Figure 29.13 The Hypothalamus Regulates Body Temperature
The first demonstration that thermoregulatory responses in mammals are controlled by the temperature of the hypothalamus was shown by Hammel et al. (1963) using dogs. The experiment shown here was done some years later on ground squirrels, which may seem to be unusual experimental animals. However, the question that drove these experiments had to do with hibernation (see Figure 29.14), an amazing adaptation for surviving winter conditions. During hibernation, the body temperature of a ground squirrel drops close to that of its environment, which might be 0° C under the snow in the mountains where these animals live. The question was whether this drop in body temperature was a failure of thermoregulation and a return to a more primitive ectothermic condition, or whether these animals had a brain thermostat that could be adjusted over a broad range of temperatures and enabled the squirrels to regulate their body temperatures at these low levels. To address this question, researchers investigated the properties of the brain thermostats of these animals when they were not hibernating. In this experiment, tiny stainless steel tubes were surgically implanted into the brains of ground squirrels on either side of the hypothalamus. By circulating water at different temperatures through these tubes and measuring the temperature of the hypothalamic tissue between the two tubes, it was possible to control the temperature of the squirrel’s brain thermostat. If the metabolic rate of the squirrel were measured at the same time, it was possible to correlate the heat production of the squirrel with the temperature of its hypothalamus. Results showed that when the hypothalamus was cooled, the metabolic rate and, thus, the body temperature of the squirrels increased;, whereas when the hypothalamus was heated, metabolic rate slowed and body temperature decreased. In accord with the previous studies of dogs, these data support the role of the hypothalamus as a thermostat to regulate body temperature.
Original Papers
Heller, H. C., G. W. Colliver, and P. Anand. 1974. CNS regulation of body temperature in euthermic hibernators. American Journal of Physiology 227: 576–582.
http://ajplegacy.physiology.org/cgi/reprint/227/3/576
Heller, H. C., and G. W. Colliver. 1974. CNS regulation of body temperature during hibernation. American Journal of Physiology 227: 583–589.
http://ajplegacy.physiology.org/cgi/reprint/227/3/583
Hammel, H. T, D. C. Jackson, J. A. J. Stolwijk, J. D. Hardy, and S. B. Strømme. 1963. Temperature regulation by hypothalamic proportional control with an adjustable set point. Journal of Applied Physiology 18: 1146–1154.
http://jap.physiology.org/cgi/reprint/18/6/1146
Links
Kimball’s Biology Pages: The Transport of Heat
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/H/HeatTransport.html
Vella, C. A. and L. Kravitz. 2004. Staying Cool When Your Body is Hot
http://www.unm.edu/~lkravitz/Article%20folder/thermoregulation.html
Stanford School of Medicine: H. Craig Heller
https://med.stanford.edu/profiles/h-craig-heller