Physiology of Smell

Introduction

The olfactory system enables rapid detection and identification of odors through specialized receptor neurons and dedicated neural pathways. Smell is closely linked to memory, emotion, appetite, and behavior, making it an essential sensory system for everyday life and survival.

• Olfaction is the sensory modality responsible for the perception of smell and is classified as a visceral sense because of its close association with gastrointestinal functions.
• The receptors for smell are specialized chemoreceptors located within the olfactory epithelium in the roof of the nasal cavity.
• In many animals, olfaction plays an essential role in locating food, recognizing mates, identifying territory, and detecting predators.
• In humans, smell enhances the flavor of food by working together with the sense of taste.
• Olfactory stimuli also influence memory, emotions, social interactions, and behavioral responses.
• Smell serves a protective function by helping detect spoiled food, smoke, toxic gases, and environmental pollutants.
• Unlike other sensory pathways, olfactory information reaches the primary olfactory cortex without first relaying through the thalamus.
• Olfactory processing also exhibits asymmetric cortical representation, reflecting specialized functional organization within the cerebral hemispheres.

Site of Olfaction

Olfactory Epithelium

• The receptors responsible for smell are located in the olfactory epithelium, a specialized sensory epithelium lining the roof of the nasal cavity.
• This epithelium extends over the superior nasal concha and the upper part of the nasal septum.
• In humans, it covers an area of approximately 2.5 square centimeters in each nasal cavity.
• The surface is coated with mucus secreted by Bowman glands, which dissolves odorant molecules and facilitates their interaction with receptor cells.
• Animals with a highly developed sense of smell possess a much larger olfactory epithelium, whereas humans have a comparatively smaller olfactory area.
• The olfactory epithelium contains approximately 10–20 million olfactory receptor neurons, along with supporting cells and basal stem cells.
• Basal cells continuously replace aging or damaged receptor neurons, making the olfactory system one of the few neural tissues capable of lifelong regeneration.
• During quiet breathing, only a small amount of inspired air reaches the olfactory epithelium because of its superior location.
• Sniffing directs a greater volume of air toward the olfactory region, increasing contact between odorants and receptor cells.
• This enhanced airflow improves odor detection and facilitates rapid identification of unfamiliar or weak smells.

Vomeronasal Organ

• The vomeronasal organ is an accessory chemosensory structure that is well developed in many mammals but is rudimentary and generally considered non-functional in adult humans.
• In animals, it detects pheromones, which are chemical signals released by members of the same species.
• Pheromones influence reproductive, social, territorial, and feeding behaviors.
• Sensory neurons from the vomeronasal organ project to the accessory olfactory bulb.
• Neural signals are then transmitted mainly to the amygdala and hypothalamus, where they regulate instinctive and reproductive behaviors.

Olfactory Receptor Cell

• The olfactory receptor cell is a specialized bipolar neuron located within the olfactory epithelium.
• Each receptor cell possesses a short dendrite ending in an expanded olfactory knob, from which 10–20 non-motile cilia project into the mucus layer.
• These cilia contain odorant receptors and are the primary sites where dissolved odorant molecules interact with the sensory neuron.
• The mucus surrounding the cilia facilitates dissolution and transport of odorant molecules to receptor proteins.
• Unlike most neurons, olfactory receptor cells undergo continuous regeneration throughout life.
• New receptor neurons arise from basal stem cells, replacing aging or damaged cells approximately every 30–60 days.
• Growth factors, including bone morphogenetic proteins, help regulate this process of neuronal renewal.
• The olfactory mucosa is unique because it is one of the few locations where neurons are directly exposed to the external environment, making it particularly vulnerable to environmental toxins, infections, and mechanical injury.

Olfactory Pathways

Olfactory Nerves

• The axons of olfactory receptor neurons bundle together to form approximately 20 olfactory nerve filaments on each side.
• These unmyelinated fibers pass through the cribriform plate of the ethmoid bone and enter the olfactory bulb.
• They represent the first-order neurons of the olfactory pathway.

Olfactory Bulb

• The olfactory bulb lies on the inferior surface of the frontal lobe, immediately above the cribriform plate.
• It serves as the first central relay station for olfactory information.
• Four principal neuronal types are present within the olfactory bulb:
  • Mitral cells
  • Tufted cells
  • Periglomerular cells
  • Granule cells
• Mitral and tufted cells function as second-order neurons.
• Periglomerular and granule cells are inhibitory interneurons that refine odor processing.

Olfactory Glomeruli

• Olfactory glomeruli are spherical synaptic structures where olfactory receptor neurons synapse with mitral and tufted cells.
• Numerous receptor neurons converge onto relatively few second-order neurons, increasing olfactory sensitivity.
• Periglomerular cells inhibit neighboring glomeruli, enhancing contrast between different odor signals.
• Granule cells produce feedback inhibition by suppressing mitral and tufted cell activity through release of gamma-aminobutyric acid.
• This inhibitory network improves odor discrimination and sharpens signal processing.

Olfactory Tract

• Axons of mitral and tufted cells form the olfactory tract, which carries olfactory information toward higher brain centers.
• Near the base of the brain, the tract divides into lateral and medial olfactory striae.

Cortical Olfactory Areas

• The lateral olfactory stria projects directly to the piriform cortex, amygdala, entorhinal cortex, anterior olfactory nucleus, and olfactory tubercle.
• Unlike other sensory systems, these primary olfactory projections reach the cerebral cortex without an initial thalamic relay.
• The piriform cortex is the principal cortical area for processing odor information and is strongly activated during sniffing.
• The amygdala links odors with emotional and behavioral responses.
• The entorhinal cortex connects olfactory input with the hippocampus, supporting odor-associated learning and memory.
• Signals from primary olfactory areas are relayed through the thalamus to the orbitofrontal cortex, where conscious odor recognition and discrimination occur.
• The orbitofrontal cortex integrates olfactory information with taste, contributing to flavor perception.
• Damage to the orbitofrontal cortex impairs the ability to identify and distinguish different odors despite preserved odor detection.
• The medial olfactory stria projects mainly to the septal region and hypothalamus, where olfactory stimuli influence autonomic and instinctive behaviors.
• Olfactory pathways also communicate with the dorsal motor nucleus of the vagus, promoting parasympathetic responses such as salivary, gastric, and pancreatic secretions during the cephalic phase of digestion.
• Through extensive connections with the limbic system, the olfactory pathway closely links smell, memory, emotion, and autonomic regulation.

Physiology of Olfaction

Mechanism of Olfaction

Role of Mucus and Odorant-Binding Protein

• Odorant molecules must dissolve in the thin mucus layer covering the olfactory epithelium before they can stimulate receptor cells.
• Dissolved odorants come into contact with the cilia of olfactory receptor neurons, where odor detection begins.
• Most odorants are small, volatile molecules that readily enter the nasal cavity during inspiration.
• Water-soluble odorants dissolve directly in the mucus and diffuse to the receptor surface.
• Lipid-soluble odorants are transported through the aqueous mucus by odorant-binding proteins.
• These binding proteins are secreted by the Bowman glands and facilitate efficient delivery of hydrophobic odorants to receptor sites.

Olfactory Transduction

• Olfactory receptor neurons express numerous odorant receptors, each belonging to the family of G protein-coupled receptors.
• Each receptor neuron expresses only one type of odorant receptor, although each receptor can recognize multiple structurally related odorants.
• Binding of an odorant activates a specialized G protein, initiating intracellular signaling.
• The principal signaling pathway stimulates adenylyl cyclase, increasing intracellular cyclic adenosine monophosphate.
• Cyclic adenosine monophosphate opens cyclic nucleotide-gated cation channels, allowing sodium and calcium ions to enter the receptor cell.
• Calcium entry subsequently activates chloride channels, causing chloride ions to leave the cell and producing further depolarization.
• Depolarization generates action potentials that travel along the olfactory nerve to the olfactory bulb.
• Olfactory pathways exhibit a low level of spontaneous activity even in the absence of odor stimulation.
• Odor presentation increases the firing frequency of receptor neurons according to the strength and quality of the stimulus.
• Within the olfactory bulb, inhibitory interneurons sharpen odor signals through lateral inhibition, improving contrast between similar odors.
• Olfactory receptor neurons expressing the same receptor type project to the same olfactory glomerulus, creating an organized spatial pattern of activation.
• Different odors activate distinct combinations of glomeruli, allowing the nervous system to distinguish thousands of odorants.

Factors Affecting Olfaction

• The effectiveness of an odor depends on its volatility, because volatile substances reach the olfactory epithelium more readily.
• Odorants with suitable water or lipid solubility are detected more efficiently than poorly soluble compounds.
• Olfactory receptors are extremely sensitive and can detect some chemicals at very low concentrations.
• Women generally demonstrate greater olfactory sensitivity than men, and sensitivity may increase during the periovulatory phase of the menstrual cycle.
• The threshold for odor detection gradually increases with advancing age, contributing to reduced olfactory function in older adults.
• Humans can recognize thousands of different odors, but they are relatively less sensitive to small changes in odor intensity.
• A substantial increase in odor concentration is usually required before a noticeable change in perceived intensity occurs.
• Olfactory adaptation develops rapidly during continuous exposure to the same odor.
• As adaptation progresses, perception of the odor gradually diminishes even though the stimulus remains present.
• This adaptation results largely from calcium-dependent regulation of cyclic nucleotide-gated channels, which reduces receptor responsiveness during prolonged stimulation.
• Rapid adaptation enables the olfactory system to remain responsive to new odorants, improving detection of changes in the surrounding environment.

Reflexes Associated with Olfaction

• A new or unfamiliar odor often triggers sniffing, which increases airflow to the olfactory epithelium and improves odor detection.
• The smell of appetizing food stimulates salivary, gastric, and pancreatic secretions, preparing the digestive system for food intake during the cephalic phase of digestion.
• Irritating airborne chemicals, such as chlorine or ammonia, activate trigeminal sensory nerve endings within the nasal mucosa rather than olfactory receptors.
• Trigeminal stimulation produces burning or irritating sensations and may evoke sneezing, lacrimation, coughing, or brief respiratory inhibition.
• Offensive odors can trigger nausea and vomiting through connections between the olfactory system and brainstem centers.

Applied Physiology

Hyposmia and Anosmia

• Hyposmia refers to reduced smell perception, whereas anosmia indicates complete loss of the sense of smell.
• Common causes include upper respiratory infections, aging, chronic rhinitis, trauma to the cribriform plate, injury to the olfactory nerve or bulb, and disorders affecting olfactory receptors.
• Kallmann syndrome is characterized by anosmia associated with hypogonadotropic hypogonadism.

Dysosmia or Parosmia

• Dysosmia refers to abnormal odor perception, while parosmia is distortion of the perceived smell of a familiar odor.
• These disorders may occur after infections, head injury, or neurological diseases.
• Olfactory hallucinations are classically associated with temporal lobe epilepsy and may precede seizure activity.

Test for Olfaction

• Clinical assessment is performed by presenting non-irritating familiar odors to each nostril separately while the patient’s eyes are closed.
• Irritant substances should be avoided because they stimulate the trigeminal nerve instead of the olfactory nerve.
• This bedside examination provides a qualitative assessment of olfactory function.
• Olfactometry offers a quantitative measurement of odor detection thresholds and olfactory sensitivity.