B&B Thermoreception Flashcards
Why is sensing of temperature important?
Helps us react to noxiously cold and hot objects
Helps us maintain constant body temperature
What is thermoregulation?
The maintenance of body temperature
What is homeostasis?
Active maintenance of a relatively constant internal environment
How do we warm our bodies?
Through metabolic activity
Shivering
Cutaneous vasoconstriction
Seeking warmth
How do we cool our bodies?
Evaporative mechanisms
Panting,sweating
Cutaneous vasodilation
Seeking cool,shady places
What is heterothermy?
Animals that vary between self-regulating their body temperature, and allowing the surrounding environment to affect it
What is a homeotherm and a poikilotherm?
H =Maintains a constant internal body temperature, usually within a narrow range of temperatures
P = An animal that varies its internal body temperature within a wide range of temperatures, usually as a result of variation in the environmental temperature
What are some factors that contribute to heat death?
Denaturation of proteins
Thermal inactivation of enzymes exceeding rates of formation
Oxygen limitation
Effects on membrane structure
Different processes in metabolic networks have different temperature dependence
What is the Q10?
Temperature increases accelerate many processes
This increase in rate caused by a 10c increase is the Q10
Q10 = (R2/R1)^10/(T2-T1) where R2 and R1 are rate values of a process measured at temperatures T1 and T2
What are typical Q10 values for biological processes?
Between 2 and 3
There is an exponential relationship between rate and temperature
Are the following processes thermosensitive?
Q10 < 4
Q10 > 7
Not particularly
Yes
What is a fever?
Increase in body temperature usually associated with an infection
What is 0 K in C
-273.15
How many joules is 1 cal?
4.184
Heat is measured in calories where 1 calorie is the energy required to raise the temperature of one gram of water by one degree Celsius.
What is the specific heat capacity?
The amount of heat required to raise the temperature of 1 kilogram of a substance by 1 kelvin
1.0 cal g-1 c-1. It is 0.8 for the mammalian body
J kg−1 K−1
To vaporise water, we need how many calories per gram?
580c/g
What is convection
Mass movement of fluid
How is heat transferred?
Conduction,radiation,evaporation
What is the equation of heat balance?
Htot = Hc + Hr + He + Hs Htot = rate of metabolic heat production Hc = rate of conductive and convective heat exchange Hr = rate of net radiation exchange He = rate of evaporative heat loss Hs = rate of heat storage in body
Thermal signals from warm and cold sensitive receptors in skin and viscera are integrated with brain temperature information from warm sensitive WHAT neurones of the central thermoregulatory network to effect activities that regulate body temperature
GABAergic preoptic neurones
What are dermatomes?
Areas of skin that send signals to the brain through spinal nerves
Give rise in sensations including temperature and pressure
They are served by different dorsal root ganglia
Describe RNA thermometers in prokaryotes
RNA structure of the riboswitch responds to temperature changes
At low temp = OFF switch
This is where the RNA conformation masks the ribosome binding site (Shine-Dalgarno sequence)
When temperature is raised = ON switch
The structure melts locally, exposing the ribosome binding site
Neurones located in the dorsal root ganglion (DRG) project to the skin where they detect changes in temperature both in the noxious and innocuous range
TRP channel expression defines at least 4 DRG neuron subclasses. What are they?
eg TRP..
TRPV1 expressing (hot nociceptors) TRPV1+TRPA1 expressing (putative hot-cold polymodal nociceptors) TRPM8 expressing (cold sensors) TRPV2 expressing (very high threshold hot nociceptors)
Warm receptors TRPV3 and TRPV4 do not appear to be expressed in DRG neurons but rather in WHAT within the skin (TRPV3) or very broadly in both neural and non neural tissues (TRPV4)
Keratinocytes
Describe TRPM8
Cold receptor channel
Receptor for methanol
First cold transducer channel described
Nonselective cation channel
Directly activated by innocuous cooling <28c, Q10 > 20
In vivo behavioural studies show involvement in thermal discrimination <25c
Still debated if its involved in cold triggered nociception
Describe TRPA1
Expressed in nociceptive cells DRGs and trigeminal neurons
Activated by pungent compounds
Behavioural response to noxious cold (<5c) in vivo
Specifically controls the responsiveness to noxious (harmful) cold, but not to innocuous cool
Are TRPM8 and TRPA1 the only cold receptors?
30-50% of isolated DRG neurons in culture are sensitive to cold but unresponsive to either agonists of TRPA1 and TRPM8.
This suggests the presence of as of yet unidentified cold receptors
What is the temperature sensitive part in the TRP channels?
Not confined to a specific domain but distributed
What is the structure for TRPs?
Crystal structure
Work as tetramers
Describe some possible mechanisms of temperature sensitivity
TRP channels are directly activated by temperature and undergo a conformational change to open a channel
A change is due to temperature in the lipid bilayer. This opens the channels. Not temperature sensitive but sensitive to a change in its environment in the lipid bilayer
Probably the way it works: Direct effect of temperature on the channel, opening it up
Gallio et al 2016 found that blocking transmission from “cold” neurons into the brain does not affect avoidance of “hot” temperature ranges and that blocking transmission from “hot” neurons into the brain does not affect avoidance of “cold” temperature ranges
What does this suggest?
This suggests that temperature is not perceived as a single percept by comparison of 2 separate inputs
But behavioural avoidance is steered separately by independent channels for hot and cold
Rather than there being a preferred temp in drosophila, there is a non avoided temp
avoids hot and cold to get temp
It is very common for one neuron to be activated which causes another to be slowed
Why?
2 possibilities
- The odorant acts as an antagonist on one neuron but an inhibitor on the other
- Odorant activates one neuron and that activation causes the other neuron to be inhibited
