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R.S. Jampel, D.X. Shi; Evidence Against the So–Called "Active Pulley Hypothesis" Is Proof That the CNS Controls Eye Movements . Invest. Ophthalmol. Vis. Sci. 2005;46(13):4677.
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The "active pulley hypothesis" (APH) divides the recti muscles and the inferior obliques (IO) into orbital layers (OL) and global layers (GL). The OL inserts on a pulley that encircles the GL. Contraction of the OL causes the pulley to shift the location of GL's muscle origin. The shift halves the angle of insertion of the GL enabling the GL to maintain a constant axis for each eye position.We maintain that no such pulleys exist and that the CNS controls eye movements.
In humans: CT scans; VOG tracking ; EOM surgery. In Macaca mulatta: brain stimulation combined with EOM, cranial, and orbital surgery; observation of the contractions of the SR, SO and LR in vivo produced by electrodes implanted in the oculomotor and trochlea nerves.
CT scans: The course of a contracted MR or LR is a straight line (Fig.). There are no connections or inflections between the orbital walls and the muscle sheathes. There are no pulley bars, "distributed pulleys", or demarcation lines splitting the muscle into distinct layers. Clinical: Comitant strabismus operations do not change the range of ocular motion or the location EOM axes. No pulleys are demonstrated at surgery. There is no coupling of the LR and IO. Physiology: Elastic tissue cannot antagonize the fast fibers of the OL. Observations of SO, SR and LR (monkey) contracting in vivo revealed no pulleys, coupling, or slipping of tendons. Tendon pathways did not shift with gaze. The SO axis remained in the same location during lateral gaze. There are no pulleys on the recti or IOs. Sherington's Law is violated by the APH. The CNS (brain) controls eye movements.
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