Olfaction: Olfactory Anatomy | Zoology Optional Notes for UPSC PDF Download

Olfactory Anatomy: Peripheral Features

• Airflow to Olfactory Epithelium: Only about 2% of inspired air reaches the olfactory epithelium due to the shape of turbinate bones.
• Air Modification: Inspired air undergoes temperature and humidity adjustments to match body temperature and near saturation during passage over nasal epithelium.
• Particulate Matter: Particulate matter in the air is deposited on nasal mucosa and directed toward the pharynx by cilia.
• Mucus Production: Glands and goblet cells within the mucosa supply mucus to the nasal epithelium.
• Cyclic Variation: Each nasal passage experiences cyclic variation in flow resistance due to the regular constriction and dilation of mucosal venous cavernous tissue, hypothalamically regulated via the Vidian nerve.

• Location: Situated at the apex of nasal cavities, the olfactory epithelium encompasses about 2-4 cm2 and contains approximately 10 x 10 receptor cells.
• Mucous Layer: Covered by a mucous layer secreted by Bowman's gland.
• Composition: Composed of receptor cells whose axons extend through the cribriform plate (perforated area of the ethmoid bone) and synapse within the olfactory bulb.
• Topographic Mapping: Loose topographic mapping from the olfactory epithelium to the olfactory bulb.
• Sustentacular Cells: Support receptor cells and provide a secretion with an unknown role and composition.
• Basal Cells: Function not clearly understood.

• Innervation of Non-Olfactory Nasal Cavity: The non-olfactory nasal cavity is innervated by free nerve endings of the ethmoid branch of the trigeminal nerve.
• Distribution: These nerve endings are also present in the epithelium of the pharynx.
• Synapses: Trigeminal nerve synapses in the trigeminal nucleus in the pons.

Understanding the peripheral anatomy and physiology of the olfactory system involves considering the airflow, mucus production, and innervation within the nasal passages and the specialized olfactory epithelium. The intricate network of cells and nerves contributes to the complex process of olfaction.

Central Olfactory Structures

• Axonal Synapses: Receptor cell axons synapse in glomeruli on mitral and periglomerular cells within the olfactory bulb.
• Convergence Ratio: Approximately 25,000 receptor axons converge to synapse in each glomerulus with 25 mitral cells, creating a 1,000:1 convergence ratio.
• Lateral Olfactory Tract (LOT): Axons of mitral cells form the lateral olfactory tract (LOT) to carry impulses toward the central nervous system (CNS).
• Connections: Mitral cell axons form axodendritic synapses, and collaterals connect to other cells within the bulb.
• Periglomerular Cells: Outnumber mitral cells about 20:1 and make horizontal connections between glomeruli.
• Granular Cells: Located in deeper layers, make dendrodendritic connections with mitral cells and with each other, as well as axodendritic connections with centrifugal neurons.
• Centrifugal Neurons: Originate in the contralateral olfactory bulb, ipsilateral anterior olfactory nucleus, and ipsilateral diagonal band of Broca, forming connections with granule cells and periglomerular cells.

• Termination Area: Olfactory fiber termination occurs in the olfactory cortex.
• Lateral Olfactory Tract Path: Courses ventrally over the prepyriform cortex toward the amygdaloid body, branching fibers spread across the cortical ventral surface to synapse in various regions.
• Axonal Connections: Mitral cell axons from the lateral olfactory tract form axodendritic synapses with pyramidal cells in the outer molecular layer of the cortex.
• Secondary Fiber Connections: Olfactory cortex structures send secondary fibers to other CNS sites, contributing to the medial forebrain bundle, hypothalamus, amygdala, and possibly the hippocampus.
• Centrifugal Fibers: Anterior olfactory nucleus sends centrifugal fibers to granule cells of the ipsilateral olfactory bulb.
• Additional Connections: Secondary olfactory connections have been reported, involving pathways originating in the amygdala and prepyriform cortex, passing through the thalamus, and terminating in the orbitofrontal cortex—an example of neocortical connections distinct from classical allocortical and limbic projections.

Understanding the central olfactory structures involves intricate networks, synaptic connections, and pathways that contribute to the complex processing of olfactory information in the central nervous system.

Physiology and Function of the Olfactory System

Psychophysiology:

• Olfactory Stimulus: Consists of airborne chemical molecules within the molecular weight range of approximately 15 to 300.
• Intensity: A function of the number of molecules of the odorous substance in contact with the olfactory epithelium.
• Perceived Increase: The rate of perceived increase in intensity with increased odorant concentration is a log function, influenced by water solubility and chemical functional groups of the odorant.
• Threshold: The stimulus concentration detected 50% of the time; difficult to study due to adaptation, cross-adaptation, individual variability, and technique-related challenges.

• Historical Puzzle: Classification of odor quality has been challenging for centuries.
• Theories: Various theories proposed, including stereochemical theory, puncture and penetration theory, molecular vibration theory, and spatio-temporal models.
• Challenges: Lack of solid evidence for many theories; unclear physicochemical attributes of molecules that make them odiferous; unknown nature of the receptor mechanism complicates understanding.

CNS Olfactory System

• Olfactory Bulb Anatomy: Glomerular level involves sensory cells firing into a synaptic neuropil of mitral and periglomerular cells; mitral cells are principal neurons.
• Data Processing: Considerable data processing at the glomerular level due to the network of periglomerular cells; odor specificity observed at this level.
• Inner Layers Processing: Interactions of sensory input data and output from the CNS occur in synapses between mitral and granule cells with axons from centrifugal cells; recurrent inhibitory loops responsible for generating oscillatory EEG.

• Unanswered Questions: How odor information is coded and transmitted to the brain is largely unknown.
• Evidence: Some evidence for mitral cell specificity, spatially organized projections, and olfactory bulb EEG patterns; influenced by habituation and learning.
• Challenges: Salient stimulus dimensions not well understood; low correlations of electrophysiological responses with stimulus properties; influence of habituation and learning on putative codes.

Brain Olfactory Mechanisms:

• Regulation of Behaviors: Structures in the brain receiving olfactory input are involved in regulating basic behaviors related to hunger, thirst, sexual activity, and sleep.
• Electrical Stimulation: Laboratory animals work to receive electrical stimulation to the medial forebrain bundle, indicating the importance of olfactory input in behavior regulation.
• Human Amygdala EEG: Recent data indicate correlations with odor qualities.

• Olfactory Influence on Animals: Olfactory stimulation strongly influences sexual behavior, social behaviors, aggression, territorial defense, and identification in nonhuman species.
• Human Olfactory Effects: Limited evidence suggests that humans can use odors to identify individuals, generate pheromonelike compounds, and exhibit correlations between olfactory acuity and menstrual variations.
• Importance of Research: Understanding the effects of odors on human behavior is crucial, given attempts to control the olfactory environment in society and the prevalence of odiferous environmental pollution.