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The Journal of Neurophysiology Vol. 87 No. 5 May 2002, pp. 2421-2433
Copyright ©2002 by the American Physiological Society
The Smith-Kettlewell Eye Research Institute, San Francisco, California 94115-1813
Miller, Joel M.,
Christopher J. Bockisch, and
Dmitri S. Pavlovski.
Missing Lateral Rectus Force and Absence of Medial Rectus
Co-Contraction in Ocular Convergence. J. Neurophysiol. 87: 2421-2433, 2002. For a given position of
the eye in the orbit, most abducens motoneurons (LRMNs) fire at higher
rates in converged gaze than when convergence is relaxed, implying that
lateral rectus (LR) muscle force will be higher for a given eye
position in convergence. If medial rectus (MR) muscle force balances LR
force, it too would be higher in convergence, that is, LRMN recording
studies predict horizontal rectus co-contraction in convergence. Three
trained rhesus monkeys with binocular eye coils and custom muscle force transducers (MFTs) on LR and MR of one eye alternately fixated near
(approximately 7 cm) and far (200 cm) targets with vergence movements
of 20-30°. Tonic muscle forces were also measured during conjugate
fixation of far targets over a 30 × 30° field. MFT
characteristics and effects on oculomotility were assessed. Contrary to
predictions, we found small (<1 g) decreases in both LR and
MR forces in convergence, for those gaze positions that were used in
the brain stem recording studies. This missing LR force
paradox (higher LRMN firing rates in convergence but lower LR forces)
suggests that motoneurons or muscle fibers contribute differently to
oculorotary forces in converged and unconverged states, violating the
final common path hypothesis. The absence of MR
co-contraction is consistent with, and supports, the missing LR force
finding. Resolution of the missing LR force paradox might involve
nonlinear interactions among muscle fibers, mechanical specialization
of muscle fibers and other articulations of the peripheral oculomotor
apparatus, or extranuclear contributions to muscle innervation.
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