functional architecture of human thermoregulation€¦ · major role in behavioral thermoregulation...
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FUNCTIONAL ARCHITECTURE OF HUMAN THERMOREGULATION
Andreas D. Flouris
FAME LaboratoryDep. of Exercise ScienceUniversity of Thessaly
Greece
THERMOREGULATION IN THE HEAT
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Afferent Input
Integration CommandBODY
TEMPERATURE
Negative Feedback Loop
Peripheral and Central Thermal Sensors
Central Thermosensors
THERMOREGULATION
Autonomic/endocrinethermoregulation– finite capacity
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Behaviouralthermoregulation– near‐infinite capacity
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
AUTONOMIC THERMOREGULATION
Reductions in skin/core temperature (cold)
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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
Heat balance
↑ rate of heat loss to environment
AUTONOMIC THERMOREGULATION
Increases in skin/core temperature (heat/work)
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Combined responses
– Heat dissipation peripheral vasodilation
sweating
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
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AUTONOMIC THERMOREGULATION
8 Kenny & Flouris, 2014
PASSIVE HEAT EXPOSURE
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Net heat load
Metabolic heat production
Evaporative heat loss
Dry heat exchange
Kenny & Flouris, 2014
Semi nude adult male30°C 36°C
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
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Compensable conditions
Uncompensableconditions
Heat balance Rate of heat storage = 0
↑ core temperature+ heat storage
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)
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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
THERMOREGULATION DURING WORK
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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
THERMOREGULATION DURING WORK
13 Kenny & Flouris, 2014
THERMOREGULATION
Autonomic/endocrinethermoregulation– finite capacity
14
Behaviouralthermoregulation– near‐infinite capacity
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
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
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)
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Hua et al, AJP‐RICP, 2005Brooks et al, Neuroimage, 2005
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)
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Craig, Ann N Y Acad Sci, 2011Craig, Nat Rev, 2009
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
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Gagge et al., Environ Res, 1967
Borg, MSSE, 1982
Gagge et al., Environ Res, 1967
THERMAL BEHAVIOUR DURING WORK
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Heat: ~425 WCatabolism:
500 W
External work: 75 W
THERMAL BEHAVIOUR DURING WORK
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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
THERMAL BEHAVIOUR DURING WORK
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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
THERMAL BEHAVIOUR DURING WORK
Changes in work intensity have a major impact on heat balance: S = M – (± W) ± (R + C+ K) – E
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Flouris & Schlader, SJMSS, 2015Flouris et al., EJAP, 2011Schlader et al., JTB, 2011
changes in work intensity affect thermoregulation and are
considered thermoregulatory behaviors
BEHAVIOURAL THERMOREGULATION
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CONSCIOUS RESPONSE TO THERMAL INPUT
What is the basis of our choices for our thermal state?
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
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45°C 10% RH
10°C 50% RH
Schlader et al., Physiol Behav, 2009
BASIS OF THERMAL DECISIONS
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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
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
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
THERMAL VS. NON‐THERMAL STIMULI
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Cap
saic
in
Men
tho
l
War
min
g
Co
olin
g
Co
ntr
ol
Schlader et al., Physiol Behav, 2011
==
THERMAL VS. NON‐THERMAL STIMULI
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Similarly Hot
Cooling
Warming
Schlader et al., Physiol Behav, 2011
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
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Schlader et al., Physiol Behav, 2011
33Core Temperature
↑ Skin Temperature
↑ Perceived Exertion
↓ Work Intensity
Thermal Perception(↑ warmth discomfort)
Cardiovascular Strain(↓ peak oxygen uptake)
Heat exposure
Flouris & Schlader, SJMSS, 2015
FUNCTIONAL ARCHITECTURE OF HUMAN THERMOREGULATION
Andreas D. Flouris
FAME LaboratoryDep. of Exercise ScienceUniversity of Thessaly
Greece