Olfaction

Olfaction represents an ancient, evolutionarily critical physiologic system. Humans have the ability to detect and discriminate at least 10,000 different odorants. The sense of smell, or olfaction, begins in a small area on the roof of the nasal cavity, which is covered in specialized mucosa. From there, the olfactory nerve transmits the sensory perception of smell via the olfactory pathway. This pathway is composed of the olfactory cells and bulb, the tractus and striae olfactoriae, and the primary olfactory cortex and amygdala. Olfaction is responsible for the detection of hazards, pheromones, and food.

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Introduction

Structure of the nose

  • The nasal passages are divided by the nasal septum in the midline.
  • Each lateral nasal wall is formed by 3 turbinates: inferior, middle, and superior.
  • The cribriform plate constitutes the roof of the nasal passage:
    • Part of the ethmoid bone
    • Perforated by the olfactory foramina (approximately 20 foramina)

Location of the olfactory epithelium

  • Along the cribriform plate
  • Along and medial to the superior turbinates
  • Can extend as far as the anterolateral middle turbinate and posterior nasal septum
  • The location of the olfactory epithelium varies among individuals.
  • Olfactory neurons are lost with age, exposure to environmental toxins, and inflammation.

Detection of smell

  • We detect smell by quickly inhaling air, which creates turbulent airflow in the nose.
  • This inhalation facilitates transfer of the odorants superiorly, where the olfactory epithelium lies.
  • The odorants then diffuse into the mucus and are bound by proteins that transport them to the olfactory receptors. 
  • Another way of detecting smell is through posterior transfer of odorants from the nasopharynx, which is important for the detection of flavor during eating and drinking.

Olfactory Epithelium and Nerve

Olfactory epithelium

The olfactory epithelium is a pseudostratified columnar epithelium overlying a lamina propria and consists of the following cell types:

  • Basal cells: 
    • Act as stem cells
    • Can differentiate into progenitors of the olfactory epithelium
  • Sustentacular cells: support olfactory neuron function 
  • Olfactory gland cells (Bowman gland): function to carry secretions to the apical epithelial surface
  • Olfactory receptor neurons: 
    • Located in an intermediate zone between basal and apical layers
    • Form the bulk of the epithelium: humans have approximately 10 million to 20 million olfactory neurons.
    • Bipolar cells:
      • Project a dendrite with a thickened end (the olfactory knob) that contains nonmotile sensory cilia to the epithelial surface
      • Project another axon that transmits signals to the brain after crossing the cribriform plate
    • Express olfactory receptors:
      • Constitute one of the largest families of G-protein–coupled receptors
      • Controlled by as many as 800 genes in humans
Diagram depicting the 1st structures of the olfactory system

Diagram depicting the 1st structures of the olfactory system:
Odorants are perceived by specialized cilia within the nasal mucosa, which send an electrical signal via the olfactory nerve through the ethmoid bone to the olfactory bulb and tract.

Image: “(b) The olfactory receptor neurons are within the olfactory epithelium” by OpenStax College. License: CC BY 4.0, cropped by Lecturio.

Olfactory nerve

  • Axons from the olfactory neurons form nerve bundles (fila olfactoria). Many such bundles coalesce to create the olfactory nerve.
  • Enclosed by dura and arachnoid mater
  • Shortest of the 12 cranial nerves and unique in that it can partially regenerate
  • Crosses the cribriform plate and synapses with neurons in the olfactory bulb

Olfactory Pathway

Overview

Odors are first detected at the olfactory bulb, where the information is received and transmitted posteriorly along the olfactory tract to the olfactory cortex.

Olfactory bulb

  • The main relay station of the olfactory pathway
  • Lies on top of the cribriform plate
  • The olfactory bulb consists of:
    • Glomeruli: dense tangles of axonal and dendritic branches 
    • Mitral cells: Each cell extends a primary dendrite into a single glomerulus, which divides into a tuft of branches onto which the primary olfactory axons synapse.

Olfactory tract

  • Consists of nerve fibers created by the axons of the mitral cells of the olfactory bulb
  • The tract passes underneath the medial frontal lobe inside the olfactory groove.
  • The olfactory tract ultimately divides into 2 striae:
    • The lateral stria terminates in the primary olfactory cortex of the temporal lobe.
    • The medial stria:
      • Passes through the anterior commissure to the contralateral olfactory tract
      • Terminates in limbic structures to contribute to the emotional responses brought on by the smell
Schematic diagram of the olfactory pathway from the nasal mucosa to the primary and secondary olfactory cortex

Schematic diagram of the olfactory pathway from the nasal mucosa to the primary and secondary olfactory cortex:
Note the ipsilateral and contralateral projections of the middle and lateral olfactory striae.

Image by Lecturio.

Olfactory cortex

The olfactory cortex is unique in receiving direct sensory inputs from the mitral cells of the olfactory bulb. This process differs from the classic sensory pathway, where sensory information is 1st relayed in the thalamus before reaching the neocortex.

  • Located on the base of the frontal lobe and medial aspect of the temporal lobe 
  • Divided into several structures along the anterior–posterior axis:
    • Anterior olfactory nucleus 
    • Olfactory tubercle
    • Piriform cortex:
      • Located below the lateral olfactory stria
      • Includes the anterior part of the parahippocampal gyrus and uncus
    • Olfactory amygdala:
      • Located within the temporal lobe, in front of the temporal horn of the lateral ventricle
      • Includes the prepiriform and periamygdaloid areas
      • Associated with emotional responses to smell, such as fear
    • Lateral entorhinal cortex
  • From the olfactory cortex, olfactory information is secondarily relayed via the mediodorsal nucleus of the thalamus to the: 
    • Insular cortex: 
      • Located inside the Sylvian fissure
      • Believed to be the site where olfactory and taste information integrate to produce the sense of flavor
    • Orbitofrontal cortex: 
      • Located on the base of the frontal lobe
      • Lesions of this cortical region can result in an inability to distinguish different odors.
  • Odor information is also sent to the orbitofrontal cortex and portions of the hypothalamus and brain stem to trigger appetite, salivation, and gastric contraction.
Schematic view of the human olfactory system

Schematic view of the human olfactory system:
The primary and secondary olfactory cortices are represented in blue and green, respectively.
Amyg: amygdala
Ento: entorhinal cortex
Hipp: hippocampus
OFC: orbitofrontal cortex
PC: piriform cortex
Thal: thalamus

Image: “Schematic view of the human olfactory system” by Saive, Royet, and Plailly (adapted from Royet et al.). License: CC BY 3.0

Clinical Relevance

  • Anosmia: temporary or permanent loss of the sense of smell. Anosmia can be due to numerous factors, including inflammation within the nasal mucosa (rhinitis) from allergy or postviral infection; blockage of nasal passages; trauma to the orbit or nasal cavity, such as fracture of the cribriform plate, which may lead to disruption of the olfactory nerve; a pathologic lesion (i.e., tumor or inflammation) of the temporal lobe; or degenerative CNS disease such as Alzheimer, Huntington, or Parkinson disease. Treatment depends on the reversibility of the underlying condition. 
  • Dysosmia: qualitative alteration or distortion of the perception of smell. Dysosmia can be classified as parosmia and phantosmia. Parosmia is an unpleasant odor perception. Phantosmia is the perception of an odor when no odorant is present. Dysosmia can be associated with prior head trauma, chronic nasal congestion, and chronic sinusitis. Dysosmia is relatively common and has been reported in as many as 6% of adults. There is no established treatment, although medications such as antiepileptics, migraine medications, and benzodiazepines have been used with various degrees of success. In extreme cases, surgical resection of the olfactory mucosa is offered.

References

  1. Lafreniere, D. (2021). Taste and olfactory disorders in adults: evaluation and management. UpToDate. Retrieved September 14, 2021, from https://www.uptodate.com/contents/taste-and-olfactory-disorders-in-adults-evaluation-and-management
  2. Klinger, E. (2017). Development and organization of the evolutionarily conserved three-layered olfactory cortex. eNeuro 4(1):ENEURO.0193-16.2016. https://doi.org/10.1523/eneuro.0193-16.2016 
  3. Pinto, J.M. (2011). Olfaction. Proc Am Thorac Soc 8:46–52. https://doi.org/10.1513/pats.201005-035RN 
  4. Purves, D., Augustine, G.J., Fitzpatrick, D., et al. (2001). The olfactory bulb. In: Neuroscience, 2nd ed. Sunderland, MA: Sinauer Associates.

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