Eye and Vision

lecture: 4/5

I. The Eyes & Vision — General

1. Approximately what percentage of the body's sensory receptors are in the eyes?

2. What is the approximate size of the eyeball?

1. Approximately 70%

2. Approximately 1 inch in diameter

1. What protects the eyeball?

2. Approximately how much of the cerebral cortex is used to process visual information?

1. A bony orbit surrounded by a layer of fat

2. Approximately half of the cerebral cortex

II. Accessory Structures

A. Eyebrows

1. Where are the eyebrows located?

2. What are the eyebrows?

3. What are two functions of the eyebrows?

1. On the supraorbital margins

2. Short, coarse hairs

3. They protect against sunlight by providing shade and block sweat

B. Eyelids

• What is another name for the eyelids?

  • What separates the eyelids?

    • What are the two ends of the eyelids called?

      • What does the medial commissure contain?

• Palpebrae

  • The palpebral fissure

    • The lateral and medial commissures

      • The lacrimal caruncle

C. Eyelashes

1. Where are the eyelashes located?

2. What is special about the innervation of the eyelashes?

3. What does stimulation of the eyelashes trigger?

1. On the margins of both eyelids

2. They are richly innervated

3. Blinking

D. Conjunctiva

• What is the conjunctiva?

  • What does the conjunctiva secrete?

• A mucus membrane

  • Mucus

E. Lacrimal Apparatus

1. Where are the lacrimal glands located?

2. How do lacrimal glands release their secretions?

• What happens when you blink according to the lecture?

1. Over the lateral part of the eye

2. Through excretory ducts

• Blinking spreads the lacrimal secretions across the eye toward the medial commissure

III. Extrinsic Eye Muscles

1. What are the functions of the extrinsic eye muscles?

• What are the four extrinsic eye muscles listed in this lecture?

1. They allow the eye to track moving objects, maintain shape, and maintain direction of movement

• Lateral rectus

• medial rectus

• superior rectus

• inferior rectus

1. What is strabismus?

2. How can extrinsic eye muscles cause strabismus?

1. Misalignment of the eyes

2. Improper muscle function can cause the eyes to point in different directions

IV. The Eyeball — Three Layers

A. Fibrous Layer

1. What is the fibrous layer?

2. What are the two structures of the fibrous layer?

1. The outermost layer of the eyeball

2. The sclera and cornea

Sclera

1. What is the sclera?

• What color is the sclera?

  • What is the sclera an anchoring site for?

1. The posterior and bulk of the fibrous layer

• Opaque white

  • The extrinsic muscles

Cornea

1. Where is the cornea located?

2. What is a characteristic of the cornea?

• What does the lecture state about immunity of the cornea?

1. At the anterior part of the eye, where it forms a clear bulge

2. It is clear

• It has no immunity

B. Vascular Layer

1. What is the vascular layer?

2. What structures are associated with the vascular layer?

1. The middle layer of the eyeball

2. The choroid, ciliary body, and iris

Choroid

• What is the function of the blood vessels in the choroid?

  • What do melanocytes in the choroid provide?

    • What does the pigment in the choroid prevent?

      • Where is the choroid located?

• They nourish the other layers

  • Pigment to absorb light

    • Light reflection

      • In the posterior portion of the eye

Ciliary Body

• What is the ciliary body?

  • What controls the shape of the lens?

    • What holds the lens in place?

• A thickened ring of tissue encircling the lens

  • The ciliary muscles

    • The ciliary zonule

Iris & Pupil

1. What is the iris?

2. What is the pupil?

1. The colored ring around the pupil

2. The opening for light

C. Sensory Layer

1. What is the sensory layer of the eyeball?

2. Where is the retina located?

1. The retina

2. It is the inner layer of the eyeball

Pigmented Layer

1. What is the pigmented layer of the retina?

2. What is the function of the pigment in the pigmented layer?

1. A single-cell-thick layer containing pigment like the choroid

2. It has the same function as the pigment in the choroid

Neural Layer

• What is the neural layer of the retina?

  • What types of cells are found in the neural layer?

    • Approximately how many photoreceptor cells are present?

• A transparent layer containing neurons

  • Photoreceptor cells, bipolar cells, and ganglion cells

    • Millions of photoreceptor cells

1. What do bipolar cells do?

2. What do ganglion cells do?

1. They link photoreceptors to ganglion cells

2. They create action potentials, and their axons leave the eye as the optic nerve

V. Important Eye Structures

Optic Disc

1. What is the optic disc?

2. What is another name for the optic disc?

1. The location where the optic nerve exits the eye

2. The blind spot

Macula Lutea

• Where is the macula lutea located?

  • What does the macula lutea contain?

• Lateral to the blind spot

  • A small pit containing the fovea centralis

VI. Photoreceptors — Rods vs. Cones

Rods

• How do rods compare with cones in number and light sensitivity?

  • Do rods provide sharp images?

• What type of vision do rods provide?

• Where are rods located mainly?

• Rods are more numerous and more light-sensitive than cones

  • No

• Peripheral vision

• Mainly in the periphery of the eye

Cones

• What type of light do cones respond to?

  • What type of vision do cones provide?

    • Where are cones highly concentrated?

• Bright light

  • Color vision and detail

    • In the fovea centralis

VII. Lens

1. What is the function of the lens?

2. What is the shape of the lens?

• What are two physical characteristics of the lens?

1. It precisely focuses light on the retina

2. Biconcave

• It is flexible and elastic

VIII. Cataracts

1. What are cataracts?

• What factors listed in the lecture may result in cataracts?

1. Clouding of the lens

• Aging

• diabetes

• heavy smoking

• exposure to intense light

IX. Light

1. What is light?

• What are packets of light energy called?

• What do rods and cones respond to?

1. Electromagnetic radiation

• Photons

• Different wavelengths of light

X. Light Refraction

• What is light refraction?

• Where is light refracted as it passes through the eye?

  • What does the lens do to refracted light?

• The bending of light as it passes from one medium to another

• When entering the cornea, entering the lens, and leaving the lens

  • It refracts light so that the focal point is on the retina

XI. Focusing

Distance Vision

1. What are light rays like during distant vision?

2. How much lens adjustment is needed for distant vision?

• What is the far point of vision?

1. They are parallel

2. Minimal lens adjustment is needed for focus

• The distance that does not require the lens to focus

Near Vision

1. What is accommodation?

2. What is the maximum point of accommodation called?

1. A process in which the ciliary muscles change the shape of the lens

2. The near point

1. What happens to the pupil during near vision?

2. What is convergence?

1. The pupil constricts through the pupillary reflex

2. Both eyes rotate medially

XII. Refraction Problems

Myopia

• What is myopia?

  • What causes myopia according to the lecture?

    • Where is the focal point in myopia?

      • What type of lens corrects myopia?

• Nearsightedness

  • A long eyeball

    • In front of the retina

      • A concave lens

Hyperopia

• What is hyperopia?

  • What causes hyperopia according to the lecture?

    • Where is the focal point in hyperopia?

      • What type of lens corrects hyperopia?

• Farsightedness

  • A short eyeball

    • Behind the retina

      • A convex lens

Astigmatism

1. What causes astigmatism?

2. How may astigmatism be corrected?

1. Uneven curvature of the cornea or lens

2. Glasses/contacts or eye surgery

XIII. Phototransduction

1. What is the sequence of cells involved in phototransduction according to the lecture?

2. Where are photoreceptor cells located?

1. Photoreceptor cell → bipolar cell → ganglion cell (→ Optic nerve)

2. Embedded in the retina

1. What does the bipolar cell do?

2. What happens to the ganglion cell axon?

1. It connects the photoreceptor to the ganglion cell

2. It forms the optic nerve

XIV. Visual Pathway Through the Brain

• What forms the optic nerve?

  • Where does the optic nerve exit the eye?

• Axons of ganglion cells

  • Through a weak spot in the sclera

• What happens at the optic chiasm?

  • What do the optic tracts carry?

• The medial optic nerve fibers cross

  • Information from the same half of the visual field

• Where are the lateral geniculate nuclei located?

  • What happens after the lateral geniculate nuclei?

• In the thalamus

  • Axons from the thalamic neurons connect to the primary visual cortex

• Where is the primary visual cortex located?

• What does the primary visual cortex do?

Visual Pathway?

• In the occipital lobe

• It interprets visual stimuli into perceived images

Optic nerve → Optic chiasm → Optic tracts → Lateral geniculate nuclei → Primary visual cortex

XV. Depth Perception

1. What contributes to depth perception?

2. What does the visual cortex do with the two visual fields?

1. Overlap of the two visual fields

2. It fuses the two fields together

Eyeball Layers

Fibrous → ??

Vascular → Sensory/Retina

Retina

Photoreceptor → ??

Bipolar → Ganglion → Optic nerve

Rods vs. Cones

• Rods → ??

• Cones → ??

• peripheral vision + more light sensitive

• color + detail + fovea

Focusing

1. Myopia → ??

2. Hyperopia → ??

3. Astigmatism → ??

1. long eyeball → focal point in front → concave lens

2. short eyeball → focal point behind → convex lens

3. uneven cornea/lens curvature

Visual Pathway

Optic nerve → ???

optic chiasm → optic tracts → lateral geniculate nuclei → primary visual cortex

Fovea

Small pit in macula lutea → ?

highly concentrated cones

Conditions

Cataracts → ?

clouding of lens → aging, diabetes, heavy smoking, intense light