Stress can be described as a response or reaction to a physical or mental event in the environment. Generally stress is perceived as a negative state of mind but can also be as a reaction to positive events such as experiences that make us feel happy, like a welcome surprise, enjoyable holiday, or successful achievement at work. In fact, the correct term for positive stress is ‘eustress‘, whilst the negative form is ‘distress‘.  However, in this modern age the balance between the state of eustress and distress has become far too much weighted in what we term generally as stress, and that is of the negative state.

As humans we are designed to function with a certain amount of stress but it seems that our reactions to this have somehow become unwired and rewired in a completely different way. If you think about it, walking down a busy highway or driving a car down a high street we are under a constant bombardment of stress. The traffic noise, crowds of people, different smells, checking our phones for emails, and generallly having to be under a constant state of concentration, these are all creating a permanent state of stress that our predecessors on this earth was not exposed to. Yes, they certainly would have had a heightened state of awareness when out hunting for food, but they sure wouldn’t have returned to the cave to continue this stress whilst on a laptop! They would have cooked and enjoyed the food and perhaps relaxed afterwards. We have become too wired, in a permanent state of stress, and this has detrimental effects on our general health and wellbeing. To first understand this we need to understand what happens in the body when stress occurs. As this is such a huge topic it is easier to break it down into a more digestible series that will later be linked to the lifestyle and wellbeing section where there will be stress avoidance techniques and ideas on how to reduce your exposure to stress.

What is the physiological response to stress?

When we perceive there to be a stressor, or life threatening situation we have to be able to react quickly so we activate the sympathetic nervous system (SNS). Acetylcholine, a neurotransmitter, is released at synapses within the preganglionic neurons and in response to this stimulus, postganglionic neurons release norepinephrine which activates adregernic receptors on the peripheral target tissues. These adregernic receptors are what exert the effects of what is often referred to as the ‘fight or flight’ response, which in short  includes dilating the pupils, increasing heart rate, mobilizing energy, and diverting blood flow from non-essential organs to skeletal muscle.

There are two exceptions to the adregernic response; the adrenal medulla, and strangely acetylcholine targets the postganglionic neurons of the sweat glands, where it stimulates the release of sweat on the palms and plantar surfaces of the feet, and the axilla, or underarm area.

The adrenal medulla of the adrenal glands are derived embryonically from the same tissue as the sympathathetic nervous system, this arising from the neural crest. It is richly innervated by preganglionic sympathetic fibres and with its cluster of neuronal cell bodies is in fact considered a glanglion, and therefore extension, of the sympathetic nervous system. At this preganglionic synapse of neurons with the chromaffin cells of the adrenal medulla, norepinephrine and epinephrine is released directly into the bloodstream having a longer effect than direct sympathetic nervous stimulation. Norepineprhine is both a neurotransmitter and a hormone. If released in the nervous system it is a neurotransmitter and if secreted by the adrenal medulla into the bloodstream it is referred to as a hormone. It has stimulatory or inhibitory effects dependant upon the receptor on the cell. It acts upon smooth mucsle cells and that have alpha or beta cells dependant upon the required action. For example, blood vessels in our gastrointestinal tract contain alpha cells and when norepinephrine binds to the receptors on the site it constricts blood flow, conversely where we need more blood flow such as to the heart, the blood vessels contain beta cells that when activated cause vasodilation.

Following this initial reaction the system know as the Hypothalamic-Adrenal-Pituitary Axis is activated that creates a cascade from the Hypothalamus down to the adrenal glands to produce cortisol.


The Hypothalamus releases Cortico Releasing Hormone (CRH) that in turn stimulates the Anterior Pituitary Gland to release Adrenocorticotropic Hormone, this stimulates the Adrenal Cortex to release Cortisol. Providing the stressor is dealt with, levels return to normal via the negative feedback loop to the pituitary gland and the hypothalamus signalling the reduction in the need for the hormone production. This can clearly be seen in the diagram opposite (Brian M Sweis, 2012).

Under normal circumstances this whole intereaction of neurotransmitters and hormones plays out well, but the issue comes when there is a constant stimulation of these pathways. This can lead many different issues as the pathways become desensitised and damaged, over or under production of hormones can occur leading to a whole myriad of dysfunction and disease processes.

These are discussed further in Part 2 – The Physiological Effects of Continued Stress.



Picture reproduced with permission from:
BrianMSweis – Own work, CC BY-SA 3.0,