'Stress Response Series' Part 3: Stress and the HPA axis

As you may recall from a previous blog, 'How do we respond to stress?':

• healthy stress is essential for our survival
• acute stress engages an immediate short-term response to a stressful event in order to protect us from immediate danger
• ongoing exposure to stress results in long-term chronic stress responses that severely impact our physiological and mental health, resulting in inflammation, chronic illness and neurodegenerative conditions.

As explored here, polyvagal theory - our system's ability to respond to our need for survival through connection or defense - gives us one way of understanding the complex cascade of neurobiological processes involved in managing stress.

It is, however, also essential to consider the role of the hypothalamus, the pituitary gland and adrenal glands, an interactive network commonly referred to as the 'HPA axis'.

{Please note:

1. The purpose of this discussion is to provide a brief and succinct overview, in order to understand the basic function and impact of stress on our mental, hormonal and physical health.
2. This is only a brief and simplified summary of a complex process}.

What is the HPA axis?

The HPA axis is the interconnected feedback loop that is our first responder in combatting stress.

It is directly linked to our general health, and, more specifically to the health of our:

• hormones,
• immune system, and
• digestion.

Let's take a look.

The hypothalamus

The hypothalamus is responsible for keeping the body in a state of balance, or homeostasis, by coordinating inputs between the endocrine system (hormones) and the autonomic nervous system.

The function of the hypothalamus is to regulate:

• body temperature
• blood pressure
• hunger and thirst
• sleep
• hormones
• daily physiological cycles
• sexual behavior
• emotions

The pituitary gland

The pituitary gland, is the master gland of the endocrine system. It releases hormones and controls other endocrine glands that tell your body what to do, and when to do it. For example, the pituitary gland releases prolactin to stimulate the mammary glands to produce breast milk.

The adrenal glands

The adrenal glands (also part of the endocrine system) produce hormones that regulate metabolism, immune system, blood pressure and the stress hormone, cortisol. These glands are also linked to sympathetic and parasympathetic nervous systems.

Simply, put, when the hypothalamus responds to a stressor, it sends a message (via the corticotropin releasing hormone) to the pituitary gland, which alerts the adrenal cortex to release cortisol.

Cortisol then increases the activity of the nervous system to mobilise it into a sympathetic fight / flight response to protect us from danger, resulting in:

• increase heart rate, oxygen supply, blood sugar levels and glucose in order to respond to the stress
• dilation of pupils to enhance vision
• inhibition of digestion (saliva glands, stomach acid and digestive activity stops) to conserve energy

What happens when the protective acute stress response becomes chronic?

As seen in the diagram above, there is a feedback loop that continues to activate the processes within the HPA axis until the acute stress is over.

In states of chronic stress, the activation of the mobilised response is maintained, resulting in the ongoing release of cortisol, leading to chronically high or low levels of cortisol in the body.

Either way, the homeostasis is disrupted.

If you recall from Part 1 of the Stress Response Series, in the face of stress we will typically with fight or flight (run away or fight), or immobilisation (collapse).

Let's explore the impact of these responses to chronic stress.

1. A mobilisation (fight / flight)response would elicit:

• increased energy to the heart, lungs and muscles
• increased immune response and brain activity increased
• elevated blood sugar levels, blood pressure
• less flexibility in blood vessels
• a depletion of brain resources, including feel-good neurochemicals
• impacts on hippocampus (affecting memory)
• increased fear response, further exacerbating the stress response due to changes in the amygdala

2. An immobilisation response (collapse) response would impact:

• compromised immune response
• shallowness of breath
• slowing or shutting down of systems generally
• long term functions such as reproduction and general growth and repair
• over time this is not good for our bodies

Ultimately this leads to long term health issues that impact:

• the cardiovascular system
• the immune system
• digestion (If your gut is not working properly, your body will not be able to absorb the nutrients you need in order for your body to function).
• mood and mental health
• the endocrine system
• and even neurodegenerative brain disease.