Eye Anatomy: Oculomotor and VOR

Eye Anatomy and Function

• The oculomotor nucleus controls extraocular muscles responsible for eye movement.
• The Edinger-Westphal nucleus controls parasympathetic pupil constriction and lens accommodation via the ciliary ganglion.
• The Edinger-Westphal nucleus sends preganglionic parasympathetic fibers to the ciliary ganglion.
• Postganglionic fibers from the ciliary ganglion go to the pupillary sphincter (for constriction) and ciliary muscle (for lens accommodation).
• Both the oculomotor and Edinger-Westphal nuclei are located in the midbrain.
• The Edinger-Westphal nucleus sits posteriorly in the periaqueductal gray.

Vestibulo-Ocular Reflex (VOR)

• The brain doesn't need signals from the eye for the VOR.
• Head movement is detected by semicircular canals in the inner ear.
• The signal travels via the vestibular nerve (CN VIII) to the vestibular nuclei in the brainstem.
• Vestibular nuclei send signals through the medial longitudinal fasciculus (MLF) to CN III and CN VI nuclei.
  • CN III controls the medial rectus muscle (eye adduction).
  • CN VI controls the lateral rectus muscle (eye abduction).
• The nuclei are in the brainstem, but the nerves exit the brainstem to reach the eye muscles.
• Origin: Semicircular canals of the inner ear
• Main location: Vestibular nuclei in the brainstem (pons/medulla)
• Pathway: Vestibular nuclei → via MLF → to cranial nerve nuclei (III, IV, VI) → move the eyes

Cranial Nerves and Eye Muscles

• CN III (Oculomotor): Originates in the midbrain (at the level of the superior colliculus) and controls most extraocular muscles and pupil constriction (via the Edinger-Westphal nucleus).
• CN IV (Trochlear): Originates in the midbrain (just below the CN III nucleus) and controls the superior oblique muscle.
• CN VI (Abducens): Originates in the pons (near the 4th ventricle) and controls the lateral rectus muscle (abducts the eye).

Visual Fields and Image Formation

• The cornea bends light, and the lens accommodates for focus.
• The lens changes shape via ciliary bodies.
• Refraction inverts and reverses the image on the retina:
  • The upper visual field projects to the lower retina.
  • The right visual field projects to the left retina.
• Light from the upper visual field is bent downward by the lens and lands on the inferior retina.
• Light from the left side hits the right side of the retina (after being bent inward).
• The brain corrects the inverted and reversed image.
• Retina sends the flipped map to the brain via the optic nerve, optic chiasm, and optic tract to the visual cortex.
• The visual cortex in the occipital lobe reinterprets the flipped image.

Retina Structure and Function

• The retina has 10 layers of photoreceptors (rods and cones).
• Pupil constriction (small pupil) increases depth of focus
• Pupil dilation (large pupil) results in a blurry image.
• The optic disc is the blind spot because it has no photoreceptors.
• The fovea has the sharpest vision, many cones, and the highest visual acuity.
• Supporting glial cells are Müller cells
• Epithelial cells are pigmented
• Horizontal cells are needed for contrast.
• Amacrine cells are needed for motion and timing.
• The pathway: light (photons) → horizontal cells → bipolar cells → amacrine cells → retinal ganglion cells → optic nerve.
• Horizontal and amacrine cells are interneurons.
• The peripheral retina has many rods which are made of rhodopsin, are sensitive to dim light and same frequencies throughout, creating scotopic vision.
• The fovea has many cones, allowing us to see colors, and uses iodopsin.
• Cones has 1:1 bipolar cells ratio, creating 3 types of cones: L (red), M (green), and S (blue).
  • L-cones are the most abundant and absorb low frequencies.
  • This trichromacy (LMS) allows for color vision.

Phototransduction Pathway

• Light hits photopigment, activating it, and 11-cis-retinal changes to all-trans-retinal.
• Activated opsin stimulates transducin (a G-protein).
• Transducin activates phosphodiesterase (PDE).
• PDE breaks down cGMP into GMP.
• Decreased cGMP causes Na⁺ channels to close
• In the dark, cGMP keeps Na⁺/Ca²⁺ channels open (the “dark current”), leading to depolarization and release of glutamate.
• With light, cGMP levels drop and channels close, leading to hyperpolarization and less glutamate release.

Visual Pathways

• Bipolar cells, ganglion cells, and LGN (in the thalamus) are all considered neurons.
• In the ganglion, parvo and magno pathways happen:
  • 70% is projected as a parvocellular pathway (color, high spatial resolution).
  • The magno pathway processes colorless information and is high in motion (high temporal resolution).
• The medial visual field is ipsilateral.
• The lateral visual field is contralateral.
• Magno and parvo pathways are part of the LGN.
• The optic chiasm is where decussation happens.

Visual Field Processing in the Brain

• The upper visual field (e.g., a bird flying above) hits the lower retina and projects to the lingual gyrus (below the calcarine sulcus).
• The lower visual field projects to the upper retina and then to the cuneus (above the calcarine sulcus).
• The calcarine sulcus is located in the primary visual cortex in the occipital lobe.
• Stereopsis is looking at near objects
• The primary visual cortex has two pathways:
  • The ventral pathway (color and shape).
  • The dorsal pathway (motion and spatial arrangement).

Pupillary Light Reflex - Dilation

• The afferent component starts in the posterior hypothalamus, responding to emotional stimuli (fear/arousal). The pathway is then:
  • A preganglionic sympathetic neuron descends to the intermediolateral cell column at the T1 level in the spinal cord.
  • Fibers synapse at the superior cervical ganglion (in the sympathetic chain).
  • Postganglionic sympathetic fibers wrap around the internal carotid artery and enter the skull.
  • They join the ophthalmic division (V1) of the trigeminal nerve and enter the orbit.
  • They travel via the long ciliary nerves to the dilator pupillae muscles of the iris, causing pupil dilation.
• The Afferent pathway acts on the visual pathway to go to primary visual cortex with activation of superior colliculus+ pretectal area- oculomotor nuclear comples.

Pupillary Light Reflex - Constriction

• The efferent pathway will involve parasympathetic to constrict pupillae muscle+ ciliary muscle

Accommodation (Near Triad)

• Eyes converge (adduct).
• Increased refraction, bending of light increases.
• Pupil constricts, the iris becomes large and increases in depth field so image focus improves.

Other Eye Structures and Functions

• Red light has a wavelength of 700nm.
• The cornea is innervated by the trigeminal nerve V1.
• The iris separates the anterior and posterior chambers of the eye.
• The sclera is the white, outer portion of the eye.
• Dilation is sympathetic (hypothalamus).
• Constriction is parasympathetic (cranial nerve 3).

Convergence and Divergence

• Convergence: Flat lens, little bending, light rays are parallel, ciliary muscles relax, object is far away. Medial rectus (adduction) activation.
• Divergence: Fat lens, more bending, focuses on near objects.

Rods vs. Cones

• Rods: Nighttime vision (many rods: 1 bipolar cell).
• Cones: Daytime vision (1 cone: 1 bipolar cell).

Visual Field Processing

• The temporal retina receives contralateral information from the medial visual field.
• The nasal retina receives ipsilateral information from the lateral visual field and decussates.

Head and Eye Movement Coordination

• Turning the head to the left activates the left horizontal canal in the inner ear, causing the eyes to move to the right.
• Turning the head to the right activates the right horizontal canal in the inner ear, causing the eyes to move to the left.
• Medial VN = semicircular canal (rotational acceleration) and activates the contralateral side
• Lateral VN = otolith organ (linear acceleration)
• Abducens = 6 activates the ipsilateral side

Visual Cortex and Pathways

• The primary visual cortex is called the striated cortex.
• The region around the striated cortex is called the peristriate cortex.
• Information from the primary visual cortex is sent to the parietal lobe (motion and spatial vision, night vision, no color) and the temporal lobe (object recognition, day vision with color).
  • Occipital Lobe to Parietal Lobe = Dorsal Pathway
  • Occipital Lobe to Temporal Lobe = Ventral Pathway
• Alexia: lesion in the left inferior temporo-occipital region.
• Prosopagnosia: lesion in the right inferior temporo-occipital region.
• Stroke affects only one side (ipsilateral).
• Optic nerve/optic tract lesions mean the issue came from the other side.

Visual Pathway Lesions

• Optic Nerve Lesion: Unilateral blindness.

  • Clue: Sudden loss of vision in one eye.
  • Consideration: Multiple sclerosis, optic neuritis, ischemia.
  • Buzz: Unilateral vision loss.

• Optic Chiasm Lesion: Bitemporal Hemianopia.

  • Clue: Bumping into things on both sides.
  • Consideration: Pituitary tumor, craniopharyngioma.
  • Buzz: Peripheral vision loss in both eyes, "Tunnel vision".

• Optic Tract Lesion: Contralateral Homonymous Hemianopia.

  • Clue: Loss of left visual field in both eyes.
  • Consideration: Stroke, tumor, MS.
  • Buzz: Right optic tract = left visual field loss, Contralateral hemianopia.
    • Example: Left visual field loss = right optic tract lesion

• Meyer’s Loop (Lower Fibers): Contralateral Superior Quadrantanopia.

  • Clue: Missing the top corner of the visual field.
  • Consideration: Temporal lobe lesion, HSV encephalitis, epilepsy surgery.
  • Buzz: "Pie in the sky", Temporal lobe stroke.
    • Example: Temporal lobe stroke= pie in sky

• Upper Fibers (Parietal Lobe): Contralateral Inferior Quadrantanopia.

  • Clue: Inability to see in the lower quadrant of both eyes.
  • Consideration: Parietal lobe infarct, MCA stroke.
  • Buzz: "Pie on the floor", Parietal lesion.
    • Example: MCA stroke= lower quadrant of both eyes get affected: contralateral inferior quadrantanopia

• Occipital Cortex Lesion: Contralateral Homonymous Hemianopia with Macular Sparing.

  • Clue: Loss of half the visual field but central vision is intact.
  • Consideration: PCA stroke, trauma, posterior infarct.
  • Buzz: Macular sparing, Cortical visual loss.
    • Example: PCA stroke= macular sparing= contralateral homonymous hemianopia with macular sparing