Olfaction – The Sense Of Smell

Introduction

  • Sensation arising from nasal cavity following stimulation of the olfactory epithelium by volatile compounds.
  • One of neglected sense  (Limited accessibility, limited theoretical knowledge, belief that it’s not important in man)
  • Helps to protect from harmful substances such as environmental contaminants, spoilt food. It is livelihood of cooks, wine tasters, firefighters, chemists etc.
  • Unique in CNS

Only part in direct contact with environment

Ability to regenerate lost neurons

Potential damage by airborne pollutants

  • Pathway projects to limbic system so it also associated with emotions, thus has a social value too.

Anatomy

Nasal Passage

1) Due to shape of turbinates 2% of inspired air reaches olfactory epithelium

2) On sniffing 20%

3) Particulate matter in inspired air is deposited on nasal mucosa and can be swept by cillia to olfactory epithelium

Olfactory Neuro-Epithelium

2-4 cm2 located in the nasal vault/apex which covers

  • Part of superior turbinate
  • Superior part of nasal septum
  • Majority of cribriform plate
  • 10×106 receptor cells
  • Covered by mucus layer secreted by Bowman’s gland
  • Has a yellowish hue: cause of color is in dispute. It is either from
  1. a) Granules of bowmans gland
  2. b) Carotenoids in mucosa
  3. c) Phospholipids or breakdown products
  4. d) Secondary to lysosomes in base of supporting cells

Composition Olfactory Epithelium

Olfactory sensory neurons (OSN)/receptor cells

  • Bipolar sensory cells,Round cell body
  • 10-23 cillia on surface, 200 Microns long, a) immotile
  • Covered by membrane with inter-membranous particles specific to this region
  • Life span of olfactory receptor neuron is one month
  • Has apoptosis regulated cell death

Sustentacular /supporting cells ensheath the receptor neuron

  • High level of P450 and detoxifying enzymes role in removing odorant
  • Maintain the extra cellular potassium level needed for neuronal activity
  • Support the receptor cells

Basal cells

  • Small polygonal cells in contact with the underlying basement membrane
  • Stem cells for receptor and sustentacular cells
  • Role in regeneration after disease
  • Replace the neuroepithelium every 40 days

Lamina propria contains the olfactory nerve and mucus secreting bowmans gland

Unmylineated axons of receptor cells extend through the cribriform plate and synapse with olfactory bulb

Become covered by schwann cells

One schwann cell contains 5-10 fibers occasionally 100

Central Olfactory Structures

Olfacoty Bulb

  • 6-16mm long
  • Axons of 15000 receptor cells converge on one mitral cell/tufted cells
  • Mitral/tufted cell project a single primary dendrite to glomerulus
  • Each glomerulus has 25 mitral cells (1000:1 convergence)
  • Periglomerular cells have horizontal connections between glomeruli
  • Granule cells connect to the mitral cells and inhibit them
  • Axons of mitral cells form the Lateral olfactory tract(LOT

Carry impulse towards the CNS

  1. Amygdyla
  2. Pre-pyriform cortex
  3. Anterior olfactory nucleus
  4. Entorhinal cortex
  5. Hippocampus, hypothalamus, thalamus

Olfactory Cortex

  • Lateral olfactory tract courses ventrally over the prepyriform cortex reaches amygdaloid body
  • Branches from above also reach anterior olfactory nucleus, prepyriform cortex and nucleus of lateral olfactory tract
  • Olfactory cortex structures send secondary fibers to limbic system
  1. a) amygdyala, hippocampus, hypothalamus, thalamus
  2. b) Other secondary connections also exist to orbit frontal cortex

Physiology

Olfactory Stimulus

  • Airborne chemical molecules
  • Intensity of stimulus is proportional to molecules in contact with epithelium
  • Rate of being perceived is different for different odour
  1. a) Depends on water solubility
  2. b) Chemical group

Olfactory Threshold

  • Defined as stimulus concentration detected 50% of time
  • Adaptation can occur with multiple presentations
  • Cross-adaptation is also known

Theories for olfaction

Stereochemical theory – Receptor have different shape to receive complimentary odorous chemical

Puncture and penetration – Odorous chemical enter the receptor and cause depolarization

Molecular vibration – Vibrational frequency of molecules determine the odour

Spatio-temporal theory – Pattern of spread of chemical across the epithelium

No theory has been fully accepted

Chemical enters—absorbed in olfactory mucosa—carried on a binding globulin to cilia—Ca channels opens based on cAMP and IP3

CNS Olfactory System:

Olfactory Bulb

  • Mitral cell are neurons with primary function of transmitting sensory data
  • Due to extensive network considerable processing of data occurs
  • Processing is odour specific
  • Due to inhibiting mechanism lot of temporal sharpening of input data occurs
  • Synaptic connections form loops which generate EEG that can be measured with electrodes in olfactory bulb

Thus main function is to perform integration and higher level data processing to aid olfactory perception and control guided behavior.

Evidence suggest that specificity in the mitral cells of the olfactory bulb corresponding to projections from olfactory receptors

Brain Olfactory Mechanism

  • Cortex also know to be involved in regulation of basic behavior
  • Hypothalamus intimately related to hunger, thirst, sexual activity and sleep
  • Amygdala related to social and sexual activity

These area receive impulse from olfactory cortex and may have an effect on there functioning

Causes of olfactory Disturbance:

Nervous – Alzheimer’s, downs, M. sclerosis, parkinsonism

Congenital – Kallamans syndrome

Metabolic – CRF, Liver disease

Trauma – Head injury , post surgical

Inflammatory – Sinusitis, URI, Allergy, Wagner’s Granulomatosis

Endocrine – Adrenal  insufficiency, Cushing’s, Hypothyroid, DM

Neoplasm – Neuroblastoma, Anterior skull base tumor

Degenerative – Age

Infections – viral , Influenza, HIV

Others – Environmental pollutants, Psychiatric Medicines

Disorders of Smell

Anosmia : Loss of smell

Hyposmia : Partial loss

Parosmia : Smel perversion

Author Name:

Dr Prasun Mishra

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