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Randy H. Kardon, Susan C. Anderson, Jan Full, Pieter Poolman; The Light Induced Electromyogram (EMG): A Reflex Pathway Integrating the Melanopsin Retinal Ganglion Cell, Trigeminal Sensory Nucleus and Facial Nerve. Invest. Ophthalmol. Vis. Sci. 2011;52(14):269.
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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 may comprise the afferent arm of this reflex through projections to thalamus and the trigeminal nucleus (Noseda et al 2010) and may explain the paradoxical photosensitivity in patients blinded by photoreceptor loss. To characterize this reflex, the EMG, pupil movement and skin conductance were recorded to increasing red and blue light stimuli.
Eight subjects were tested under both scotopic and photopic conditions using red (640nm) and blue (485nm) Ganzfeld, full field light, one second in duration, over a 6 log unit range of intensity (0.5 log unit steps). Time-stamped, computerized recording of the pupil, orbicularis and procerus muscle EMG, and skin conductance were measured simultaneously.
The EMG showed a linear increase with log unit light intensity in parallel with pupil responses, but with a threshold approximately 3 log units less sensitive. At the brightest light intensities, an involuntary reflex blink was also triggered. Similar to pupil responses, the EMG showed a sustained, prolonged response to bright blue light stimuli, consistent with intrinsic activation of melanopsin containing retinal ganglion cells. Subjects with the largest light-induced EMG responses also had the largest changes in skin conductance
This study provides the first physiological evidence in humans that the light induced EMG response is mediated by melanopsin containing retinal ganglion cells, providing input to the trigeminal sensory nucleus, which then stimulates efferent output from the facial nucleus (as evidenced by EMG responses of orbicularis and procerus muscles) and sympathetic nervous system (as evidenced by increases in skin conductance). This unique light reflex may be useful in monitoring afferent light input affected by retinal and optic nerve disorders and also in diagnosing photosensitivity due to either ocular or central causes.
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