Olfactory 1 ppt

The Chemical Senses

Overview

  • Dr. James Dillon discusses two primary chemical senses: Smell (olfaction) and Taste (gustation).

  • Learning outcomes include understanding:

    • Specialized sense organs involved in olfaction/gustation at molecular and cellular levels.

    • Coding for olfactory/gustatory information.

    • Brain structures processing these sensory inputs in the CNS.

  • Supporting Text: Neuroscience Purves, 6th Edition, Principles of Neurobiology by Liqun Luo.

Sensory Systems: General Features

  • Sensory systems consist of multiple layers involving different types of energy stimuli:

    • Light (Vision)

    • Chemical (Taste & Smell)

    • Mechanical (Hearing, Balance, Touch)

  • Key processes in sensory systems:

    • Transduction - Conversion of stimuli into signals.

    • Encoding - Transformation of signals into meaningful information.

    • Processing - Interpretation of the encoded signals, leading to sensation and perception, which includes factors such as modality, intensity, duration, and location.

Functional Categories of Sensory Receptors

Types of Receptors

  • Exteroceptors: Respond to external stimuli.

  • Proprioceptors: Provide sensations regarding body position and movement.

  • Interoceptors: Monitor internal body conditions.

Categories of Mechanoreceptors

  • General Mechanoreceptors: Include various types such as:

    • Superficial: Merkel's discs, Meissner's corpuscles.

    • Deep: Ruffini's end organs, Pacinian corpuscles.

  • Special Mechanoreceptors: Hair cells in cochlea or otolith organs.

  • Nociceptors: Sensitive to pain stimuli, categorized into:

    • Thermal nociceptors, Mechanical nociceptors, Polymodal nociceptors, Silent nociceptors.

The Chemical Senses

Chemoreceptors

  • Chemoreceptors bind to chemicals in the environment to generate signals.

  • Thought to be evolutionarily conserved through various organisms.

  • Olfaction: Detects airborne molecules (odorants).

  • Gustation: Involves chemical and physical qualities of ingested substances.

    • Important for behavior in lower organisms and stimulates gastrointestinal systems in higher organisms.

Olfactory System

Olfactory Sensation

  • Thresholds vary by molecule:

    • Ethanol: 2mM

    • 2-trans-6-cis nonadenial: 0.07nm

  • Smell interpretation is concentration-dependent (e.g., inc concentration may change scent perception).

  • Involves activation patterns of various olfactory receptor neurons, referred to as across-fibre pattern coding.

Anatomy of Olfactory Reception

  • Olfactory Epithelium: Contains sensory receptors and neurons project to the olfactory bulb.

  • Olfactory Neurons: Bipolar, unmyelinated sensory neurons with specialized cilia.

    • Mucus from Bowman’s gland aids in concentrating chemicals for odor detection.

    • Neurons have a short lifespan (6-8 weeks) and are prone to damage.

Mechanisms of Odor Transduction

  • The process begins with stimulation of olfactory cilia.

  • Nobel Prize (2004) awarded to Richard Axel and Linda Buck for discovering GPCRs as chemoreceptors.

    • 3-5% of the human genome dedicated to GPCRs; many functional odorant genes exist:

      • 400 in humans, 1000 in dogs, 1200 in mice.

    • Each olfactory receptor neuron (ORN) expresses one receptor gene, contributing to the diversity of smell perception.

Odorant Receptor Gene Expression

  • ORNs expressing the same receptor send projections to the same glomerulus, promoting convergence and amplification of sensory signals.

  • A single glomerulus may contain significant numbers of mitral cells and olfactory cells.

Encoding of Odor Signals

  • Chemical composition causes unique spatial patterns in the olfactory bulb.

  • Across-fibre coding: The pattern of activity among many sensory neurons encodes specific smells.

  • Perception of smells is complex and defined by patterns of response across multiple neurons rather than single receptor responses.

Central Processing of Olfactory Signals

  • The olfactory bulb's major tract is the lateral olfactory tract, targeting the piriform cortex.

  • Responses in the piriform cortex are organized spatially, with a complex mapping of different odors.

  • The unique combination of active glomeruli reflects the distinctiveness of each odor.

Summary of Olfactory Encoding

  • Majority of olfactory neurons respond to multiple odorants, encoding odors through an active pattern across many neurons - reinforcing across-fibre coding.

  • Review of learning outcomes to ensure understanding of:

    • Specialized organs in olfaction.

    • How olfactory information is coded.

    • Brain structures involved in processing sensory information.