Allostasis, perception, and epigenetics are three components of an individual’s stress response that are vital to understanding stress. Allostasis is an umbrella term that refers to balance in the bodily systems, perception is our personal understanding of different situations, and epigenetics is how the environment influences the expression of our genes. If we can understand the biological systems involved in allostasis, the effect different perceptions have on interpretation, and how are genes can be expressed differently depending on our epigenome we can better understand how the body responds and copes with both acute and chronic stress.
To understand allostasis, a good place to begin is by explaining homeostasis. Homeostasis is the concept of independent bodily systems using negative feedback loops in order to maintain an internal balance in localized areas of the body. For example, the bodies temperature reliably stays at a temperature of about 98.6 degrees. Allostasis is a similar concept but it is more of an umbrella term that is not as localized as homeostasis. Dr. Selye tested rats that were distressed and because of that distress they had an enlarged adrenal cortex, shrinking of the thymus, and ulcers. All these different systems acting together was explained by allostasis or as it is sometimes referred to as “stability through change” (P. Steffen, personal communication, January 10, 2012).
Whenever we encounter a stressor our allostatic balance is disrupted. In order to respond to this stressor and to restore balance our body has a stress response that can be simplified into five parts: the body needs to get energy, move that energy where it is most needed, shut down long term functions, pain perception is blunted, and cognitive function is increased (Sapolsky, 2004).
When a stressor is acting upon something or someone, such as a lion chasing a zebra, that zebra has to get energy quickly in order to escape alive. By utilizing hormones, like cortisol, the body is able to locate energy storage in the form of glucose and mobilize that energy. That energy is then moved to parts of the body that have the most need. The zebra will then experience a number of physiological responses such as increased heart rate, blood pressure, breathing rate, and the kidneys will retain water (P. Steffen, personal communication, January 10, 2012).
Now that the body has the energy and is mobilizing that energy to where it is most needed there are a number of biological processes that get shut down. In the type of emergency that this zebra is experiencing processes like growth, digestion, reproduction, and long term immune function are not necessary. Indeed they may never be necessary again if the zebra isn’t lucky. So the body shuts these systems off temporarily (P. Steffen, personal communication, January 10, 2012).
The last two stress responses include a blunting of pain reception and increased cognitive functioning. This is vital in a situation where the zebra may take a wrong step and twist its’ ankle. The pain is not perceived as strongly as it would be in normal circumstances so that the zebra can continue to run. Even swelling, which would normally occur, is inhibited. And while all of this is happening the zebra will have increased cognitive functioning by having an increase of oxygen and glucose to the brain. This is so that the zebra will be able to remember and focus more acutely at the circumstance at hand (P. Steffen, personal communication, January 10, 2012).
To understand these five steps more fully it is also important to understand what is happening inside the body. The sympathetic nervous system is the first system to respond to a stressor and it does so specifically through the sympathetic adrenal medullary (SAM) system. When stressed the brain recognizes the need for “fight or flight” and sends a message down the spinal chord via the adrenergic nerves to the adrenal medulla. The adrenal medulla releases epinephrine while at the same time norepinephrine is released by nerve cells. Epinephrine ensures that all cells in the body are equally stimulated while norepinephrine works on glands and muscles (Pinel, 2011). As shown in figure 1.1 these two hormones create important responses throughout the body; dilated pupils, less saliva flow, increased heart beat, relaxes airways, inhibits digestion, inhibits gut motility and secretions, and relaxes the bladder (see Appendix A). All of this is what makes it possible for the body to get energy as quickly and efficiently as possible (P. Steffen, personal communication, January 19, 2012).
At the same time as the SAM systems response the Hypothalamic pituitary adrenal (HPA) axis is also acting upon the body in reaction to a stressor. The HPA axis includes the hypothalamus, the pituitary gland, and the adrenal cortex. First the hypothalamus releases CRH, a hormone that regulates the pituitary gland. CRH leads the pituitary gland to release ACTH, a hormone which in turn acts upon the adrenal cortex. The adrenal cortex then triggers the release of a steroid hormone called glucocorticoid. A common glucocorticoid is cortisol which was discussed previously as part of getting energy and mobilizing it in the body. The cortisol works as a negative feedback to stop the hypothalamus and pituitary gland from continuing to release CRH and ACTH (Sapolsky, 2004).
In this case, where the zebra is fleeing from a lion, the chase will be brief. When the chase is done the parasympathetic system takes over and things return to normal. Stress that comes on quick and stops just as suddenly is known as acute stress. This is a beneficial type of stress that can improve performance and aid survival and eventually results in a return to allostatic balance. With humans, however, there is sometimes tendency to ruminate on stressful events. In such cases, stress is not alleviated and it is known as chronic stress. Chronic stress is harmful to the body and while it does not directly cause disease it can make one more susceptible to disease or make the disease process worse (P. Steffen, personal communication, January 10, 2012).
One of the most interesting and amazing aspects of our biological response to stress is that there does not have to be an actual stressor present. If we perceive a situation as stressful then our body will respond accordingly. This is what makes chronic stress a possible problem for humans as opposed to animals. Humans can think about a stressful situation and prevent the recovery from an allostatic imbalance. Animals do not have this problem. Whether we are experiencing physiological stress or psychological stress, our body cannot distinguish between the two. We have what is known as a literal brain, where our perceptions can activate a positive response or a negative response to perceived stress. Our perception of stress as ongoing can cause chronic stress symptoms (P. Steffen, personal communication, January 10, 2012).
While chronic stress can have negative effects on the individual it can also alter a persons epigenetics which can harm the individuals posterity for up to four generations. Epigenetics are similar to genes except that where genes represent a relatively unchanging biological code in our bodies epigenetics consist of the expression of those genes. This expression is something that is different depending on environmental factors in an individuals life (P. Steffen, personal communication, January 12, 2012).
There are three specific studies that emphasize the importance of epigenetics. The first showed that mother rats who were fed a fatty diet had larger children and mother rats who were fed soy diets with a specific methyl mark had children that were thin. The methyl mark changed the genetic expression in the mother and the expression was passed on to her children. The second study found that the epigenetics of twins were similar when young but because of different environmental influences were much more diverse as twins got older. The third study took rats who were raised by a good mother (less stress) and switched them with a bad mother (more stress). They found that the epigenetic expression could be changed simply by changing the environment while the rats were still young. The brains in these young rats showed neuroplasticity when they changed to lead a less stressful life (P. Steffen, personal communication, January 12, 2012).
The biological factors of stress are extremely important to understand. Our bodies need to maintain a state of allostasis and the SAM system, HPA axis, and the stress response help our bodies do so. While the stress response is beneficial in acute situation it is important to remember that it can be harmful when our perceptions begin to cloud our reality and stress becomes chronic. As epigenetics show, chronic stress is not only harmful to us but to our posterity as well. This makes it vital to understand the stress response so that we can strive for less stress except when it is necessary.
References
Pinel, J. P. J. (2011). Biopsychology. Boston, MA: Allyn and Bacon
Sapolsky, R. M. (2004). Why zebras don’t get ulcers. New York: Henry Holt and Company
