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Weeks 3: Sensory Transduction Part I

Sensory System Overview

  • Overview of Sensory Systems
       - Olfaction (sense of smell)
       - Phototransduction (vision)
       - Audition (hearing)
       - Gustation (taste)
       - Somatosensation (touch, pain, temperature)
          - Includes receptors like Mrgprs, TRP channels, and Piezo channels
       - Thirst Regulation

Experimental Design: Required/Sufficient Practices

  • Emphasis on understanding experimental design in sensory science

Important Dates and Reminders

  • Next Class: Focus on Diseases
  • Homework #10: Due 4/17
  • Unit 3 Group Project: Due 4/19 (group evaluation can be submitted 24 hours late without penalty)
  • Next Perusall Assignment: #12 is due 4/20 @ 3:30 pm

Review Questions

  • Identify correct statements:
      - A. Phosphorylation of potassium channels can induce synaptic plasticity/strengthening
      - B. The pathway: Serotonin → 5-HT → Ca2+ → CaM → CREB is plausible
      - C. NGF autocrine signaling stabilizes dendritic spines via RhoGTPases
      - D. Cdc42 and RhoA engage in crosstalk with nearby dendritic spines
      - E. During synaptic plasticity, spines grow larger exclusively through actin dynamics

Learning Objectives

  • Connect learning and memory to past concepts
  • Explain sensory information transduction and encoding
       - Compare labeled line coding vs population coding
       - Graded potentials vs action potentials
  • Define key terms:
      - Transduction
      - Receptor
      - Receptor Cell
      - First-order Neuron
      - PDE
      - Mrgpr, TRP channel, Piezo
      - Heterodimer, Heterotrimer
      - Vasopressin
  • Explain ionotropic and metabotropic mechanisms in sensory transduction
      - Discuss how similar pathways are reused in different sensory systems (e.g., CamKII, GPCRs)
  • Use various molecular techniques (cell culture, transfection, electroporation) for experimental design
  • Discuss confidence levels in experimental designs: in vitro, ex vivo, and in vivo

Sensory Signal Transduction Process

  • The process of Transduction converts environmental stimuli into electrical signals
  • First-order Neurons are specialized to transmit sensory information
       - Primary Sensory Neurons: initiate sensory processing in the Peripheral Nervous System
       - Signal processing begins in either receptor cells or first-order neurons
  • Graded potentials encode strength of stimulus; larger stimuli produce larger EPSPs

Major Sensory Modalities

  • Various receptors mediate distinct classes of sensory modalities
       - External stimuli: Touch, taste, sight, sound
       - Internal sensations: Hunger, thirst, temperature, blood pressure, bladder state

Sensory Encoding Mechanisms

Labeled Line vs Population Code

  • Labeled Line Coding
       - A specific neuron or receptor encodes a single sensory quality
  • Population Coding
       - Sensation is determined by the combination of activated neurons
       - Enables differentiation without multiple distinct receptor types
  • Example: Different neurons may respond to different aspects of a single olfactory stimulus

Sensory Receptors and Their Functions

  • Sensory systems utilize diverse receptors to process various stimuli

Olfactory System

  • More than 100,000 olfactory receptors can detect over 20,000 unique odors
  • Each odorant interacts with specific GPCRs on olfactory sensory neurons
      - ORs utilize cAMP as a secondary messenger
     ## Phototransduction
  • Rhodopsin is a GPCR involved in converting light to electrical signals
      - Undergoes a conformational change when struck by light, affecting cGMP levels
  • Hyperpolarization occurs, leading to the cessation of synaptic signaling

Auditory System

  • Tip Links: Filaments connecting stereocilia, crucial for auditory transduction
      1. Sound Waves cause bending of stereocilia
      2. Opening of mechanically gated ion channels due to tip link tension
      3. K+ and Ca2+ influx results in EPSP generation
      4. Glutamate is released following sufficient cell depolarization
  • Mutations in tip-link proteins lead to hearing loss

Types of Sensory Receptors

Gustation

  • Five primary taste receptors: Sweet, Bitter, Umami, Salty, Sour
  • Myth: Distinct areas on the tongue for taste types
  • Salty and Sour employ ionotropic channels; Bitter, Sweet, and Umami use metabotropic GPCR signaling

TRP Channels and Sensory Modulation

Somatosensation

  • TRP Channels: Non-selective cation channels involved in detecting temperature, pain, and other stimuli
  • Notable Types:
       - TRPM8: Senses coolness (agonist: Menthol)
       - TRPV1: Senses heat pain (agonist: Capsaicin)

Piezo Channels

  • Mechanoreceptors: (e.g., Meissner's, Merkel cells) that sense touch and pressure
  • Integral for detecting physical forces including stretch and vibration

Osmoreception and Thirst Regulation

  • Osmoreception: Detects osmolarity changes for bodily homeostasis
       - TRP and Piezo channels facilitate osmolarity detection
       - Higher extracellular osmolarity results in cell shrinkage, triggering signaling pathways that open TRP channels
  • Vasopressin (ADH): Hormone released by the hypothalamus to regulate water retention in kidneys
  • Angiotensin II: Hormone that initiates thirst behavior through GPCR signaling in the subfornical organ

Experimental Design in Sensory Neuroscience

Sufficient vs. Required

  • Sufficient: Examining if a single gene/protein can elicit a behavior independently
  • Required/Necessary: Determining if eliminating a gene/protein abolishes the function

Experimental Techniques

  • Cell Cultures: Grow cells outside organisms for experimental observations
       - Can be primary cultures or immortalized cell lines (e.g., HEK293)
  • Transfection: Delivers genes to cells to study protein function
  • Electroporation: Involves electrical pulses to aid in nucleic acid delivery

Types of Experiments

  • In Vitro: Conducted outside of a living organism
  • In Vivo: Conducted within a living organism
  • Ex Vivo: Involves tissues extracted for experimental purposes

Summary and Key Terms

Key Terms:

  • Chemoreceptor: Receptors that detect chemical stimuli
  • TrP Channels: Channels that respond to various stimuli across sensory modalities
  • Heterodimer/Heterotrimer: Protein complexes formed by distinct receptor subunits

Questions for Review:

  • How do different sensory signals engage their unique receptors?
  • Discuss the difference in sensory signaling mechanisms and adaptation processes.
  • How are sensory pathways interconnected and differentiated based on input types?

Study Strategies

  • Compare/contrast key terms and concepts from different sensory modalities, e.g., graded vs action potentials, labeled vs population coding, etc.
  • Ensure understanding of ionotropic vs metabotropic signaling pathways in relation to sensory transduction principles.