Taste and Smell - Comprehensive Notes

Adaptation and Survival: Sensory systems enable us to detect environmental changes necessary for adaptation and survival.
Sensory Systems Covered:
- Chemical senses (Chemoreceptors)
- Light sensation (Photoreceptors)
- Sound sensation (Stretch receptors)
- Balance sensation (Mechanoreceptors and Proprioceptors)
- Touch (Nociceptors and Stretch receptors)
- Pain sensation (Nociceptors)
- Temperature sensation (Thermoreceptors)

Definition: Chemically sensitive cells that sense changes in chemicals in our environment. They are distributed throughout the body.
Taste Chemoreceptors: Located on the tongue.
Smell Chemoreceptors: Located in the nasal cavity.

Nervous System Job:
- Collect information (input) - sensory receptors.
- Integrate this information (integration/processing).
- Distribute this information (output).
All sensory receptors are connected to the CNS (Central Nervous System) by sensory neurons.

Gustation Definition: Detection of dissolved chemicals by receptor cells mostly located on the tongue.
Taste Buds: Sensory organs for taste.
Gustatory Epithelial Cells: Each taste bud contains 50-100 gustatory epithelial cells (chemoreceptors) that sense chemicals.

Location: Most taste buds are found in papillae (peg-like projections on the tongue).
Types of Papillae:
- Fungiform Papillae: Scattered over the tongue surface (1-5 taste buds each).
- Vallate Papillae: 8-12 large papillae at the back of the tongue (many taste buds).
- Foliate Papillae: Located on lateral areas of tongue (many taste buds, less with age).

Gustatory Hairs (Microvilli): Project from the ends of gustatory epithelial cells.
Taste Pore: Where food and beverage dissolved in saliva enter.
Gustatory Afferent Axons: Synapse with epithelial cells.
Basal Cells: Stem cells that eventually become taste bud cells.
Receptor Specificity: Most taste receptor cells respond almost exclusively to one type of molecule.

Mechanism:
- Chemical activation of a taste receptor cell changes the membrane potential (usually more positive).
- This results in a voltage shift called a receptor potential.
- If the receptor potential is strong enough, an action potential can occur.

Source: Typically from the taste of the cation $Na+$ .
Low Concentration: Detected by $Na+$ selective channels in epithelial cells.
High Concentration: Activates bitter and sour taste receptor cells (potassium and proton channels).

Source: $H+$ ions that dissociate from acids (low pH).
Mechanism:
- $H+$ seems to bind to and block $K+$ selective channels.
- It also seems to activate a special type of ion channel from a family of transient receptor potential channels (TRPs).
- Likely other mechanisms, but they are unknown.

Source: Elicited by alkaloids (quinine, nicotine, caffeine, morphine, and other compounds containing nitrogen).
Receptors: There are about 25 different types of bitter receptors in a family of receptors called T2Rs.
Function: These are “poison” receptors, and most cells express multiple types of T2Rs, so we can’t distinguish bitter taste types.

Source: Elicited by sugars, saccharin, alcohols, and some amino acids.
Receptor: Detected by the same receptor protein, a dimer of T1R GPCR proteins.
Distinction: We can’t distinguish sweet tastes well because of this unified receptor.

Source: Elicited by amino acids.
Receptor: Detected by a similar T1R dimer to the sweetness receptor.
Distinction: Different cells express the umami receptor than the sweet receptor, so we don’t get them confused.

Bitter: Elicited by alkaloids (quinine, nicotine, caffeine, morphine, and other compounds containing nitrogen).
Possible 6th modality related to lipids (still being studied).

Olfaction Definition: Sensation of volatile chemicals in inspired air by the olfactory epithelium.
Olfactory Epithelium: Pseudostratified columnar epithelium in the roof of the nasal cavity.

Components:
- Olfactory Receptor Cells: Sense chemicals with olfactory cilia.
- Supporting Cells: Provide nutrition, structural support, and mucous production.
- Basal Cells: Stem cells that differentiate into either neurons or supporting cells.

Structure: Bipolar neurons with one dendrite and one axon.
Dendrite Function: Has cilia that respond to odorants by inducing a receptor potential.
Signal Transmission: If the receptor potential is strong enough, it can result in an action potential that is transmitted along the axon.

Rodents: More than 1000 different odorant receptor genes.
Humans: About 350, which is why we are relatively poor smellers.
Receptor Type: Olfactory receptor proteins are GPCRs.
Second Messengers: Use second messengers, and there is increasing evidence that cAMP is the only one used by olfactory receptors.

Process:
1. Receptor is activated by odorant (volatile chemical in air).
2. Odorant receptor activates G-protein.
3. G-protein activates an enzyme called adenylate cyclase.
4. Adenylate cyclase converts ATP to cAMP.
5. cAMP opens a channel allowing $Ca^{2+}$ and $Na+$ into the cell, leading to depolarization of the membrane (graded potential).

Sensory information is transmitted by:
1. Olfactory receptor cells.
2. Olfactory nerve (CN I).
3. Olfactory tract.
4. Olfactory tubercle.
From the olfactory tubercle, it can be transmitted to:
- Olfactory orbitofrontal cortices (conscious perception and interpretation).
- Medial dorsal nucleus of the thalamus (sensory gating).
- Amygdala (emotion, sympathetic responses).
- Hippocampus (memory).

Sensory Map Definition: An arrangement of neurons that correlates with certain features of the environment.
Mapping Technique: Microelectrode recordings have been used to map the arrangement of receptor responses.
Olfactory Bulb: Neurons that respond to the same chemical are arranged spatially in groups.
Olfactory Cortex: A different subset of neurons activates different subsets of neurons. Each odor shows a different pattern of neuronal activation.