topic #10: sensory transduction ll

what are 2 examples of chemoreceptors

olfactory & taste receptors

what are the cell types in olfactory epithelium

A region in the back of the nasal cavity

1. olfactory sensory neuron

- polarized cell

-dendrite on apical pole ends in knob-like swelling which has about 2 dozen cilia, where olfactory transduction occurs

- axon on basal pole goes through cribriform plate to olfactory bulb

2. supporting cell

- has microvilli to keep mucous moving (mucous secreted by bowman's glands)

3. basal cell

- regenerate olfactory neurons

what makes the dogs sense of smell so sensitive compared to humans

- sense of smell in dogs is estimated to be from 10,000 to 100,000 times more acute than that of humans

- possess up to 300 million olfactory neurons, compared to 6 million in humans

- relative to whole brain, portion of dogs brain that analyzes smells is about 40x larger than our own

- in humans , sense of smell is relegated to small area at back of nasal cavity that is in main air flow path to lungs

* in dogs, have 2 separate air flow path ; about 88% of air flows to lungs, while other 12% enters a recess at back of nose, where it is filtered through convoluted bony maze of "turbinates" that contain vast olfactory epithelium

what does clinical evidence show about humans smell

- evidence for strong central inhibition of human olfactory pathways which may suppress sense of smell

- human subjects sometimes exhibit unusual "hyperosmic" states in which sense of smell is dramatically enhanced ("amphetamine-induced dopamine excitation)

what are features of olfactory system

- axons penetrate cribriform plate in many olfactory nerve filaments via small holes called "FORAMINA”

- OC axons & 2nd order olfactory neurons (ONs) form glomeruli in olfactory bulb

- 25,000 OC axons converge on about 100 2nd-order ONs

- much integration within & btwn glomeruli, w/ descending modulation

- anatomical arrangement of olfactory system is unique bc olfactory axons first enter cortex & later thalamus

- widespread projection to areas of brain involved in motivation, emotion, reproduction, feeding & memory

how does olfactory transduction occur

- binding of odorant to an odorant receptor , Golf is activated, adenylyl cyclase lll is stimulated, cAMP produced , Ca2+ & Na+ enter cell & cause depolarization

- Ca2+ also opens a Cl- channel causing further depolarization

- (these neurons have high intracellular Cl- [ ] so when channels open, Cl- exists cell)

- phosphodiesterase breaks down cAMP & terminates messenger system

- other pathways may involve activation of PLC, to act directly on calcium channels via IP3 production

what is gustation

*smell

- diff sensitivities for each taste

- high except bitter (salt = 10mM, sour= 2mM, sweet= 20mM, bitter = 8uM for quinine,0.1 uM for strychnine)

what is the location of taste receptor cells

- taste buds in papillae (human)

1. fungiform : anterior 2/3 of tongue

- several 100 in tongue, each contains 1-5 taste buds & each taste bud has about 100 taste cells

2. foliate: line side & posterior of tongue

- couple on each side of tongue & each contains about 600 taste buds

3. circumvallate : in back of tongue

- 9 in tongue & each contains about 250 taste buds

- epiglottis, larynx, pharynx, around throat

4. filiform papillae don't have taste receptors

taste receptor cells characteristics

- highly polarized

- 30-100 taste cells in taste bud

- transduction of signal occurs in their microvilli

- apical region specialized for detecting tastants (taste stimuli)

- basolateral region specialized for general cellular housekeeping & synaptic transmission

- many taste cells are capable of firing APs spontaneously or response to chemical or electrical stimulation

*Na+ & Ca++ currents contribute to spike depolarization

what is the taste transduction systems process

1. SALT

- involves an amiloride-sensitive Na+ channel (also known as ENaC = epithelial sodium channel)

2. SOUR (acid)

- proton (H+) current enters sour receptor cells via TRP channel, PKD2L1

-H+ may also block K+ channels, preventing K from exiting cells causing depol

3. SWEET

- receptors are T1R2-T1R3 heterodimeric GPCRs

- work through taste cell specific G protein (gustducin)

- activation of gustducin activated adenylyl cyclase producing cAMP, which closes K+ channel & cell depol

- sucrose, as well as other sugars & artificial sweeteners detected

4. BITTER

-some bitter compounds (quinie) can act isotropically, by directly blocking K+ channels

-most bitter receptors T2R GPCRs

- T2Rs expressed on tongue & soft palate as well as other tissues around body like intentistes., lungs, heart, brain , etc

- repsobile for detecting thousands of bitter ( & often toxic) compounds

5. YMAMI (savory or glutamate flavor)

- some receptors are T1R1-T1R3 heterodimeric GPCRs

- others metabotropic glutamate receptors (heteromeric or homomeric dimers of mGluR1 & mGluR4)

6. Hot chilli flavor

- trpv1 (vr1) & Trpv2 (vrl1) heat & capsaicin (hot pepper extract) receptors

- other 2nd taste modalities (swee, sour, etc)

7. carbonation taste

- carbonation = Co2 bubbles in H20

- carbonation taste = combination of acid (H+) stimulation of sour receptors & tactile stimulation of touch receptors in tongue & rest of mouth by bursting bubbles

what are thermoreceptors

*warm & cold receptors in mammalian sin

- skin temps from 1-20 C blow normal skin temp (about 34 C) activate cold receptor in mammals

*innervated by as & C fibers & relatively neumorus

- skin temps from 34-45 C increasingly activate warm receptors in mammals

* innervated exclusively by C fibers & relatively sparse in skin

what is "paradoxical cold"

sensation is caused by activation of cold receptors in skin by high (burning) temps

cold receptors are strongly activated by what

- by chemical "menthol"

- a receptor molecules was clones from trigeminal sensory neurons that is activated by menthol & cold temp

*this cold & menthol sensitive receptor = CMR1

CMR1 belong to = TRP (transient receptor pot) family of excitatory ion channels which include heat sensitive VR1 & VRL-1 channels

- both activated by noxious heat

VR1 also binds to ot pepper extract, capsaicin

what does nociceptive neurons that response to both heat & extreme cold contain what

*contain Trpv1 (heat) receptor channe;s & TRPA1 (extreme cold) receptor channels previously known as Anktm1

- might be molecular basis of "paradoxical cold" or "burning cold" sensation

what are As fibers

- myelinated nociceptors (about 20 m/sec conduction velocity)

- mediate sharp , prickling , well localized pain

- 2 main categories : high threshold mechanical (HTM) & mechanothermal

- receptive field = cluster of spots w/ total area of about 5mm^2

what are C fibers

- unmyelinated nociceptors (o.3-3.0 m/sec conduction velocity)

- mediate dull ache, diffuse pain

- polymodal nociceptors (PMNs) response to intense mechanical, thermal (>43 C) & chemical irritants

what are polymodal nociceptors

- respond to either from environment or released by damage inflamed tissue including histamine, bradykinin, prostaglandin E2 (PGE2), leukotriene C4 (LTC4)

- these substances directly activate nociceptors & can cause prolonged allodynia (pain produced by non-noxious stimulus like "light touch")

- histamine, serotonin & Ach injected into skin by "stinging nettle" plant which causes intense burning sensation lasting mins and allodynia lasting up to a couple of days

what are histamines

irritate nociceptor & cause inflammation

What is an example that has Histamine, Acetylcholine & 5-HT?

Hollow