Gustation and Olfaction

• Special senses covered include gustation (taste), olfaction (smell), and vision.

Chemical Senses and Chemoreceptors

• Gustation (taste) and olfaction (smell) are chemical senses.
• Chemoreceptors respond to aqueous chemicals: taste to saliva-dissolved substances, smell to nasal membrane fluids.

Taste Buds

• Approximately 10,000 taste buds are primarily located on the tongue.
• Taste buds reside within the papillae of the tongue mucosa.

Basic Taste Sensations

• The five basic human taste sensations that can be tasted are sweet, salt, sour, bitter, and umami.
• Sweet tastes are elicited by sugars, saccharin, alcohol, and specific amino acids
• Salt tastes are elicited by metal ions
• Sour tastes are elicited by hydrogen ions
• Bitter tastes are elicited by alkaloids like quinine and nicotine
• Umami tastes are elicited by the amino acid glutamate

Taste Physiology

• For a chemical to be tasted, it must dissolve in saliva and contact gustatory hairs.
• Binding of a food chemical depolarizes the taste cell membrane, releasing neurotransmitters.
• This process then initiates a generator potential, leading to an action potential.

Taste Transduction

• Taste stimulus energy is converted into nerve impulses.
• Sodium ion (Na+) influx is responsible for salty tastes.
• Hydrogen ions (H+) mediate sour tastes by entering cells, opening cation channels, or blocking potassium ion (K+) channels.

Gustatory Pathway

• The gustatory pathway travels to the gustatory cortex

Other Influences on Taste

• Taste relies on smell for 80% of its sensation.
• Thermoreceptors, mechanoreceptors, and nociceptors also influence taste perception.
• Temperature and texture either improves or deters from the sensation of taste.

Sense of Smell: Olfactory Epithelium

• The olfactory epithelium, located on the superior nasal concha, is the organ responsible for smell.
• Olfactory receptor cells are bipolar neurons featuring radiating olfactory cilia.
• Supporting cells surround and cushion the olfactory receptors.

Physiology of Smell

• Olfactory receptors are responsive to many different odor-causing chemicals.
• Ligand-bound proteins trigger a G protein mechanism utilizing cAMP as a secondary messenger.
• cAMP activation causes sodium (Na+) and calcium (Ca2+) channels to open, leading to membrane depolarization and an ensuing action potential

Eye and Associated Structures

• The eye contains 70% of all sensory receptors in the human body.
• The eye is largely protected by a cushion of fat and the bony orbit.
• Accessory structures include eyebrows, eyelids, conjunctiva, lacrimal apparatus, and extrinsic eye muscles.

Sensory Tunic: Retina Layers

• The retina has a two-layered membrane structure.
• The outer pigmented layer absorbs light, preventing its scatter.
• The neural layer contains photoreceptors, bipolar cells, ganglion cells, amacrine, and horizontal cells.
• Photoreceptors transduce light energy

Ganglion Cells and the Optic Disc

• Ganglion cell axons run along the inner retinal surface and exit the eye as the optic nerve.
• The optic disc is the site where the optic nerve exits, creating a blind spot due to the absence of photoreceptors.

Retina Photoreceptors: Rods

• Rod photoreceptors in the retina respond to dim light and facilitate peripheral vision.
• Cone photoreceptors in the retina respond to bright light.

Retina Photoreceptors: Cones

• Cones enable high-acuity color vision.
• Cones are highly concentrated in the fovea centralis.