In the second preparation, the right eye was examined and its iris line was rotated ~38° relative to the horizon. Stimulations (n=5) of the right CN III using 600ms 175Hz trains of pulses (350µA; 5ms) produced extraocular movements opposite to those evoked by stimulating CN IV and VI. Translation was upward (1.8° ± 0.1), but horizontal movement (13.7° ± 3.2) was toward the midline of the fixed head. Rotation (34.5° ± 0.6) also occurred instead away from the nostril.
Our results in turtle suggest that stimulation of the CN IV, an animal with more laterally oriented orbits, generates both torsion and abduction. The findings may explain why robust responses of trochlear motoneurons in turtle occur in response to horizontal rotation, an unusual characteristic of vestibulo-ocular physiology in comparison to other species.
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