Craniofacial Muscles

4 Motor Control of Extraocular Muscle
67
Fig. 4.9 Geometric
arrangement of superior
rectus and superior oblique
muscles. This arrangement
results in the signi fi cant
primary vertical or torsional
and secondary torsional or
vertical actions of the
cyclovertical muscles
with horizontal position of the eye (Robinson 1982 ) . This is largely a product of the
geometrical arrangement of the eye in the orbit and EOM insertion points (Fig. 4.9 ).
Thus, the vertical recti are approximately 23 deg temporal in each eye and the
obliques are approximately 51 deg nasal in each eye. Therefore, if the eye is turned
out toward the temple (abduction), the obliques have more torsional action, and the
vertical recti have more vertical action. If the eye is turned in towards the nose
(adduction), the obliques have more vertical action, and the vertical recti have more
torsional action.
Just like the horizontal system, the cyclovertical system is organized into agonist–antagonist
pairs. Thus, the superior rectus and inferior rectus of each eye form
an agonist–antagonist pair and the superior oblique and the inferior oblique of the
same eye form another agonist–antagonist pair. In addition, the four cyclovertical
muscles in each eye are also arranged in yoked muscle pairs that help to ensure that
binocular alignment and binocular coordination is maintained in the vertical and
torsional planes. For example, the superior rectus of one eye and the inferior oblique
of the other eye form a yoked muscle pair. Simultaneous excitation of both these
muscles will result in a coordinated elevation and same-direction torsion of both
eyes. Similarly, the inferior rectus of one eye and the superior oblique of the other
eye form a yoked muscle pair because simultaneous excitation of both sets of muscles
results in a coordinated depression and same-direction torsion of both eyes.
Cyclovertical alignment must also be maintained during head tilt. In this condition,
the yoked muscle pairs are the superior rectus and superior oblique of the lower eye
(i.e., ear nearest shoulder upon head tilt) and the inferior rectus and inferior oblique
of the higher eye (i.e., ear farthest from shoulder upon head tilt). The pairing of
these muscles results in an incyclotorsion of the lower eye and excyclotorsion of the
upper eye with minimal vertical change in either eye. The pattern of anatomical
connection from the oculomotor and trochlear nuclei to cyclovertical muscles guarantees
that pre-motor excitation of the oculomotor and trochlear nuclei on the same
side of the brain results in a torsional movement of both eyes with little vertical
component. Note that the simultaneous excitation of the yoked muscles must be