Chapter 8 Chemical sences Flashcards
taste is called
gustation
olfaction is called
smell
chemically sensitive cells are called
chemoreceptors
chemoreceptors
distributed throughout the body. Reports subconsciously and consciously about our internal state.
e.g. nerve endings in digestive organs, receptors in ateries, sensory endings in muscles.
Gustation ja oflaction have strong direct connections to
basic internal needs: thirst, hunger, sex and certain forms of memory.
Five taste qualities
saltiness, sourness, sweetness(sugar, artificial sweeteners, proteins), bitterness(K+ ions Mg2+) and umami (taste of glutamate)
Why is sensitive and versatile taste necessary
To distinguish between sources of food and toxins
How do we perceive the countless flavors of food?
taste, smell and its feel (texture and temperature) occurring simultaneously.
Tell which part of tongue is responsible for certain flavor
Tip: sweetness
back: bitterness
sides: saltiness and sourness
Most of tongue is sensitive to all basic tastes
Parts of mouth involded in taste:
tongue, palate(kitalaki), pharynx(nielu), epiglottis(kurkunkansi).
Surface of the tongue
small projections called papillae, each has 1-100 taste buds, each bud has 50-150 taste receptor cells. Also taste buds have basal cells which surround the taste cells, and gustatory afferent axons.
threshold concentration
concentrations too low will not be tasted, but at some critical concentration, the stimulus will evoke a perception of taste.
apical end, microvilli
region which is chemically sensitive part of a taste receptor cell, near the surface of the tongue. Microvilli is a thin extension that project into the taste pore, where the taste cell is exposed to the contents of the mouth.
receptor potential
appropriate chemical activates a taste receptor cell and its membrane potential chances. It causes voltage-gated calcium channel to open, triggering the release of transmitters (which depend on the taste receptor type)
Transduction
environmental stimulus causes an electrical response in a sensory receptor cell. The nervous system have myriad transduction mechanisms that make it sensitive chemicals, pressures, sounds and light.
Transduction mechanisms of salt and sour
Tastants can iteract directly with ion channels either by passing through them (Na+ and H+) or blocking them (H+ blocking the potassium channel). Influences the Ca2+ and transmitter release, serotonin.
Taste receptor proteins
About 25 types of bitter receptors, T2R proteins. Bitter receptors are dimers consisting of two different T2R receptors.
One type of sweet receptor, formed of T1R2 and T1R3.
One type of umami receptor, formed of T1R1 and T1R3.
Why don’t we confuse bitter and sweet chemicals?
They connect to different gustatory axon and their receptors are expressed in different taste cells.
Transduction mechanisms of bitter, sweet and umami.
Tastants bind directly to the G-protein-coupled taste receptor,
which activates phospholipase C,
which increases the synthesis of IP3.
IP3 triggers the release of Ca2+ from internal storage sites,
which opens taste-specific ion channel, leading to depolarization andtransmitter release.
Main transmitter is ATP, which is released from the taste cell by diffusing through ATP permeable channels.
Central taste pathways
By three cranialnerves(aivohermo) (VII, IX, X) to the medulla.
1) Gustatory axons enter the gustatory nucleus within the medulla.
2) Gustatory nucleus axons synapse on neurons of the thalamus. (VPM=Ventral posterior medial)
3) They project to the primary gustatory cortex in regions of post central gyrus and insular cortex.
ageusia
Loss of taste perception
Localized lesions of hypothalamus or amygdala can cause
an animal to either chronically overeat or ignore food, or alter its preferences for food types.
Pheromone
important signals for reproductive behaviors, identify individuals and indicate aggression or submission. Well developed in many animals.
Olfactory epithelium
three main cell types: olfactory receptor cells -sites of transduction -genuine neurons, with axons of their own supporting cells -similar to glia, help producing mucus basal cells -source of new receptor cells (cycle that lasts about 4-8 weeks)
Odorants
Chemical stimuli, which dissolves in the mucus layer before reaching receptor cells.
mucus consists
water base, dissolved mucopolysaccharides, proteins (antibodies(vasta-aine), enzymes, odorant binding proteins, Also important are odorant binding
proteins, which are small and soluble and which may help concentrate odorants in the mucus. ).
olfactory receptor neuron
single thin dendrite that end with a single knob atthe surface of the epithelium
- within a mucus layer are several long thin cilia
- opposite site of the receptor cell is a thin unmyelinated axon= constitute olcafactory nerve (cranial(kalloon liittyvä) nerve I)
How do olcafactory nerves (cranial(kalloon liittyvä) nerve I) form after leaving the epithelium?
The olfactory axons do not all come together as a single nerve bundle, as other cranial nerves.
-Small clusters of axons penetrate a thin sheet of bone called cribriform plate, then course into olfactory bulb.
Olfactory transduction
Oborants->Bind to odorant receptor proteins->Stimulate G-protein->Activate adenylyl cyclase->Form cAMP->Bind cAMP to cyclic nucleotidi-gated cation channel->open cation channels(Na+ Ca2+)->Open Ca2+ activated Cl- channels->cause current flow and membrane depolarization.
(the internal Cl- consentration must be unusually high so that Cl- depolarizes the membrane)
Olfactory signaling pathway has two unusual features:
receptor binding proteins and cAMP-gated channels
How many odorant receptors human have
350 (odorant receptor genes comprise about 3-5% of the entire mammalian genome which is a huge amount)
tell about odorant receptor genes
- each gene has a unique structure, allowing the receptor proteins to bind different odorants.
- each receptor cell seems to express very few of many types of receptor genes
Olfactory receptor proteins
- G-protein-coupled
- seven membrane-spanning alpha helices
- include variety of neurotransmitter receptors
cAMP-gated channels
channels in the cilia of olfactory cells respond directly to cAMP, they are cAMP-gated.
glomerulus
-each bulb contains spherical structures, each about 50–200 um in
diameter.
-Each glomerulus receives receptor axons
from a large region of the olfactory epithelium
How does the information get to olfactory cortex and related temporal lobe structures
This anatomy makes olfaction unique. All
other sensory systems first pass information through the thalamus before
projecting it to the cerebral cortex. The olfactory arrangement produces
an unusually direct and widespread influence on the parts of the forebrain that have roles in odor discrimination, emotion, motivation, and
certain kinds of memory
Where is the conscious perceptions of smell
Conscious perceptions of smell may be mediated by a path from the olfactory tubercle, to
the medial dorsal nucleus of the thalamus, and to the orbitofrontal cortex
when we smell those
same chemicals, we can easily tell them apart. How is the whole brain
doing what single olfactory cells cannot?
(1) Each odor is represented by the activity of a large population
of neurons; (2) the neurons responsive to particular odors may be organized into spatial maps; and (3) the timing of action potentials may be an essential code for particular odors.
How do the receptor neurons form their way to the glomeruli?
Each glomerulus receives input only from receptor cells expressing a particular receptor protein gene.
Most tastes involve some combination of the five basic tastes. What other sensory factors can
help define the specific perceptions associated with a particular food?
smell and its feel (texture and temperature) occurring simultaneously.
The transduction of saltiness is accomplished, in part, by a Na-permeable channel. Why would
a sugar-permeable membrane channel be a poor mechanism for the transduction of sweetness?
sugars are the least sweet of all of these; gram for gram, the artificial sweeteners and proteins are 10,000–
100,000 times sweeter than sucrose.
Chemicals that have sweet, bitter, and umami tastes all activate precisely the same intracellular
signaling molecules. Given this fact, can you explain how the nervous system can distinguish the
tastes of sugars, alkaloids, and amino acids?
Their
receptor proteins are expressed in different taste cells. They connect to different gustatory axons. The
activity of different gustatory axons reflects the chemical sensitivities of
the taste cells that drive them, so the messages about sweetness, umami and bitterness are delivered to the central nervous system along different transmission lines.
Why would the size of an animal’s olfactory epithelium (and consequently the number of receptor
cells) be related to its olfactory acuity?
Each receptor gene has a unique structue, allowing the receptor proteins encoded bt these genes to bind different odorants. So more receptors in the epithelium enable more different oronts to be detected.
If the olfactory system does use some kind of spatial mapping to encode specific odors, how might
the rest of the brain read the map?
With practice
and very specialized goggles, we might be able to read the “alphabet” of
odors mapped on the surface of the olfactory bulb with our eyes. This may
roughly approximate what higher regions of the olfactory system do.
