Hello everyone!! This is my new article on olfaction. It may be surprising to you to know that Smell of food determines its taste. Get ready to know HOW??
Olfaction
It is Sense of smellInteresting fact!!
Out of all 12 cranial nerves, olfactory nerve is the first and the shortest nerve and is the only cranial nerve which doesn’t relay thalamus to reach brain cortexIntroduction
The primary function of olfaction is ability to detect environmental chemicals.
Sense of smell determines flavours of foods by stimulating gustatory and somatosensory stimuli so if any food has better smell than it will make that food more tasty.
It also aids digestion by triggering normal gastrointestinal secretions.
The recent study done in older persons shows mortality risk nearly two and a half times higher in those with impaired olfaction than those with normal olfaction.
Anatomy and Physiology
Olfactory neuroepithelium
• It is linned by pseudostratified columnar epithelium
Types of cells present in olfactory neuroepithelium:
1. Bipolar sensory receptor neuron derived embryologically from the olfactory placode
extends odourant receptor-containing cilia into the mucus.
2 Supporting or sustentacular cell -
* regulates mucus production
* transports molecules across the epithelium
*detoxifies and degrades odourants.
3. Duct cell of Bowman’s glands - secretes most of mucus.
4. Microvillar cell - located at surface of epithelium and sends tufts of microvilli into nasal mucus.
5.Horizontal (dark) basal cells - stem cells within basement membrane of epithelium.
6.Globose (light) basal cells - a multipotent basal cell - give rise to neurons and non-neuronal cells including the horizontal basal cells.
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Image by Britannica https://www.britannica.com/science/olfactory-epithelium
Image from Scott’s and Brown; 8th edition
Olfaction pathway
The odourants enters the nose by either active (e.g. sniffing) or passive (e.g., diffusion) process. Than it passes through the olfactory cleft and moves from air phase to largely aqueous phase of the olfactory mucus. It gets diffuse or transported by specialized proteins (odourant binding proteins) to the receptors in olfactory neuroepithelium. Approximately 6 million receptor cell axons ultimately coalesce into 30–50 fascicles to form olfactory fila. Than it traverse cribriform plate and pia matter.
It synapses with second-order neurons in glomeruli of the olfactory bulb (ventral surface of the frontal lobes directly over the cribriform plate). Neurotransmitter used by olfactory receptor to excite olfactory bulb are primarily glutamate and used as modulator of olfactory nerve input is dopamine.
Image by Katalin Gallatz; Slide playerhttps://slideplayer.com/slide/14461438/
Olfactory bulb
Mitral and Tufted cells – They are main afferent second-order neurons
Olfactory bulbs also contain:
*periglomerular cells – It contains inhibitory neurons connecting one glomerulus to another.
*granule cells - It has no axons and make reciprocal synapses with the lateral dendrites of the mitral and tufted cells inhibiting those cells
At these synapses, mitral or tufted cells excite the granule cell by releasing glutamate and granule cells in turn inhibit the mitral or tufted cell by releasing GABA which is known as Reverberating’ circuits (negative and positive feedback)
Signal transduction
• chemical energy of receptor converting into a neural signal
• complex cascade- activation of G proteins and various second messenger enzymes
Primarily the signal is conveyed to ipsilateral side of brain cortex but some cortical projections from primary to secondary (i.e. orbitofrontal) cortex ultimately relay through the thalamus. And some contralateral projections via anterior commissure arises from pyramidal cells of the anterior olfactory nucleus (AON)
AON is a structure whose cells are primarily in the rostral olfactory peduncle
Image by Michaels; Quizlet
https://quizlet.com/36829041/cn-i-olfactory-pathways-olfaction-michaels-flash-cards/
Olfactory memory
I guess everyone knows smell of rose.
Can everyone recall the smell of rose??
Answer is Yesss!!
This is due to olfactory memory which let us recall those smell which we have smelled it before.
Amygdala, neocortex and hippocampus plays role in olfactory memory.
The Amygdala is situated in the anterior temporal lobe and lies beneath the primary olfactory cortex and is involved in the formation of memories of emotional experiences. It is connected by various pathways to other parts of the brain mostly to the basal forebrain which contains magnocellular cells which provide extensive input into the neocortex and hippocampus. There are also direct projections to the hippocampus from the amygdala, which are involved in the integration of various sensations into memory.
Image by Nawaf AlKhudairy
Olfactory disorders
• Anosmia - inability to detect qualitative olfactory sensations (i.e., absence of smell function)
• Partial anosmia - ability to perceive some, but not all, odours
• Hyposmia or microsmia - decreased sensitivity to odours
• Hyperosmia - increased sensitivity to common odours;
• Parosmia: any olfactory sensation takes a foul odour
• Cacosmia: constantly present foul odour
• Olfactory agnosia - inability to recognize an odour sensation, even though olfactory processing, language, and general intellectual functions are essentially intact
stroke patient
associated with lesions of the right inferior temporal lobe
prosopagnosia (agnosia for familiar faces)
• Phantosmia - dysosmic sensation perceived in the absence of an odour stimulus (a.k.a. olfactory hallucination)
Olfactory reference syndrome- depression related disorder when someone believes a smell is present (hallucination) and persistently gives this smell personal reference to outside events, despite contradicting evidence
• Heterosmia – a condition where all odours smell the same
• Presbyosmia – a decline in smell sense with age
• Osmophobia – a dislike or fear of certain smells
Olfactory loss in covid19
• Prevalence – 68% to 85.6%
Possible mechanism by which corona virus impair our olfaction are as follows:
- conductive impairment due to obstructive inflammation of olfactory cleft blocking odors reaching olfactory cleft
- sensorineural impairment due to damage to olfactory nerve
- direct damage by virus to olfactory receptor
- virus target angiotensin converting enzyme 2 receptors found on sustentacular and basal cells of nasal epithelium and destroys it
- virus is capable of invading CNS through olfactory bulb or can cause peripheral neuropathy
So if you are suffering from covid19 and you have lost your smell than don’t worry it will become normal with 2 – 3 weeks
Thanks for reading. I hope you enjoyed it.
Reference
- Scott’s and Brown; 8th edition
My previous article
2.https://hive.blog/lovesniper/@doctorshrestha/introduction-of-myself
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