sensory systems

Introduction

  • Speaker: Dr. Sonja Pyott

  • Institution: Department of Biology and Marine Biology, University of North Carolina

  • Focus: Sensory systems in animals, specifically cochlea and hair cells using confocal techniques.

Overview of Sensory Systems

  • Animal Sensory Systems:   - Examines how different species perceive and respond to stimuli.   - Sensory Receptors: Specialized neurons that transduce external/internal stimuli into neural activity.     - Inner Hair Cells and Outer Hair Cells: Modified cilia that play critical roles in sensory reception.   

Types of Sensory Stimuli

  • Animals respond to various stimuli based on sensory systems:   - Questions posed: How do sensory cells recognize and respond to stimuli?   - Variance in stimuli and perception across species, including humans.

Sensory Receptors

  • Definition:   - Specialized neurons that convert stimuli into neural activities.

  • Components include:   - Inner Hair Cells: Primary receptors in hearing, connected to auditory nerve.   - Outer Hair Cells: Amplify sound; associated with modified cilia.
      - Supportive Structures: Include supporting cells and basal lamina.

Nervous System Communication

  • Neurons:
      - Fast, targeted communication through electrical signals.
      - Synapses:     - Neurons release neurotransmitters at synaptic junctions for targeted messaging.   

Sensory Transduction Mechanisms

  • Types of Mechanisms:   - Two main categories of sensory transduction:     1. Ionotropic Receptors: Directly gated ion channels, reacting to stimuli by changing membrane potential.
        2. Metabotropic Receptors: G protein-coupled receptors (GPCR) that alter cellular responses indirectly.

  • Receptor Potential:   - Change in membrane potential caused by a sensory stimulus.   - Can lead to:     - Triggering neurotransmitter release onto afferent neurons.     - Generation of action potentials sent to the central nervous system (CNS).

Transmitting Information to the CNS

  • Information encoding through sensory cells

  • Information dispatched in an ordered manner toward CNS.

Overview of Chemoreception

  • Chemoreception: Sensation of chemicals responsible for taste and smell.   - Taste involves taste cells on the tongue that transduce dissolved chemicals into neural signals.

  • Categories of Mammalian Taste:
      1. Sour
      2. Bitter
      3. Salty
      4. Sweet
      5. Umami (MSG and aspartate).

Taste Cell Mechanisms

  • Each taste cell expresses proteins specific to one type of taste.

  • Saltiness: Involves ionotropic Na+ channels that facilitate membrane depolarization when activated, leading to neurotransmitter release.

  • Metabotropic Taste Transduction:   - Sweet, umami, and bitter compounds activate GPCRs leading to Na+ channel opening and neurotransmitter release.
        - Notable receptors include T1R2 (sweet) and T1R1 (umami) with multiple T2R genes for bitter tastes.

Olfactory Receptors and Smell

  • Olfactory Receptor Neurons (ORNs):   - Located in the olfactory epithelium; detect volatile chemicals through microvilli.
      - Each ORN typically expresses a single type of GPCR, binding to specific chemical ligands allowing varied responses.

  • Combinatorial Coding:
      - Different odorants stimulate groups of ORNs. Even with limited GPCRs (~400 in humans), combinations can yield a diverse range of odor detection.

Overview of Mechanoreception

  • Mechanoreception: Excited by mechanical stimuli (touch, pressure, vibration).   - Involves ion channel proteins that open when mechanically stretched.

Mechanoreception in Insects

  • Bristle displacement causes depolarization in mechanoreceptors via stretch-activated cation channels.

Mechanoreception in Humans: Hearing and Balance

  • Mechanoreceptors in structures such as semicircular canals react to body movement.

  • In humans, cochlear hair cells respond to sound vibrations.

  • Inner hair cells convert sound waves into neurotransmitter release to auditory nerves for brain communication.

Mechanotransduction in Cochlea

  • As sound vibrates the basilar membrane, it displaces stereocilia on inner hair cells.

  • Tip Links: Structures connecting stereocilia facilitate opening and closing of stretch-gated channels.

  • Changes in membrane potential from these movements results in neurotransmitter release corresponding to sound frequency.

Photoreception: Vision and Detection of Light

  • Photoreceptors: Transduce light energy into neural signals; found in various structures (e.g., eyes, skin, brain).   - Mammalian eyes focus light through a lens and feature photoreceptor cells: rods and cones.

  • Rod Cells: Function in dim lighting; high sensitivity.

  • Cone Cells: Responsible for color vision, functioning well in bright light with higher acuity.

Photopigments in Vertebrate Vision

  • Composition includes retinal bound to opsin proteins, crucial for visual processes.

Phototransduction in Rods

  • Light exposure causes retinal to change structurally, activating opsin and closing Na+ channels, leading to photoreceptor hyperpolarization.

  • In absence of light, photoreceptors release higher levels of neurotransmitter signaling (dark current).

Human Color Vision

  • Three types of cones, each with specific photopigments that react to distinct wavelengths (short, medium, long).

  • Color Deficiency: Conditions such as protanopia (red-green deficiency) indicate alterations in cone pigments or their absorption spectra.

Contextual Influences in Color Perception

  • Color perception can vary significantly; examples include the infamous dress that appears in different colors to different observers.

  • Variance in photopigment availability leads to differing visual experiences among species.

Summary of Sensory Systems

  • Types of Sensory Systems Covered:
      I. Overview of sensory systems
      II. Chemoreception (taste and smell)
      III. Mechanoreception (touch and hearing)
      IV. Photoreception

References and Figures

  • Various figures and illustrations were referenced throughout these presentations to support understanding of complex mechanisms and concepts.

Additional Notes

  • Sensory transduction is vital for interaction with the environment, influencing behavior and survival.

  • The study of sensory systems reveals intricate biological adaptations in different species.