Shiver me muscles: why do you shake when you’re cold?

Posted by Susan Lubejko on August 4, 2022 in Neuroscience, Perception, Stress | Leave a comment

As a proud mid-Atlantic East Coaster, I thought I was relatively well adapted to colder winter climates. After being in sunny San Diego for a few years, however, I have realized that this is NOT so when a slight breeze invokes a shiver in my spine, or sitting outside on a 55 degree day can send me into full body shakes (thank goodness for outdoor dining spots with heaters). If you are also prone to shivers, you may have asked yourself, why do we shake in response to the cold? What brain mechanisms allow us to process the environmental temperature and our own body temperatures while producing the repetitive movements in our muscles? And what’s the deal with other causes of similar quaking, like fear and anxiety? As is usually the case, our brains have built amazing mechanisms to deal with these uncomfortable situations, which we will explore here.

How does your brain make you shiver in the cold?

Perhaps the most recognizable form of shivering is our response to cold temperatures. To better understand how shivering works, it is helpful to first address how the body and brain sense temperature and “thermoregulate” in the first place. Thermoregulation is the technical term for the process of controlling body temperature, which involves the brain sensing the temperature at many sites inside and outside of the body and coordinating a response to increase or decrease core body temperature accordingly. The skin, as well as certain areas of the spinal cord and brain, is home to specialized sensory nerve cells that sense hot or cold temperatures. Like the sensory cells in our skin that respond to touch, these cold- or heat-sensing neurons send signals to the spinal cord to report a change in environmental temperature. From the spinal cord, this temperature information is sent along two pathways. The first pathway ends in the cerebral cortex, the wrinkled surface of our brains that processes our more complex thoughts and sensory information, which determines our conscious perception of the outside temperature (“it’s chilly out there!”). The second pathway involves areas of the brain that unconsciously help us regulate our internal body temperature. From the spinal cord and after a stop in the brainstem, thermoregulatory temperature information arrives in the preoptic area of the hypothalamus (POA). In addition to thermoregulation, the hypothalamus plays a role in a host of other bodily processes, such as thirst, hunger, and sleep [1]. 

Once this environmental temperature information arrives in the POA and is processed alongside information about current internal body temperature, the POA acts through 4 major routes to increase or decrease body temperature: control of blood flow in the skin, sweating, production of heat by brown adipose tissue (also known as “brown fat,” a hot topic in health sciences that we will briefly discuss later), and shivering. To induce shivering, POA signals back to other portions of the hypothalamus, through another stop in the brainstem, and on to the spinal cord, this time evoking activity in the spinal cord neurons that control the movement of our muscles. These movement-controlling neurons signal to the muscles to cause the rapid and repetitive muscle contractions that we recognize as shivering [1]. In a scientific study using rats to confirm the role of the POA in shivering, researchers found that activation of neurons in the POA directly causes shivering even at normal environmental temperatures, while silencing the activity in the POA prevents shivering in cool environments [2]. These results show that the POA is the major driver of shivering activity.

How does shivering warm you up?

Although we now know how the body and brain work together to interpret cold signals and send responses back to the muscles to induce shivers, how does shivering actually warm us up? Muscle contraction requires the use of ATP, the main energy molecule of our cells. The production of ATP relies on fuels, such as carbohydrates and fats, to be broken down in the body. All of the chemical reactions that must take place in order to produce and utilize ATP to cause muscle contractions involve the release of a tiny bit of heat. This production of heat, called thermogenesis, at scale with our large muscles is actually pretty effective at producing heat that can shift core body temperature. While the best way to increase thermogenesis and warm up is by voluntarily moving or exercising, shivering provides an effective source of thermogenesis when movement isn’t the best strategy [3]. 

Another type of shivering that uses similar mechanisms is febrile shivering, which is shivering produced by fever. You have probably experienced an illness at some point that produces a fever to increase core body temperature in an attempt to kill off invading pathogens. Often, fever is accompanied by chills and shivers. This seems counterintuitive – why do we feel cold when a fever is the result of our body temperature going up? This is because fever occurs when fever-triggering molecules – called pyrogens – are released from the pathogens that make us sick or our immune cells tasked with fighting off these invaders. Pyrogens are directly sensed by special receptors on POA neurons in the hypothalamus, and these signals tell the POA to increase the core body temperature in an attempt to damage invading pathogens. We feel chills during fever due to the fact that core body temperature is now lower than the new target set by the hypothalamus. Shivering is thought to be triggered through the same mechanisms as those above that are responsible for cold-induced shaking: the POA signals through the brainstem to the spinal cord to trigger repetitive movements in the muscles. The resulting shivers then help as a mechanism for increasing body temperature in fever [1,4].

As an interesting aside, not all of us use the same mechanisms of thermoregulation to increase internal body temperature. Interestingly, babies don’t shiver. Instead, body temperature increase in babies can be achieved by one of the other mechanisms under POA control mentioned above: increased thermogenesis in brown adipose tissue (BAT). For a long time, it was thought that brown fat disappears in adulthood. In recent years, however, brown fat became popular in the media after it was discovered that adult humans do, in fact, store brown fat and researchers questioned its utility in weight loss. A bit different from regular white adipose (fat) tissue, brown fat cells have a larger number of mitochondria (classically, the powerhouse of the cell), which is where ATP is made. Differently from other cells, however, brown fat cells respond to chemical signals from the nervous system by disrupting the production of ATP. As a consequence of how mitochondria usually store charged particles to make ATP, this disruption actually leads to the production of a lot of heat. Babies don’t shiver, but they do have far more BAT for their size than human adults and can rely on this process to regulate body temperature [5,6].

Other things that may make you shiver

Cold temperatures and fever, however, aren’t the only stimuli that can cause shivering. Sometimes, shivering can occur due to purely psychological reactions, termed psychogenic shivering. You may have experienced psychogenic shivering if you’ve ever found yourself quaking from fearful or anxiety-producing situations. Perhaps you are a person that experiences shivers in response to particular music or ASMR videos, or even may shudder involuntarily when peeing (ladies, ask a guy friend – they are much more likely to experience this). Are all of these varieties of shivers governed by the same brain and body mechanisms? Although there is far less research on these other causes for shivering, the answer is likely no. 

Fear- or anxiety-produced shivering is probably caused by entry into “fight or flight” mode as a behavioral response to stress and the accompanying flood of the neurochemical noradrenaline throughout the brain and body. Music- (or other pleasure-) induced chills and shivers are likely caused by the activation of the circuitry in the brain that processes rewarding experiences. Although there are no scientific studies on pee shivers, some doctors have their theories on this phenomenon that could involve the thermoregulatory brain machinery, but also may involve aberrant signals in the input and output to the parts of our nervous system that regulate involuntary behavior. Ultimately, there are many different mechanisms in the nervous system that may result in the same shaky output.

Although shivering may not always be the ideal response to cold (refer to me spilling my drink while dining outside in relatively moderate weather) or stress (such as me nearly dropping the microphone while giving a speech at my sister’s wedding), the science behind shivering paints an extraordinary picture of how our bodies actually use biology, chemistry, and physics to unconsciously preserve our wellbeing. Who knew you were so good at all of those science subjects?

References

1. Tan CL, Knight ZA (2018) Regulation of body temperature by the nervous system. Neuron, 98:31-48
2. Nakamura K, Morrison SF (2011) Central efferent pathways for cold-defensive and febrile shivering. Journal of Physiology, 589(14):3641-3658
3. Haman F, Blondin DP (2017) Shivering thermogenesis in humans: origins, contribution, and metabolic requirement. Temperature, 4(3):217-226
4. Balli S, Sharan S. Physiology, Fever. [Updated 2021 Aug 28]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK562334/
5. Cannon B, Nedergaard J (2004) Brown adipose tissue: function and physiological significance. Physiological Reviews, 84:277-359
6. Dawkins MJR, Scopes JW (1965) Non-shivering thermogenesis and brown adipose tissue in the human newborn infant. Nature, 206:201-202

Cover Image Source: PBS.org