synergist
A synergist is a muscle that moves an eye in the same direction as another muscle of that eye. The superior rectus and inferior oblique are ipsilateral synergists for elevation, although they are not synergists for horizontal or torsional movement.1
Two muscles can be synergists for one action and antagonists for another. The superior oblique and inferior rectus both depress the eye, so they are synergists for depression. The superior oblique intorts the eye and the inferior rectus extorts it, so the same pair are antagonists for cyclorotation.2
Synergistic muscles in the two eyes move both eyes in the same direction and are called yoke muscles.2 Hering's law relates the innervation of an agonist in one eye to its yoked muscle in the other eye, which is also called the contralateral synergist.1
More detail
After a muscle palsy, the contralateral synergist of the palsied muscle overacts because of Hering's law of equal innervation. The overaction is very marked when the palsy is of recent onset.1
Sources (8)
- Pickwell's Binocular Vision Anomalies, ch. 17
- von Noorden and Campos, Binocular Vision and Ocular Motility, ch. 4
- American Academy of Ophthalmology. 2020. Neuro-Ophthalmology/Orbit: Basic and Clinical Science Course. San Francisco: American Academy of Ophthalmology.
- Ansons, A.M. and Davis, H. eds. 2014. Diagnosis and management of ocular motility disorders. 4th ed. Oxford: Wiley-Blackwell.
- Noorden, G.K. and Campos, E.C. 2002. Binocular vision and ocular motility: theory and management of strabismus. 6th ed. St. Louis: Mosby.
- Rutstein, R. and Daum, K. 1997. Anomalies of Binocular Vision: Diagnosis & Management. St. Louis: Mosby.
- Rowe, F.J. 2012. Clinical orthoptics. 3rd ed. Oxford: Wiley-Blackwell.
- Wong, A.M. 2004. Eye movement disorders. New York: Oxford University Press.
Reviewed October 2026