Chapter 3 – Eye and Vision

Structure of the Human Eye (Front)

The human eye is a complex organ responsible for vision, protected and maintained by several external and internal structures.

• Eyelids (upper and lower):

  • Protect the exposed surface of the eye from mechanical damage.

  • Prevent excessive light from entering the eye, protecting photoreceptors.

  • Blinking spreads tears over the eye to keep it moist and remove dust particles.

• Eyelashes: Shield the eye from dust particles.

• Tear glands:

  • Keep the exposed surface of the eye moist.

  • Wash away dust particles.

  • Lubricate the eye during blinking.

  • Maintain moisture for atmospheric oxygen to dissolve and diffuse into the cornea.

• Conjunctiva:

  • Thin, transparent layer covering the sclera.

  • Secretes mucus to keep the exposed surface moist.

Structure of the Human Eye (Interior)

The internal anatomy of the eye includes several fluid-filled chambers and specialized tissues that contribute to vision.

• Aqueous humour:

  • Clear, watery fluid between the cornea and the lens.

  • Maintains eye pressure and provides nutrients (amino acids, glucose, oxygen).

  • Transports metabolic waste away from the cornea.

• Vitreous humour:

  • Clear, jelly-like fluid between the lens and retina.

  • Thicker than aqueous humour.

  • Maintains the shape of the eye and prevents collapse.

  • Acts as a shock absorber.

  • Keeps the retina properly connected to the back of the eye.

• Cornea:

  • Colorless, clear, transparent structure at the front of the eye.

  • Provides the greatest refraction of light due to the difference in refractive index between air and cornea.

• Iris:

  • Contains melanin (genetically determined).

  • Amount of melanin determines eye color.

• Sclera (outermost layer):

  • Tough, fibrous layer composed of collagen fibers.

  • White and opaque; continuous with the cornea.

• Choroid (middle layer):

  • Rich in blood vessels, providing nutrients and oxygen to the sclera and retina.

  • Pigmented black (high melanin) to prevent internal reflection of light.

• Retina (innermost layer):

  • Light-sensitive layer where images are formed.

  • Contains several layers of cells, including photoreceptors.

Photoreceptors

Photoreceptors are specialized cells in the retina that detect light and initiate the process of vision.

• Types: Rods and Cones

• Location: Deep within the retina to prevent damage and receive nutrients from the choroid.

• Ratio: 1:1 ratio of cone to ganglion cells; many:1 ratio of rod to ganglion cells (rods are more sensitive to light).

• Structure:

  • Outer segment: Receives stimulus/light.

  • Inner segment: Contains cell organelles.

  • Synaptic terminal: Transmits chemical signals to bipolar cells, forming a synapse.

Comparison of Structure

Comparison of Type of Vision

Additional info: Rods have more disc layers, increasing their surface area for photopigment absorption, making them more sensitive to motion. Cones have more synaptic terminals, enabling finer visual processing (e.g., color).

Fovea (Yellow Spot)

The fovea is a small depression in the retina where visual acuity is highest.

• Contains a high density of cones and is responsible for sharp central vision.

• Located in the center of the macula region of the retina.

• Essential for activities requiring detailed vision, such as reading and driving.

Blind Spot (Optic Disc)

The blind spot is the region of the retina where the optic nerve exits the eye; it contains no photoreceptors.

• Appears as a depression or pit near the nose.

• Lack of photoreceptor cells means it cannot detect light, creating a natural blind spot in the visual field.

• The brain compensates for this gap by integrating information from the other eye.

Two Types of Vision

• Central vision: Processed by the fovea, responsible for sharp, detailed, and color vision.

• Peripheral vision: Processed by rods outside the fovea, responsible for detecting motion and vision in dim light.

Focusing and Accommodation

Focusing is the process by which the eye forms a clear image on the retina. Accommodation refers to the eye's ability to change focus from distant to near objects by altering the shape of the lens.

• Cornea: Provides the greatest refraction of light due to the change in medium (air to cornea).

• Lens: Fine-tunes focus by changing shape, controlled by ciliary muscles and suspensory ligaments.

• Accommodation for distant objects: Ciliary muscles relax, lens flattens.

• Accommodation for near objects: Ciliary muscles contract, lens becomes more rounded.

Example: Reading a book requires accommodation as the lens becomes more convex to focus on close text.

Common Eye Problems

• Myopia (nearsightedness): Difficulty seeing distant objects clearly.

• Hyperopia (farsightedness): Difficulty seeing near objects clearly.

• Presbyopia: Age-related loss of near focusing ability.

Colour Perception in Humans (RGB Cones)

Color vision is mediated by three types of cones, each sensitive to different wavelengths (red, green, blue).

• All cones absorb light at different wavelengths.

• Electrical signals are generated and sent to the brain.

• The brain integrates these signals to perceive color.

Example: The combination of signals from red, green, and blue cones allows humans to perceive a wide range of colors.

Summary Table: Presence of Photoreceptors