Study Notes on Extraocular Eye Muscles

Introduction to Extraocular Eye Muscles

• Presented by David Gonsalves, Part Three of a series.
• Focuses on the innervation and isolated actions of extraocular eye muscles.

Learning Objectives

• Identify the six extraocular eye muscles capable of moving the globe.
• Understand their neural innervation.
• Appreciate the isolated actions of each muscle.

Extraocular Eye Muscles

Overview of Muscle Positions

• Reference to a previous image featuring:

  • Medial side (nose)
  • Lateral side (zygomatic bone)
  • Apex located at the sphenoid bone.

• Annulus of Zinn:

  • A fibrous ring at the apex that serves as the origin point for four muscles:
  • Superior Rectus
  • Medial Rectus
  • Inferior Rectus
  • Lateral Rectus

• Rectus: Derives from a word meaning straight; these muscles attach directly to the eye.

Muscle Origins and Insertions

• Importance of understanding muscle origins and insertions for eye movement.
• Levator Palpebrae Superioris:
  • Origin not involved in globe movement; responsible for retracting the eyelid.
• Superior Oblique Muscle:
  • Originates from the sphenoid bone, narrowing into a tendon that passes through a trochlea and attaches to the sclera.
• Inferior Oblique Muscle:
  • Origins similar in approach.

Compartmentalization of Muscles

• The annulus divides the orbit into compartments for muscle arrangement.
• Differentiates between intraconal and extraconal spaces.
• Muscles involved:
  • Superior Rectus
  • Medial Rectus
  • Inferior Rectus
  • Lateral Rectus
  • Oblique Muscles

Innervation of Eye Muscles

Neural Structures in the Orbit

• Sensory branches from V1, the trigeminal nerve, enter through the supraorbital fissure.
• Important nerves include:
  • Lacrimal Branch: Innervates the lacrimal gland.
  • Frontal Nerve: Splits into Supraorbital and Supratrochlear nerves.
  • Trochlear Nerve: Innervates the superior oblique muscle via the trochlea.

Major Nerves Innervating Eye Muscles

• Optic Nerve: Runs to the back of the eye, enclosed in dura.
• Ophthalmic Artery: Supplies blood, with a branch to the central retinal artery.
• Autonomic Nerves:
  • Sympathetic: From superior cervical ganglion along the internal carotid.
  • Parasympathetic: From Edinger Westphal nucleus along the oculomotor nerve.

Cranial Nerves Involved

• Oculomotor Nerve (Cranial Nerve III): Divided into:
  • Superior division (controls Levator Palpebrae, Superior Rectus)
  • Inferior division (controls Inferior Rectus, Medial Rectus, Inferior Oblique)
• Trochlear Nerve (Cranial Nerve IV): Innervates the Superior Oblique.
• Abducens Nerve (Cranial Nerve VI): Innervates the Lateral Rectus.
• Mnemonic for Muscle Innervation:
  • Lateral Rectus = 6, Superior Oblique = 4, others = 3.

Muscle Actions

Reference Position for Eye Movements

• All muscle actions considered from a reference position with gaze straightforward (similar to anatomical position).

Actions of Rectus Muscles

• Superior Rectus:
  • Primary action: Elevate the eye.
  • Secondary: Adduct and medially rotate (intorsion).
• Inferior Rectus:
  • Primary action: Depress the eye.
  • Secondary: Adduct and slightly laterally rotate (extorsion).
• Medial Rectus:
  • Primary action: Adduct the eye.
• Lateral Rectus:
  • Primary action: Abduct the eye.

Combined Forces in Movement

• Each muscle also contributes to secondary and tertiary movements due to the muscle's angle of pull rather than a direct line of action.

Actions of Oblique Muscles

• Superior Oblique Muscle:
  • Primary action: Depression and abduction of the eye.
  • Secondary action: Medial rotation.
• Inferior Oblique Muscle:
  • Primary action: Elevation and abduction of the eye.
  • Secondary action: Lateral rotation.

Diagrams and Videos for Visualization

• Reference to an external video to elucidate the isolated actions of the muscles for better understanding.