1. WHAT IS MEAN BODY TEMPERATURE? Very different for different animals WHY? Regional differences - core body temperature Hottest region - lumen of stomach + upper part small intestine - distinctly hotter than aortic blood (Grayson, 1983) - not due - digestive or absorptive functions - nor to metabolism

2. Estimates of body temperature (steady-state conditions) Tb = (a1 x Tcore) + (a2 x Tskin) a factors = empirically determined contributions of core & shell to mean body temperature Some estimates : 9 : 1 and 6 : 4 Varies with thermal environment and investigator Tb = 0.67 Tre + 0.33 Tskin Which thermal sensors are important?

3. How do you determine mean surface temperature? Humans: Tskin = 0.07Tfeet + 0.32Tlegs + 0.18Tchest + 0.17Tback + 0.14Tarms + 0.05Thand + 0.07Thead Periphery - skin, mucous surfaces (mouth, nose) Core - spinal cord, hypothalamus, medulla oblongata, + organs Difficult - determine by experiment - which are important sites

4. Chicken - exposed cold Ta - shivers before change in deep body temperature Suggests - heat production - controlled by thermal receptors in skin. Kuhnen and Jessen - 1988 - goat Change - skin temperature - alone - alters MR

5. 2 Populations of Temperature Sensitive Structures (skin, spinal cord, hypothalamus) 1. Warm sensors - activity increases with local temperature increase No discharge at skin temperature below 30 C Receptors discharge with increase temperature from 30 >> 35 C But little sustained activity Further increase - skin temperature >> increased sustained discharge that changes with temperature Peak activity - 45-47 C Activity ceases at higher temperature

6. 2.Cold Sensors - activity decreases over same temperature range Most active at 15 - 35 C & peak at 20 - 30 C =========================================================== PARADOXICAL RESPONSE Most cold receptors >> no discharge above 35 C But may get discharge near noxious level (43 - 47 C) (ex. sudden chill - stepping into hot shower) Also paradoxical warm receptor response - BUT not as common

7. Proportions - cold & warm sensors - vary with site In general - more warm sensors - hypothalamus more cold sensors - cutaneous regions Exceptions - scrotal region - high concentration warm sensors Sheep & rats - local heating scrotum >> powerful heat loss effector activity If sustained >>> decrease in body temperature Heating scrotum - ram - above 36 C >> panting & decrease core body temperature (~ 2 C) Similiar effects - heating mammary skin - ewe

8. General facts about skin temperature Not homogenous - large difference in skin temperature in different parts of the body May account for regional differences by averaging surface area BUT - density of skin thermoreceptors are different in different areas Larger weight to face and less to extremities Face - approx. 20% of total skin signal

9. Skin temperature - represents interaction between convective heat exchange to surface by blood + removal of heat (geometry, external insulation, ambient temperature) Virtually all skin areas - including trunk - constrict at cold Ta (Exception = human forehead - no vasoconstriction at cold Ta)

10. CENTRAL RECEPTORS Under normal conditions - trunk sites do not vary more than 0.5°C Arterial temperature (e.g., carotid) - contain core temperature sensitive sensors Hypothalamus - contains many neurons and thermoregulatory circuits Preoptic Anterior Hypothalamus (POAH) still a black box

11. During heat stress and exercise - may maintain brain temperature below trunk In panting carnivores - blood supply passes through carotid rete A net-like structure - base of brain Numerous small arteries embedded in cavernous sinus Rete = heat exchanger

12. Cooled venous blood - evaporating surfaces (e.g., nasal mucosa) arterial blood entering brain running antelope - brain temperature ~3°C below carotid temperature Also goat and sheep maintenance of lower brain temperature also found in species without carotid rete (rabbit, squirrel, monkey, man) Nonpanting animals - cool venous blood - sweating face Site of exchange with arterial blood - unknown