Thermoregulation and Circadian Rhythms Part 1: The Body’s Thermostat

My project is about the interaction of thermoregulation and circadian rhythms in the brain.  Each of my three blog entries will focus on a specific piece of this.  In this first entry I will describe how we regulate our body temperatures, the second will focus on circadian rhythms and the last entry will bring it all together and describe what we know about their interaction.  I think that is enough foreshadowing, on to the task at hand: the body’s thermostat.

Present in most living organisms is the desire for homeostasis, the ability to resist changes in the external environment in an effort to maintain a constant internal condition.  As warm blooded animals one of our most important homeostatic functions is maintaining a stable internal body temperature.  If a person’s body temperature is altered too greatly serious disability or death could occur.  Humans are able to sustain a set body internal body temperature regardless of their environment by regulating two sets of behaviors: heat loss and heat production.  If temperature is too high the body will engage in heat loss behaviors such as sweating, flattening of body hair, and directing blood flow to the skin surface.  Beyond the simple physiological responses people also actively seek out cooler places.  In contrast, if temperature were too low the body would shut down the heat loss behaviors and switch on the heat production behaviors.  These include diverting more blood flow internally, shivering and raising hairs on the body in an effort to trap heat and preserve it.  The key to regulating body temperature, therefore, is controlling these processes, switching them on and off in balance like a thermostat.  The question then becomes where and how does this occur.

Our thermostat is located in the hypothalamus a region deep within the brain that is responsible for most homeostatic processes.  Specifically, we are dealing the pre-optic/anterior hypothalamus.  The importance of this area in thermoregulation was discovered after whole animal studies were conducted by sticking a probe into the anterior hypothalamus.  As the probe’s temperature was raised, the animal panted and exhibited other heat loss behaviors.  Since then the system has been studied closely and the most prevailing theory on how our “thermostat” functions is called Hammel’s Model.

The anterior and pre-optic regions of the hypothalamus have two different types of neurons.  Warm-sensitive neurons will increase their firing rate by at least .8 impulses per second per degree Celsius.  The higher the temperature the more action potentials the neuron will fire.  These neurons comprise about 30% of the neurons in this area.  Temperature insensitive neurons however maintain a relatively constant firing rate and comprise the other 70% of pre-optic/anterior neurons.  Hammel’s model theorizes that these two types of neurons work antagonistically to create the thermostat like effect we see with heat loss and heat production behaviors.  According to Hammel’s theory an “effector neuron” (one that controls heat loss or heat production behaviors) would receive inputs from both an insensitive neuron and a warm-sensitive neuron.  In the case of heat loss the warm sensitive neuron would be sending an excitatory signal while the temperature insensitive neuron would be sending the inhibitory signal.  At our normal set point temperature the two inputs would cancel each other out and the behaviors would be at their normal level.  If temperature became too high, however, the excitatory signals from the warm sensitive neurons would overpower the inhibitory signals causing the heat loss behaviors to be acted upon.  For heat production behaviors the type of signal is switched.  This is a lot clearer if you look at a picture so I’ve included on in the entry.  It’s from a review article by Jack Boulant published in 2006.

Hammel's model

So, our internal body temperature will change if the baseline firing rate of either warm sensitive or temperature insensitive neurons is changed by a neurotransmitter, second messenger protein or other drug.  For my project, a big question becomes what chemical or protein involved in circadian rhythms could be affecting these neurons to cause the daily temperature fluctuations common in most people.  I’ll be back soon with that answer.

For more information see:

Neuronal basis of Hammel’s model for set-point thermoregulation

Jack A. Boulant

Department of Physiology and Cell Biology, College of Medicine, Ohio State University, Columbus, Ohio