Excerpt from Extraocular Muscles, Actions

Please click here to view the full topic text: Extraocular Muscles, Actions

• Movement of the eye nasally is adduction; temporal movement is abduction.

• Elevation and depression of the eye are termed sursumduction (supraduction) and deorsumduction (infraduction), respectively.

• Incycloduction (intorsion) is nasal rotation of the 12-o'clock position on the vertical meridian; excycloduction (extorsion) is temporal rotation of the 12-o'clock position on the vertical meridian.

• The primary muscle that moves an eye in a given direction is known as the agonist.

• A muscle in the same eye that moves the eye in the same direction as the agonist is known as a synergist, while a muscle in the same eye that moves the eye in the opposite direction of the agonist is an antagonist.
  • For example, in abduction of the right eye, the right lateral rectus muscle is the agonist, the right superior and inferior oblique muscles are the synergists, and the right medial, superior, and inferior recti are the antagonists.

  • By Sherrington law, increased innervation to any muscle (agonist) is accompanied by a corresponding decrease in innervation to its antagonists.

Binocular eye movements are either conjugate (versions) or disconjugate (vergences).

• Versions are movements of both eyes in the same direction (eg, right gaze in which both eyes move to the right).
  • Dextroversion is movement of both eyes to the right, and levoversion is movement of both eyes to the left.

  • Sursumversion (supraversion) and deorsumversion (infraversion) are elevation and depression of both eyes, respectively.

  • Yoke muscles are the primary muscles in each eye that accomplish a given version; for right gaze, the right lateral rectus and left medial rectus muscles are the yoke muscles.
    • Each extraocular muscle has a yoke muscle in the opposite eye to accomplish versions into each gaze position.

    • By Herring law, yoke muscles receive equal and simultaneous innervation, the magnitude of which is determined by the fixing eye.

    • Since the magnitude of innervation to yoke muscles is determined by the fixing eye, the angle of deviation between eyes (strabismus) may vary depending on which eye is fixing. The primary deviation is misalignment with the normal eye fixing. When the cause of deviation is acquired muscle paresis, as the paretic eye fixates, the ensuing secondary deviation is typically larger than the primary deviation. When the cause of deviation is congenital strabismus, the primary and secondary deviations are typically about the same.

• As opposed to versions (in which both eyes move in the same direction), vergences are movements of the eyes in opposite directions.
  • Convergence is movement of both eyes nasally, and divergence is movement of both eyes temporally.

  • Vertical vergence movements also may occur—one eye moving upward or the other downward relative to the contralateral eye.

  • Accommodative convergence is convergence of the eyes stimulated by accommodation or focusing on a near target.

  • Abnormality of the ratio of accommodative convergence to accommodation may cause certain types of strabismus.

  • Fusional convergence and divergence are optomotor reflexes that are designed to position the eyes such that the image of regard falls on the fovea of each eye. This motor fusion is important to avoid diplopia (double vision).

  • The field of action of an extraocular muscle is the direction of rotation of the eye when that muscle contracts. This term also indicates the gaze position in which the effects of a muscle are demonstrated most easily.

  • Knowledge of fields of action is important; strabismus often increases in the field of action of a weak eye muscle.
  Please click here to view the full topic text: Extraocular Muscles, Actions