OLFACTION

Introduction

  • Olfaction: The sense of smell.

  • Gustation: The sense of taste.

Odor and Odorants

  • Odor: The translation of a chemical stimulus into a smell sensation.

  • Odorant: A molecule defined by its physiochemical characteristics, which can be converted by the nervous system into the perception of smell.

    • For odorants to be smelled, they must possess the following characteristics:

    • Volatile: Capable of floating through the air.

    • Small: Must have a small molecular size.

    • Hydrophobic: Repellent to water.

Olfactory Physiology

The Human Olfactory Apparatus

  • The sense of smell is integrated with another organ, the nose.

    • Primary Purpose of the Nose:

    • To filter, warm, and humidify the air we breathe.

    • Anatomical Components:

    • Small Ridges: Assist in air flow.

    • Olfactory Cleft: A narrow space at the back of the nose where air flows and where the main olfactory epithelium is found.

    • Olfactory Epithelium: A secretory mucous membrane whose primary function is to detect odorants in inhaled air.

Breathing vs. Sniffing

  • Sniffing: Directs air to the olfactory mucosa.

Cells of the Olfactory Epithelium

  • The olfactory epithelium contains three types of cells:

    • Supporting Cells: Provide metabolic and physical support for olfactory sensory neurons.

    • Basal Cells: Precursor cells that develop into olfactory sensory neurons.

    • Olfactory Sensory Neurons (OSNs):

    • Main cell type within the olfactory epithelium.

    • Small neurons located under a watery mucous layer.

Perception of Smells

Olfactory Sensory Processing

  • Key Structures Involved:

    • Olfactory Bulb: Where olfactory information is first processed; contains granular, mitral, and tufted cells.

    • Olfactory Nerve: Cranial nerve I; transmits signals from olfactory sensory neurons.

    • Cribriform Plate: Bony structure with tiny holes that separates the nose from the brain.

Olfactory Signal Transduction

  • Cilia (plural of cilium): Hairlike protrusions on the dendrites of OSNs containing receptor sites.

    • First structures involved in olfactory signal transduction.

  • Olfactory Receptor (OR): The part of the cilia where odorant molecules bind, requiring multiple molecules to initiate an action potential.

    • Typically, 7 to 8 odorant molecules are needed for an action potential to occur, and it takes about 40 nerve impulses for a smell sensation to be perceived.

Neurophysiology of Olfaction

Processing Paths

  • Primary Olfactory Cortex: Includes various interconnected brain structures (amygdala, parahippocampal gyrus, and entorhinal cortex) where olfactory information is first processed.

  • Entorhinal Cortex: Provides sensory association input into the hippocampus and receives direct projections from olfactory regions.

Limbic System Connection

  • The limbic system, comprising the olfactory cortex, amygdala, hippocampus, and piriform cortex, is involved in emotion and memory, making olfaction unique among the senses due to its direct connection to emotions.

Genetic Basis of Olfactory Receptors

  • Buck and Axel (1991): Identified approximately 1000 different genes coding for olfactory receptors, most mammals share a similar set while some exist as non-functional pseudogenes.

  • Pseudogenes Distribution:

    • Dogs and mice have about 20% pseudogenes.

    • Humans have between 60% and 70% pseudogenes.

    • Variability in pseudogenes among individuals leads to differing sensitivities to smells.

Theories of Olfactory Perception

  • Shape-Pattern Theory: The dominant theory explaining how specific odors are perceived based on the fit between an odorant shape and receptor shape, resulting in specific firing patterns in the olfactory bulb.

Importance of Patterns

  • Despite having approximately 1000 olfactory receptors, a wide variety of scents can be detected through the patterns of activity across these receptors.

  • Intensity of Odorant: Different concentrations affect which receptors are activated, and the temporal order of activation of receptors also plays a critical role.

Olfactory Coding

  • Olfactory Population Coding: Uses responses from a large population of receptors to encode stimuli.

  • Olfactory Temporal Coding: Suggests that the timing of neuronal spikes encodes the quality of odors.

Odor Mixtures

  • We generally perceive odors as mixtures rather than pure odorants, processing these components through:

    • Analysis: Detecting individual components (similar to auditory perception).

    • Synthesis: Experience of combining components (similar to color perception).

  • Olfactory perception primarily operates on a synthetic basis.

Complex Smells

  • Distinct complex smells, like bacon and rose, stimulate specific parts of the physiochemical and perceptual space indicating a commonality across sensory modalities.

Olfactory Identification and Psychophysics

Identification Challenges

  • Assigning verbal labels to smells can be difficult, leading to phenomena such as:

    • Tip-of-the-Nose Phenomenon: Inability to name a familiar odorant, lacking lexical access to its name; highlights disconnection between language and olfactory perception.

Disconnection Between Olfaction and Language

  • Olfactory processing does not integrate in the thalamus before cortical processing, leading to a split where olfactory processing primarily occurs in the right hemisphere and language processing occurs in the left hemisphere.

Assessing Odor Pleasantness

  • Individuals are generally adept at judging odors as pleasant or unpleasant, a learned response potentially influenced by cultural interactions.

  • Though human memory for odors is noted to have long-lasting retention compared to other sensory modalities.

Odor, Emotion, and Memory

Memory Associations with Odors

  • Odor-evoked memories tend to be linked with more intense emotional responses than memories triggered by other sensory experiences.

  • Aromatherapy: The premise that odors can affect mood, performance, and well-being.

  • An example includes infants being drawn to areolar gland secretions that encourage them to feed.

Individual Differences in Olfaction

Sensitivity Factors

  • Various factors influence olfactory detection thresholds:

    • Gender Differences: Women generally exhibit lower thresholds than men, particularly during the ovulatory phase.

    • Professional Sensitivity: Individuals like perfumers or wine tasters can identify up to 100,000 different odors.

    • Age Effects: By age 85, approximately 50% of individuals may lose their sense of smell (anosmia).

Anosmia

  • Anosmia: The total inability to smell, often a result of sinus illness or head trauma.

    • Significant blows to the head may fracture the cribriform plate leading to damage in olfactory neurons.

    • Anosmia results in pronounced impacts on taste along with smell and affects an estimated 5% of the population.

Olfactory Adaptation

  • The sense of smell acts as a change detector:

    • An example includes initial perceptions of scent decreasing with continuous exposure (e.g., fresh bread's scent fades over time).

  • Receptor Adaptation: A biochemical process that occurs after sustained odorant exposure, where receptors become unresponsive to the odor, halting detection.

Pheromones and Chemosignals

Pheromones

  • Pheromone: Chemicals emitted by individuals that trigger physiological or behavioral responses in others of the same species, important for communication in social insects and mammals.

    • For example, certain pheromones indicate female fertility or provoke male sexual behavior.

    • In studies, male rhesus monkeys ignore female companions in heat if they cannot smell properly.

Human Response to Pheromones

  • McClintock Effect: Observed synchronization of menstrual cycles among women living in close proximity, first identified by Martha McClintock.

    • Questions arise regarding whether this effect arises from smell, touch, or dietary influences.

Chemosignals in Humans

  • A more neutral term than pheromone for humans is chemosignal, encompassing various chemicals emitted that may influence mood, behavior, hormonal status, and sexual arousal.

  • Research has shown that lap dancers earn significantly more tips when they are in the ovulatory phase of their menstrual cycle, indicating a potential influence of chemosignals on attraction.