Physio chemical senses
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Transcript of Physio chemical senses
Dr. Mohanad R. Alwan
Several types of chemoreceptors are present:
Taste buds - receptors of taste (gustatation) Olfactory receptors- receptors of smell (olfaction) Cutaneous nociceptors -Irritating chemicals on
skin Muscle sensors - burning during heavy exercise,
acidity receptors Circulatory sensors - oxygen & CO2 receptors Digestive tract sensors - receptors for various
ingested substances
Taste and smell (chemical receptors)tend to influence each other
Taste (gustation)
Taste buds- epithelial cells that act likeneurotransmitters
Innervated by cranial nerves
Taste (‘gustation’) and smell (‘olfaction’) are often referred
to as the chemical senses
They are mediated in the first instance by receptors that
are stimulated by chemical substances. Receptors are
called ‘chemoreceptors’
Taste: Chemicals dissolve in our mouth (must be water
soluble) and stimulate the taste buds in the oral cavity
(tongue, soft palate, cheek, etc.).
Gustation = Taste
Basic tastes:◦ Sweetness - outer tip of tongue◦ Saltiness - lateral edge of tongue behind tip◦ Sourness - lateral edge of tongue behind
saltiness region◦ Bitterness - back edge of tongue toward throat◦ Umami - taste of glutamate - not localized
Tastes triggers are not unique◦ many chemicals can trigger each receptor type
Loss of taste is ageusia
Four major sensations of taste
Sweet- G-protein activates second messengersystem
Bitter- also G-protein activated specific receptor
Sour- proton channel (acids taste sour)
Salty- sodium channels
Tastes seem to be enhanced in different partsof the tongue
The tongue is the major organ of gustationThere are also some taste buds on the
palate, pharynx, and epiglottis
Papillae - contain taste buds◦Fungiform papillae (mushrooms) - top surface of
tongue◦Foliate papillae (ridges) - rear, lateral margin of
tongue◦Vallate papillae (pimples) - rear, medial margin of
tongue
Only 1% of the epithelial cells on tongue’s surface are taste buds
Contain 50 to 75 sensitive cells eachTaste-sensitive cells have a limited lifetime,
and are constantly being replaced. Taste pore permits substances to enterThree cell types in each taste bud:◦Taste receptors – chemoreceptors◦Basal cells - source of new taste
receptors◦Gustatory afferent fibers - conduct
action potentials to brain
Come from the division and differentiation of the basal cells
Not neurons in the strict sense, but have synapses with gustatory afferent fibers
Arrival of chemical molecules on surface leads to change in membrane potential - receptor potential
Most taste receptors respond to at least two of the basic tastes
Sometimes taste receptors generate action potentials
Transduction differs for each type of taste receptor
Afferents◦Anterior 2/3 of tongue - VII (facial nerve)◦Posterior 1/3 of tongue - IX
(glossopharyngeal nerve)◦glottis, epiglottis, pharynx, palate - X (vagus
nerve)All afferent fibers end in gustatory nucleus
in the medulla
Taste fibers proceed along several pathways to the
medulla oblongata or brain stem, then to the
thalamus, and finally to the taste area on the anterior
cortex.
For taste sensation, gustatory nucleus neurons send
fibers to the ventral posterior medial nucleus (VPM) of
thalamus
VPM sends fibers to the primary gustatory cortex
(Brodmann's area 43 in ventral parietal lobe
For autonomic functions, gustatory nucleus sends
fibers to the many brainstem regions involved in
swallowing, salivation, gagging, vomiting, digestion,
respiration and to the hypothalamus and amygdala
• appetite and food preferences
receptor cells in olfactory epithelium
sense of smell transmitted directly to
cerebral cortex (all others go through
thalamus)
receptors in medial temporal lobesand in limbic system (emotionalresponse to odors)
Molecular basis for smell is complex-
humans can distinguish up to 10,000
different odors
Smell (olfaction)
Inside each side of the nose is an air chamber, the nasal cavity.
Air comes in through the nostril and flows down, around the rear of the roof of the mouth, into the throat.
When you sniff, air swirls up into the top of the cavity.
A small patch of about 10 million specialized olfactory (smelling) cells are in the cavity
Loss of smell is anosmia
Smell: Volatile (gaseous) chemicals are inhaled into the nasal
passages (or enter via the mouth) where olfactory receptors
line the membranes
Taste and smell are closely linked in that they are both usually
involved in many activities such as food seeking and sampling
(flavour involves both)
VOMERONASAL SYSTEM
Smell also conveys important non-nutritive information
such as the presence of prey, predators and in some
species mates (pheromones regulate sexual activity)
- Taste aids in the regulation of nutrients and enables the
organism to ‘test’ substances prior to ingestion (important
for identifying both nutrious foods and harmful
substances)
Olfactory epithelium in roof of nasal cavity
Olfactory receptors – chemoreceptorsSupporting cells - secrete mucousBasal cells - generate new olfactory
receptorsSize of the olfactory epithelium is a
measure of keenness of smell humans have about 10 sq. cm dogs have 170 sq. cm and dogs may also
have 100 times the olfactory receptor density
The olfactory sensors are located on each side of the inner nose.
True neurons: Dendrite ends in knob with multiple cilia
(containing receptors) at surface of epithelium Odor particles in the air stick to the cilia Unmyelinated axon leaves base of epithelium
thru cribiform plate and ends in olfactory bulb Born, live, and die with a 4-8 week cycle - only
neurons regularly replaced throughout life The olfactory cells produce nerve signals, which
travel to the olfactory bulb
Odorant binds to G-protein coupled receptor protein◦ There are 500 - 1000 different olfactory receptor
proteins coded genetically Effector protein is adenylyl cyclase cAMP produced and binds to cation channel
(sodium + calcium) Calcium influx opens calcium-sensitive
chloride channel Calcium and chloride cause membrane
depolarization (receptor potential) Action potentials in olfactory receptor fibers
A pre-processing center that sorts the signals before they travel along the olfactory tract to the brain
Axons from receptor cells project to the olfactory bulbs
Here the glomeruli, receive signals from 26,000 receptors
The olfactory bulbs on either side are cross-connected.
Nerve fibers project from the olfactory bulb to
the olfactory tubercle
Olfactory tubercle neurons project to medial
dorsal nucleus of thalamus
Thalamic neurons project to orbitofrontal cortex
reach the olfactory areas in the neocortex for
the sensation of smell
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