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Pieter Poolman, Lauren S. Talman, Susan C. Anderson, Andrew F. Russo, Ana Recober, Jan M. Full, Randy H. Kardon; Electromyographic Evidence of a Fast Response Functional Pathway between Retinal Ganglion Cells and Trigeminal Sensory Nucleus. Invest. Ophthalmol. Vis. Sci. 2012;53(14):4840.
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The Photo-Blink Reflex protects the eye after a bright flash of light (Mukuno et al 1983, Malin 1982, Yasahura et al 1982, Ozaki 1976). Melanopsin containing retinal ganglion cells (RGCs) may comprise the afferent arm of this reflex through projections to thalamus and the trigeminal nucleus (Noseda et al 2010). Okamoto et al (2010) have recently proposed that a slow trigeminal response could be mediated by changes in vessel size in the choroid in response to bright light and might help to explain how bright light causes pain. This study compared smooth muscle (iris-pupil light reflex) and skeletal muscle (electromyogram-EMG) responses to light stimuli varying in temporal frequency to determine if the photic-EMG can occur at a rate that is greater than what could be accounted for by vascular smooth muscle changes.
Five subjects were tested under photopic conditions using red (640nm) and blue (485nm) Ganzfeld, full field light at 1, 10 and 275cd/m2. Each stimulus consisted of a sequence of fifteen light bursts, 50ms in duration, separated by dark intervals to obtain 1, 3, 5 and 10Hz trains. The time between each train of light was 60s. Conditions of color, intensity, and temporal frequency were presented in random order. Time-stamped, computerized recording of the pupil, and orbicularis and procerus EMG were collected simultaneously.
The smooth muscles of the pupil could track the 1Hz stimulus, with significant suppression at 3Hz, and with extinction of the effect at 5 and 10Hz. However, EMG amplitudes at 5 and 10Hz were on average still between 30-40% of their respective amplitudes for the 3Hz stimulus, even though the muscles were less likely to completely relax during the higher frequency sequences.
This study provides the first physiological evidence in humans for a fast 10Hz light-induced EMG response. The photic-EMG appears to be mediated by RGCs which provide input to the trigeminal sensory nucleus, which then stimulates the facial nucleus, giving rise to the orbicularis and procerus EMG responses. Trigeminal input from vascular dilation/contraction cycles of smooth muscle in the choroidal vessels cannot explain the fast 5 and 10Hz response exhibited by facial muscles, and supports the notion of a separate, faster functional pathway between RGCs and the trigeminal sensory nucleus that could not be explained by changes in vascular tone leading to trigeminal activation.
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