Lecture 19: Thermoregulation Flashcards
Thermoregulation
Maintenance of body temperature within a normal range
Endotherms
Generate own internal body heat, e.g. humans
Homeotherms
Keep body within narrow temperature range despite wide ambient temperature range e.g. humans
Body temp measure
Internal core temp is what our body tries to maintain; rectal most accurate
Body temp variation
Temp varies w/ time of day, menstrual cycle, metabolic/physical activity, age (extremes)
Newborn/elderly thermoregulation
Newborns: high surface area:volume, thin skin, limited sweat/shivering
Elderly: atrophied thermoreceptors, sk. muscle, sweat glands -> reduced temp sensing, heat generation, heat dissipation
System components of thermoregulation
- Thermal sensors
- Afferent paths
- CNS integration sensors
- Efferent paths
- Target organs
Heat transfer modes
- Radiation
- Conduction
- Convection
- Evaporation
Heat elimination mechanisms
Skin circulation, sweat glands (primarily through skin), behavior
Heat generation mechanisms
Sk. muscle, brown fat, behavior; heat production linked to metabolism
Radiation
Heat loss by EM rays; depends on temp difference between skin and environs
Conduction
Heat transfer through direct contact; direction depends on temperature difference
Convection
Heat transfer through direct contact with moving fluid (air, water). Depends on temp difference, heat transfer capacity of fluid, and fluid velocity
Evaporation
Heat transfer by vaporizing liquid covering any surfaces. Depends on vapor P gradient (defines ability to evaporate) i.e. partial P of H2O at skin vs in air. Also depends on air velocity; stagnant air -> H2O accumul. -> higher air vapor P of water
Peripheral thermoreceptors
Special sensory neurons (free nerve endings) over entire skin surface
2 types: 1) warmth, 2) cold
Skin thermoreceptor innervation
Density varies; coarse in trunk/limbs, fine in acral skin (face, lips, fingers)
Distinct sensors innervate separate warm/cold skin areas. Tune reflex sensitivity to facilitate feedforward reflexes; metabolism change varies with skin temp.
Warmth thermoreceptors
Increase firing with increasing temperature, up to 44-46 C
Cold thermoreceptors
Increase firing with decreasing temperature, down to 24-28 C
Skin thermoreceptor ion channels
ThermoRs have ion channels at neuron ends; warmth/cold opens channels -> depolar -> APs. Channels also have ligand binding sites e.g. menthol/capsaicin for cooling/warming feeling
Core thermoreceptors
Especially concentrated in pre-optic area, anterior hypothalamus. Warmth receptors only. Facilitate reflex responses.
Skin and core signal integration
Thermogenic command neurons and warm-sensing core neurons coordinate to activate production/inhib. dissipation or vice versa
Thermal effectors
- Cutaneous circulation
- Sweat glands
- Skeletal muscle
- Brown fat
Vasodilation in acral skin
Vasodilation occurs w/ symp. adrenergic tone withdrawal -> increased AVA flow
Vasodilation in non-acral skin
Symp. ACh (not NE) activates sweat glands to sweat more and produce kallikrein -> bradykinin (potent vasodilator)
Vasoconstriction of skin
NE stim. -> vasoconstriction for all skin
Brown adipose tissue adrenergic innervation
NE binds to β-3 and α-1 receptors to activate brown fat heat generation
BAT α-1 response
Alongside T-4 hormone, increases 5’-deiodinase activity to convert T-4 to T-3 which increases UCP expression
BAT β-3 response
β-3 increases lipase activity to breakdown TAGs for FA import to mitochondria; FAs stimulate UCP expression in IMM
UCP
O2 dependent heat production in IMM; has higher affinity for H+ than ATP synthase, so mitochondria switch to heat production over energy
Exercise hyperthermia
Increase in metabolism increases thermogenesis, which is then sensed and reflex response occurs. Results in initial period where production outpaces dissipation; opposite occurs when stopping.
Fever
Increase in thermal set point stimulated by pyrogens released by immune cells. More pyrogens -> more fever. At moderate levels, promotes immune proliferation + migration.
Pyrogens
Fever-inducing factors from immune cells which cross BBB and stimulate endothelial prostaglandin E2 production
Prostaglandin E2
Synth. stim. by pyrogens; inhibit warmth sensing neurons in core, raising the set point.
Fever response and ambient temperature
In hot environs for fever, turning off heat dissipation is most effective. In cold, can only activate thermogenesis -> sufficiently cold environs will result in no fever
Exercise vs fever hyperthermia
Exercise: heat rise due to delayed, incomplete thermoregulatory compensation for heat load
Fever: regulated elevation in core temp by the thermoregulatory system.
