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Primal sense for humans as well as animals

§ Most ancient of senses

§ Subjective phenomenon

§ Poorly developed in human beings

Importance of Smell

Eliciting Salivary & Gastric secretions

§ HYGEINE Monitor

§ Social Information

§ Influencing Sexual Behavior §Affecting emotional state

Important for enjoyment & selection of food

§ Warning of harmful substances or places

§ Important means by which our environment communicates with us

§ Physiologically, smell & taste are related

§ Anatomically, smell & taste are different

Olfactory Mucous Membrane

Yellowish pigmented specialized nasal mucosa

§Total surface area = 5cm2 –Very small as compared to macrosmatics

Olfactory Epithelium

Olfactory cells

§ Supporting cells (Sustentacullar cells)

§ Basal cells

§ Microvillar cells

§Bowman’s glands

Olfactory Cells

Specialized bipolar neurons

n Derived from CNS

n Short thick dendrite with an expanded end

–   ‘OLFACTORY ROD’ – 6-12 cilia project

Ciliary plasma membrane contains ODORANT RECEPTORS

n Basal region gives rise


Continual degeneration & replacement

§ Olfactory renewal process is regulated by BMP (bone morphogenic protein)



§ Volatile

§ Water soluble

§ Lipid soluble



§ 1000 in number

§ 7 transmembrane protein

Few odorants hyperpolarize olfactory cells

Importance of Indirect Mechanism –Greatly multiplies the excitatory effect of even the weakest odorant

Olfactory Bulb

Anterior growth of brain tissue

§ Different cells

§ Mitral cells

§ Tufted cells

§ Periglomerular cells

§ Granule cells

Olfactory Glomeruli

25,000 olfactory cell axons terminate on dendrites of 25 mitral & 60 tufted cells

Inhibitory interneurons

Periglomerular cells

Granule cells

Significance of Inhibitory cells

Lateral inhibition of neighbouring glomeruli – sharpens olfactory signals

Olfactory Tract

§Axon of mitral & tufted cells

Functions of Olfactory Cortex

§ Very Old Olfactory System

§ More primitive responses to olfaction

§ Less Old Olfactory System §Learned control of food intake

§Newer System

§Conscious perception & analysis of odor

§Odor discrimination

§Activation is greater on right side

The portion of the brain involved in olfaction are supposed to be the oldest structure

Centrifugal fibers

Sharpens the capability of distinguishing one odour from another §Olf. acuity is best in 3rd-5th decade of life

§ Female > Male

Trigeminal Innervation of General Nasal Mucous Membrane

§ V nerve innervates respiratory mucous membrane along nasal & oropharyngeal passages

§ Responds to




Commonly encountered trigeminal stimulants

§ Allyl isothiocyanate (mustard)

§Capsiacin (hot chille powder)

§Diallyl sulfide (onion)

Primary Sensations of Smell








§100 – 1000 pri. Sensations

Olfactory Threshold

§Minimum concentration of a substance that can be smelled

§Methyl mercaptan – <500pg/L of air

Olfactory Discrimination

§Recognize > 10,000 odors §Determination of differences in the intensity of any given odor – poor §Conc. must be changed 30% before difference can be detected

§Occurs at two levels

§At receptor level

§At olfactory glomeruli level


§Contraction of lower part of nare

§Semi-reflex response to new odor §Sniffing activate the piriform cortex

Adaptation of Smell

§Occurs when continuously exposed to particular odor

§Perception of odor decreases §Eventually ceases

§Some occurs at level of RECEPTOR

§Most occurs at level of CNS (centrifugal fibres)


§Cul de sac

§Narrow tubular entrance – empties into Nasal Cavity

§In a pit on ant. 3rd of nasal septum §Walls of tube lined with Neuroepithelium ( has microvilli but no cilia )

Not well developed in humans §Concerned with perception of odors – PHEROMONES


§Substances produced by an animal that act at a distance to produce hormonal, behavioral or other physiological changes in another animal of same species

§There is close relationship b/w this & sexual function

Bonding b/w mother & infant §Important in relation to suckling behavior

Restricted parts of olfactory bulb mature earlier

§Early maturing Glomeruli at birth – lie b/w main bulb & accessory olfactory bulb Activated by PHEROMONES associated with breast feeding

Applied Physiology





§Anosmia + hypogonadotropic hypogonadism = KALLMANN’S SYNDROME


§Upper RTI – sinusitis, common cold §Nasal polyps


§Head trauma, damage to the ethmoid bone


§Reduced ability to smell & detect odors

§Upto 4 million affected in USA §Very early sign of Parkinsonism

PHANTOSMIA (Olfactory Hallucination)

§smelling odors that are NOT really present §damage to the nervous tissue in the olfactory system

§viral infection

§brain tumor §



§exposure to toxins or drugs

§Epilepsy affecting the olfactory cortex


§Distorted sense of Olfaction


§Accidental damage to Olfactory system

§Burning flesh

§Garbage etc.

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