Sensory Systems: Lecture Notes

Review Clicker Question

  • Question: True or False: At "A" extracellular Na+ > intracellular Na+; At "B" extracellular Na+ < intracellular Na+

Lecture 4 - Sensory Systems

Learning Objectives for this Lecture

  • a. General principles of sensory perception

  • b. Cellular and molecular basis of olfaction (sense of smell)

  • c. Cellular and molecular basis of vision

Information Flow in the Nervous System

  • Example Inquiry: What is your name? What is my name?

  • Response: My name is Bill.

Overview of Sensory Systems

  • Sensory modalities include:

    • Touch

    • Vision

    • Smell

    • Hearing

  • Information Environment:

    • We are surrounded by various forms of information.

    • Some information is useful while other is not.

    • Animals evolved different systems to extract information from their environments.

    • No animal, including humans, is completely aware of its environment.

    • Example: Honey bees, sharks, sea turtles, rays, homing pigeons, migratory birds, tuna, and salmon can sense the Earth's magnetic field, which humans cannot.

Mechanism of Sensory Systems

  • General Principles:

    • All stimuli represent forms of energy.

    • A sensory receptor converts the stimulus energy into changes in membrane potential in the nervous system.

    • The term sensory receptor can refer to:

    • The receptive cell

    • The sensory organ

    • A molecular component of the sensory cell.

  • Basic Functions of Sensory Pathways:

    1. Sensory Reception: Capture the energy of the sensory signal.

    2. Transduction: Convert the sensory signal into changes in membrane potential (language of the nervous system).

    3. Transmission: Send sensory signals to the brain.

    4. Perception: Become aware of input.

  • Most sensory cells are modified neurons specialized in detecting various stimuli, such as pressure, heat, light, etc.

Types of Sensory Receptors

  • Categories of Sensory Receptors:

    • Mechanoreceptor: Respond to mechanical pressure or distortion.

    • Chemoreceptor: Detects chemical stimuli.

    • Photoreceptor: Detects light.

  • Mechanism: Sensory receptors convert stimuli (light, touch, etc.) into the nervous system language through changes in membrane potential, via opening of ion channels either directly or indirectly.

Example of a Sensory System: Olfaction (Smell)

Step 1: Sensory Reception

  • The membrane of sensory cells has odorant receptors, which represent the largest gene family (≈ 1000 members) in the mammalian genome.

  • These receptors differ from taste receptors, which are limited to a few dozen genes located in the mouth.

Dog's Olfactory Sensitivity

  • Dogs are approximately 10,000 times more sensitive to odors than humans.

  • They possess up to 300 million olfactory receptors in their noses, whereas humans have around six million.

  • The proportion of a dog's brain devoted to analyzing smells is 40 times greater than that in humans.

Step 2: Transduction

  • Membrane Potential (mV) Changes:

    • Initial resting potential: 0 mV

    • Change with odorant: Na+ enters the sensory cell.

  • Concept: The entry of an odor molecule causes a transduction signal interpreted by the nervous system as a change in membrane potential.

Clicker Question on Metabotropic Sensory Receptors

  • For metabotropic sensory receptors:

    • a. The receptor and ion channel are a single molecule.

    • b. Exemplified by the touch receptor.

    • c. Both olfaction and vision utilize metabotropic receptors.

    • d. Metabotropic sensory receptors are interneurons.

    • e. TRPV1 receptor is metabotropic.

Step 3: Transmission

  • Key Concept: The strength of sensory input is encoded by the frequency of action potentials, not the action potentials' size.

    • Example:

    • Weak smell: low frequency

    • Strong smell: high frequency.

Step 4: Perception

Sensory Processing: Amplification and Adaptation

  • Amplification: Strengthening of a sensory signal during transduction.

  • Sensory Adaptation: Decrease in responsiveness to continuous stimulation.

Vision

Importance of Light

  • Light provides significant information.

    • Simple visual systems in invertebrates detect changing light or direction of light.

    • Complex eyes (compound and single-lens) form images of the environment.

  • Light emission or reflection provides information on objects' chemical composition, perceived as color.

  • Photoreceptors: Specialized cells that detect light, classified as modified neurons.

Structure of the Human Eye

  • Components of the eye:

    • Choroid: Nourishes the neurons, a thin pigmented layer.

    • Lens: Transparent disk of protein located behind the iris.

    • Aqueous Humor: Watery substance in front of the lens.

    • Vitreous Humor: Jelly-like substance behind the lens.

    • Iris: Muscular structure that controls pupil diameter (light entry).

    • Optic Nerve: Carries visual information from the eye to the brain.

  • Key structures include:

    • Sclera

    • Suspensory ligament

    • Cornea

    • Pupil

    • Retina

    • Fovea

    • Central artery and vein of the retina

    • Optic disk

Retina Composition

  • Neurons in the Retina:

    • Photoreceptors (Rod and Cone cells)

    • Amacrine cells

    • Horizontal cells

    • Bipolar cells

    • Ganglion cells

    • Pigmented epithelium

  • Functionality of Photoreceptors:

    • Rods: Sensitive to light, do not distinguish color.

    • Cones: Allow for color vision; humans possess three cone types (S, M, L) sensitive to blue, green, and red light, respectively.

    • Cones allow perception of the full color spectrum through overlapping sensitivities.

Synaptic Processing in the Retina

  • Photoreceptors synapse onto:

    • Bipolar Cells: Transmit signals from photoreceptors to ganglion cells.

    • Horizontal Cells: Involved in visual processing, enhance edge detection via lateral inhibition.

    • Amacrine Cells: Participate in processing visual motion.

  • Processing Information:

    • Retina initiates processing of visual information before it reaches the brain’s interpretation.

    • A single ganglion cell integrates signals from multiple rods and cones, defining the receptive field; smaller receptive fields lead to sharper images.

Transduction in Photoreceptors (Rhodopsins)

  • Rhodopsin: Molecule that absorbs light and undergoes conformational changes.

  • Composed of:

    • Protein (opsin)

    • Light-absorbing group (11-cis-retinal), covalently bound within the opsin molecule.

  • Activation Process: When 11-cis-retinal absorbs a photon, it converts to all-trans-retinal, changing the opsin's conformation which signals light detection.

Relationship of Retinal to Vitamin A

  • Retinal Derivation: Derived from Vitamin A.

    • Chemical Formulas:

    • 11-cis-retinal

    • all-trans-retinal (converted form).

  • Importance of Vitamin A in visual function.

Sensory Transduction in the Eye

  • Initiation: Conversion of cis-retinal to trans-retinal by light initiates the transduction pathway.

  • Mechanism Insight:

    • Trans-retinal activates rhodopsin.

    • Activation leads to G protein signaling, hydrolyzing cyclic GMP (cGMP).

    • In darkness, cGMP keeps Na+ channels open; light causes reduced levels of cGMP, closing these channels, leading to hyperpolarization of rods and cones.

Dark and Light Responses in the Retina

  • Dark Condition:

    • Rhodopsin inactive.

    • Na+ channels open.

    • Rod cells are depolarized, consistently releasing glutamate to bipolar cells.

  • Light Condition:

    • Rhodopsin active.

    • Na+ channels closed.

    • Rod cells hyperpolarized, ceasing glutamate release.

    • Bipolar cells react (either depolarized or hyperpolarized) based on glutamate receptor binding.

  • Ganglion and Receptive Fields:

    • Ganglion cells receive cumulative information from groups of rods and cones, defining receptive fields.

    • Smaller receptive fields yield sharper imagery.