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
- a) Granules of bowmans gland
- b) Carotenoids in mucosa
- c) Phospholipids or breakdown products
- 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
- Amygdyla
- Pre-pyriform cortex
- Anterior olfactory nucleus
- Entorhinal cortex
- 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
- a) amygdyala, hippocampus, hypothalamus, thalamus
- 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
- a) Depends on water solubility
- 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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