How stress affects the nervous system
The triune brain
The brain can be considered to be composed of three layers
- Neo-cortex - the seat of cognitive thought
- The limbic system - the emotional brain
- The brainstem - the primitive animal brain
Understanding the differences and interrelationships between these three layers of the brain is important if you are to understand the stress response.
Read this article on the triune brain, which describes the evolution, form and function of these layers of the brain.
What are the key functions and characteristics associated with each layer of the brain?
The emotional brain
According to Goleman (1996) there are two structures within the limbic system which are strongly associated with the emotional response the amygdala, and the hippocampus. He describes the Amygdala as "the seat of all passion", and the hippocampus as "the storehouse of emotional memory".
LeDoux a neuroscientist has been quoted as saying "The hippocampus is crucial in recognising a face as that of your cousin. But it is the amygdala that adds that you really don't like her" (LeDoux as cited in Goleman, 1996)
The conscious mind
The neocortex is the "seat of rationality & abstract thought". This is involved with strategy and planning, logical and creative thinking, as well as feelings about ideas, art, etc. (Goleman, 1996) This neurological structure is more developed in human beings than any other creature except perhaps dolphins.
Have a look at this page which compares the neocortexes of different species.
The biological name of the human race - homo sapiens means "the thinking species" (Goleman, 1996). This is perhaps an old idea, if the function of the neocortex is rationality and abstract thought, then surely other species must have this capacity, however it's interesting that our biological name identifies us with our thoughts.
And in general we do identify ourselves with our thoughts. We consider ourselves to be rational, and that our actions are based in rational thought. What neuro-science tells us is that we actually have two minds - an emotional mind and a rational mind, and that often our emotional mind is the one who is in control on a subconscious level.
The neurological component of the fight or flight response
Open up the following page in a new window - http://www.cns.nyu.edu/home/ledoux/ LeDoux lab Stress response then resize both that window and the window you're reading this in so that you can see the image, and can also read this text.
When a new stimulus is perceived the information package associated with this stimulus is relayed to the thalamus. From there it travels simultaneously to
- the amygdala and hippocampus (providing an immediate response)
- The sensory cortex (providing a slower, more considered response)
The nervous system decides whether the stimlus is benign, threatening, or if it is unsure.
If the stimuli is not immediately recognised as something which is benign
- the locus ceruleus is activated & the person becomes alert & attentive to the environment
- the pre-frontal cortex of the brain is activated and is engaged in the attempt to determine the source and nature of the stimuli
If that stimuli is perceived to be a threat, the amygdala simulates the activation of
- the sympathetic nervous branch of the autonomic nervous system
- the brainstem
- the hypothalamus causing release of corticotrophin-releasing factor (CRF)
and the fight or flight response is triggered.