O F G Y L O S IO Y L H LP E S M KASHMEERA.N.A II SEM MSc ZOOLOGY ROLL NO:37
Important for enjoyment &selection of food .Flavours are combinations oftaste and smell (smell contributionis about 80 %)Gives warning of harmfulsubstances or places
Primary Sensations of SmellBased 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. PutridIn recent years, specific studies of the genes that encode for the receptor proteins, suggest the existence of at least 100 primary sensations of smell
• The nose contains 10–100 million olfactory receptors contained within an area called the olfactory epithelium.• olfactory epithelium lies in the superior part of each nostril.• In each nostril, the olfactory membrane has a surface area of about 2.4 square centimeters.• The olfactory epithelium consists of three kinds of cells: olfactory receptors, supporting cells / sustentacular cells basal cells
Olfactory receptors• The receptor cells for the smell sensation are the olfactory cells .• They are actually bipolar nerve cells derived from the CNS .• There are about 100 million of these cells in the olfactory epithelium.• The mucosal end of the olfactory cell forms a knob .• From knob 4 to 25 olfactory hairs (olfactory cilia), project into the mucus that coats the inner surface of the nasal cavity.• These projecting olfactory cilia form a dense mat in the mucus.• These cilia react to odours in the air and stimulate the olfactory cells
Supporting cells/sustentacular cellsThe receptor cells in the olfactory epithelium are interspersed amongsustentacular cells or supporting cells.Supporting cells are columnar epithelial cells. They provide physical support, nourishment and electrical insulation forthe olfactory receptors,They help detoxify chemicals that come in contact with the olfactoryepithelium.
Basal 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 smallBowman’s glands that secrete mucus onto the surface of the olfactory membranemucus is carried to the surface of the epithelium by ducts.The secretion moistens the surface of the olfactory epithelium and dissolvesodourants so that transduction can occur.
MECHANISM OF EXCITATION OF OLFACTORY CELLS.Cilium is the portion which respond to the olfactory chemical stimuli.The odourant substance on coming in contact with olfactory surfacefirst diffuse in to the mucus which covers the cilia. Then binds with a receptor protein that protrudes through the ciliarymembrane.This receptor is a long molecule, it threads its way through themembrane 7 times, folding inward and outward.Odourant binds with portion of receptor and coupled to G-PROTEIN.G-PROTEIN –a combination of 3 subunits.
On excitation of receptor,an alpha subunit breaks awayfrom G-PROTEIN and activates adenylcyclase.Activated cyclase converts many molecules of intracellularadenosine-tri-phosphate into cyclic-adenosinemonophosphate(cAMP).This cAMP activates another near by membrane protein,agated sodium ion channel.Allows large number of sodium ions to pour into receptorcell cytoplasm.Sodium ions helps in exciting the olfactory neuron andtransmitting action potential in to the CNS through anolfactory nerve.
Mechanism of olfactory cell stimulation Odourant + receptor protein ↓ Activation of G protein ↓ Activation of adenylate cyclase ↓ ATP → cAMP ↓ Opening of Na+ channels ↓ Na+ influx ↓
Physical factors affect the degree ofstimulation. Only volatile substances that can be sniffed intothe nostrils can be smelled. The stimulating substance must be at leastslightly water soluble so that it can pass throughthe mucus to reach the olfactory cilia. The substance should be at least slightly lipidsoluble, because lipid constituents of the ciliumsolubleitself are a weak barrier to non-lipid-solubleodourants.
Cribiform plate separates nasal cavity and cranial cavity.Olfactory bulb lies above cribiform plate.Small nerves from olfactory membrane in nasalcavity pass through the smallperforations 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.Tract.
Short axons from the olfactory cells terminate in multiple globular structureswithin the olfactory bulb called glomeruli.Each bulb has several thousand such glomeruli,each of which is the terminus forabout 25,000 axons from olfactory cells.Each glomerulus also is the terminus for dendrites from about 25 large mitralcells and about 60 smaller tufted cells, the cell bodies of which lie in the olfactorybulb superior to the glomeruli.These dendrites receive synapses from the olfactory cell neurons, the mitral and tufted cells send axons through the olfactory tract to transmitolfactory signals to higher levels in the central nervous system.Some research has suggested that different glomeruli respond to differentodours.
Olfactory pathways into the Central Nervous System
The olfactory tract enters the brain at the anterior junction between themesencephalon and cerebrum;there, the tract divides into two pathways, one passing medially into the medialolfactory area of the brain stem, and the other passing laterally into the lateralolfactory area.The medial olfactory area represents a very old olfactory system, whereas thelateral olfactory area is the input to(1) A less old olfactory system and (2) a newer system
Olfactory pathwayOlfactory receptor cell Olfactory nerve Olfactory bulb Olfactory TractMedial Olfactory area Lateral Olfactory area Prepyriform cortex Thalamus Septal Nuclei Pyriform Cortex Amygdala Hypothalamus Limbic system Limbic system Orbitofrontal (primitive parts) (hippocampus) Cortex(Very Old (Less Old Olfactory (Newer System)Olfactory System) System) 25
Very Old Olfactory System More primitive responses to olfaction salivation, liking lips and primitive emotional drives to smellLess Old Olfactory SystemLearned control of food intake Aversion to food that have caused nausea and vomiting.Newer SystemConscious perception & analysis of odour Odour discrimination