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)