N375: Special Senses II

What is the main refractive surface of the eye that focuses incoming light onto the lens?

Cornea

What is the retina?

Neuroepithelium at the back of the eye, sends APs to optic nerve

Which clear fluid maintains intraocular pressure in the anterior chamber?

Aqueous humour

What part of the eye adjusts the amount of light entering by changing pupil diameter?

Iris

What cranial nerve causes sphincter pupillae muscles to contract?

Oculomotor CN III

What is the function of the lens?

Focus light appropriately to the back of the eye

Which jelly-like substance fills the posterior cavity and helps maintain the eye’s shape?

Vitreous humour

Which structure transmits visual signals from the retina to the brain?

Optic nerve (CN II)

What region of the retina provides the highest visual acuity?

Fovea

Which cells detect color and fine detail in bright light?

Cones

What layer of blood vessels supplies oxygen and nutrients to the outer retina?

Choroid

What is the tough, protective outer layer of the eyeball? (homolog to dura mater)

Sclera

Which neural cells in the retina send visual information through the optic nerve?

Retinal ganglion cells (RGCs)

What part of the eye contains photoreceptors and converts light into neural signals?

Retina

What kind of lesion?

Optic nerve lesion

What kind of lesion?

Optic chiasm lesion

What kind of lesion?

Optic tract lesion

Where is the fovea located?

In the center of the macula lutea on the retina.

Which photoreceptors are most concentrated in the fovea?

Cones (almost no rods).

What is the name for the region near the center of the retina

Macula

Which photoreceptors are present in the macula?

Mix of cones and some rods

What are the 3 cranial nerves that travel through the superior orbital fissure?

CN III + CN V + CN VI

How many extraocular muscles move the eyeball?

Six (four rectus, two oblique).

Mnemonic for cranial nerve supply to extraocular muscles.

LR6 SO4, all others III.

Muscle that elevates the upper eyelid.

Levator palpebrae superioris (CN III).

Superior/Inferior/Medial Rectus + inferior oblique– nerve?

CN III.

Lateral Rectus – nerve?

CN VI (Abducens).

Superior Oblique – nerve?

CN IV (Trochlear).

Looking right – which muscles contract?

Right Lateral Rectus (VI) + Left Medial Rectus (III).

Looking left – which muscles contract?

Left Lateral Rectus (VI) + Right Medial Rectus (III).

Define abduction.

Moving away from midline.

Define adduction.

Moving toward midline.

Parasympathetic vs sympathetic effect on pupil.

Parasympathetic = constrict; Sympathetic = dilate.

Function of superior rectus?

Elevates, adducts, and intorts the eye

Function of inferior rectus?

Depresses, adducts, and extorts the eye

Function of medial rectus?

Adducts the eye

Function of lateral rectus?

Abducts the eye

Function of superior oblique?

Depresses, abducts, and intorts (down + in)
💡 Used for reading (down and in gaze)

Function of inferior oblique?

Elevates, abducts, and extorts (up + out)

Describe the parasympathetic pathway of CN III.

Midbrain → through superior orbital fissure → synapse in ciliary ganglion → short ciliary nerves → sphincter pupillae + ciliary muscle

Function of sphincter pupillae?

Constricts pupil (miosis)

Function of ciliary muscle?

Changes lens curvature for accommodation (near focus)

Which system dilates and which constricts the pupil?

Sympathetic = dilate; Parasympathetic = constrict

What’s unique about the trochlear nerve? 

Only CN that exits posteriorly from brainstem and crosses before exiting. Superior oblique

What oculomotor muscle does CN VI innervate?

Lateral rectus

What oculomotor muscle does CN IV innervate?

Superior oblique

Where does CN VI travel?

Long path between pons and medulla → superior orbital fissure

Result of CN VI lesion?

Eye cannot abduct → pulled medially (medial strabismus) → diplopia (double vision)
Ipsilateral lesion

Result of CN III lesion?

Loss of all except superior oblique + lateral rectus → eye points down and out + dilated pupil
Ipsilateral lesion

Result of CN IV lesion at the nerve?

Ipsilateral superior oblique paralysis → eye drifts up + in → vertical diplopia

Result of CN IV lesion at the nucleus?

Contralateral symptoms (fibers cross before exiting)

Where is the primary visual cortex (V1) located?

In the calcarine sulcus of the occipital lobe.

What is special about the topographical organization of V1?

The fovea occupies a disproportionately large area → high acuity & detail.

What are the receptors of the visual system?

Rods and cones in the retina (convert light → AP).

Which visual field is processed in the right V1?

The left visual field (left temporal retina + right nasal retina).

Which visual field is processed in the left V1?

The right visual field (right temporal retina + left nasal retina).

Lesion in the optic nerve causes what?

Ipsilateral blindness in that eye (anopsia).

What happens to the pupillary light reflex if the optic nerve is damaged?

No light detection → no constriction in either pupil.

Lesion at the optic chiasm causes what?

Bitemporal (bilateral) hemianopsia → tunnel vision.

Common cause of optic chiasm compression?

Pituitary gland tumor pressing upward on the chiasm.

Lesion in the optic tract causes what?

Contralateral homonymous hemianopsia (loss of opposite visual field in both eyes).

What happens with an oculomotor nerve lesion?

Ipsilateral loss of pupillary light reflex + eye deviates down and out.

What is the dorsal visual stream (“where” pathway)?

V1 → posterior parietal cortex; processes spatial information and movement.

Damage to the dorsal stream causes what?

Optic ataxia (difficulty using vision to guide movement, e.g., reaching for objects).

What is the ventral visual stream (“what” pathway)?

V1 → inferotemporal cortex; processes object characteristics like shape and color.

Damage to the ventral stream causes what?

Visual agnosia (inability to recognize objects).

Describe the pupillary light reflex pathway.

Optic tract → both pretectal nuclei → bilateral Edinger-Westphal nuclei → CN III → ciliary ganglia → sphincter pupillae muscles.

Is the accommodation reflex consensual?

No, it is non-consensual (each eye acts independently).

What is the pathway for the accommodation reflex?

V1 → visual association cortex → oculomotor nucleus → ciliary ganglion → 3 actions.

What are the three actions in the accommodation reflex?

1. Accommodation – ciliary muscles contract → lens thickens

2. Convergence – medial rectus pulls eyes inward

3. Pupillary constriction – via sphincter pupillae