Special Sense - Vision

Structure of the eyeball

• 3 layers/tunics (superficial → deep)

  • 1. Fibrous tunic

  • 2. Vascular tunic (Uvea)

  • 3. Sensory tunic (Retina)

• Lens

• Internal cavity (ant. & pos. to lens)

  • Anterior segment → aqueous humor

  • Posterior segment → vitreous humor

1. Fibrous tunic

Composed of:

• Sclera

• Cornea

Fibrous tunic - labeled

• Protective; superficial

• Sclera:

  • Whites of eyes

  • Muscles attach

• Cornea:

  • Clear

  • Light passed through

  • Refract (bend) light

Extrinsic muscles of the eye

• Attach to the fibrous tunic

• Originate on surface of orbit (bone) & insert on sclera

• 6 muscles:

  • Superior rectus muscle

    • A: Elevation of eyeball

    • I: Oculomotor nerve (CN lll)

  • Inferior rectus muscle

    • A: Depression of eyeball

    • I: Oculomotor nerve (CN lll)

  • Medial rectus muscle

    • A: Adduction of eyeball

    • I: Oculomotor nerve (CN lll)

  • Lateral rectus muscle

    • A: Abduction of eyeball

    • I: Abducens nerve (CN Vl)

  • Superior oblique muscle

    • A: Depression & abduction of eyeball

    • I: Trochlear nerve (CN lV)

  • Inferior oblique muscle

    • A: Elevation & abduction of eyeball

    • I: Oculomotor nerve (CN lll)

Extrinsic muscles of the eye - labeled

To help remember:

• LR6SO4AO3

  • Lateral Rectus → CN Vl (6) / Abducens N.

  • Superior Oblique → CN lV (4) / Trochlear N.

  • All Others → CN lll (3) / Oculomotor N.

Extrinsic muscles of the eye - lateral & superior views

2. Vascular Tunic (Uvea)

Composed of:

• Choroid

• Ciliary body

• Iris → central opening = pupil

Vascular Tunic - labeled

• Choroid:

  • Highly pigmented - absorbs light

  • Highly vascularized

• Ciliary body:

  • Muscle

  • Change shape of lens

• Iris:

  • Colored part

  • Pupil:

    • Central opening

    • Light enters

Oculomotor nerve (CN lll) - parasympathetic

• Iris = change pupil diameter

• Parasympathetic:

  • Stimulation causes circular muscles to contract, which constricts the pupil

• Sympathetic:

  • Stimulation causes radial muscles to contract, which dilates the pupil

Lens

• Used to focus light on the retina by changing shape

• Relaxed ciliary muscles = far accommodation = narrower lens

• Contracted ciliary muscle = near accommodation = wider lens

Retina (sensory tunic)

• Composed of photoreceptors

• Optic nerve (CN ll) is made up of the axons of the retinal ganglion cells

Sensory Tunic (Retina) - labeled

Interior cavity

• Anterior segment:

  • Anterior to lens

  • Filled w/ aqueous humor

    • Liquidy; replaced often

• Posterior segment:

  • Posterior to lens

  • Filled with vitreous humor

    • Gelatinous

    • Not replaced

    • Shock absorber

Lacrimal Apparatus

• Lacrimal gland

  • CN Vll - Parasympathetic function

• Nasolacrimal duct

Vision

Visual pathway (right side example)

• Optic nerve coming from left eye: all info from left eye (both visual fields)

• Optic chiasm: partial crossover of peripheral info

• Optic tract on right side: left visual field only

• Right primary visual field: left visual field

Visual pathway - order summary

1. Photoreceptors

2. Ganglion cells (axons for the optic nerve / CN ll)

3. Optic chiasm (partial crossover of peripheral info)

4. Optic tract

5. Thalamus

6. Primary visual cortex

The visual info from the left eye travels in which structure?

Left optic nerve

The visual info from the left visual field travels in which structure?

Right optic tract

Peripheral vision crosses over in which structure?

Optic chiasm