Eye Movement Muscles

Extraocular Muscles

• Four muscles control the movement of the eye in the four cardinal directions: up, down, left, and right.

Muscle Movements

• Medial rectus (MR) moves the eye inward, toward the nose (adduction).
• Lateral rectus (LR) moves the eye outward, away from the nose (abduction).
• Superior rectus (SR) primarily moves the eye upward (elevation), secondarily rotates the top of the eye toward the nose (intorsion), and tertiarily moves the eye inward (adduction).
• Inferior rectus (IR) primarily moves the eye downward (depression), secondarily rotates the top of the eye away from the nose (extorsion), and tertiarily moves the eye inward (adduction).
• Superior oblique (SO) primarily rotates the top of the eye toward the nose (intorsion), secondarily moves the eye downward (depression), and tertiarily moves the eye outward (abduction).
• Inferior oblique (IO) primarily rotates the top of the eye away from the nose (extorsion), secondarily moves the eye upward (elevation), and tertiarily moves the eye outward (abduction).

Muscle Relationships

• Primary muscle that moves an eye in a given direction is known as the “agonist.”
• Muscle in the same eye that moves the eye in the same direction as the agonist is known as a “synergist.”
• Muscle in the same eye that moves the eye in the opposite direction of the agonist is the “antagonist.”
• Sherrington’s Law states that increased innervation to any agonist muscle is accompanied by a corresponding decrease in innervation to its antagonist muscle(s).

Effects of Muscle Impairment

• If all extraocular muscles are affected, the affected eye will be turned outward and downward due to the unopposed action of the lateral rectus and superior oblique muscles.