Special Senses: Olfaction and Gustation

• Study notes for the special senses

General vs. Special Senses

• General sensory receptors are widely distributed throughout the body and handle tactile, temperature, and pain sensations.
• Special sensory receptors are located in complex sense organs in the head, dealing with vision, smell, taste, hearing, and equilibrium.

Chemical Senses

• Olfaction (smell) and gustation (taste) are the two chemical senses.
• Chemoreceptors are the receptors for these senses and detect chemicals dissolved in aqueous solutions.
• Airborne chemicals are dissolved in nasal membranes for smell.
• Food chemicals are dissolved in saliva for taste.

Olfaction (Smell)

• Odors are detected by chemoreceptors in the nasal cavity.
• Olfactory impulses are sent along olfactory tracts to the olfactory cortex located in the temporal lobe.

Gustation (Taste)

• Gustatory cells, basal cells, and supporting cells comprise taste receptors.
• Gustatory cells are taste receptor cells, with microvilli extending through a taste pore.
• Basal cells are stem cells that give rise to new gustatory cells every 10 days.
• Supporting cells provide structural support to the gustatory cells but are not involved in taste sensation.

Basic Taste Sensations

• The five basic taste sensations are:
• Sweet: sugars, alcohol, saccharin
• Sour: acids (hydrogen ions in solution)
• Salty: metal ions; sodium chloride tastes "saltiest"
• Bitter: alkaloids such as nicotine, caffeine, and nonalkaloids such as aspirin
• Umami: amino acids glutamate and aspartate, characteristic of "beef" taste and found in MSG and aging cheese

Taste Detection and Processing

• A tastant must dissolve in saliva, diffuse into a taste pore, and contact microvilli to be detected.
• Impulses are sent to the gustatory cortex in the parietal lobe and the limbic system.
• These regions determine our appreciation of what we are tasting.

Gustatory Cortex

• The gustatory cortex is located in the insular cortex and the frontal operculum of the brain.

Structures of the Eye and Their Functions

• Accessory structures protect and aid eye function and include:
• Eyebrows
• Eyelids
• Conjunctiva
• Lacrimal apparatus
• Extrinsic eye muscles
• The wall of the eyeball has three tunics:
• Fibrous tunic(outer layer): sclera, cornea
• Vascular tunic(middle layer): choroid, ciliary body, iris
• Nervous tunic layer(inner layer): retina

Key Structures to Identify and Label

• Retina
• Choroid
• Ciliary body
• Cornea
• Iris
• Optic nerve
• Optic disc (blind spot)
• Pupil
• Lens
• Anterior cavity
• Sclera
• Posterior cavity
• Eyelids
• Eyebrows
• Eyelashes
• Conjunctiva
• Lacrimal caruncle
• Lacrimal gland
• Tarsal glands
• Ciliary processes
• Ciliary muscles
• Ciliary zonule
• Posterior segment (vitreous humor)
• Anterior segment (aqueous humor)
• Extrinsic eye muscles

Additional Information on Structures

• Tarsal glands produce meibum, preventing evaporation of the eye's tear film.
• Lacrimal glands continuously produce and secrete tears that drain, cleaning, nourishing, and lubricating the eye's surface and are located on the lateral side of the orbit.

Extraocular Muscles

• Levator palpebrae superioris: Elevates the upper eyelid
• Superior rectus: Moves the eye upward
• Inferior rectus: Moves the eye downward
• Medial rectus: Moves the eye inward (adduction)
• Lateral rectus: Moves the eye outward (abduction)
• Superior oblique: Rotates the eye inward and downward
• Inferior oblique: Rotates the eye outward and upward

Macula Lutea

• The macula lutea is the center of the posterior portion of the retina and the visual axis of the eye.
• Macular degeneration causes damage to the macula, leading to loss of central vision but retention of peripheral vision.

Fovea Centralis

• The fovea centralis is a depression in the center of the macula lutea, containing the highest concentration of cones.
• The fovea centralis is the area of highest visual acuity.

Autonomic Nervous System Regulation of Pupil Diameter

• Bright light and close vision:
• Circular muscles (pupillary constrictor muscles) contract, causing the pupil to constrict.
• Dim light and distant vision:
• Radial muscles (pupillary dilator muscles) contract, causing the pupil to dilate.

Receptors for Vision

• The receptors for vision, rods and cones, are located in the retina.
• Rods are highly sensitive to light and crucial for peripheral and low-light vision (scotopic vision).
• Cones are responsible for color vision and high visual acuity (photopic vision) in bright light conditions.

Eye Detection of Light and Visual Pathway

• Photoreceptor cells (rods and cones) in the retina convert light into electrical signals.
• The electrical signals travel through the optic nerve to the brain for processing.

Refraction in Image Formation

• Refraction is the bending of light rays caused by a change in the speed of light as it passes from one transparent medium to another at an oblique angle.
• Lenses refract light because they are curved on both sides.

Convex Lenses

• Convex lenses are thicker in the center than at the edges and converge light rays.
• The image formed by a convex lens is inverted, upside down, and reversed from left to right.

Concave Lenses

• Concave lenses are thicker at the edges than in the center and diverge light rays.
• Concave lenses prevent light from focusing.

Accommodation

• The lens can adjust its curvature to allow for fine focusing.