Lecture 22

Definition: Gustation refers to the sense of taste, which involves the detection of chemicals present in food and drink via chemoreceptors located in the oral cavity.
Importance: This sense allows humans to evaluate the contents of the substances we consume and is enhanced by olfactory senses (smell).

Papillae and Taste Buds:
- Papillae on the tongue contain taste buds, functioning as sensory units.
- Taste buds are located on the anterior tongue surface and soft palate, housing gustatory cells, which are the taste receptor cells responsible for detecting tastants.
- Each taste bud has an onion-shaped structure that consists of various gustatory cells and supporting cells.

Foliate Papillae:
- Leaf-like structure located on the lateral boundaries of the tongue.
- Not well developed in humans; mainly functional during infancy.
Filiform Papillae:
- Short, thread-like projections primarily on the anterior two-thirds of the tongue.
- Do not contain taste buds; they detect texture and assist in food manipulation.
Circumvallate Papillae:
- Arranged in an inverted ‘V’ shape at the back of the tongue, around 10 to 12 present.
- They contain the greatest concentration of taste buds, surrounded by deep narrow depressions.
Fungiform Papillae:
- Mushroom-shaped structures scattered across the tongue, primarily on its anterior margin.
- Have about 300 taste buds.

Taste Pores:
- Open to the oral cavity, allowing interactions between dissolved tastants and gustatory cells.
Components of Taste Buds:
- Comprised of gustatory cells (taste receptor cells), basal cells (stem cells), and transitional cells (support cells).
- Nerve fibers are located between these cells, with a life span of approximately 10 days for taste receptor cells.

Composed of specialized neuroepithelium with dendritic endings extended as gustatory microvilli (taste hairs) through taste pores to contact saliva.
Types of Gustatory Cells:
- Type I: Respond to sodium ions (Na+) - associated with salty taste.
- Type II: Transducer cells for sweet, umami, and bitter tastes via G-protein coupled receptors (GPCRs).
- Type III: Responsive to sour stimuli, detecting hydrogen ions (H+).
- Type IV: Stem cells for generating new gustatory cells.

Gustatory cells interact with sensory neurons from the facial nerve (CN VII) and glossopharyngeal nerve (CN IX).
Innervation:
- CN VII: Anterior 2/3 of the tongue.
- CN IX: Posterior 1/3 of the tongue.
- CN X: Innervates the epiglottis/lower pharynx.
Axons project to the medulla oblongata, synapsing with secondary neurons that project to the thalamus, and subsequently to the cortical gustatory area, limbic system, and hypothalamus for emotional responses related to taste.

Salty:
- Triggered by sodium salts; sodium ions pass through specific ENaC channels located on Type I cells.
Sour:
- Caused by hydrogen ions (H+); blocks potassium channels on Type III cells, causing depolarization and neurotransmitter release.
Sweet:
- Triggered by specific configurations of glucose; involves GPCRs on Type II cells to transduce the signal.
Umami:
- Activated by amino acids like glutamate, also using GPCRs on Type II cells.
Bitter:
- Involves a diverse array of alkaloids, activating numerous GPCRs on Type II cells specific for bitter compounds.

G-Protein Coupled Receptors:
- Sweet, umami, and bitter receptors belong to this superfamily.
- Main receptors:
- Bitter: T2Rs.
- Sweet: T1R2 + T1R3.
- Umami: T1R1 + T1R3.
- Salty: ENaC (specific for sodium detection).
- Sour taste receptors identified through PKD2L1.

Taste perception is influenced by olfactory receptors; carbon dioxide and fat receptors may also play roles.
The common 'tongue map' myth originates from a misinterpretation of sensitivity data published in 1901.

Spherical Structure:
- Recursed into the orbit cushioned by orbital fat.
Composed of three principal layers:
1. Fibrous Tunic: Sclera and cornea.
2. Vascular Tunic: Iris, ciliary body, choroid.
3. Nervous Tunic: Retina and optic nerve.

Characteristics:
- Wavelength determines energy, amplitude defines intensity of light waves transmitted by photons.

Eyebrows: Prevent sweat from dripping into the eyes.
Eyelashes: Prevent foreign objects from reaching the eye surface.
Eyelids: Movable covering consisting of fibrous core, muscles, and conjunctiva.
- Tarsal Glands: Produce sebum to prevent overflow.

Specialized epithelium forming a continuous lining over the external eye and internal eyelids.
Contains goblet cells for lubrication and is highly vascularized for scleral nutrient supply.

Produces and drains tears, lubricating and cleansing the eye.
Components include the lacrimal gland, lacrimal puncta, lacrimal canaliculi, and the lacrimal sac draining into the nasal cavity.

Sclera: Protective white part of the eye.
Cornea: Transparent lens allowing light passage and refraction, supplied by lacrimal fluid and aqueous humor.

Highly vascularized with intrinsic muscles.
Choroid: Nutrient supply to the retina.
Ciliary Body: Contains ciliary muscles that adjust lens shape for accommodation.
Iris: Regulates light entry through pupil via two muscle groups (sphincter and dilator pupillae).

Layers:
- Pigmented Layer: Provides Vitamin A and absorbs excess light.
- Neural Layer: Contains photoreceptors, bipolar cells, and ganglion cells.

Photoreceptors: Rods for dim light (high sensitivity) and cones for bright light and color vision (lower sensitivity).
Ganglion Cells: Convey signals to the CNS through optic nerve.

Controls light transduction with opsin and retinal. - Types:
- Rhodopsin in rods and various photopsins in cones.

Depends on the stimulation ratio of cone types; examples illustrate how specific wavelengths are perceived.

Define areas where photonic inputs affect ganglion cells’ responses; variations affect acuity.

Determines acuity based on the number of converging photoreceptor cells on ganglionic cells.

Adjustment to low light levels after high light exposure; involves rods and cones with varying adaptation speeds, leading to photopic and scotopic vision types.