Chapter 17: The Special Senses – Olfaction, Gustation, and Vision

Chapter 17: The Special Senses

Introduction to the Special Senses

The special senses are specialized sensory systems that provide information about the external environment and internal body status. These include olfaction (smell), gustation (taste), vision, equilibrium (balance), and hearing. Each sense relies on distinct sensory organs and neural pathways for perception and discrimination.

• Olfaction: Detection of airborne chemicals (odors).

• Gustation: Detection of dissolved chemicals (tastes).

• Vision: Detection of light and color.

• Equilibrium: Sensing body position and movement.

• Hearing: Detection of sound waves.

17-1 Olfaction: The Sense of Smell

Olfactory Organs and Pathways

Olfaction is the sense of smell, mediated by specialized olfactory organs located in the nasal cavity on either side of the nasal septum. These organs contain the olfactory epithelium, which houses olfactory sensory neurons, supporting cells, and basal epithelial (stem) cells.

• Olfactory epithelium: Contains olfactory sensory neurons that detect dissolved chemicals.

• Supporting cells: Provide structural and metabolic support.

• Basal epithelial cells: Stem cells that replace worn-out olfactory neurons (process slows with age).

• Bowman's glands: Produce mucus to trap odorants and protect the epithelium.

Olfactory Pathway to the Cerebrum

Structure	Function
Olfactory epithelium	Detects odorants
Olfactory nerve fibers	Transmit signals through cribriform plate
Olfactory bulb	Initial processing of olfactory information
Olfactory tract	Relays information to the brain
Olfactory cortex	Perception of smell (direct, not via thalamus)

Olfactory Receptors and Signal Transduction

• Olfactory receptors are modified neurons with cilia-shaped dendrites that increase surface area for odor detection.

• Odorant molecules bind to receptor proteins, initiating an action potential.

• Olfactory information is unique in that it bypasses the thalamus and goes directly to the olfactory cortex.

Olfactory Discrimination

• Humans can distinguish thousands of chemical stimuli.

• Dogs have a much greater olfactory receptor surface area, making their sense of smell over 10,000 times more sensitive than humans.

• Olfactory receptors are frequently replaced, but the total number of neurons declines with age.

17-2 Gustation: The Sense of Taste

Gustatory Organs and Taste Buds

Gustation provides information about foods and liquids consumed. The sensory organs for taste are taste buds, which contain gustatory epithelial cells (taste receptor cells), basal epithelial cells (stem cells), and supporting cells.

• Taste buds: Located on the superior surface of the tongue, pharynx, and larynx; associated with lingual papillae.

• Gustatory epithelial cells: Extend microvilli (taste hairs) through the taste pore; survive about 10 days before replacement.

Types of Lingual Papillae

Papilla Type	Features
Filiform papillae	Provide friction; do not contain taste buds
Fungiform papillae	Contain about five taste buds each
Vallate papillae (circumvallate)	Contain as many as 100 taste buds each
Foliate papillae	Have taste buds

Taste buds are found on the surface of the circumvallate, foliate, and fungiform papillae.

Structure of Taste Buds

• Each taste bud is composed of taste cells joined near the apical surface with tight junctions.

• Microvilli extend into the taste pore to interact with dissolved chemicals.

• Basal cells replace taste cells every ~10 days.

Gustatory Discrimination

Type of Primary Taste	Molecule(s) that trigger this taste receptor
Sour	Presence of H+ in food
Salty	Presence of sodium in food
Sweet	Organic molecules with varied structures; includes natural and artificial sweeteners
Bitter	Nitrogen-containing compounds
Umami	Amino acids, especially glutamate (found in meats and parmesan cheese)

Gustatory Pathways

• Sensory nerve fibers transmit taste information to the gustatory cortex in the brain.

• Gustatory cortex is located in the insular region of the cerebrum.

17-3 Structures of the Eye

Accessory Structures of the Eye

The eye is protected and supported by several accessory structures, including the eyelids, superficial epithelium, and lacrimal apparatus.

• Eyelids: Continuation of skin; blinking lubricates and cleans the eye surface.

• Eyelashes: Prevent foreign material from reaching the eye.

• Lacrimal apparatus: Produces, distributes, and removes tears; secretions contain lysozyme (antibacterial enzyme).

Layers of the Eye

Layer	Main Structures	Functions
Fibrous Layer	Sclera, Cornea	Protection, muscle attachment, focusing
Vascular Layer (Uvea)	Iris, Ciliary body, Choroid	Blood supply, lens shape control, pigment
Inner Layer (Retina)	Pigmented layer, Neural layer	Light absorption, photoreception

Fibrous Layer

• Sclera: "White of the eye"; dense fibrous connective tissue covering most of the ocular surface.

• Cornea: Transparent anterior portion; continuous with the sclera.

Vascular Layer (Uvea)

• Iris: Pigmented anterior ring; controls pupil diameter.

• Ciliary body: Controls lens shape for focusing.

• Choroid: Vascular layer surrounding inner layers posterior to the ora serrata.

Inner Layer (Retina)

• Pigmented layer: Absorbs light passing through the neural layer.

• Neural layer: Contains supporting cells and neurons; outermost part contains photoreceptors (rods and cones).

Photoreceptors

• Rods: Highly sensitive to light; do not discriminate colors.

• Cones: Provide color vision; clustered in the fovea centralis (site of sharpest color vision).

• Visual axis: Line from an object to the fovea.

Histological Organization of the Retina

• Layers include nuclei of ganglion cells, rods and cones, and bipolar cells.

• Photoreceptors are closest to the choroid, farthest from the posterior cavity.

Summary Table: Key Structures and Functions of the Special Senses

Sense	Organ	Receptor Type	Pathway
Olfaction	Nasal cavity	Olfactory sensory neurons	Olfactory bulb → Olfactory tract → Olfactory cortex
Gustation	Tongue (taste buds)	Gustatory epithelial cells	Sensory nerve fibers → Gustatory cortex
Vision	Eye (retina)	Rods and cones	Optic nerve → Visual cortex

Example: The process of olfactory transduction involves odorant molecules binding to receptor proteins on olfactory cilia, generating an action potential that is transmitted directly to the olfactory cortex for perception.

Additional info: The study notes above expand on the original slides by providing definitions, structural details, and pathway summaries for each special sense covered in the provided materials.