Sense Organs

A sense organ is a specialized structure composed of cells or tissues that detect, interpret, and transmit stimuli to the central nervous system. Sensory receptors are classified based on the type of stimulus they detect:

• Mechanoreceptors: Detect mechanical changes.
• Thermoreceptors: Respond to temperature changes.
• Chemoreceptors: React to chemical stimuli.
• Photoreceptors: Detect light.

The five main sense organs in mammals are:

1. Eyes: Enable vision by detecting light.
2. Skin: Senses touch, pressure, pain, and temperature.
3. Ears: Detect sound and assist with balance.
4. Tongue: Contains taste buds for identifying flavors.
5. Nose: Detects smells using specialized receptors.

The Eye

The eye is the organ of vision and is roughly spherical in shape. It is protected by external structures, including:

• Eye sockets: Provide structural protection.
• Eyelids and eyelashes: Block dust and particles.
• Lacrimal glands: Produce tears to clean and moisten the surface.
• Conjunctiva: A thin membrane covering the eye's surface for added protection.

Additional protective features include:

• Eyebrows: Prevent sweat from dripping into the eyes.
• Eyelashes: Shield against dust and particles.
• Tears: Contain antibacterial enzymes and keep the eyes moist.

Parts of the Eye

1. Sclera: A tough, white outer layer that protects and maintains the eye's shape. At the front, it transitions into the transparent cornea, which allows light to enter and focuses it on the retina.
2. Choroid: A pigmented, vascular layer that absorbs light to prevent reflection and supplies nutrients. Key components include:
• Iris: Regulates the size of the pupil to control light entry.
• Pupil: The central opening that adjusts for light entry.
• Ciliary muscles: Change the lens's shape to focus on objects at different distances.
• Suspensory ligaments: Secure the lens in position.
• Lens: A transparent, biconvex structure that refracts light onto the retina.
3. Retina: A light-sensitive layer where images are formed. It contains:
• Cones: Detect color and function in bright light, containing the pigment iodopsin.
• Rods: Function in dim light and detect black-and-white, containing the pigment rhodopsin (vitamin A derivative).
4. Fovea: The most sensitive part of the retina for sharp vision.
5. Blind Spot: The point where the optic nerve exits the eye, devoid of photoreceptors.
6. Optic Nerve: Transmits visual signals to the brain for interpretation.
7. Aqueous Humor: A watery fluid between the cornea and lens that maintains shape and refracts light.
8. Vitreous Humor: A gel-like substance that fills the eye and supports its structure.

Mechanism of Vision

Light rays enter through the cornea, which bends them toward the aqueous humor, pupil, and lens. The lens further refracts the rays, focusing them on the retina, particularly the fovea. Photoreceptors in the retina convert the light into nerve impulses, which travel via the optic nerve to the brain for processing.

Accommodation

Accommodation allows the eye to focus on objects at varying distances by adjusting the lens's shape:

For Near Objects:

• Ciliary muscles contract, loosening suspensory ligaments.
• The lens becomes more convex, shortening the focal length.

For Distant Objects:

• Ciliary muscles relax, tightening suspensory ligaments.
• The lens flattens, lengthening the focal length.

Eye Defects

1. Myopia (Nearsightedness): Distant objects appear blurry due to light focusing in front of the retina. Corrected with concave lenses.

2. Hypermetropia (Farsightedness): Nearby objects appear blurry as light focuses behind the retina. Corrected with convex lenses.

3. Presbyopia: Age-related loss of lens elasticity, corrected with bifocal or reading glasses.
4. Astigmatism: Uneven cornea curvature causes blurry vision. Corrected with cylindrical lenses.
5. Cataracts: Clouding of the lens, often treated with surgery.
6. Color Blindness: Inherited inability to distinguish certain colors due to missing or defective cones.

Skin

The skin acts as a general sense organ, detecting touch, pressure, pain, and temperature through sensory receptors located throughout the body.

• Touch receptors: Detect light pressure.
• Pain receptors: Sense injury or damage.
• Pressure receptors: Respond to heavy pressure.
• Temperature receptors: Detect heat and cold.

The Ear

The ear is responsible for hearing and balance. It has three main sections:

1. Outer Ear: Includes the pinna, auditory canal, and eardrum.
2. Middle Ear: Contains ossicles (malleus, incus, stapes) and the Eustachian tube.
3. Inner Ear: Includes the cochlea and semicircular canals.

Mechanism of Hearing

Sound waves enter through the auditory canal, vibrate the eardrum, and are amplified by the ossicles. The cochlea converts these vibrations into nerve impulses, which are sent to the brain for interpretation.

Balance

The semicircular canals detect head movement through fluid motion, which stimulates sensory cells. Nerve impulses are sent to the brain to coordinate balance.

The Tongue

The tongue contains taste buds that detect four primary tastes: sweet, sour, salty, and bitter. Taste chemicals dissolve in saliva, activating sensory cells that send signals to the brain.

The Nose

The nose detects smells using olfactory receptors that respond to airborne chemical particles. Signals are transmitted to the brain for interpretation.