Sensory Systems

Sensory Receptors and Organs

• Sensory organs contain sensory receptors (dendrites that react to stimuli).

• Five major types of sensory receptors:

  1. Mechanoreceptors – Touch

  2. Thermoreceptors – Temperature variations

  3. Pain Receptors (Nociceptors) – Pain detection

  4. Chemoreceptors – Chemical detection

  5. Photoreceptors – Light detection

• Sensory receptors are distributed throughout the body but are more concentrated in some areas.

General vs. Special Senses

• General Senses: Touch (includes multiple receptors spread across the body).

• Special Senses: Sight, hearing, smell, taste, and balance (receptors are clustered in specific organs).

The Eye and Vision

External Anatomy of the Eye

• Eyelid – Protects the eye

• Eyelashes – Trap debris

• Conjunctiva – Secretes mucus to lubricate the eye

• Lacrimal Apparatus – Produces tears that contain mucus, antibodies, and lysozyme (antibacterial enzyme)

Internal Anatomy of the Eye

• Three Layers of the Eye:

  1. Sclera – White, fibrous outer layer

  2. Choroid – Pigmented, vascular layer including the iris and pupil

  3. Retina – Contains photoreceptors (rods and cones)

How Light Enters the Eye

• Light passes through the cornea, pupil, and lens before reaching the retina.

• Iris adjusts pupil size to control light entry.

• Lens focuses light onto the retina.

Photoreceptors and Image Processing

• Rods – Low-light vision, highly sensitive

• Cones – Color detection, concentrated in the fovea centralis (sharpest vision point)

• The optic nerve transmits signals to the brain.

• Blind Spot – Where the optic nerve exits, lacking photoreceptors.

• Vision Problems:

  • Myopia (nearsightedness) – Eye too long

  • Hyperopia (farsightedness) – Eye too short

  • Corrective lenses adjust light refraction.

The Ear: Hearing and Balance

Structures of the Ear

1. Outer Ear – Auricle (pinna), external auditory canal

2. Middle Ear – Tympanic membrane (eardrum), ossicles (malleus, incus, stapes)

3. Inner Ear – Cochlea (for hearing), semicircular canals (for balance)

Hearing Process

1. Sound waves enter the ear and vibrate the tympanic membrane.

2. Vibrations pass through the ossicles (malleus, incus, stapes).

3. Cochlea transforms mechanical vibrations into nerve impulses using the Organ of Corti.

4. The auditory nerve sends signals to the brain for interpretation.

Balance and Equilibrium

• Static Equilibrium: Maculae in the cochlea help maintain head position at rest.

• Dynamic Equilibrium: Semicircular canals detect motion via fluid movement and send signals to the vestibular nerve.

Smell and Taste

Smell (Olfaction)

• Olfactory receptors in the upper nasal cavity detect chemical molecules.

• Continuous exposure to a smell leads to olfactory accommodation (reduced sensitivity over time).

Taste (Gustation)

• Taste buds contain chemoreceptors known as gustatory cells.

• Types of Papillae:

  • Circumvallate Papillae

  • Filiform & Fungiform Papillae

  • Foliate Papillae

• Gustatory cells transmit signals to the brain via three facial nerves.

Key Takeaways

• Sensory receptors detect stimuli such as touch, temperature, pain, chemicals, and light.

• The eye contains photoreceptors that allow vision, and the lens adjusts to focus light.

• The ear processes sound through vibrations and maintains balance using fluid-filled structures.

• Smell and taste are detected by chemoreceptors, with olfactory accommodation reducing sensitivity over time.