ABSTRACT
The familiar notion that extraocular muscle paths are little constrained except at origins and insertions was called into question by biomechanical modeling (Miller and Demer, 1992). Magnetic resonance imaging, having previously shown that the paths of rectus muscle bellies remain fixed in the orbit during large ocular rotations (Miller, 1989), was then used to show that they remained fixed across large surgical transpositions of their insertions (Miller et al., 1993), confirming that pulleys near the globe equator couple muscle paths to the orbital wall. Autopsy and immunohistochemical studies showed that these soft rectus muscle pulleys were sleeves composed of dense bands of collagen, elastin and smooth muscle (Demer et al., 1995a), suspended from the orbit and adjacent extraocular muscle sleeves by bands of similar composition. Evidence has been found of sympathetic, parasympathetic, and nitroxidergic neurotransmission, and innervation has been traced to the superior cervical ganglion in monkey (Demer et al., 1995b).
Muscle actions under a pulley model (Miller et al., 1995) are different from those under traditional models (Miller and Robinson, 1984; Robinson, 1975), therefore different control signals are required. The pulleys and associated tissues act, we believe, as a mid-orbital suspension, passively constraining the globe’s rotational freedom to approximate Listing’s Law. Tonic innervation of pulley smooth muscle stiffens musculo-orbital coupling, and perhaps pulley innervation is modulated to refine binocular coordination or move Listing’s Plane.
