Transcript الشريحة 1

The Chemical Senses
Taste & Smell
Taste Sensation
Objectives
1.Describe the location, structure & afferent
pathway of the taste receptors.
2. Name the basic taste sensations.
3. Describe the cell of taste bud.
4.Identify the cranial nerves that transmit
taste information to the cerebral cortex.
Taste sensation is mainly a function of
the taste buds in the mouth.
There are four primary sensations of taste
1. Sour.
2. Sweet.
3. Salty.
4. Bitter.
The primary sensations of taste
1- The sour taste
It is caused by the acids & the intensity of the
taste sensation is proportional to the logarithm
of the hydrogen ion concentration (more acidicstronger sensation).
2- The salty taste
It is caused by the ionized salt, the cations
mainly responsible for salty taste, & also the
anions to the lesser extent.
3- The sweet taste
a list of some of the types of chemicals that cause
this taste includes:
Sugars,
aldehyds, esters , haloginated acid
Glycols,
ketons, aminase, inorganic salts
of lead & beryllium, Alcohols, amides , sulfonic
acids
4. Bitter taste
Substances that cause bitter taste are almost entirely
organic substances:
1.Long chain organic substances containing nitrogen.
2.Alkaloids (quinine, caffeine, strychnine & nicotine).
The bitter when it occurs in high intensity usually
causes the person or animal to reject the food, & this
is an important function of the bitter taste sensation
because many of the deadly toxins found in poisonous
plants are alkaloids.
The taste bud & its function
It is composed of about modified epithelial
cells:
1. Some of which
(sustentacular) cells.
are
supporting
2. The others are taste cells which are
continually being replaced by mitotic
division from the surrounding epithelial
cells.
The outer tips of the taste cells are arranged
around minute taste pore.
From each of taste cells tip several
microvillus or taste hairs (2-3 microns)
protrude outward into the taste pore to reach
the cavity of the mouth, these microvillus is
believed to provide receptor surface for taste.
Among taste cells is a branching terminal
network of several taste nerve fibers
stimulated by taste cells.
Adults have approximately (3000-10000 )taste buds.
Location of taste buds
Large number of taste buds are on the wall of
the troughs that surround the circumvallate
papillae which form V-line towards the
posterior of the tongue.
1. Moderate number of taste buds are on the
fungiform papillae over the front surface of
the tongue.
3.Moderate numbers are on the foliate
papillae located in folds along the lateral
surfaces of the tongue.
4.Additional taste buds are located on the
palate, & few on the tonsillar pillars &
other points around nasophanynx.
Location of taste sensation
1. Sweet & salty sensation: are
principally on the tip of the tongue.
located
2. Sour taste: on two lateral sides of tongue.
3. Bitter taste: on the circumvallate papillae an
posterior tongue and soft plate.
Specificity of taste buds for primary
taste stimuli:
a- Each taste bud usually responds to only
one of the four primary taste stimuli when
the taste substances is in low concentration.
b- But at high concentration, most buds can
be excited by 2, 3 even 4 of the primary taste
stimuli as well as by a few other taste stimuli
that do not fit into the “primary” categories.
Mechanism of stimulation of taste buds
The microvilli on the tip taste believed to
provide the receptor surface for taste.
The membrane of the taste cell, like that of
other sensory receptors cells, is negatively
charged on the inside with respect to the
outside.
So application of a taste substance to the taste
hairs causes partial loss of this negative
potential, means the taste cell depolarized
getting receptor potential.
Receptor potential is initiated by:
1.Binding of the taste chemicals to
protein receptor molecules that
protrudes
through
the
villus
membrane.
2.This open ion channels (sodium ions)
which allow sodium ions to enter and
depolarize the cell.
Adaptation of Taste
This adaptation is about one half by
taste buds & the extreme degree of
adaptation occurs in the C.N.S.
Transmission of taste signals into the
central nervous system:
1- Taste impulses from anterior two thirds of the
tongue pass into the chorda tympani into the facial
nerve.
2- Taste sensations from the circumvallate papillae on
the back of the tongue & from other posterior regions
of the mouth are transmitted through the glossopharyngeal nerve.
3- Finally few taste signals are transmitted from the
base of the tongue & other parts of the pharyngeal
region by vagus.
On each side, the taste nerve fibers in these
three nerves unite in the medulla oblongata to
form the tractus solitorius. Second order
neurons are located in the nucleus of this
tract. Their axons cross midline & join medial
lamniscus ending in specific nuclei in
thalamus.
Impulses then relayed from there to the taste
projection area in the cerebral cortex at the
lower tip of post central gyrus in the parietal
cortex where it pass deep in the Sylvain
fissure.
Smell Sensation
Objectives
1. Describe the locations, structures, &
afferent pathways of smell receptors.
2.Explain how olfactory receptors are
activated, & the olfactory mechanism.
3.Describe the structures & functions of
central olfactory centers.
The olfactory membrane lies in:
1.Superior part of each nostril.
2.Medially it folds down over the surface of the
superior septum.
3. Laterally it folds over the superior turbinate .
4.Even over the small portion of the upper
surface of the middle turbinate.
The surface area of olfactory membrane
in each nostril is 2.4 seq.cm & it is
always wet by the secretion of alveolar
glands (Bowman’s glands) which are
present just below the olfactory mucosa.
These secretions dissolve the odorous
material inorder to make the stimulation
of the olfactory receptors possible.
The olfactory cells
The receptor cells for the smell sensation
are the olfactory cells which are actually
bipolar nerve cells derived originally
from the central nervous system itself.
There are about 100 million of these cells
in the olfactory epithelium interspersed
among sustentacular cells.
Olfactory hairs
The mucosal end of the olfactory cell forms
a knob from which (6-12) olfactory hairs or
cilia project into mucus that coats the inner
surface of the nasal cavity.
These olfactory cilia form a dense mat in
the mucus & these cilia react to odors in
the air & then stimulate the olfactory cells.
Stimulation of olfactory cells
The portion of olfactory cell respond to olfactory
chemical stimuli is the cilia.
So olfactory chemical substances combine with
(receptor portion) on the cilia & protrude inside
ciliray membrane.
This odorous or olfactory chemical causes
excitation of the receptor & this cause an alpha
subunit of a G-protein to be separated & the Gprotein is attaching to receptor protein.
This alpha subunit activate adenyl
cyclase that produce cyclic AMP from
ATP (adinosin monophosphate from
adenosine triphosphate).
This cyclic AMP activate another
membrane protein (a gated sodium
channel that allow large number of
sodium ions to pour into receptor cell
cytoplasm. Sodium ions contribute to the
inside positivity of receptor to get
excitation.
Physical factors affect degree of
stimulation:
1.Only volatile substances that can be
sniffed into nostril can be smelled.
2.Stimulating substance must be slightly
water soluble to pass through mucus to
reach receptors.
3.At least slightly fat soluble because of
lipid constituent of cilia.
Adaptation:
Smell sensation adapt within a minute when one
enters a strong odorous atmosphere.
1.Fifty percent (50%) of this adaptation in the first
second by the receptors.
2.The other 50% adaptation by the central nervous
system by strong inhibitory signals on transmission
in the olfactory bulb.
Olfactory cells send axons into the
olfactory bulb in the brain to end on
dendrites from mitral cells in a structure
called glomerulus.
Approximately 25000 axons enter each
glomerulus & synapse with the mitral
cells that in turn send signals into the
brain.
From the mitral cells…
The major pathway of transmission from
mitral cells the olfactory signals pass to the
brain.
The fibers from the cells travel through the
olfactory tract & terminate either primarily
or after really neurons in three principal
areas of the brain:
1.The medial olfactory area.
2.The lateral olfactory area respectively.
3.Through thalamus to orbitofrontal cortex.
1- The medial olfactory area:
very old olfactory area. Consists of group of nuclei
located in mid-basal portion of the brain anterior to the
hypothalamus. It serve the basic olfactory reflexes.
2- The lateral olfactory area:
less old olfactory system. Composed mainly of
prepyriform & pyriform cortex plus cortical portion of
amygdaloid nuclei. From these areas signals pathways
pass into almost all portion of limbic system specially to
hippocampus. This system provides automatic but
partially learned control of food intake & rejection of
toxic & unhealthy foods.
3- The newer pathway:
Passes through the thalamus & then to the orbito
frontal cortex. This newer system used for conscious
perception of olfaction.