Lecture 19: Thermoregulation

Question 1

Thermoregulation

Answer 1

Maintenance of body temperature within a normal range

Question 2

Endotherms

Answer 2

Generate own internal body heat, e.g. humans

Question 3

Homeotherms

Answer 3

Keep body within narrow temperature range despite wide ambient temperature range e.g. humans

Question 4

Body temp measure

Answer 4

Internal core temp is what our body tries to maintain; rectal most accurate

Question 5

Body temp variation

Answer 5

Temp varies w/ time of day, menstrual cycle, metabolic/physical activity, age (extremes)

Question 6

Newborn/elderly thermoregulation

Answer 6

Newborns: high surface area:volume, thin skin, limited sweat/shivering
Elderly: atrophied thermoreceptors, sk. muscle, sweat glands -> reduced temp sensing, heat generation, heat dissipation

Question 7

System components of thermoregulation

Answer 7

1. Thermal sensors
2. Afferent paths
3. CNS integration sensors
4. Efferent paths
5. Target organs

Question 8

Heat transfer modes

Answer 8

1. Radiation
2. Conduction
3. Convection
4. Evaporation

Question 9

Heat elimination mechanisms

Answer 9

Skin circulation, sweat glands (primarily through skin), behavior

Question 10

Heat generation mechanisms

Answer 10

Sk. muscle, brown fat, behavior; heat production linked to metabolism

Question 11

Radiation

Answer 11

Heat loss by EM rays; depends on temp difference between skin and environs

Question 12

Conduction

Answer 12

Heat transfer through direct contact; direction depends on temperature difference

Question 13

Convection

Answer 13

Heat transfer through direct contact with moving fluid (air, water). Depends on temp difference, heat transfer capacity of fluid, and fluid velocity

Question 14

Evaporation

Answer 14

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

Question 15

Peripheral thermoreceptors

Answer 15

Special sensory neurons (free nerve endings) over entire skin surface
2 types: 1) warmth, 2) cold

Question 16

Skin thermoreceptor innervation

Answer 16

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.

Question 17

Warmth thermoreceptors

Answer 17

Increase firing with increasing temperature, up to 44-46 C

Question 18

Cold thermoreceptors

Answer 18

Increase firing with decreasing temperature, down to 24-28 C

Question 19

Skin thermoreceptor ion channels

Answer 19

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

Question 20

Core thermoreceptors

Answer 20

Especially concentrated in pre-optic area, anterior hypothalamus. Warmth receptors only. Facilitate reflex responses.

Question 21

Skin and core signal integration

Answer 21

Thermogenic command neurons and warm-sensing core neurons coordinate to activate production/inhib. dissipation or vice versa

Question 22

Thermal effectors

Answer 22

1. Cutaneous circulation
2. Sweat glands
3. Skeletal muscle
4. Brown fat

Question 23

Vasodilation in acral skin

Answer 23

Vasodilation occurs w/ symp. adrenergic tone withdrawal -> increased AVA flow

Question 24

Vasodilation in non-acral skin

Answer 24

Symp. ACh (not NE) activates sweat glands to sweat more and produce kallikrein -> bradykinin (potent vasodilator)

Question 25

Vasoconstriction of skin

Answer 25

NE stim. -> vasoconstriction for all skin

Question 26

Brown adipose tissue adrenergic innervation

Answer 26

NE binds to β-3 and α-1 receptors to activate brown fat heat generation

Question 27

BAT α-1 response

Answer 27

Alongside T-4 hormone, increases 5’-deiodinase activity to convert T-4 to T-3 which increases UCP expression

Question 28

BAT β-3 response

Answer 28

β-3 increases lipase activity to breakdown TAGs for FA import to mitochondria; FAs stimulate UCP expression in IMM

Question 29

UCP

Answer 29

O2 dependent heat production in IMM; has higher affinity for H+ than ATP synthase, so mitochondria switch to heat production over energy

Question 30

Exercise hyperthermia

Answer 30

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.

Question 31

Fever

Answer 31

Increase in thermal set point stimulated by pyrogens released by immune cells. More pyrogens -> more fever. At moderate levels, promotes immune proliferation + migration.

Question 32

Pyrogens

Answer 32

Fever-inducing factors from immune cells which cross BBB and stimulate endothelial prostaglandin E2 production

Question 33

Prostaglandin E2

Answer 33

Synth. stim. by pyrogens; inhibit warmth sensing neurons in core, raising the set point.

Question 34

Fever response and ambient temperature

Answer 34

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

Question 35

Exercise vs fever hyperthermia

Answer 35

Exercise: heat rise due to delayed, incomplete thermoregulatory compensation for heat load
Fever: regulated elevation in core temp by the thermoregulatory system.