Extraocular muscles:

The extraocular muscles are a group of six muscles that regulate the eye movements. Depend on the position of the eye at the time of muscle contraction, these muscles aid in controlling the eye movement.

Extraocular muscles Mnemonics

• Medial rectus (MR),
• Lateral rectus (LR),
• Superior rectus (SR),
• Inferior rectus (IR),
• Superior oblique (SO), and
• Inferior oblique (IO).

Extraocular Muscles

Extraocular muscles movements:

Eye muscles work together with other eye muscles, of the same eye and the opposite eye, to move the eyes in various directions. However, a given extraocular muscle, if working on its own in isolation (without other extrinsic eye muscles in play), would move the front of an eye in a specific direction or directions, as follows:

• 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)
  • 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)
  • 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)
  • 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)
  • tertiarily moves the eye outward (abduction)

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 the muscle in the same eye that moves the eye in the opposite direction of the agonist is the “antagonist.” According to “Sherrington’s Law,” increased innervation to any agonist muscle is accompanied by a corresponding decrease in innervation to its antagonist muscle(s).

Nerve innervations of the Extraocular muscles:

Extraocular muscles are innervated by a specific cranial nerve (C.N.):

• Superior oblique (SO)—cranial nerve IV (Trochlear)
• Inferior oblique (IO)—cranial nerve III (Oculomotor)
• Superior rectus (SR)—cranial nerve III (Oculomotor)
• Inferior rectus (IR)—cranial nerve III (Oculomotor)
• Medial rectus (MR)—cranial nerve III (Oculomotor)
• Lateral rectus (LR)—cranial nerve VI (Abducens)