Hypothalamic-Pituitary-Adrenal System

We all know what stress feels like, but have you ever wondered what happens inside your body when stressed? How does our body prepare us to handle the stressors we encounter in life? Maybe you have read about the fight-or-flight system, but there is more to learn about the body’s stress response.

• First, we will explore the function of the hypothalamic-pituitary-adrenal (HPA) system.
• Next, we will look at abnormalities in the HPA system.
• We will look at the HPA system and depression.
• Then, we will examine the HPA system and stress.
• Finally, we will discuss the effects of HPA system dysfunction.

Fig. 1 - Stress can not only have a psychological effect on you but a physiological one too.

Hypothalamic-Pituitary-Adrenal System: Function

Several areas of the brain connect to each other and work together in our response to stress. The hypothalamic-pituitary-adrenal system (HPA axis) is the body’s response system to chronic stress (long-term stress). The HPA axis does not react as fast as the sympathomedullary pathway, the body’s response to acute stress.

Here is an overview of the HPA axis and how it functions:

This 5 stage process takes about 20 minutes.

Hypothalamic-Pituitary-Adrenal System: Abnormality

Barring certain illnesses or complications, the HPA axis will develop and function normally. However, certain physiological conditions can cause an abnormal HPA system reaction. Prolonged or intense stress or trauma can have the same result.

Heim et al. (2000) found that women sexually abused during childhood had higher levels of HPA responses to stress. The finding suggests that CRH hypersecretion, which leads to hyperactivity of the hypothalamic-pituitary-adrenal system, is a consequence of childhood abuse.

Newcomer et al. (1999) gave participants either high cortisol levels to produce blood sugar levels similar to those undergoing major surgery or lower cortisol levels similar to those undergoing minor surgery. The participant’s given high cortisol levels were worse at recalling prose passages, which shows the effects of stress on the hypothalamic-pituitary-adrenal system on memory.

Kiecolt-Glaser et al. (1984) took blood samples from 75 students one month before their examination (control condition) and on the first day of their exams (an experimental stress condition). They also completed questionnaires about levels of loneliness, number of life events and psychiatric state. They found the activity of natural killer (NK) hormone and T cells, which help fight infection, were lowered in the stress condition.

These studies show that those with higher stress hormone levels and those who have experienced significant trauma may develop a compromised HPA system function.

Fig. 2 - Research has shown the negating effects of stress.

Hypothalamic-Pituitary-Adrenal System: Depression

Stress affects our mood and contributes greatly to the development of depression. Our HPA system manages a cascade of stress hormones, so it’s no surprise to find that HPA dysfunction contributes to depression.

Noradrenaline and serotonin are important in the regulation of mood and emotional behaviour.

Increased cortisol from an overactive HPA axis results in hyperactivity in the amygdala, an area of the brain responsible for regulating our emotions. This can lead to a reduction in serotonin production, which leaves us more vulnerable to depression and the effects of stress.

An overactive HPA system and depression can have physical consequences as well. Loneliness and problems such as depression are associated with a decreased immune system.

Kiecolt-Glaser et al. (1991) assessed depression, immune function and health in a group of people caring for a relative with Alzheimer’s disease (long-term stress group) and a control group. The stress group had significantly lower immune system functioning and higher instances of infectious illness and depression.

Hypothalamic-Pituitary-Adrenal System: Stress

Research has highlighted the importance of the HPA system in adaptive human behaviour. People who have their adrenal glands surgically removed need hormone supplements to combat stressors. This shows the vital role of the HPA system in dealing with stress. However, an overactive HPA axis can adversely affect our health and well-being.

When we encounter a stressor, our bodies react in a way that best prepares us to handle the immediate situation. Stressors trigger our body’s fight-or-flight response, amping us to face a threat (fight) or flee from it (flight). Our bodies do this by releasing adrenaline. Our muscles engage, and our heart rate and breathing increase, so we are better equipped to handle the threat.

As we saw above, stressors trigger our HPA axis, resulting in a release of cortisol. Most cells in the body have cortisol receptors. So while cortisol is a stress hormone, it plays an important role in bodily functions.

Our bodies need a balanced level of cortisol to function properly. An overactive HPA axis compromises this function by flooding our system with a constant supply of excess cortisol.

Prolonged exposure to chronic stress has negative effects on health, suggesting the HPA system may be wearing us down and hindering survival. This is because our stress response is old; it reacts the same way it would have done thousands of years ago. Now, it may not be equipped to deal with the modern stressors we face today.