Copy of The senses - the eye

Special Senses Overview

• Special senses include smell, taste, sight, hearing, and equilibrium.

• General senses include touch, temperature, pressure, and pain.

The Eye and Vision

• Sensory Receptors: Approximately 70% of all sensory receptors are located in the eyes.

• Nerve Fibers: Each eye contains over a million nerve fibers.

• Protection: The eye is mostly enclosed within a bony orbit, surrounded by a cushion of fat for protection.

Accessory Structures of the Eye

External Protective Structures

• Eyelids and Eyelashes: Protect the eye from debris and injury.

• Conjunctiva: A mucous membrane that lubricates the eyeball.

• Lacrimal Apparatus: Includes the lacrimal gland that releases a dilute salt solution onto the surface of the eye. It contains antibodies and lysozyme.

• Extrinsic Eye Muscles: Six muscles that attach to the outer surface of the eye, responsible for eye movements.

Glands Associated with Eyelids

• Tarsal (Meibomian) Glands: Sebaceous glands at the eyelid edges that produce an oily secretion to lubricate the eye. Infection of the oil duct results in a sty.

• Ciliary Glands: Located between eyelashes, lubricate the eye with sweat.

Conjunctivitis and Types

• Conjunctivitis: Inflammation of the conjunctiva, causing reddened and irritated eyes.

• Pink Eye: A contagious infection caused by bacteria or viruses characterized by redness and discharge.

• Allergic Conjunctivitis: Similar to viral conjunctivitis but accompanied by nasal congestion and sensitivity to light; not contagious.

Eye Anatomy

Structure of the Eye

• Sclera: Tough white connective tissue layer; visible as the white of the eye.

• Cornea: Transparent front portion; allows light entry and can repair itself.

• Aqueous Humor: Watery fluid in the anterior segment; maintains intraocular pressure and provides nutrients.

• Vitreous Humor: Gel-like substance in the posterior segment, helping maintain the eye's shape.

Vascular Layer

• Choroid: Blood-rich layer providing nutrients; modified into the ciliary body (smooth muscle for lens attachment) and iris (controls pupil size).

Sensory Layer: Retina

• Photoreceptors: Contains millions of photoreceptors: rods for dim light and peripheral vision; cones for color vision (blue, green, red).

• Fovea Centralis: Area of highest cone density; responsible for high-resolution vision.

• Optic Disc: The blind spot lacking photoreceptors where the optic nerve exits.

Vision Conditions

Common Vision Issues

• Cataracts: The lens becomes hard and opaque, leading to hazy vision; treatment involves surgical lens replacement.

• Glaucoma: Increased pressure in the eye due to improper drainage of aqueous humor, potentially causing blindness.

• Myopia (Nearsightedness): Distant objects appear blurred, corrected with concave lenses.

• Hyperopia (Farsightedness): Near objects are blurred; corrected with convex lenses.

• Astigmatism: Blurry images resulting from irregular corneal shape.

Eye Reflexes

• Convergence: Eyes aim toward nearby objects, controlled by extrinsic muscles.

• Photopupillary Reflex: Pupils constrict in bright light to protect photoreceptors.

• Accommodation Reflex: Pupils constrict when viewing close objects, controlled by iris muscles.