---start physio.lec.1.9.97-----

Dr. George Gomez of the something centre will talk about Chemosensory 
Systems. see that section of large physio handout.

Olfaction, Gustatory system, vomeronasal system, trigeminal 
chemoreception.

Chemical senses: all animals have a form of this. it's very primitive. 
bacteria even have chemoreception.

today: we'll be discussing mammal chemoreception. 

first, OLFACTION:

many people ignore the sense of smell, mainly because to move around in 
environment you can rely on the sense of sight which is quite accurate, 
and also the sense of hearing, which like sight is linear and accurate. 
so in animals like people, olfaction takes a secondary role. but it plays 
a very important role in eating! if you hold your nose shut and eat 
something, you can't identify the food, only sweet, sour, salty, bitter, 
and something else. so olfaction is quite important for feeding. people 
who lose sense of smell often become inappetant. additionally, in "lower" 
animals eg hamsters and mice, olfaction is involved in social 
interaction. mice may identify animals of similar or different genetic 
strains by olfaction. mice will identify the genetic strain of an animal 
based on urine odor alone, preferring to mate w/mice of different genetic 
strain. 

re: pigs, pork cooking: about half of humans can smell androstenone, a 
pig pheromone, and it gives pork that really strong smell. makes pigs 
nuts.

the nostril leads into the nasal cavity which contains three large folds 
of tissue aka turbinates, which increase surface area which contacts air, 
warming and humidifying air, and also which are lined with olfactory 
epithelium. when you breathe in and out, air stays in lower part of 
cavity, but if you sniff, it goes up to the top, where lots of olfactory 
cells are present. olfactory info is sent from epithelium to brain. 
[note: hard to type when sneezing!]

3 types of cells:

olfactory neuron- has oflactory cilia extending from dendritic knob at 
surface, site of odor detection, and has olfactory axon sending info back 
to brain. cell body sits inside epithelium. mucus lines olfactory epi, 
keeps epi from dessicating, and has lots of proteins in it which protect 
epi from junk whch comes in, and which help to solubleize odorants. 
neurons live 2-4 weeks before being replaced.

respiratory cells: surround the olfactory neurons. epi cells with motile 
cilia, constantly beating, moving mucus around.

basal cells: progenitor neurons. these neurons regenerate. this is 
unusual. will form new neuron and rewire new axon into brain.

entire olfactory epi regenerates in 120 days.

Dr Gomez will try to explain how a chemical signal is transduced into 
biological signal.

the role of the chemoreceptor is to translate chemical signal into action 
potential. in general, ifyou look at the cell membrane in the olfactory 
cilia, on that cell membrane you have a receptor molecule. that receptor 
molecule can temporarily interact with odorants, and will activate a G 
protein. the G protein will hydrolyze GTP to GDP, activating an enzyme 
(adenylate cyclase or phospholipase C) catalyzing the production of a 
second messenger molecule which will open an ion channel, causing an 
influx of cations, causing the cell to depolarize. in the olfactory 
system, ca++ is one of the ions that comes in, and it triggers the 
opening of more channels. so the opening of the cation channels causes 
depolarizion which triggers action potentials.

recent investigations have focused on the receptor molecules. the 
receptor interacts wth the odorants. in 1991 - it was known before that G 
proteins were involved but no one knew what receptor was - they cloned a 
7 transmembrane domain receptor which is homologous to other mammalian 
receptors linked with G proteins. so the N terminus is in the 
extracellular space, and there are 7 transmembrane domains, and then the  
tail is intracellular. there are two hypervariable loop regions which are 
extracellular, which interact with the odorant, causing a conformational 
change, such that the tail interacts wth the g protein. then you get 
enzyme activation and second messenger production and opening of ion 
channels.

now experiments were done to measure cAMP and IP3 production in the 
presence of odorants, which showed that within about 15 msec of odorant 
exposure, you rapidly increase the amount of second messenger molecules, 
and and then that concentration rapidly drops by 150 msec.

so say you use different odorants and measure this stuff...for any given 
odorant, you get accumulation of EITHER cAMP OR IP3, never both. and 
there is no seeming functional reason to have two separate pathways, they 
just do. even substances within the same odor category (eg, different 
floral odorants) produce one or the other, not all in the same category 
produce the same second messenger.

when the second messenger is produced, those molecules bind the cation 
channels and open them up. so you get an influx of Na+ and Ca++ which 
happens in the dendrite and depolarizes the cell. the whole cell will 
depolarize and then generate an action potential. the AP travels to the 
brain via the axon. 

[slide: action potential of salamander olfactory cell]

again, realize that as cell depolarizes, it must reach threshold 
potential first and then will start action potential. ultimately second 
messenger concentration goes down, ion channels close, cell gets 
repolarized. 

if you increase the concentration of stimulus (odorant in olfactory 
system), you begin to fire a larger number of action potentials. so there 
is a concentration or dose response function - higher stimulus 
concentration yields larger response, eg higher frequency of action 
potential. afterwards when the high frequency response occurs, it will 
stop quickly (he was vague about that...)

another thing is, there are some cells that if you give some odor, they 
will fire APs, and others, if you give odor, will STOP firing APs. so 
some cells are waiting for absence of odor. some of these cells are 
inhibitory.

some cells only respond to one stimulus. some respond to a few. some 
respond to many. if you think about an odor, you have to be able to 
identify what the odor is, how strong it is, and is it still there or has 
it left. you look for quality, intensity, and duration. those are the 
things you need to know to figure out if you should leave or not. :)

so we think quality info is decoded by NOT by individual cells. if you 
give a particular odor, you get a response from a particular subset of 
cells. so it seems that quality info is decoded by the ENTIRE epithelium, 
by the pattern of response. in insects, it's one receptor per smell. but 
in mammals, there are more odors we can detect, and we need to be able to 
smell new fragrances, mixed fragrances, etc. this way it's more flexible 
and a bit sloppy but it expands the number of odors and kinds of odors we 
can respond to. 

the number of APs signals the intensity. 

the duration is kind of ignored. because after you smell something for a 
bit, you stop smelling it. if you sit next to someone, you may smell 
their perfume. then you stop smelling it. then the person leaves, and you 
notice that the smell is gone. you pick up CHANGES in odor more than 
constantly picking up background odors.

[see handout diagram]
olfactory neurons in epithelium - anywhere from 1 to 10 million in human 
nose. the axons of these all together form olfactory nerve which passes 
through cribiform plate (CP) and goes into olfactory bulb. in the 
olfactory bulb, the axons come together in a little clump called a 
glomerulus (G). a mitral cell (M) sends a dendrite into the glomerulus. a 
bunch of different mitral cells send dendrites into the glomeruli. 
between the glomeruli are periglomerular(pg) cells which exchange info 
between glomeruli. between mitral cells are granule cells which talk to 
two mitral cells. this is poorly understood, but when you try to record a 
response from a mitral cell, the info is integrated at the glomeruli, and 
the mitral response will change based on stimuli given to whole 
epithelium. when you activate a glomerulus, the periglomerular cells are 
also activated and they inhibit the glomeruli around it.

so the mitral cells send their output to the rest of the brain. first 
into the limbic system, which is responsible for many emotional and 
memory responses. therefore attention has been focused on odor, emotion, 
and memory. odor is a strong memory stimulus. a lot of "gut reactions" 
are mediated by the olfactory system. some people who lose sense of smell 
have diminished emotional and memory responses.

that is the sense of smell in a nutshell. :)

note: dr gomez says the brain is a scary thing. :)
note: odor quality is dependent on which cells are activated. the pattern 
of cells which respond indicate what odor is smelled. the wierd thing is, 
though, if you remove 90% of the cells, it still works. so that's 
strange.

----break----

VOMERONASAL organ aka VMO is closely related to olfactory system. it 
mediates mostly sexual and reproductive responses. if you remove the 
vomeronasal nerve from hamsters and mice, they will lose a lot of sexual 
response. VMO located above palate (check anatomy book if confused). the 
receptors are neuronal with axons and dendrites, but instead of cilia 
they have microvilli. the vomeronasal nerve does not go to the olfactory 
bulb but rather back into the accessory olfactory bulb, behind the 
olfactory bulb. the primary function seems to be to mediate reproductive 
responses. horse flehmen response is result of VMO stimulation. now, 
there is no known VMO stimulus, so it is hard to do physiological 
studies. it's not known what molecules stimulate it, in other words. 
researchers have looked at VMO cells and think there is a similar 7 TMD 
receptor as in olfactory epi, but no evidence of cAMP has been found. VMO 
in snake is primary olfactory organ. molecules are picked up by tongue, 
and tongue is retracted and two ends fit into VMO grooves or something 
like that. there is more to be said about it but dr gomez says we don't 
want to know :)
for innervation of VMO system, see handout fig 2. the axons go directly 
to accessory olfactory bulb and then into hypothalamus. no projection 
into the neocortex. the information of VMO is generally unavailable to 
conscious system. adult humans may not have a functional VMO system. 
there is definitely a fetal human VMO, but no evidence exists for adult.

TASTE or GUSTATORY system. can detect five different stimuli.
Sweet, salty, sour, bitter, and umami which is japanese for savory, and 
is the "taste" of MSG. glutamate is a neurotransmitter. it has its own 
unique transduction system and it is a "taste sensation"

taste is localized to the tongue. across the tongue are papillae which 
are covered with 20-50 taste buds per papilla. the front of the tongue is 
innervated by CN VII chorda tympani branch, and is covered with fungiform 
papillae. the middle grooved part of tongue has foliate papillae and the 
back of the tongue has circumvallate papillae, and these are innervated 
by glossopharyngeal nerve IX. hard palate, soft palate, buccal wall, 
sublingual organ and epiglottis also have some taste buds. see handout 
for diagram and innervations.

vagus innervates epiglottis and vagus goes to different part of brain, 
and can stimulate vomit reflex. so if you eat something gross, it has to 
get to the epiglottis to stimulate that.

the taste bud itself looks like an orange. it has a small pore opening to 
the surface called the taste pore, which gives accss to the taste cells - 
there are about 50-100 taste cells per taste bud. the taste cells are 
epithelial, not neuronal. they are innervated at the basal region by 
afferent taste fibers. the taste cells secrete neurotransmitter onto the 
nerve.

taste molecule===>depolarization--note, taste cells are not neuronal, but 
will fire action potentials====>Ca++ increase in cell===>NT release 
(mostly serotonin)===>generation of AP===>sent to taste nerve/brain

SALT taste:

NaCl outside the cell...Na+ will enter the channel, causing 
depolarization, etc.

SOURtaste:

harder.
the taste cells have a resting potential. while the cell is at rest there 
is constant K+ and Na+ conductance. When sour taste, H+ comes and blocks 
K+ conductance- blocks the K+ leak channels. this will cause 
depolarization.

UMAMI:
it's the glutamate, not the sodium. lots of AAs give this taste, 
actually. not salty, rather savory. it's umami. not many folks look at it 
since it isn't a primary component of most foods. popular in japan, in 
many seaweeds and japanese mushrooms. 

possible mechanism:
glutamate opens a Na+ Ca++ channel eg, glutamate directly gates the ion 
channel.
other possibility: 
glutamate triggers a 7 TMD receptor causing IP3 production

there is evidence for both

BITTER:
many compounds taste bitter. KCl tastes bitter, quinine, many AAs, etc. 
so there are likely multiple mechanisms. some people think blocking of 
Na+ channel occurs and it may. also receptor mediated:

bitter ligand is linked to receptor gustducin (similar to transducin) 
which activates PDE which breaks down cAMP causing the cAMP gated Na+ 
Ca++ channel to open. Increased Ca++ causes NT release
(note: cAMP keeps Na+ Ca++ channels CLOSED in bitter cells, opens them in 
sweet cells_

or

bitter ligand activates phospholipase C, causing production of IP3, which 
causes Ca++ to be released from bound state. increased Ca++ causes NT 
release.

note that the increased Ca++ will cause the voltage gated ion channels to 
open as well, causing action potential and NT release.

SWEET:

sugar or non sugar can stimulate

sugar -->activates adenylate cyclase-->produces cAMP-->activates Na+ Ca++ 
channel OR activates protein kinase A aka PKA which activates K+ channel. 
either depolarizes cell, causing AP and NT release.

non-sugar sweet molecule- activates phospholipase C, activates IP3, 
increases Ca++ level, etc.

note: AP in taste cell only causes NT release. the AP is NOT propagated 
to the nerve cell.

so. the branches of chordae tympani etc send branches into base of brain 
into (see p 10 handout) the nucleus of the solitary tract NST. this is 
where taste is first decoded. this is hindbrain. NST sends fibers into 
the parabrachial nucleus, PBN, and into higher brain regions from there. 
now, each taste bud will respond to all 5 stimuli. there isn't really 
spatial separation. it just so happens that a majority of the responses 
in front region are to salt or sweet, but it isn't EXCLUSIVE. back of 
tongue mostly responds to bitter or umami, and sour all over the tongue. 


TRIGEMINAL CHEMORECEPTION:

trigeminal nerve has three branches. 
Ophthalmic branch - innervates cornea and front of nasal cavity
Maxillary branch: rear nasal cavity and maxilla
Mandibular branch: lower jaw and tongue

any sensation you might get in mouth that isn't quite taste but is 
chemical, is detected by trigeminal. eg, spiciness, menthol - burning, 
cooling, acidity. anatomically it isn't exciting. there are naked, 
unmyelinated nerve fibers sent into epithelium. they just sit a couple of 
microns, about a mm below the skin.
so, mildly spicy food takes about 20 sec to reach nerve. the more 
capsaicin you eat, the more response you get. so if you eat a LOT of 
mildly spicy food, you feel like it is getting spicier, because you are 
getting sensitized to it. same with acidity, coolness, etc. even after 25 
min of exposure, perceived intensity will be 50% stronger than initial 
stimulus. 

why does this occur? in general, trigeminal stimuli are irritating. 
noxious stuff. since the stimulus has to get through all epi layers to 
reach nerve, you need a higher concentration - 10-100 mmolar. 

how do these stimuli create an AP? these are free nerve endings so hard 
to experiment. several possibilities.

for capsaicin and alcohols etc, these things will form pores in the 
membrane, allowing ions to flow in, causing depolarization. also you 
might see direct gating of ion channels, but they haven't been isolated 
yet. it is suspected that this is the case, however.

CARBONATION: CO2 can enter epithelium. there is carbonic anhydrase in the 
ECF, which catalyzes the rxn CO2 + H2O <===> HCO3===>H+ + CO3-
can stimulate trigeminal nerve causing tingly sensation at low 
concentration or burning at high concentration.


---end----