The Brain and Our Stress Response

The HPA Axis

The HPA axis is the classical neuroendocrine system that responds to stress and whose final product, corticosteroids, targets components of the limbic system, particularly the hippocampus. New research is beginning to show that the HPA axis should instead be thought of as the body's energy regulator, as it is ultimately responsible for controlling virtually all of the hormones, nervous system activity and energy expenditure in the human body, as well as modulating the immune system.

The HPA axis is a grouping of responses to stress by the brain and the pituitary and adrenal glands. First, the hypothalamus (a central part of the brain) releases a compound called corticotrophin releasing factor (CRF), which was discovered in 1981. The CRF then travels to the pituitary gland, where it triggers the release of a hormone, ACTH. The ACTH is released into the bloodstream and causes the cortex of the adrenal gland to release the stress hormones, particularly cortisol, which is a corticosteroid. Cortisol affects the availability of the fuel supply (carbohydrate, fat, and glucose metabolism), which is needed to respond to stress. However, if cortisol levels stay increased for too long, then muscle breakdown, a decreased inflammatory response, and suppression of the immune (defense) system occur.

Because they suppress the immune system, corticosteroids in measured doses are used to treat many illnesses that are characterized by an overactive immune system, such as asthma and inflammatory bowel disease. For the same reason, they are used to help reduce the chances that our body will immunologically reject a transplanted organ. Corticosteroids also can cause fluid retention and high blood pressure. Therefore, it is critical that the response to corticosteroids be carefully controlled (modulated). This control is usually accomplished by a feedback mechanism in which increased cortisol levels feeding back to the hypothalamus and pituitary turn off production of ACTH. In addition, extremely high levels of cortisol can cause depression and psychosis, which, however, disappear when the levels return to normal.

Locus Coeruleus

The locus coeruleus has many connections to other parts of the brain, particularly areas that bring in and process sensory information. (The senses include sight, hearing, smell, taste, and touch.) The locus coeruleus secretes nor-epinephrine and stimulates other brain centers to do the same. It is like the pacemaker (controls the tempo) of the brain. Thus, it increases arousal (heightened awareness, alertness) and vigilance (watchfulness, carefulness), and adjusts (modulates) the action of the autonomic nervous system. The autonomic nervous system regulates blood flow, heart rate, blood pressure, and breathing (respiration). It can also temporarily shut down the gastrointestinal (GI) and sexual systems until the crisis is over. These initial reactions, to get our blood flowing, heart pumping, and muscles energized, occur very quickly and automatically.

The HPA axis and the locus coeruleus systems are linked through the hypothalamus and the brain area known as the limbic system. The limbic system is the control area for emotion and the processing area for memory. These linkages are critical. For example, if you see the bushes rustling, your locus coeruleus immediately starts things (the stress response) rolling. However, then, if you see that it is not a mountain lion, but a golden retriever in the bush, your memory of the tameness of the dog will turn off the stress response.

The Limbic System

Functions
sets the emotional tone of the mind
filters external events through internal states (emotional coloring)
tags events as internally important
stores highly charged emotional memories
modulates motivation
controls appetite and sleep cycles
promotes bonding
directly processes the sense of smell
modulates libido

Problems
moodiness, irritability, clinical depression
increased negative thinking
perceive events in a negative way
decreased motivation
flood of negative emotions
appetite and sleep problems
decreased or increased sexual responsiveness
social isolation

This part of the brain is involved in setting a person's emotional tone. When the deep limbic system is less active there is generally a positive, more hopeful state of mind. When it is heated up, or overactive, negativity can take over.

The deep limbic system, especially the hypothalamus at the base of the brain, is responsible for translating our emotional state into physical feelings of relaxation or tension. The front half of the hypothalamus sends calming signals to the body through the parasympathetic nervous system. The back half of the hypothalamus sends stimulating or fear signals to the body through the sympathetic nervous system. The back half of the hypothalamus, when stimulated, is responsible for the fight or flight response, a primitive state that gets us ready to fight or flee when we are threatened or scared. This "hard-wired response" happens immediately upon activation, such as seeing or experiencing an emotional or physical threat. In this response the heart beats faster, breathing rate and blood pressure increases, the hands and feet become cooler to shunt blood from the extremities to the big muscles (to fight or run away) and the pupils dilate (to see better). This "deep limbic" translation of emotion is powerful and immediate. It happens with overt physical threats and also with more covert emotional threats. This part of the brain is intimately connected with the prefrontal cortex and seems to acts as a switching station between running on emotion (the deep limbic system) and rational thought and problem solving with our cortex. When the limbic system is turned on - emotions tend to take over. When it is cooled down, more activation is possible in the cortex. Current research on depression indicates increased deep limbic system activity and shut down in the prefrontal cortex, especially on the left side.