functional architecture of human thermoregulation · 2015-05-28 · functional architecture of...
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FUNCTIONAL ARCHITECTURE OF HUMAN THERMOREGULATION
Andreas D. Flouris
FAME LaboratoryDep. of Exercise ScienceUniversity of Thessaly
Greece
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THERMOREGULATION IN THE HEAT
2
Afferent Input
Integration CommandBODY
TEMPERATURE
Negative Feedback Loop
Peripheral and Central Thermal Sensors
Central Thermosensors
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THERMOREGULATION
Autonomic/endocrinethermoregulation– finite capacity
3
Behaviouralthermoregulation– near‐infinite capacity
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THERMOSENSORS
Transient receptor potential (TRP) ion channels– expressed in pain‐ and temperature‐sensitive neurons– 30 proteins divided into 6 sub‐families– central axons project to lamina I– signals are carried to the hypothalamus, the brainstem, and the insular cortex
4
Flouris & Schlader, SJSMM, 2015Romanovsky, AJP‐RICP, 2007Craig, Nat Rev, 2009
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AUTONOMIC THERMOREGULATION
Reductions in skin/core temperature (cold)
5
Combined responses
↓ rate of heat loss to the environment
↑ rate of metabolic heat production
Heat balance
– Heat conservation peripheral vasoconstriction
– Heat generation shivering thermogenesis
non‐shivering thermogenesis
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Heat balance
↑ rate of heat loss to environment
AUTONOMIC THERMOREGULATION
Increases in skin/core temperature (heat/work)
6
Combined responses
– Heat dissipation peripheral vasodilation
sweating
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AUTONOMIC THERMOREGULATION
Each effector response is characterised by a mean body temperature onset threshold beyond which it increases proportionally to the change in core and/or skin temperature Bligh, JAP, 2006
7
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AUTONOMIC THERMOREGULATION
8 Kenny & Flouris, 2014
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PASSIVE HEAT EXPOSURE
9
Net heat load
Metabolic heat production
Evaporative heat loss
Dry heat exchange
Kenny & Flouris, 2014
Semi nude adult male30°C 36°C
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PASSIVE HEAT EXPOSURE
Peripheral vasodilation serves to decrease the temperature gradient between the skin and the environment, thus attenuating the rate of dry heat gain– evaporation is the only means by which the body can lose heat in hot conditions
10
Compensable conditions
Uncompensableconditions
Heat balance Rate of heat storage = 0
↑ core temperature+ heat storage
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EVAPORATION
Water evaporation from the skin and the membranes of the respiratory tract is a vital means of dissipating heat– conversion of water molecules from fluid to gas requires large amounts of energy (~600 kcal/L)
11
Rest Work% kcal/min % kcal/min
Conduction & Convection 20 0.3 15 2.2Radiation 60 0.9 5 0.8Evaporation 20 0.3 80 12Total 100 1.5 100 15
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THERMOREGULATION DURING WORK
12
At the beginning of work, heat production rises rapidly due to increased metabolism primarily in the working muscles
The mechanisms of heat dissipation react with significantdelay and so body temperature rises at the start of work
When/If heat dissipation reaches heat production (i.e., attainment of heat balance), core temperature will stabilize
Flouris & Cheung, EJAP, 2010
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THERMOREGULATION DURING WORK
13 Kenny & Flouris, 2014
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THERMOREGULATION
Autonomic/endocrinethermoregulation– finite capacity
14
Behaviouralthermoregulation– near‐infinite capacity
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THERMAL BEHAVIOUR
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THERMOSENSORS
Transient receptor potential (TRP) ion channels– expressed in pain‐ and temperature‐sensitive neurons– 30 proteins divided into 6 sub‐families– central axons project to lamina I– signals are carried to the hypothalamus, the brainstem, and the insular cortex
16
Flouris & Schlader, SJSMM, 2015Romanovsky, AJP‐RICP, 2007Craig, Nat Rev, 2009
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CEREBRAL LOCI OF THERMAL FEELINGS
In addition to the hypothalamus and the brainstem, lamina I also activates the insular cortex
Integration within the insulagenerates the template for a “feeling”– a combined representation of homeostatically salient features of the individual’s internal and external environment
17Craig, Nat Rev, 2009Craig, Ann N Y Acad Sci, 2011
Subjective cooling
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CEREBRAL LOCI OF THERMAL FEELINGS
In addition to the hypothalamus and the brainstem, lamina I also activates the insular cortex
Integration within the insulagenerates the template for a “feeling”– A: activation due to innocuous cooling (neck; hand)
– B: activation due to noxious heating(face; arm; leg; overlap)
18
Hua et al, AJP‐RICP, 2005Brooks et al, Neuroimage, 2005
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NEURAL BASIS OF THERMAL BEHAVIOUR
The preoptic anterior hypothalamus does not play a major role in behavioral thermoregulation
The cerebral neural pathways important for behavioural thermoregulation have little to dowith triggering thermoeffector responses (i.e., autonomic thermoregulation)
19
Craig, Ann N Y Acad Sci, 2011Craig, Nat Rev, 2009
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MEASURES OF THERMAL PERCEPTION
Thermal comfort: subjective indifference with the thermal environment
Thermal sensation: relative intensity of the temperature being sensed
Perceived exertion: subjective perception of effort
20
Gagge et al., Environ Res, 1967
Borg, MSSE, 1982
Gagge et al., Environ Res, 1967
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THERMAL BEHAVIOUR DURING WORK
21
Heat: ~425 WCatabolism:
500 W
External work: 75 W
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THERMAL BEHAVIOUR DURING WORK
22
Cycling (most efficient physical task)~20% of energy used for work
Whipp & Wasserman, JAP, 1972
Heat: ~400 W
Catabolism: 500 W
External work: 100 W
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THERMAL BEHAVIOUR DURING WORK
23
Heat: ~400 W
Catabolism: 500 W
External work: 100 W
↓ performance & health
Schlader et al., JTB, 2011Ely et al, MSSE, 2010Tatterson et al, JSMS, 2000
↑ core temperature
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THERMAL BEHAVIOUR DURING WORK
Changes in work intensity have a major impact on heat balance: S = M – (± W) ± (R + C+ K) – E
24
Flouris & Schlader, SJMSS, 2015Flouris et al., EJAP, 2011Schlader et al., JTB, 2011
changes in work intensity affect thermoregulation and are
considered thermoregulatory behaviors
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BEHAVIOURAL THERMOREGULATION
25
CONSCIOUS RESPONSE TO THERMAL INPUT
What is the basis of our choices for our thermal state?
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BASIS OF THERMAL DECISIONS
Shuttle‐box model:– freedom of movement between two thermally extreme environments
– thermoregulatory behavior = moment at which a conscious decision is made Tre Tsk thermal comfort time in chamber
26
45°C 10% RH
10°C 50% RH
Schlader et al., Physiol Behav, 2009
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BASIS OF THERMAL DECISIONS
27
1
2
3
4
5
6
7
8
9
10
29.0
30.0
31.0
32.0
33.0
34.0
35.0
36.0
37.0
38.0
0 10 20 30 40 50 60 70 80
Thermal Com
fort
Tempe
rature ( °C)
Time (min)
Tc
Ts
ThC
Schlader et al., Physiol Behav, 2009
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BASIS OF THERMAL DECISIONS
High probability of exit at Tsk:– cold to hot: 29.6 to 26.4°C– hot to cold: 34.1 to 36.2°C
Behaviour was mainly driven by Tsk and not by Tc
28 Schlader et al., Physiol Behav, 2009
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THERMAL VS. NON‐THERMAL STIMULI
Cycling protocol: – RPE clamped at 16 (hard – very hard)– liquid conditioning garment: 55°C
Conditions:– control– thermal face cooling– non‐thermal face cooling (menthol)– thermal face heating– non‐thermal face heating (capsaicin)
29 Schlader et al., Physiol Behav, 2011
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THERMAL VS. NON‐THERMAL STIMULI
30
Cap
saic
in
Men
tho
l
War
min
g
Co
olin
g
Co
ntr
ol
Schlader et al., Physiol Behav, 2011
==
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THERMAL VS. NON‐THERMAL STIMULI
31
Similarly Hot
Cooling
Warming
Schlader et al., Physiol Behav, 2011
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THERMAL VS. NON‐THERMAL STIMULI
Facial temperature and thermal perception are capable modulators of behaviouralthermoregulation and work output
Physical (thermal) temperature change is not a necessary requirement for the initiation of thermoregulatory behaviour
32
Schlader et al., Physiol Behav, 2011
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33Core Temperature
↑ Skin Temperature
↑ Perceived Exertion
↓ Work Intensity
Thermal Perception(↑ warmth discomfort)
Cardiovascular Strain(↓ peak oxygen uptake)
Heat exposure
Flouris & Schlader, SJMSS, 2015
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FUNCTIONAL ARCHITECTURE OF HUMAN THERMOREGULATION
Andreas D. Flouris
FAME LaboratoryDep. of Exercise ScienceUniversity of Thessaly
Greece