During these hot summer days, lying in the shadow puffing and sweating, my arms and legs pulling down like bags of sand, it is sometimes difficult to believe that my brain is still functioning fine. How do we manage to keep our head cool, even on hot days like these?
Like most animals, our body temperature is 37 degrees Celsius (98.6 Fahrenheit), small changes above or below this temperature lead to changes in the rate of chemical reactions, malformations of proteins and other unpleasant consequences (Sukstanskii & Yablonskiy, 2006).
Of all organs, the brain is probably the most vulnerable to heat. The delicate neural activity patterns can change dramatically when the brain gets too warm, leading to neuronal death and seizures (Bazille et al., 2005; Kiyatkin, 2010). The body has therefore developed clever strategies to keep our brain at the right temperature.
The main executor of these strategies is the preoptic area, a region that lies at the bottom of our brain and is part of the hypothalamus. The preoptic area receives information mainly from thermoreceptors in the skin (Morrison & Nakamura, 2011) and in the preoptic area itself, but also from thermoreceptors in the abdomen and spinal cord.
Some of these sensory areas are specialized in detecting decreases in body temperature, whereas other parts are better in detecting that the body is heating up. For example, the skin and the spine contain more cold-sensitive receptors whereas the local thermoreceptors in the preoptic are mostly heat-sensitive. This specialization was also shown functionally in a study that investigated how chickens respond to hot temperatures and found that regardless of the way in which the chickens are heated up (for example by selectively radiating their lower body), the chickens will only start panting in response to heat when both their abdominal temperature and the temperature of the preoptic area (areas known to be more heat sensitive) are raised (Richards, 1970).
The preoptic area thus weighs its inputs depending on their origin in order to make the right decision. And on a day like today that decision would be quite straightforward: the body needs to cool down!
To accomplish this, the preoptic area does a number of things. Firstly, it signals to the sweat glands in the skin to increase sweat production. Water uses heat energy to change into the gaseous state (this is also called heat of vaporization), and thus sweating cools down the skin. When the air is humid, less water can evaporate, and thus less heat can be lost. This is why humid heat feels so much hotter than dry heat.
In order to cool down the rest of the body, the preoptic area signals to the veins in our skin to widen (this process is called vasodilation). By dilating the veins and thus increasing the blood flow to our skin, more blood can be cooled down by the effects of sweating. This cooled blood is then used to cool the inside of our body.
Thus, in order to keep our head cool, what we really need is cold blood. This feels paradoxical, since we relate increased blood flow to increased temperature. A red face is usually very warm, and when our hands become white they feel very cold. However, inside the body, especially in our brain, it often works the other way around. Our brain tissue is on average 0.4 degrees Celsius (0.7 Fahrenheit) warmer than the blood in our veins (Kiyatkin, 2010). An increase in blood flow to a specific brain area (like we see on BOLD-fMRI) thus means that this area will actually become cooler instead of hotter.
The brain has many mechanisms to adjust the blood flow to specific areas (Kiyatkin, 2010), and since neural activity depends strongly on temperature, it seems quite likely that is uses small temperature changes to affect neural activity. Next to regulating the temperature of the blood by sweating and vasodilation, the brain thus also scrupulously regulates local temperatures within the brain, to ensure the perfect environment for its brain cells. If only I could have a thermostat like that in my house!
One last way in which we cool down our bodies is by changing our behavior. Like most animals on a warm day, people seek the shadow and reduce their activity levels (since muscle activity produces a lot of heat). Although animal studies have shown that local heating or cooling of the preoptic area induces behaviors that are in line with the preoptic temperature changes (Boulant, 2000), there are also studies showing that animals without a preoptic area still accurately adjust their behavior to the outdoor temperature (Morrison & Nakamura, 2011). Behavioral regulation as a result of temperature thus seems to be a more complex process, probably involving many different brain areas.
Now that we know the biology, it should be no problem to keep our head cool! Drink plenty of water, so you can continue to sweat away, lie back in you chair, eat some ice cream (just don’t freeze your brain) and let the cool blood flow!
- Bazille, C., Megarbane, B., Bensimhon, D., Lavergne-Slove, A., Baglin, A.C., Loirat, P., Woimant, F., Mikol, J., Gray, F. (2005). Brain damage after heat stroke. J Neuropathol Exp Neurol., 64(11), 970-975.
- Boulant, J.A. (2000). Role of the preoptic-anterior hypothalamus in thermoregulation and fever. Clin Infect Dis, doi: 10.1086/317521.
- Childs, C. (2008). Human brain temperature: regulation, measurement and relationship with cerebral trauma: part 1. J. Neurosurg., 22(4), 486-496.
- Kiyatkin, E.A. (2010). Brain temperature homeostasis: physiological fluctuations and pathological shifts. Front Biosci, 1(15), 73-92.
- Morrison, S.F., Nakamura, K. (2011). Central neural pathways for thermoregulation. Biosci, 16, 74-104.
- Richards, S.A. (1970). The role of hypothalamic temperature in the control of panting in the chicken exposed to heat. J Physiol. 211(2): 341–358.
- Sustanskii, A.L., Yablonskiy, D.A. (2006). Theoretical model of temperature regulation in the brain during changes in functional activity. PNAS, 103(32), 12144-12149.