1 Sense of Smell. Smell (=Olfaction) 2 Normal individual can discriminate more than 10.000 odors,...
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- Slide 1
- 1 Sense of Smell
- Slide 2
- Smell (=Olfaction) 2 Normal individual can discriminate more
than 10.000 odors, but not highly developed in human as in some
animals. Olfactory receptors in nose are specialized endings of
renewable afferent neurons Olfactory mucosa: 3 cm 2 of mucosa in
ceiling of nasal cavity.
- Slide 3
- Significance of olfaction 3 Olfaction is related to taste (in
severe rhinitis loss of specific taste of some foods). The
receptors of olfaction are chemoreceptors (moderately adapting) and
they are true neurons & close to external environment.
Olfaction may produce changes in GIT motility & secretion.
Olfaction may produce Sexual & Psychogenic effects. Olfaction
has a Protective function (dangerous odors as gases or fires).
- Slide 4
- 4 Olfactory mucosa contains 3 cell types: 1- Olfactory receptor
cells: true bipolar neurons derived from CNS. 2- Supporting cells:
Secrete mucus 3- Stem Basal cells: Precursors of new olfactory
receptor cells (replaced about every two months)
- Slide 5
- 5 Olfactory receptor cells: Afferent neuron whose receptor
portion is in olfactory mucosa in nose and afferent axon traverses
into brain. Receptor portion is enlarged knob-like bearing several
long cilia. Cilia contains binding sites for attachment of odorant
molecule. Axons of olfactory receptor cells collectively form
olfactory nerve
- Slide 6
- 6 Odorant substances must be volatile to come in contact with
olfactory epithelium and must be water soluble to be dissolved in
mucous and to cross the cell membrane. The olfactory mucus contains
odorant binding proteins that transmits & concentrate the
odorant substances to the receptors. Human nose contains approx. 5
millions olfactory receptors * Physiological information about
olfaction:
- Slide 7
- 7 There are 1000 different odorant receptors encoded in 1000
different genes and this explains the ability of normal individual
to discriminate more than 10.000 odors. Afferent signals are sorted
according to action of glomeruli within olfactory bulb.
- Slide 8
- Mechanism of stimulation of olfactory receptors: Chemical
theory: Copyright 2009, John Wiley & Sons, Inc. RMP of
olfactory receptors is -55 mv and at this level continuous impulses
arise from these receptors at a rate of 1: 3/ sec. Stimulation of
olfactory receptors by odorant substance this potential from -55 to
-30 mv (depolarization) No. of impulses up to 20/sec. When odorant
substances come in contact with the olfactory area, they diffuse
into mucous layer and bind with receptor protein on cilia. Such
binding activate G protein activate adenyl cyclase cAMP open Na +
channels depolarization (receptor potential) nerve impulse through
olfactory nerves olfactory bulb olfactory tract primary olfactory
area of the cerebral cortex
- Slide 9
- Olfactory transduction Copyright 2009, John Wiley & Sons,
Inc.
- Slide 10
- Olfactory Pathway 10 Olfaction is the only sense which has no
area in neocortex unlike other sensations but it is detected mainly
in limbic system. The first order neurons: are the receptor cells
in the olfactory mucosa. Their axons unit to from "Olfactory nerve"
(1st cranial nerve), these axons penetrate cribriform plate of
ethmoid bone to enter the "Olfactory bulb".
- Slide 11
- 11 The second order neurons: are "mitral & tufted cells" in
the olfactory bulb and they synapse with axons of olfactory cells
forming "Olfactory glomeruli", each glomerulus contains axons of
25,000 receptor cells that synapse with 25 mitral and 60 tufted
cells. Different glomeruli respond to different odors. The axons of
tufted and mitral cells pass posteriorly to form "Olfactory tract"
which enter the brain then divides into three stria.
- Slide 12
- Processing of Scents in Olfactory Bulb 12
- Slide 13
- Vomeronasal organ (VNO) 13 A structure on each side of the
nasal septum of human and more common in animals. It is a chemical
sensory center for detection of pheromones (chemical signals that
carry information between individuals of the same species). It is
responsible for good or bad feeling between people. Binding of
pheromones to receptors on neurons in VNO action potential travels
through non olfactory pathway to limbic system.
- Slide 14
- Smell Abnormalities Anosmia: Absence or marked loss of smell
sensation Hyposmia (diminished olfactory sensitivity) Dysosmia
(distorted sense of smell) Parosmia: smelling of non existing odors
usually is Psychogenic. Cacosmia: receiving a bad smell due to
Chronic sinusitis, foreign body in the nose or lung abscess.
Several dozens different anosmias have been detected in humans.
They are presumably due to absence or disrupted functions of one of
the many members of odorant receptor family.
- Slide 15
- 15 Thank You
- Slide 16
- References Human physiology by Lauralee Sherwood, seventh
edition Text book physiology by Guyton &Hall,11 th edition Text
book of physiology by Linda.s contanzo,third edition