Know your Neurobiology: Stress – Moving out of Homeostasis Part 2

Following on from our last blog post looking at Stress we thought we’d go over the neurobiology of the stress response.

Monitoring for stressors is a constant activity of the Autonomic Nervous System(ANS) and associated structures. Stress, when registered, initiates an ancient, carefully orchestrated and instantaneous sequence of neurotransmitter & hormonal changes. These create physiological and behavioural responses. Mostly our responses go unnoticed but when we perceive greater threat, we can display more obvious, fight, flight, freeze responses.

Stress is part of everyday life. The key is to create adaptive responses to solve the stressful situation quickly and efficiently and bring us back to baseline, into homeostasis. The efficiency of our arrival at homeostasis defines our resilience capacity. The adaptive response becomes the new coping strategy for that stress trigger. We want such strategies to be helpful, not only in the moment, but also in the long term.

The ANS has two components, the sympathetic and parasympathetic nervous systems; these constantly ebb and flow as they turn up or down the volume of their influence in any moment.

Here we will look at:

  1. The brain structures associated with the stress response
  2. The neurobiology of the sympathetic nervous system response
  3. The neurobiology of the parasympathetic response
  4. Stephen Porges’ ‘Poly-Vagal theory’ of stress response
  5. How to support a return to homeostasis and build resilience.

1. Structures of the Stress Response

2. The Neurobiology of Stress – Parasympathetic response

  • The SNS acts to enhance survival by providing a burst of energy, increased alertness, muscle power, speeding up some vital functions, vigilance to lookout for stressors etc.
  • Chronic stress is when the sympathetic responses keep running.
  • Stressors that are long-lasting and intensive can lead to persistent change in the stress response as well as the structures and function of the brain.
  • Long term too much cortisol impacts immune, cardiovascular, endocrine &nervous systems. It increases the likelihood of mental health challenges, anxiety, depression, substance use etc.
  • Stressors can instigate epigenetic changes to gene expression.

3. The Neurobiology of Stress – Parasympathetic Nervous System (PNS) response

  • The PNS actions slow the body’s responses down helping to “rest and digest’.
  • The vagus nerve is the main component of the PNS & oversees mood, immune function, digestion, heart rate; also it connects the gut to brain. Gut bacteria can have beneficial effect on mood and anxiety, partly by affecting the activity of the vagus nerve. There is now greater recognition of the role the vagus nerve system plays in stress responses.
  • The ventral branch of the vagus nerve has the volume turned up during homeostatic moments. This is when we feel everything is OK, we feel comfortable and safe and can engage in positive activities, social interaction, exploration and learning.
  • The dorsal branch of the vagus nerve is thought to go beyond the response of the ventral branch to slow the body’s responses to a point of ‘shut down’. At times this may be a needed response but is suggestive of an extreme stress response by the body and brain. The diagram and bullet points below summarise Stephen Porges’ Poly Vagal Theory relating to SNS and PNS.

4. Poly Vagal Theory – Stephen Porges

Porges suggests we have three levels of response to stress situations:

  • Ventral Vagal activation where we use Social Communication, co-regulation to support parasympathetic homeostasis. This is an oxytocin heavy process. Food may take the place of social.
  • Sympathetic response is activated when stressors move us beyond homeostasis. This results in resolution behaviours – potentially ‘Fight & Flight’ type behaviours– get us to mobilise to find a solution to the resolve the stressor. If this does not bring us back into homeostasis, if the stressors are too huge/chronic – eg traumatic or the stress response continues to be activated over time – then the final place we can go is Dorsal Vagal (below).
  • Dorsal Vagal activation where the mobilisation hasn’t worked and now the sense is that the way to survive is to shut down, to ‘Freeze’. This results in withdrawal behaviours and lack of physical and emotional response.

5. Supporting Homeostasis

Many of the sensory qualities of positive social interactions (touch, auditory, smell, vision, vestibular – in time movements & proprioception – moulding your body to others, copying positive muscle movements) seem to promote activation of the ventral vagal part of the PNS. However somatosensory sensations seem to be especially good at releasing oxytocin, and especially in early life. Suckling, massage, kisses, etc are particularly potent activities. Not only do these buffer stress in the moment but they also affect the capacity for homeostatic resolution later in life. Early oxytocin release and ventral vagal activation create resilience – enabling switching off of the stress response quickly and efficiently.

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