The Eye and Vision - Detailed Notes

Special Senses Part III - The Eye and Vision

Vision and Light

• Vision is the perception of light emitted or reflected from objects.
• Visible light is electromagnetic radiation with wavelengths from 400 to 750 nm.

External Anatomy of Eye

• Key external features include:
  • Superciliary ridge
  • Eyebrow
  • Eyelashes
  • Palpebral fissure
  • Pupil
  • Superior palpebral sulcus
  • Upper eyelid
  • Iris
  • Sclera
  • Lateral commissure
  • Tarsal plate
  • Medial commissure
  • Lower eyelid
  • Inferior palpebral sulcus

Eyebrows and Eyelids

• Eyebrows:
  • Provide facial expression.
• Eyelids (palpebrae):
  • Block foreign objects.
  • Help with sleep.
  • Blink to moisten the eye.
• Eyelashes:
  • Help keep debris from the eye.

Conjunctiva

• Transparent mucous membrane that lines the eyelids and front of the sclera.
• Richly innervated and vascular, which allows it to heal quickly.

Lacrimal Apparatus

• Tears flow across the eyeball.
• Functions of tears:
  • Wash away foreign particles.
  • Diffusion of O2 and CO2.
  • Contains bactericidal enzyme.

Extrinsic Muscles of the Eye

• Superior, medial, and inferior rectus muscles, plus the inferior oblique are controlled by the oculomotor nerve.
• Lateral rectus is controlled by the abducens nerve.
• Superior oblique is controlled by the trochlear nerve.

Tunics of the Eyeball

• Fibrous layer:
  • Sclera.
  • Cornea.
• Vascular layer:
  • Choroid.
  • Ciliary body.
  • Iris.
• Internal layer:
  • Retina.
  • Optic nerve.

Optical Components

• Structures refract light to focus on the retina.
• Cornea:
  • Transparent cover on the anterior surface of the eyeball.
• Lens:
  • Changes shape to help focus light.
  • Rounded with no tension.
  • Flattened due to the pull of suspensory ligaments.

Cataracts and Glaucoma

• Cataract:
  • Clouding of the lens.
  • Caused by aging, diabetes, smoking, and UV light.
• Glaucoma:
  • Death of retinal cells due to elevated pressure within the eye.
  • Caused by obstruction of the scleral venous sinus.
  • Symptoms include colored halos and dimness of vision.

Neural Components

• Includes the retina and optic nerve.
• Retina:
  • Forms as an outgrowth of the diencephalon.
  • Attached at the optic disc and at the ora serrata.
  • Pressed against the rear of the eyeball by vitreous humor.

Detached Retina

• Caused by a blow to the head or lack of vitreous humor.
• Results in blurry areas in the field of vision.
• Disrupts blood supply and can lead to blindness.

Formation of an Image

• Light passes through the lens to form an inverted image on the retina.
• Pupillary constrictor:
  • Parasympathetic stimulation narrows the pupil.
• Pupillary dilator:
  • Sympathetic stimulation widens the pupil.
  • Active when light intensity changes or gaze shifts from a distant object to a nearby object.

Principle of Refraction

• Light striking the lens or cornea at a 90 degree angle is not bent.

Effects of Corrected Lenses

• Hyperopia:
  • Farsightedness (eyeball too short).
  • Corrected with convex lenses.
• Myopia:
  • Nearsightedness (eyeball too long).
  • Corrected with concave lenses.

Light and Dark Adaptation

• Light adaptation (walk out into sunlight):
  • Pupil constriction and pain from overstimulated retinas.
  • Color vision and acuity below normal for 5 to 10 minutes.
• Dark adaptation (turn lights off):
  • Dilation of pupils occurs.
  • Takes 20 to 30 minutes.

Duplicity Theory

• Explains why we have both rods and cones.
• A single type of receptor cell is incapable of providing high sensitivity and high resolution.
• Sensitive night vision and high-resolution daytime vision use different cell types and neuronal circuitry.

How Photoreceptors Work

• Rods recycle rhodopsin using:
  • Bleaching.
  • Regeneration.

Color Vision

• Primates have well-developed color vision.
• Cones are named for absorption peaks of photopsins.
  • Blue cones peak sensitivity at 420 nm.
  • Green cones peak at 531 nm.
  • Red cones peak at 558 nm (orange-yellow).

Color Blindness

• Hereditary lack of one photopsin.
• Red-green color blindness is common (lack either red or green cones).
• Incapable of distinguishing red from green.

Stereoscopic Vision (Stereopsis)

• Depth perception is the ability to judge distance to objects.
• Requires 2 eyes with overlapping visual fields.

Visual Projection Pathway

• Rods and cones synapse with bipolar cells, which synapse with ganglion cells.
• Axons of ganglion cells form optic nerves.

Visual Projection Pathway

• Optic nerves cross at the optic chiasm.
• Some axons stay on the same side of the brain, others cross over.
• Each side of the brain sees objects on the side where it has motor control over limbs.
• First stop: Thalamus.
• Next stop: Primary visual cortex (in occipital lobe).

Visual Information Processing

• Some processing occurs in the retina, including adjustments for contrast, brightness, motion, and stereopsis.
• Visual association areas add color and motion information.
• Ventral visual stream (temporal lobe):
  • Identification and significance of objects.
  • Visual memories.
• Dorsal visual stream (parietal lobe):
  • Location of objects.
  • Motor response.