Craniofacial Muscles

4 Motor Control of Extraocular Muscle
55
implementation of Listing’s law (Demer 2004 ) . Subsequent neurophysiological and
electrical stimulation studies in the abducens nerve by Angelaki and colleagues
showed that there was indeed no neural implementation of Listing’s law in the brain
and therefore Listing’s Law must be implemented mechanically (Ghasia and
Angelaki 2005 ; Klier et al. 2006, 2011 ) . Although it appears that the pulleys can
obviate the necessity for central control of torsion required by Listing’s law, the
brain needs to provide control signals for torsion that deviates from Listing’s law
such as during convergence, the vestibulo-ocular re fl ex, and head-free gaze shifts
(Crawford et al. 1999 ; Demer et al. 2003 ) .
The initial studies by Miller and colleagues suggested that the pulley structures
stabilized muscle paths in the posterior orbit (Miller et al. 1993 ) . The functional
signi fi cance of preventing muscle sideslip is that the EOM force vector remains
constrained. In the event of rectus muscle sideslip, perhaps due to pulley malposition,
the rectus EOM force vector could be misdirected into the orthogonal plane
resulting in problems of binocular coordination such as A or V patterns of strabismus
(Oh et al. 2002 ) . Demer suggests that many of the cases of strabismus could in
fact be of biomechanical origin due to pulley problems (Demer 2001, 2004 ) .
However, other studies in monkeys with a developmental strabismus induced by
sensory methods have clearly demonstrated a neural origin for strabismus and a
pivotal neural role in maintaining the state of strabismus including the A and V patterns
(Das and Mustari 2007 ; Das 2011 ; Joshi and Das 2011 ) . An example is provided
in Fig. 4.3 . Corroborating these reports have been studies that examined
muscle anatomy of monkeys with sensory strabismus that determined that the pulley
structure is in fact normal (Narasimhan et al. 2007 ) . Thus, it appears that the
etiology could be important in understanding the role that EOM pulleys and
motoneuron control of EOM might play in determining eye alignment or eye movement
properties in disease states.
4.2.2 Modern Approaches to Modeling of the Plant
The nonlinear properties of the oculomotor plant tissue make it dif fi cult to formulate
models using conventional linear control systems theory and lumped elements (see
Fig. 4.1 ). Lumped element models tend to oversimplify the complexity of the EOM
and other plant tissue. A few laboratories have attempted to create plant models
using modern approaches. The fi rst such attempt was that by Miller (Orbit 1.8 software,
Eidactics Inc.). The Orbit software is a sophisticated biomechanical model of
the eye plant that allows the user to modify many parameters including strength of
innervation of each muscle, muscle stiffness and contractility, pulley stiffness, pulley
locations, etc. The primary use proposed for this software is to simulate expected
outcomes from strabismus surgeries (Demer et al. 1996 ; Clark et al. 1998a, b ) .
Although more sophisticated than control systems models, one disadvantage of this
model is that it only simulates static eye positions, not dynamic eye movements. An
alternative to Orbit 1.8 is the SEE++ software developed by Haslwanter, Buchberger,