Eye 2025 Student Notes

Special Senses

• Definition: Special senses are crucial for gathering unique sensory information essential for interaction with the environment.

  • Types of special senses:

    • Smell

    • Taste

    • Sight

    • Hearing

    • Equilibrium

• Receptors:

  • Utilize complex sensory organs (e.g., eye and ear).

  • Localized clusters of receptors like taste buds and olfactory epithelium.

• Functional Link:

  • Each special sense influences motor output in the nervous system. For example, visual cues can lead to reflexive movements to avoid hazards.

  • Sensory information is integrated in specialized areas of the cerebrum.

Part I: The Eye and Vision

• Sensory Receptors:

  • 70% of all sensory receptors are located in the eyes.

  • Each eye transmits information through over 1 million nerve fibers to the brain.

• Accessory Structures of the Eye:

  • Extrinsic eye muscles

  • Eyelids

  • Conjunctiva

  • Lacrimal apparatus

External and Accessory Structures

• Eyelids:

  • Meet at medial and lateral commissures (canthus).

• Eyelashes:

  • Tarsal glands produce oily secretions for lubrication.

  • Ciliary glands located between eyelashes.

• Conjunctiva:

  • Membrane lining eyelids and eyeball, connects with cornea.

  • Secretes mucus for eye lubrication and moisture.

Lacrimal Apparatus

• Components: Lacrimal gland and ducts.

  • Lacrimal Gland: Produces lacrimal fluid (tears), located laterally.

  • Tear Drainage: Drains tears across the eye into lacrimal canaliculi, the lacrimal sac, and nasolacrimal duct into the nasal cavity.

• Tear Composition:

  • Dilute salt solution

  • Mucus

  • Antibodies

  • Lysozyme (enzyme that destroys bacteria)

• Functions of Tears:

  • Cleansing, protection, moisture, and lubrication of the eye.

Extrinsic Eye Muscles

• Structure:

  • Six muscles attached to the outer eye surface.

  • Responsible for gross eye movements.

  • Most are controlled by cranial nerve III (oculomotor) except for lateral rectus (CN VI) and superior oblique (CN IV).

Rectus Muscles of the Eye

Oblique Muscles of the Eye

Internal Structures of the Eyeball

• Layers/Tunics:

  • Fibrous Layer: Outer layer (sclera + cornea).

  • Vascular Layer: Middle layer (choroid, ciliary body, iris).

  • Sensory Layer: Inner layer (retina).

• Humors: Fluids (aqueous humor and vitreous humor) fill the interior of the eyeball.

• Lens: Divides the eye into anterior and posterior segments.

Anatomy of the Eye (Sagittal Section)

• Ciliary Body: Focuses the lens.

• Iris: Regulates light entering the eye.

• Pupil: Opening in the iris.

• Retina: Contains photoreceptors for light detection (rods and cones).

• Fovea Centralis: Area of sharpest vision located lateral to the optic disc.

Fibrous Layer: Sclera and Cornea

• Sclera:

  • Whitish, connective tissue layer, known as "the white of the eye."

• Cornea:

  • Transparent, central anterior part; allows light entry.

  • Easily repairs itself. Only human tissue transplantable without rejection risk.

Vascular Layer

• Choroid:

  • Nutrient-rich layer containing pigment to prevent light scattering.

  • Anteriorly modified into:

    • Ciliary Body: Attaches to the lens via ciliary zonule.

    • Iris: Controls light entry, providing color.

Sensory Layer: Retina

• Layers of Retina:

  • Outer Pigmented Layer: Absorbs light.

  • Inner Neural Layer: Contains photoreceptors:

    • Rods: Detect dim light, peripheral vision.

    • Cones: Provide detailed color vision; densest in the fovea centralis.

Light Pathway and Vision Physiology

• Light Refraction:

  • Light is bent by the cornea, aqueous humor, lens, and vitreous humor.

• Accommodation: The lens adjusts shape to focus on nearby objects.

• Image Formation on Retina:

  • Real images are reversed, upside down, and smaller than the object.

Visual Pathways to the Brain

• Optic Nerve: Carries impulses from the retina.

• Optic Chiasma: Where optic nerve fibers cross.

• Optic Tracts: Pathways to the thalamus.

• Visual Cortex in Occipital Lobe: Where image interpretation occurs.

Visual Field and Binocular Vision

• Each eye has a slightly different view leading to depth perception and three-dimensional vision.

Common Eye Conditions

• Emmetropia: Normal vision where focal point is on the retina.

• Myopia (Nearsightedness): Occurs when focal point is in front of the retina; results from an elongated eyeball.

• Hyperopia (Farsightedness): Objects focus behind the retina; can occur from a short eyeball or lazy lens.

• Astigmatism: Blurred vision resulting from uneven cornea/lens curvatures.

Eye Reflexes

• Convergence: Medial eye movement when focusing on close objects.

• Photopupillary Reflex: Pupil constriction in response to bright light.

• Accommodation Pupillary Reflex: Pupils constrict for close viewing.