Lecture on Special Senses

Introduction to Special Senses

• There are five types of special senses:
  • Taste: Detected by chemoreceptors in the mouth.
  • Smell: Detected by chemoreceptors in the nose.
  • Vision: Detected by photoreceptors in the eyes.
  • Hearing: Detected by mechanoreceptors in the ear.
  • Balance: Also involves mechanoreceptors in the ear.
• Special senses have receptors located in the head region.

Types of Receptors

• Chemoreceptors: Respond to chemical stimuli; involved in taste and smell.
• Mechanoreceptors: Respond to mechanical stimuli like pressure; involved in hearing and balance.
• Photoreceptors: Respond to light stimuli; involved in vision.

Smell (Olfaction)

• Unique Pathway: Smell bypasses the thalamus and is directly transmitted to the cerebral cortex.
• Olfactory Epithelium: Sensory receptor organ located at the nasal cavity roof.
• Olfactory Cells:
  • Sensory Neurons: Bipolar neurons with dendrites & axons.
  • Supporting Cells: Maintain receptor cells.
  • Basal Cells: Stem cells that replace olfactory cells every 30-60 days.
• Olfactory Pathway:
  • Olfactory neurons synapse with mitral cells in the olfactory bulb.
  • Axons form the olfactory tract, transmitting information to the CNS.
  • Information processed in the olfactory cortex and limbic system (triggers emotions).

Taste (Gustation)

• Five Basic Tastes: Sweet, salty, sour, bitter, and umami (meaty flavor).
• Taste Buds: Located in papillae on the tongue; house gustatory cells.
• Gustatory Cells: Contain microvilli with chemoreceptors and are replaceable.
• Pathway:
  • Taste information is transmitted by cranial nerves (VII, IX, X) to the medulla.
  • Synapse with secondary neurons, continue to thalamus, then to insula and limbic system.
  • Insula processes taste perception; limbic system attaches emotions to tastes.

Vision

• Dominant Sense: 70% of body receptors are in the eyes; 40% of cerebral cortex processes visual information.
• Eye Structure:
  • Eyeball houses and protects photoreceptors.
  • Accessory structures (muscles, eyelids, eyelashes) protect and clean the eye.
• Photo-receptors: Do not regenerate; sensitive to UV light.

Eye Accessory Structures

• Eyebrows: Prevent sweat from entering eyes.
• Eyelids: Protective covering; made of tarsal plates, muscle, and skin.
  • Tarsal Plates: Framework of the eyelid.
  • Orbicularis Oculi: Muscle encircling the eye, innervated by CN VII.
  • Levator Palpebrae Superioris: Muscle lifting upper eyelid, innervated by CN III.
• Eyelashes: Prevent objects from entering the eye.
• Glands:
  • Tarsal Glands: Secrete oil to prevent eyelid sticking.
  • Ciliary Glands: Secrete lipids to slow evaporation.
  • Lacrimal Caruncle: Secrete oily white substance; trap dust.
• Conjunctiva:
  • Palpebral Conjunctiva: Covers internal eyelid.
  • Bulbar Conjunctiva: Covers sclera, not cornea; maintains transparency for light passage.