Gustatory and Olfaction Notes pt 1

Chapter 1: Gustatory and Olfaction

Glial Cells Supporting Neuronal Functions

  • Glia: Non-neuronal cells that support various functions of neurons.

    • Astrocytes:

    • Most numerous type of glia.

    • Fill spaces between neurons.

    • Influence neuronal growth and overall health.

    • Myelinating glia: Cells responsible for myelination of axons.

    • Oligodendroglia: Myelinate axons in the central nervous system (CNS).

    • Schwann cells: Myelinate axons in the peripheral nervous system (PNS).

    • Ependymal cells:

    • Line the ventricles of the brain and the central canal of the spinal cord.

    • Contribute to the regulation of cerebral blood flow and act as a cushion or buffer to protect the brain from impacts.

  • The three primary types of glia:

    1. Astrocytes.

    2. Oligodendroglia.

    3. Microglia.

Gustatory System

  1. Initiation

    • Begins with taste receptors located on the tongue.

  2. Transmission of Taste Information

    • Information is transmitted via cranial nerves:

      • Cranial Nerve VII: Facial nerve.

      • Cranial Nerve IX: Glossopharyngeal nerve.

      • Cranial Nerve X: Vagus nerve.

    • These nerves transmit taste info to the Nucleus of the Solitary Tract (NST) in the brainstem.

  3. Relay of Information

    • NST relays sensory information to the thalamus.

    • Thalamus sends signals to the insular and frontal gustatory cortices.

  4. Perception

    • Taste perception occurs in the frontal gustatory cortex.

  5. Emotional Connections

    • NST connects with the amygdala and hypothalamus, integrating emotional contexts and homeostatic aspects of taste.

  6. Tongue Anatomy

    • Taste receptors are randomly distributed on the tongue.

    • Papillae types:

      • Circumvallate.

      • Foliate.

      • Fungiform.

  7. Taste Bud Structure

    • Microvilli: Tiny, finger-like projections on the apical surface of taste cells; these projections extend into the taste pore and contain receptors that detect tastants (chemicals present in food).

    • Taste pore: A small opening at the top of the taste bud that allows microvilli of taste cells to interact with dissolved tastants.

    • Taste cells: Sensory receptor cells in taste buds detecting five taste modalities: sweet, sour, salty, bitter, and umami.

    • Basal cells: Supportive cells at the base of the taste bud that differentiate into new taste cells, aiding in regeneration.

Types of Taste Receptor Cells

  • Salty:

    • Detection involves sodium (Na⁺) activating amiloride-sensitive channels.

    • Entry of Na⁺ leads to depolarization and opens voltage-gated Na⁺ channels, ultimately releasing ATP as a neurotransmitter.

  • Sweet, Umami, and Bitter:

    • Compounds activate G-protein-coupled receptors, leading to phospholipase C (PLC) activation. PLC releases Ca²⁺ from intracellular stores, which triggers depolarization via TRPM5 channels allowing Na⁺ passage and the release of ATP.

  • Sour:

    • Produced through detection of protons (H⁺) binding to OTOP1 channels;

    • H⁺ influx causes depolarization and closes K⁺ channels, leading to further depolarization due to K⁺ accumulation inside the cell.

    • Influx of Ca²⁺ triggers serotonin release, which binds to afferent nerve fibers.

Afferent Axon Pathway for Taste

  • Taste afferents send information to the brain via labeled lines, where each sensory neuron transmits info about a specific taste type.

  • Taste receptor cells mainly express receptors for a single taste, whereas bitter and sour are broadly tuned.

  • Taste receptors converge onto primary taste axons that relay information to the gustatory nucleus.

Coding in the Taste System

Labeled Lines

  • This sensory coding principle implies specific sensory neurons dedicated to transmitting information about a particular taste.

Population Coding

  • This sensory coding strategy represents information by combined activity of multiple neurons, enabling nuanced perception of tastes.

Topographic, Distributed, and Temporal Coding

  • Unlike most sensory cortices which are organized topographically, the olfactory and gustatory cortices utilize distributed representations allowing for flexible coding.

  • Flavor involves the experience of food beyond basic taste modalities; it includes attributes such as creaminess and temperature.

Olfactory System

Neural Pathway of Olfactory Perception

  1. Initiation: Olfactory processing starts in the olfactory epithelium.

  2. Axonal Projection: Axons project to the olfactory bulb.

  3. Cortical Pathways: Two pathways emerge from the olfactory bulb:

    • Direct pathway to the piriform cortex.

    • Indirect pathway to the orbitofrontal cortex, routing via the thalamus.

  4. Connected Regions: The piriform cortex connects to the olfactory tubercle, amygdala, entorhinal cortex, and hippocampus. These regions project to thalamus, hypothalamus, and orbitofrontal cortex.

Olfactory Epithelium Structure

  • Olfactory Receptor Cells (ORC): Bipolar cells with dendritic processes forming olfactory cilia embedded within mucus.

  • Supporting cells.

  • Basal cells: Stem cells of the olfactory epithelium.

  • Bowman’s glands: Produce mucus covering the olfactory epithelium.

  • Lamina propria: Tissue layer between olfactory epithelium and cribriform plate.

  • Odorants: Evoke depolarizing currents when binding to cilia, indicating that each olfactory receptor neuron (ORN) expresses a single gene for receptors scattered across zones in the olfactory epithelium.

Signal Transduction

  • Olfactory signal transduction involves G-protein-coupled receptors (GPCRs). These activate G-proteins, stimulating adenylyl cyclase which produces cAMP, enabling influx of Na⁺/Ca²⁺ ions leading to depolarization.

  • Lateral inhibition: Enhances the signal-to-noise ratio in olfactory sensory pathways.

Accessory Olfactory Bulb

  • Vomeronasal organ (VNO): Processes pheromones important for behaviors such as mating and predator recognition, sending projections to the medial amygdala and other areas for emotional regulation.

Chapter 2: Vision

Overview of Human Vision

  • The human eye detects wavelengths between 380 and 750 nanometers (visible light).

Components of the Eye

  • Pupil: Circular aperture controlling light entry to the eye.

  • Iris: Colored structure surrounding the pupil, regulating its size.

  • Aqueous Humor: Fluid between the cornea and lens, nourishing and maintaining intraocular pressure.

  • Cornea: Transparent outer layer focusing incoming light (70% of focusing).

  • Lens: Flexible structure behind the iris, focusing light onto the retina (30% of focusing).

  • Retina: Light-sensitive layer at the back of the eye containing photoreceptors.

  • Sclera: Tough outer covering providing protection and structure.

  • Choroid: Vascular layer providing oxygen and nutrients.

  • Optic Nerve: Transmits visual information to the brain.

  • Fovea: Area of highest visual acuity, containing a high density of cones.

  • Optic Disc: Point of exit for optic nerve fibers, creating a natural blind spot in vision.

Resolution vs Sensitivity

  • Resolution: Ability to distinguish fine details; higher resolution results in sharper images.

  • Sensitivity: Ability to detect low levels of light; systems with high sensitivity function better in dim conditions.

  • An example of sensitivity vs resolution involves pupil dilation, affecting the clarity of the visual field.

Effects of Excess Light

  • Melanin: Absorbs excess light in choroid and iris to protect tissues from UV damage.

  • Adaptations for light sensitivity in animals (e.g., cats) feature additional rod cells, larger pupils, and a reflective layer behind the retina.

Retinal Structural Support

  • Retinal Pigment Epithelium (RPE): Supports photoreceptors, recycles visual pigments, and maintains blood-retina barrier.

Photoreceptors

  • Cones: Responsible for color vision, functioning optimally in bright light.

  • Rods: Highly sensitive to light; enable low-light vision but do not detect color.

  • Bipolar Cells: Intermediate neurons transmitting signals from photoreceptors to ganglion cells.

  • Ganglion Cells: Neurons whose axons form the optic nerve, sending information to the brain.

  • Horizontal Cells: Integrate input from multiple photoreceptors.

  • Amacrine Cells: Modulate bipolar and ganglion activity, crucial for motion detection.

  • Light travels through layers before reaching photoreceptors; this backward layout limits resolution.

Foveal Properties

  • The fovea has only cone photoreceptors, allowing for sharp, colored vision with minimal scattering due to its structure.

Visual Transduction

  1. Dark Adaptation Processes: Pupil dilation and restoration of cGMP levels improve sensitivity in low-light conditions.

  2. Phototransduction Cascade: Light converts retinal in rhodopsin (visual pigment) to its active form, reducing cGMP levels, closing Na⁺/Ca²⁺ channels, and inducing hyperpolarization.

  3. Light Adaptation Mechanism: Light exposure reduces intracellular Ca²⁺ levels which activates guanylate cyclase, resetting conditions for continued light perception.

Theories of Color Vision

Trichromatic Theory (Young–Helmholtz Theory)

  • Humans perceive color with three types of cone photoreceptors:

    • S-cones: Sensitive to short wavelengths (blue).

    • M-cones: Sensitive to medium wavelengths (green).

    • L-cones: Sensitive to long wavelengths (red).

  • Color perception arises from comparative activation levels of these cones.

Opponent-Process Theory (Ewald Hering’s Theory)

  • Represents color information in opposing pairs:

    • Red vs. Green

    • Blue vs. Yellow

    • Black vs. White (light/dark)

  • Explains why combinations like reddish-green cannot be perceived due to opposing activation.

    • Evidence: Afterimages and color contrast effects.

ON- and OFF-Center Bipolar Cells

In Darkness:

  1. Photoreceptor releases GLU.

  2. OFF-center bipolar cells activated by AMPA-type receptors, causing depolarization and GLU release to ganglion cells.

  3. ON-center bipolar cells become hyperpolarized due to mGLU-Rs (metabotropic GLU receptors).

In Light:

  1. Photoreceptor stops releasing GLU.

  2. OFF-center bipolar cells return to resting (hyperpolarized); no GLU released.

  3. ON-center bipolar cells depolarize and activate ganglion cells.

Visual Processing in the V1 Cortex

  • The visual system organizes functionally into columns, maintaining consistency within receptive fields as electrodes are penetrated.

  • Cells exhibit varying complexity in receptive fields, aiding in detailed motion detection and visual perception.

  • Different types include:

    • Midget Ganglion Cells: High spatial resolution; essential for color discrimination.

    • Parasol Ganglion Cells: Sensitive to motion and contrast changes; lower spatial resolution.

    • Small Bistratified Ganglion Cells: Intermediate properties; encode blue-yellow color opponency.

    • IpRGCs: Non-image-forming vision functions, regulating circadian rhythms.

Summary of Visual Pathways

  • Maintain retinotopic mapping through the retina, LGN, and V1 cortex.

  • Retinal ganglion cell responses are preserved in the LGN, divided mainly into magno (M) and parvo (P) pathways, each tailored to specific visual functions like movement detection and color vision.

  • Cortical responses are organized to reflect orientation preferences, and this complexity contributes to robust visual processing.