Craniofacial Muscles

4 Motor Control of Extraocular Muscle
69
observed for a conjugate eye movement to the same orbital position. This meant that
the lateral rectus muscle was innervated more strongly in convergence than for conjugate
gaze, although the eye is at the same orbital position. In order to balance the
increased lateral rectus innervation during vergence, the medial rectus must also be
innervated more strongly (i.e., increased fi ring in oculomotor neurons) during convergence
to a particular orbital position than for conjugate movement to the same
orbital position. This was found to be mostly true (Miller et al. 2011 ) . These observations
lead to the prediction of co-contraction of the medial and lateral muscles
during vergence due to the increased forces in convergence compared to conjugate
eye movements. However, in two studies that used implanted strain gauges directly
onto the rectus muscles, no increased force in the medial and lateral rectus was
observed during convergence (Miller et al. 2002, 2011 ) . 2
So as of now, the “missing force” remains an unsolved paradox. There is therefore
a disconnect between the predictions from the recordings of motoneurons in
the abducens and oculomotor nuclei and the force measurements at the muscle.
These fi ndings have led some to propose abandoning the “ fi nal common path”
hypothesis since the path for vergence and conjugate eye movements do not appear
to be the same (Miller 2003 ; Miller et al. 2011 ) . It may be that the answer lies in
the complexity of the muscle structure. Other than the points mentioned already
about the different fi ber types and the different motoneuronal subtypes, it is also
known that there are complex serial and parallel connections between muscle
fi bers, and at the molecular level, there is a lot of variation in expression of myosins
along the muscle fi bers (McLoon and Wirtschafter 2003 ; McLoon et al. 2004,
2011 ) . There is also evidence for substantial muscle remodeling over time. Any of
these factors could affect the relationship between motoneuron fi ring and the force
generated by the motor unit. As suggested by Miller et al. ( 2011 ) , one resolution to
the paradox would be if neurons that have large differences in the conjugate and
vergence sensitivities do not contribute much towards an eye movement due to
innervation of an inherently weak muscle fi ber. Additional investigation into both
muscle structure and innervation of individual muscle fi bers is necessary to resolve
this rather thorny issue.
2
Studies from labs that examined motoneuron responses during combined saccade-vergence
movements have shown that many abducens motoneurons and medial rectus motoneurons surprisingly
encode a binocular signal (i.e., encode movements of either eye) although they might be
expected to only encode movements of the eye that they project to. For the purposes of this chapter,
it should be noted that the fi nding of unequal sensitivities for vergence and conjugate eye movements
is exactly equivalent to the fi nding of binocular encoding in motoneuronal activity by these
other studies. The reason that the two fi ndings are equivalent is that a simple linear mathematical
transformation can transform a conjugate/vergence representation of motoneuronal responses into
a right eye/left eye representation (King and Zhou 2002 ; Sylvestre and Cullen 2002 ) .
Conjugate = (right eye + left eye)/2
Vergence = Left eye − right eye
Right eye = Conjugate − vergence/2
Left eye = Conjugate + vergence/2