PHYSIOLOGY OF

PHYSIOLOGY OF

SMELLSMELL

KASHMEERA.N.AKASHMEERA.N.A

II SEM MSc ZOOLOGYII SEM MSc ZOOLOGY

ROLL NO:37ROLL NO:37

Important for enjoyment & Important for enjoyment & selection of food .selection of food .

Flavours are combinations of taste Flavours are combinations of taste and smell (smell contribution is and smell (smell contribution is about 80 %)about 80 %)

Gives warning of harmful Gives warning of harmful substances or placessubstances or places

Primary Sensations of SmellPrimary Sensations of Smell

Based on psychological studies, one attempt to classify these sensations is the following:

• 1. Camphoraceous• 2. Musky• 3. Floral• 4. Pepperminty• 5. Ethereal• 6. Pungent• 7. Putrid

In recent years, specific studies of the genes that encode for the receptor proteins, suggest the existence of at least 100 primary sensations of smell

Anatomy of Olfactory Anatomy of Olfactory ReceptorsReceptors

• The nose contains 10–100 million olfactory receptors The nose contains 10–100 million olfactory receptors contained within an area called the olfactory epitheliumcontained within an area called the olfactory epithelium..

• olfactory epithelium lies in the superior part of each olfactory epithelium lies in the superior part of each nostril.nostril.

• In each nostril, the olfactory membrane has a surface In each nostril, the olfactory membrane has a surface area of about 2.4 square centimeters.area of about 2.4 square centimeters.

• The olfactory epithelium consists of three kinds of cells:The olfactory epithelium consists of three kinds of cells: olfactory receptors, olfactory receptors, supporting cells / sustentacular cellssupporting cells / sustentacular cells basal cellsbasal cells

Olfactory receptors

• The receptor cells for the smell sensation are the olfactory cells .The receptor cells for the smell sensation are the olfactory cells .

• They are actually bipolar nerve cells derived from the CNS . They are actually bipolar nerve cells derived from the CNS .

• There are about 100 million of these cells in the olfactory epithelium.There are about 100 million of these cells in the olfactory epithelium. • The mucosal end of the olfactory cell forms a knob .The mucosal end of the olfactory cell forms a knob .

• From knob 4 to 25 olfactory hairs (olfactory cilia), project into the From knob 4 to 25 olfactory hairs (olfactory cilia), project into the mucus that coats the inner surface of the nasal cavity.mucus that coats the inner surface of the nasal cavity.

• These projecting olfactory cilia form a dense mat in the mucus.These projecting olfactory cilia form a dense mat in the mucus.

• These cilia react to odours in the air and stimulate the olfactory These cilia react to odours in the air and stimulate the olfactory cellscells

Supporting cells/sustentacular cellsSupporting cells/sustentacular cells

The receptor cells in the olfactory epithelium are interspersed among The receptor cells in the olfactory epithelium are interspersed among sustentacular cells or supporting cells.sustentacular cells or supporting cells.

Supporting cells Supporting cells are columnar epithelial cells.are columnar epithelial cells.

They provide physical support, nourishment and electrical insulation for They provide physical support, nourishment and electrical insulation for the olfactory receptors, the olfactory receptors,

They help detoxify chemicals that come in contact with the olfactory They help detoxify chemicals that come in contact with the olfactory epithelium.epithelium.

Basal cellsBasal cells• Basal cells are stem cells located between

the bases of the supporting cells.

• They continually undergo cell division to produce new olfactory receptors, which live for only a month or so before being replaced.

• This process is remarkable - olfactory receptors are neurons, and mature neurons are generally not replaced.

• The olfactory renewal process is carefully regulated - a bone morphogenic protein (BMP) exerts an inhibitory effect.

• [ BMPs are a large family of growth factors originally described as promoters of bone growth]

Spaced among the olfactory cells in the olfactory membrane are many small Bowman’s glandsBowman’s glands that secrete mucus onto the surface of the olfactory membrane

mucus is carried to the surface of the epithelium by ducts.

The secretion moistens the surface of the olfactory epithelium and dissolves odourants so that transduction can occur.

Physiology of OlfactionPhysiology of Olfaction

MECHANISM OF EXCITATION OF OLFACTORY CELLS.MECHANISM OF EXCITATION OF OLFACTORY CELLS.

Cilium is the portion which respond to the olfactory chemical stimuli.Cilium is the portion which respond to the olfactory chemical stimuli.

The odourant substance on coming in contact with olfactory surfaceThe odourant substance on coming in contact with olfactory surfacefirst diffuse in to the mucus which covers the cilia.first diffuse in to the mucus which covers the cilia.

Then binds with a receptor protein that protrudes through the ciliary Then binds with a receptor protein that protrudes through the ciliary membrane.membrane.

This receptor is a long molecule, it threads its way through the This receptor is a long molecule, it threads its way through the membrane 7 times, folding inward and outward. membrane 7 times, folding inward and outward.

Odourant binds with portion of receptor and coupled to Odourant binds with portion of receptor and coupled to G-PROTEIN.G-PROTEIN.

G-PROTEIN –a combination of 3 subunits.G-PROTEIN –a combination of 3 subunits.

Odourant receptor

heptahelical receptor/

serpentine receptor/

G protein-linked receptor (GPLR)

On excitation of receptor,an alpha subunit breaks away On excitation of receptor,an alpha subunit breaks away from G-PROTEIN and activates adenylcyclase.from G-PROTEIN and activates adenylcyclase.

Activated cyclase converts many molecules of intracellular Activated cyclase converts many molecules of intracellular adenosine-tri-phosphate into cyclic-adenosine adenosine-tri-phosphate into cyclic-adenosine monophosphate(cAMP).monophosphate(cAMP).

This cAMP activates another near by membrane protein,a This cAMP activates another near by membrane protein,a gated sodium ion channel.gated sodium ion channel.

Allows large number of sodium ions to pour into receptor Allows large number of sodium ions to pour into receptor cell cytoplasm.cell cytoplasm.

Sodium ions helps in exciting the olfactory neuron and Sodium ions helps in exciting the olfactory neuron and transmitting action potential in to the CNS through an transmitting action potential in to the CNS through an olfactory nerve.olfactory nerve.

Odourant + receptor Odourant + receptor

proteinprotein

Activation of G proteinActivation of G protein

Activation of adenylate Activation of adenylate

cyclasecyclase

ATP ATP cAMP cAMP

Opening of NaOpening of Na++ channels channels

NaNa++ influx influx

depolarizationdepolarization

Mechanism of olfactory cell stimulationMechanism of olfactory cell stimulation

Physical factors affect the degree of Physical factors affect the degree of stimulation.stimulation.

Only volatile substancesvolatile substances that can be sniffed into the nostrils can be smelled.

The stimulating substance must be at least slightly water solublewater soluble so that it can pass through the mucus to reach the olfactory cilia.

The substance should be at least slightly lipid lipid solublesoluble, because lipid constituents of the cilium itself are a weak barrier to non-lipid-soluble odourants.

Cribiform plate separates nasal cavity and cranial cavity.Cribiform plate separates nasal cavity and cranial cavity.

Olfactory bulb lies above cribiform plate.Olfactory bulb lies above cribiform plate.

Small nerves from olfactory membrane in nasalcavity pass through the small Small nerves from olfactory membrane in nasalcavity pass through the small perforations in the cribiform plate to enter olfactory bulb in the cranial cavity.perforations in the cribiform plate to enter olfactory bulb in the cranial cavity.

Olfactory nerve fibers leading from olf.bulb are called Cranial nerve I or olf. Olfactory nerve fibers leading from olf.bulb are called Cranial nerve I or olf. Tract.Tract.

Short axons from the olfactory cells terminate in multiple globular structures Short axons from the olfactory cells terminate in multiple globular structures within the olfactory bulb called within the olfactory bulb called glomeruli.glomeruli.

Each bulb has several thousand such glomeruli,each of which is the terminus for Each bulb has several thousand such glomeruli,each of which is the terminus for about 25,000 axons from olfactory cells. about 25,000 axons from olfactory cells.

Each glomerulus also is the terminus for dendrites from about 25 large Each glomerulus also is the terminus for dendrites from about 25 large mitral mitral cellscells and about 60 smaller and about 60 smaller tufted cellstufted cells, , the cell bodies of which lie in the olfactory the cell bodies of which lie in the olfactory bulb superior to the glomeruli. bulb superior to the glomeruli.

These dendrites receive synapses from the olfactory cell neurons,These dendrites receive synapses from the olfactory cell neurons,

the mitral and tufted cells send axons through the olfactory tract to transmitthe mitral and tufted cells send axons through the olfactory tract to transmitolfactory signals to higher levels in the central nervous system.olfactory signals to higher levels in the central nervous system.

Some research has suggested that different glomeruli respond to different Some research has suggested that different glomeruli respond to different odours.odours.

Olfactory pathways into theOlfactory pathways into theCentral Nervous SystemCentral Nervous System

The olfactory tract enters the brain at the anterior junction between the The olfactory tract enters the brain at the anterior junction between the mesencephalon and cerebrum;mesencephalon and cerebrum;

there, the tract divides into two pathways, one passing medially into the there, the tract divides into two pathways, one passing medially into the medialmedialolfactory areaolfactory area of the brain stem, and the other passing laterally into the of the brain stem, and the other passing laterally into the lateral lateral olfactory area. olfactory area.

The medial olfactory area represents a very old olfactory system, whereas the The medial olfactory area represents a very old olfactory system, whereas the lateral olfactory area is the input to lateral olfactory area is the input to

(1) A less old olfactory system and (2) a newer system(1) A less old olfactory system and (2) a newer system

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Olfactory pathway

Olfactory TractOlfactory Tract

Medial Olfactory areaMedial Olfactory area Lateral Olfactory areaLateral Olfactory area

Septal NucleiSeptal Nuclei

HypothalamusHypothalamusLimbic systemLimbic system(primitive (primitive parts)parts)

Prepyriform Prepyriform cortexcortex Pyriform CortexPyriform CortexAmygdalaAmygdala

LimbicLimbic systemsystem(hippocampus)(hippocampus)

ThalamuThalamuss

Orbitofrontal Orbitofrontal CortexCortex

Olfactory receptor cellOlfactory receptor cell Olfactory nerveOlfactory nerve

Olfactory Olfactory bulbbulb

(Very Old (Very Old Olfactory Olfactory System)System)

(Less Old (Less Old Olfactory System)Olfactory System)

(Newer (Newer System)System)

LIMBIC SYSTEMLIMBIC SYSTEM

Very Old Olfactory SystemVery Old Olfactory SystemMore primitive responses to olfaction

salivation, liking lips and primitive emotional drives to smell

Less Old Olfactory SystemLess Old Olfactory SystemLearned control of food intake

Aversion to food that have caused nausea and vomiting.

Newer SystemNewer SystemConscious perception & analysis of odour

Odour discrimination

Thank You

Take time toTake time to smell the flowers…. smell the flowers….