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The Neuron

• Definition: A neuron is a cell in the body that transmits information through electrical (nerve impulse) and chemical signals.

• Characteristics of Neurons:

  • All neurons are electrically excitable, displaying voltage gradients across their membranes.

  • These voltage gradients are controlled by ion channels.

The Glial Cells

• Definition: Glial cells are the most abundant cells in the brain.

• Functions:

  • Traditionally regarded as a "support system" for neurons.

  • Recent understanding identifies them as active participants in CNS signaling, particularly through a process known as gliotransmission.

Brain Reference Planes

• Slices of the brain categorized by reference planes:

  • Sagittal Plane: Also known as the y-z plane.

  • Transverse Plane: Also referred to as the axial or x-y plane.

  • Coronal Plane: Known as the x-z plane.

  • Spatial orientations:

  • Lateral: Toward the side.

  • Medial: Toward the middle.

  • Caudal / Posterior: Toward the back.

  • Dorsal / Superior: Toward the top.

  • Ventral / Inferior: Toward the bottom.

  • Rostral / Anterior: Toward the front.

Why Do We Taste?

• Function of Taste: Acts as a chemical gatekeeper, providing an assessment of the nutritional value or potential toxic consequences of food.

• Nature of Taste:

  • Hardwired biological response but remains plastic, indicating the capability for learning and experience adaptation.

Drug vs. Test Taste Reactivity

• Concept: The biologic/physiological response to orosensory flavor can be altered through experience.

• Example of Conditioned Taste Aversion (CTA):

  • Conditioning allows an organism to learn to reject specific tastes due to negative experiences (e.g., if a sweet flavor is associated with nausea from the drug lithium chloride).

Taste vs. Flavor

• Flavor Definition: The sensory impression of a substance as determined by taste, smell, texture, and temperature, potentially influenced by visual input; can be thought of as a Gestalt.

• Taste Definition: The sensation produced when a substance reacts chemically with taste buds in the mouth (often referred to as orosensory).

The Textbook Overgeneralization

• Misconceptions about taste categories:

  • Bitter is often overly generalized without nuance.

  • Many sour taste distinctions are common.

  • Sweetness is primarily perceived at the front of the tongue, while salty may also be detected there.

The Taste System (Gustatory)

• Taste Receptor Cells (Gustatory Cells):

  • Express receptors, which are protein molecules responding to specific molecules (ligands) that bind to them, potentially altering cellular reactions.

  • Components include:

  • Cell Membrane Receptors: Membrane or surface receptors that regulate ion flow.

  • Nuclear Receptors: Located in the cell nucleus.

  • Ion Channels: Protein channels that form pores to regulate ion flow (

  • Ions: $Na^+$, $K^+$, $Ca^{2+}$, $Cl^-$) based on the electrochemical gradient.

Taste Buds and Papillae

• Definition of Taste Buds: Goblet-like structures composed of approximately 50 taste and supporting cells.

• Afferent Nerve Terminals: Sensory neurons in the cranial nerve ganglia that convey taste signals.

Types of Papillae

• Fungiform Papilla: Located at the front of the tongue; mushroom-shaped, responds to sweet, salty, umami, and bitter tastes.

• Foliate Papilla: Located on the posterior lateral sides; responds to sour and sweet tastes.

• Circumvallate (Vallate) Papilla: Located on the posterior medial side; responds to bitter, sour, and sweet tastes.

• Filiform Papilla: Do not contain taste buds; provide sensation of touch.

Basic Categories of Taste

1. Sweet:

   • Receptors: T1R2+3 (heterodimer), T1R3 (homodimer).

   • Also known as TASRs.

2. Bitter:

   • Receptors: T2Rs (approximately 25 types).

   • Most sensitive taste; examples include coffee, beer, olives, and quinine.

3. Umami:

   • Receptors: T1R1+3; responds to monosodium glutamate (MSG).

   • Often referred to as "protein taste."

4. Salt:

   • Mechanism: $Na^+$ ions enter cells through specific ion channels, present throughout the tongue.

5. Sour:

   • Detects proton ions (hydronium ions) from acids through specialized ion channels.

   • Notably mediated by the Otop1 gene which encodes the ion channel.

6. Fat (Potential 6th Taste):

   • Taste vs. mechanosensory; receptors like GPR120 and CD36 respond to fatty acids.

Oral Sensation: Pathways to the Brain

• Afferent Nerves involved in oral sensory cues:

  • Chorda Tympani (VII): Carries signals from the anterior tongue (front and middle).

  • Trigeminal Nerve (V): Carries tactile and pain signals from the anterior tongue.

  • Glossopharyngeal Nerve (IX): Responsible for mid and posterior taste, particularly bitter and sour.

  • Vagus Nerve (X): Involves the epiglottis and related structures.

Brain Involvement in Taste Processing

• Nucleus Tractus Solitarius (NTS):

  • Located in the medulla oblongata; receives sensory information from orosensory, gastrointestinal, and cardiovascular domains.

• Rostral NTS: Considered the "gustatory nucleus"; first brain region to receive taste-related information.

Taste Pathways in the Brain

• Taste-related information relay:

  • From the NTS, signals relay through the ventral posteromedial thalamus to the insula (gustatory cortex/insular cortex).

• Noteworthy Species Differences:

  • In rodents, an additional relay occurs through the parabrachial nucleus (PBN) in the pons.

Integration in the Brain

• From the insular cortex, taste-related information connects to the orbitofrontal cortex (OFC), part of the prefrontal cortex.

• The OFC modulates taste-evoked signals based on motivational states (hunger, satiation) and integrates different sensory modalities (taste, olfaction, visual, and memory) for flavor representation.

Individual Differences in Taste

• Supertasters vs. Tasters:

  • Nontasters: (~25% of the population).

  • Medium Tasters: (~50% of the population).

  • Supertasters: (~25% of the population); detect intense unpleasant tastes, especially from compounds like PROP (6-n-propylthioracil).

PROP Tasting and Genetics

• PROP Description: Used to treat hyperthyroidism and binds to T2Rs.

• Genetic Variance: Individual differences in PROP tasting related to the TAS2R38 receptor gene.

  • Supertasters experience heightened bitterness from PROP compared to others.

The Supertaster Phenomenon

• Benefits:

  • Reduced obesity risk; tend to avoid high-fat and sugary foods.

• Challenges:

  • Increased likelihood of avoiding bitter compounds; lower preferences for specific alcoholic beverages, cigarettes, and coffee.

• Health Implications:

  • Supertasters may be at higher risk for certain cancers (e.g., breast, colon) due to lower vegetable intake, leading to a non-diverse diet.