Transcript smell

‫بسم هللا الرحمن الرحيم‬
‫﴿و ما أوتيتم من العلم إال قليال﴾‬
‫صدق هللا العظيم‬
‫االسراء اية ‪58‬‬
Dr. Abdel Aziz M. Hussein
Lecturer of Physiology
Member of American Society of Physiology
• The olfactory (smell) sensation is a chemical sense
involving receptors which are sensitive to volatile
chemical substances
• It is located on the superior nasal concha and the
upper third of the nasal septum.
• Its total area in both sides is about 5 cm2.
• Olfactory mucosa is distinguished from the
surrounding respiratory mucosa by:
1. Its distinctive yellow-brown pigments
2. The presence of many tubular Bowman's glands
3. The absence of rhythmic ciliary movements that
characterize the respiratory mucosa
• The olfactory mucosa is innervated by;
1. Olfactory (cranial I) nerves which are stimulated by
the odorant substances
2. Branches of the trigeminal (cranial V) nerve which
are stimulated by the irritative effect of some odors as
peppermint, menthol, and chlorine.
• They are responsible for initiating sneezing,
lacrimation, respiratory inhibition, and other reflex
responses to nasal irritants.
• consists of 3 types of cells;
a) Olfactory receptor cells;
• Bipolar neurons which have:
1. A short thick dendrite with an expanded end called an
olfactory rod. Each rod has 10-20 cilia
2. Axons → pierce the cribriform plate of the ethmoid bone and
enter the olfactory bulbs.
• Each olfactory mucosa contains 100 million olfactory cells
• Their life span is about 60 days.
b) Supporting or (sustentacular) cells.
c) Basal cells:
• Are stem (progenitor) cells for production of new receptor
• The odorant substance must be;
1. Volatile in order to be sniffed into the nose
2. Sufficiently water soluble in order to pass through
mucous layer
3. Lipid soluble in order to penetrate the cells membrane
• Olfactory cells are stimulated when air blasts upwards
into the superior region of the nose.
• Amount of air reaching this region is greatly increased
by sniffing (contraction of the lower part of the nares on
the septum, deflecting the airstream upward)
• Odorant substance diffuses into the mucus that
covers the cilia, then binds with odorant-binding
proteins (OBP) that trap and concentrate the
substances and transfer them to the olfactory
• Binding the odour to the receptor activates Golf →
stimulates adenylate cyclase → produce cAMP→
cAMP opens Na, Ca channels → depolarization and
generation of receptor potential which at threshold
levels will fire an action potential
• 3 order neurons;
1) First order neurons:
• Olfactory receptors are the first order neurons.
• Each has a single nonmyelinated axon which must
pass through the cribriform plate to end in the
glomeruli in the olfactory bulb.
2) Olfactory bulb and 2nd order neurons:
• 2nd order neurons are mitral and tufted cells present
in the olfactory bulbs.
• Dendrites synapse with the axons of the olfactory
receptors in globular structures called the olfactory
• Each glomerulus contains the synapses of about
25,000 olfactory receptors and 100 of 2nd order
• About 60 glomeruli in each bulb.
• Axons of the mitral and tufted cells form the olfactory
tract that leaves the bulb to the olfactory cortex.
• 2 types of inhibitory interneurons present in the bulb:
1. Periglomerular cells → short inhibitory feedback
loops among the glomeruli.
2. Granule cells → reciprocal synapses with neuronal
processes of the mitral and tufted cells.
• Form long inhibitory feedback loop involving the
olfactory cortex.
The axons of the mitral and tufted cells form the
olfactory tract which enters the brain between the
cerebellum and mesencephalon
• They divide into 2 tracts that terminate on medial and
lateral olfactory areas
1. Medial olfactory area
2. Lateral olfactory area
• A) The medial olfactory area:
• The medial olfactory area consists of a group of nuclei
located in the immediately anterior to the
hypothalamus e.g. septal nuclei, and primitive
portions of limbic system.
• This olfactory area is concerned with:
1. Primitive responses to olfaction, such as licking the
lips, salivation caused by the smell of food.
2. Primitive emotional drives associated with smell.
B) The Lateral Olfactory Area:
1) To Prepyriform and pyriform cortex and
amygdaloid nuclei → almost all portions of the limbic
system, especially the hippocampus
• concerned with:
1. Learning to like or dislike certain foods.
2. Behavioral responses to food e.g. development of
absolute aversion to foods
3. Olfactory conditioned reflexes
B) The Lateral Olfactory Area:
2) To the dorsomedial thalamic nucleus →
orbitofrontal cortex.
• This system is responsible for the conscious
perception of olfaction i.e. analysis, interpretation
and significance of the odour.
• Adaptation of olfactory receptors is;
1. Specific i.e. occurs for this odour only and not for the
other odours
2. Rapid i.e. occurs within few minutes (about 50% in
the first second thereafter, they adapt very little and
very slowly).
3. It is mainly central due to large number of
centrifugal nerve fibers
4. Peripheral mechanism of adaptation is also present
and believed to be due to an odorant eating enzymes
in the mucus layer
1. Stimulation of all GIT secretions e.g. salivary
secretion through conditioned reflexes
2. Determination of the flavor of food with taste
3. Selection of the type of food i.e. separation of
undesirable foods from those are nutritious.
4. Control of sexual functions particularly in
animals. Smell is acute in women particularly
at the time of ovulation.
• Man can recognize more than 10,000 different
• Determination of differences in the intensity of any
given odor is poor.
• The concentration of an odor-producing substance
must be changed by about 30% (it is 1% in visual
receptors) before a difference can be detected.
• So, concerned mainly with detection of the presence
or absence of the odours rather than with detection
of their intensities.
• Methyl mercaptan, one of the substances in garlic, can
be smelled at a concentration of less than 500 pg/L of air.
1. Anosmia: complete absence of the sense of smell
2. Hyposmia: diminished olfactory sensitivity. More
than 75% of humans over the age of 80 have an
impaired ability to identify smells.
3. Dysosmia: distorted sense of smell e.g.
• Olfactory hallucinations due to lesion in the
prepyriform cortex.
• Hypersensitivity for both smell and taste as in
Addison diseases
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