sense of smell
DESCRIPTION
For SSC-II & HSSC-IITRANSCRIPT
SENSE OF SMELL
EXPERT VISION ACADEMY
Smell
Chemical Senses: Taste and Smell
Both senses use chemoreceptors Stimulated by chemicals Taste has four types of receptors Smell can differentiate a large range of
chemicals Both senses complement each other and
respond to many of the same stimuli Olfaction (smell) and gustation (taste) Both project to cerebral cortex & limbic
system Evokes strong emotional reactions
Olfactory organs•Contain olfactory receptors and supporting epithelial cells•Cover parts of nasal cavity and a portion of the nasal septum
Olfactory receptors•Chemoreceptors•Respond to chemicals dissolved in liquids
Olfactory Epithelium•Located on roof of nasal cavity•Contain olfactory receptor cells with columnar supporting cells•Covered by mucous to trap airborne molecules
Sense of Smell
Olfactory Epithelium
6
Cells of the Olfactory Membrane
Olfactory receptors bipolar neurons with
cilia or olfactory hairs Supporting cells
columnar epithelium Basal cells = stem cells
replace receptors monthly
Olfactory glands produce mucus
Sense of Smell
Air entering the nasal cavity must make a hairpin turn to stimulate the olfactory receptors before entering the respiratory passageway below
Sniffing also brings the air superiorly across the olfactory
epithelium intensifies the smell
Olfactory tract
Olfactory bulb
(a)
Nasalconchae
Route ofinhaled air
Olfactoryepithelium
Mitral cell (output cell)
Olfactorygland
Olfactorytract
Olfactoryepithelium
Filaments of olfactory nerve
Cribriform plate of ethmoid bone
Lamina propria connective tissue
Basal cell
Supporting cell
Dendrite
Olfactory cilia
Olfactory bulbGlomeruli
Axon
Olfactory receptor cell
Mucus
Route of inhaled aircontaining odor molecules(b)
Olfactory Cells
Dendrites of each olfactory cell called olfactory cilia
extend into the nasal cavity
Olfactory axons project upward through the foramina in the
cribriform plate of the ethmoid bone of the skull synapse on neurons within the olfactory nerve
Physiology of Smell
Series of events the chemical must be volatile
it must be in the gaseous state as it enters the nasal cavity
the chemical must be water soluble so that it can dissolve in the fluid containing the olfactory
epithelium the dissolved chemicals stimulate the olfactory
receptors by binding to protein receptors in olfactory cilium membranes
the generation of APs in the olfactory cells an impulse travels through the olfactory cell axons to
the olfactory nerve where the smell sensation is transmitted to the brain
OLFACTORY PATHWAY
As air enters the nasal passageway the level of odor-producing chemicals dissolve in the mucus membrane
With in the mucus membrane the olfactory cilia capture the chemicals
After reaching a threshold level it is passed to the olfactory nerves located in the olfactory bulb
The impulse passes through the olfactory tract and into the thalamic and olfactory centers of the brain for interpretation, integration, and memory storage.
The sense of smell can create powerful and long-lasting memories. That often persist from early childhood to death
Olfactory Nerve Pathways
Olfactory receptor fibers synapse with neurons in the olfactory bulbs (cranial nerve I).
Impulses travel along the olfactory tracts Impulses are interpreted in olfactory cortex. Olfactory receptor neurons are in direct contact with
the environment and can be replaced if damaged. Olfactory receptors undergo sensory adaptation
rapidly Sense of smell drops by 50% within a second after
stimulation
Transmission of smell signals to CNS
Abnormalities
Anosmia – absence of sense of smell
Hyposmia – diminished olfactory sensitivity
Dysosmia – distorted sense of smell
More than 75% of humans over the age of 80 have an impaired ability to identify smells
Anosmia
Absence of the sense of smell
Trauma Colds or allergies producing excessive
mucus Polyps causing blockage 1/3 are from zinc deficiency