Human Eye Anatomy and Conditions Quiz

Sense of Vision Learning Outcomes

• Students should be able to describe the accessory structures of the eye and their functions.
• Students should be able to describe the anatomy of the eye.
• Students should be able to describe the anatomy of eye cavities and their chambers.
• Students should be able to discuss the refraction of light and image formation.
• Students should be able to describe the stimulation of photoreceptors.
• Students should be able to discuss the visual pathway to the brain.

Introduction

• More than half of the sensory receptors in the human body are located in the eyes.
• A large portion of the cerebral cortex is dedicated to processing visual information.
• The eye is the organ of sight located in the orbital cavity and is supplied by the optic nerve (2nd cranial nerve).

Accessory Organs of the Eyes

• Eyebrows/eyelashes - protect the eyeballs from foreign objects, sweat, and direct sunlight.
• Eyelids - protect the eyes from excessive light, injury, and keep the cornea moist.
• Conjunctiva - a thin mucous membrane that lines the inner aspect of the eyelids and the anterior surface of the eyeball.
• Lacrimal apparatus - produces and drains tears, moisturizing the cornea and conjunctiva.
• Extraocular muscles - six muscles attached to the sclera that move the eyeballs in all directions.

Eyelashes and Eyebrows

• Eyelashes - shield the eyeballs from foreign particles, sweat, and sun glare.
• Eyebrows - redirect sweat away from the eyes and contribute to facial expressions.

Eyelids

• Function:
  • Protect the eyes from excessive light.
  • Protect from injury.
  • Help to keep the cornea moist.
• Orbicularis muscle - closes the lids.
• Levator palpebrae superioris muscle - raises upper eyelids.

Conjunctiva

• A thin mucous membrane lining the inner aspect of the eyelids and reflected onto the anterior surface of the eyeball.

Conjunctivitis

• Inflammation of the conjunctiva.

Lacrimal Apparatus

• A group of structures that produce and drain tears.
• Located in the upper outer corner of each orbit.
• Comprises:
  • Lacrimal gland
  • Lacrimal canal
  • Lacrimal sac
  • Nasolacrimal ducts
• Tears contain salt, water, and lysozyme.

Lacrimal Apparatus (Flow of Tears)

• Tears from the lacrimal gland flow into the lacrimal puncta.
• Tears are drained through superior and inferior lacrimal canals into the lacrimal sac.
• The right and left nasolacrimal ducts empty into each side of the nose.

Lacrimal Apparatus (Clinical Application)

• Watery eyes - occur due to blockage of the nasolacrimal ducts.
• Blocked nasolacrimal ducts - caused by nasal mucosa inflammation (e.g., cold).
• Overproduction of lacrimal fluid - occurs during emotional responses (crying) and tears spill over eyelid edges, draining into the nasal cavity.

Extraocular Muscles of the Eye

• The six extrinsic muscles are attached to the sclera and move the eyeballs in various directions.

Anatomy of the Eye

• The walls of the eyeball are made up of three layers of tunics:
  • Fibrous Tunic - sclera and cornea
  • Vascular Tunic (Uvea) - choroid, ciliary body, and iris
  • Nervous Tunic - retina

Anatomy of the Eye (Internal Cavity)

• Filled with fluids called humors.
• Lens separates the internal cavity into anterior and posterior segments.
  • Anterior segment contains aqueous humor.
  • Posterior segment contains vitreous humor.

Fibrous Tunic

• Outermost layer of the eyeball.
• Composed of sclera and cornea.
• Protects the intraocular contents.
• Scleral venous sinus (canal of Schlemm): an opening at the junction of the sclera and cornea that drains aqueous humor.

Fibrous Tunic (Sclera)

• Known as the "white" of the eye.
• Maintains the shape of the eyeball.
• Provides attachment sites for extrinsic muscles.

Fibrous Tunic (Cornea)

• Continuously from sclera.
• Convex anteriorly, acting as a refracting surface for light rays.
• A common visual corrective procedure, LASIK, is performed on the cornea.

Vascular Tunic

• Middle layer of the eyeball.
• Composed of choroid, ciliary body, and iris.
  • Choroid - lines between sclera and retina, contains blood vessels to nourish the retina. Contains melanocytes - producing melanin to absorb light.
  • Ciliary body - consists of ciliary processes secreting aqueous humor and ciliary muscles that adjust lens shape for near/far vision.
  • Iris - colored portion; adjusts pupil size to control light intake.

Choroid

• Located between sclera & retina.
• Contains blood vessels to nourish the retina.
• Melanocytes produce melanin causing the dark brown color - absorbs light rays, preventing reflection and scattering.

Ciliary Body

• Consists of ciliary processes (secrete aqueous humor) and ciliary muscle (changes the shape of the lens).

Iris

• Colored portion of the eyeball.
• Pupil - black hole; adjusts to regulate the amount of light reaching the retina.

Sensory Tunic (Retina)

• Innermost layer of the eyeball.
• Thin, delicate membrane.
• Receives light, converts it into nerve impulses.
  • Optic disc - blind spot where optic nerve exits eyeball
  • Macula lutea - area rich in cones, the exact center of the posterior retinal portion.
  • Fovea centralis - point of sharpest vision due to high cone concentration.

Retina (Rods)

• Specialized for black-and-white vision in dim light.
• Allow us to discriminate between shades of dark and light.
• More than cones.
• Contains rhodopsin.

Retina (Cones)

• Specialized for color vision and sharpness of vision (high visual acuity) in bright light.
• Three types of cones: blue, green, and red.
• Concentrated in the fovea centralis.

Retina (Clinical Application)

• Retinal detachment - retina peels away from its underlying supporting tissue, resulting in visual distortions or blindness.
• Diabetic retinopathy - blood vessels may block or damage, causing damage to the retina; the main cause of blindness in people under 65.
• Age-Related Macular Degeneration (AMD) - leads to vision loss in the center of the visual field due to damage to the macula.

Interior of the Eyeball

• Contains anterior and posterior chambers that contain aqueous and vitreous humor, respectively.

Anterior & Posterior Chamber

• Anterior cavity: Space between cornea and lens. Contains 2 chambers:

  • anterior - between cornea and iris
  • posterior - between iris and lens

• Contains aqueous humor.

  • Secreted by ciliary glands.
  • Supplies oxygen, nutrients, and removes waste from the cornea, lens, and lens capsule.
  • Maintains eye shape.
  • Drains through the scleral venous sinus (canal of Schlemm) into the blood stream.

• Posterior cavity (vitreous chamber):

  • Large region between lens and retina.
  • Filled with vitreous humor.
  • Formed during embryonic development.
  • Not replaced thereafter
  • Vitreous humor: soft, colorless, transparent, jelly-like substance maintains eye shape and holds the retina against the choroid.

Aqueous Humor (Clinical Application)

• Glaucoma - aqueous humor does not drain properly, leading to increased intraocular pressure that damages the optic nerve, causing blindness if untreated.

Lens

• Biconvex, avascular, colorless, and transparent.
• Located behind the iris and pupil.
• Its thickness is controlled by the ciliary muscle.
• Function: focus light rays onto the retina (refraction).

Lens (Clinical Application)

• Cataracts - lens becomes hard and opaque.
  • Common complication of diabetes mellitus.
  • Caused by smoking and UV exposure to sunlight.
  • Treated by replacing the natural lens with a plastic one.

Pathway of Light and Refraction

• Normal image formation depends on:
  • Refraction of light waves
  • Accommodation of the lens
  • Constriction of the pupil
  • Convergence of the two eyes

Refraction

• Bending of light rays by the refractive media (cornea, aqueous humor, lens, vitreous humor) in the eye to focus light rays onto the retina.
• In a normal eye, the refractive media bends light rays from objects at least 20 feet away to be focused on the retina.

Refraction (cont)

• 75% of total light refraction occurs at the cornea.
• Lens further refracts light rays so they come into exact focus on the retina.
• In order to focus light after bending by cornea, the shape of the lens must adjust.

Accommodation

• Adjustment in the lens shape to focus the image on the retina.
• For objects less than 20 feet away:
  • Ciliary muscles contract.
  • Suspensory ligaments become loose or relaxed.
  • Lens becomes bulged (thicker).
• For objects farther than 20 feet (far vision):
  • Ciliary muscles relax.
  • Suspensory ligaments become taut.
  • Lens becomes flat.

Pupillary Response

• Pupil - opening in the center of the iris to allow light into the eye.
  • Composed of a radial muscle (radiating from the center of the iris) and a circular muscle.
  • Contraction of circular muscles constrict the pupil, limiting light intake.
  • Contraction of radial muscles dilate the pupil, allowing more light intake.

Convergence

• Inward movement of the eyes to direct both eyes at the viewed object.
• The closer the object, the greater the degree of convergence, which is coordinated by extrinsic muscles, helping maintain binocular vision.

Refraction Abnormalities

• Myopia (nearsightedness): image is focused in front of the retina, corrected via concave lens.
• Hyperopia (farsightedness): image is focused behind the retina, corrected via convex lens.
• Astigmatism: irregular curvature of the cornea or lens affecting the sharpness of vision.

Photoreceptors

• Rods and cones: Light-sensitive cells in the retina that convert light energy into neural signals.

Photoreceptors (Rods)

• Thin cells with rod-like projections.
• Specialized for black-and-white vision in dim light.
• More abundant than cones.

Photoreceptors (Cones)

• Specialized for color vision in bright light.
• Three types of cones: blue, green, and red.
• Densely concentrated in the fovea centralis.
• Intermediate colors are perceived by activation of more than one type of cone.

Photoreceptors (Clinical Application)

• Night blindness- reduced rhodopsin, vitamin A deficiency.
• Colorblindness- inherited absence or deficiency of one type of cone photopigment; more common in males. (e.g., red-green colorblindness).

Optic Nerve, Optic Chiasma, and Optic Tract

• Optic nerve - cranial nerve carrying impulses from retina to brain.
• Optic chiasma - point where optic nerve fibers cross in the brain.
• Optic tract - extends from the optic chiasma to the thalamus, for visual awareness.

Visual Pathway

• After light stimulation, rods and cones trigger electrical signals to ganglion cells.
• Impulses are conveyed through the retina to the optic nerve, and then to the optic chiasma.
• From there, impulses travel through the optic tract to the thalamus, and then through optic radiations to the visual cortex in the occipital lobe.

Recap/Quiz Points

• Conjunctiva
• Lacrimal gland
• Tears (mucus, antibodies, lysozyme)
• Eye layers (Fibrous, Vascular, Nervous)
• Structures in Fibrous tunic (Sclera, Cornea)
• Structures in Vascular tunic (Choroid, Ciliary body, Iris)
• Structure in Nervous tunic (Retina)
• Lens; separates anterior and posterior segments
• Biconvex lens shape
• Sclera
• Cornea
• Aqueous humor
• Vitreous humor
• Glaucoma
• Accommodation
• Night blindness
• Rhodopsin
• Rods

Test your knowledge on the anatomy and functions of the human eye, including conditions such as cataracts and diabetic retinopathy. This quiz covers essential functions of eye components and common visual impairments. Enhance your understanding of how the eye works and what can affect its performance.