5.4 Stress and the Brain Flashcards
Describe the General Adaptation Syndrome response to stress
The alarm phase - Where the brain detects a stress signal which triggers the sympathetic system to enact fight or flight responses and activates the HPA axis causing the release of stress hormones like cortisol and adrenaline.
The resistance phase - Where the parasympathetic system tries to bring the physiology back to baseline levels, but if the stressor remains, the body will remain in a state of alert
The exhaustion phase - Where energy resources following the body trying to recover from this prolonged period of stress. The body can not fight the stress and there are feelings of tiredness, anxiety and being unable to cope.
Describe allostasis and the allostatic load
Allostasis is maintaining stability and homeostasis through changing physiological or behavioural conditions in response to stress. Allostasis can only be sustained for a limited period of time and if the stress is chronic, the body will go into allostatic overload. Allostatic load is the price the body pays for allostasis following stress.
Describe the general outline of both the rapid and delayed response to stress
The initial rapid response to stress is the stimulation of the sympathetic nervous system. Causing a range of fight or flight responses including stimulating the adrenal medulla to release catecholamines
The more delayed response to support the sympathetic system in times of more prolonged stress is via the neuroendocrine system. The hypothalamic-pituitary-adrenal axis is activated. This releases steroid hormones glucocorticoids (e.g cortisol) and mineralocorticoids (e.g aldosterone), these provide more energy for prolonged activity and have anti-inflammatory actions.
What are some different brain regions that can modulate the hypothalamic-pituitary-adrenal (HPA) axis.
Amygdala and NTS can activate the HPA in stress response. But the NTS is mostly following a homeostatic imbalance whereas the amygdala is following fear and threat detection.
Prefrontal Cortex and Hippocampus can deactivate the HPA in stress response, following a cortisol negative feedback loop.
Describe the role of the amygdala in the stress response
The amygdala processes emotions, especially fear and threat detection. It receives sensory information from the thalamus and cortex. In times of stress, it detects danger and emotional stimuli. It sends input to the hypothalamus to trigger sympathetic responses and the activation of the HPA axis.
Describe the activation of the hypothalamic-pituitary-adrenal axis in response to stress
The emotional or physiological stress causes the amygdala to signal to the hypothalamus. The paraventricular nucleus is activated and parvocellular neurosecretory neurons secrete corticotropin releasing hormone (CRH). The CRH travels via the blood vessels in the hypophyseal portal system to the anterior pituitary gland and stimulates the anterior pituitary to release adrenocorticotropic hormone (ACTH) into the blood stream. The ACTH then stimulates the adrenal cortex to release cortisol.
What is cortisol
Cortisol is the primary glucocorticoid. It acts throughout the body to activate energy sources and suppress the immune response. It is a lipophilic molecule and can dissolve through lipid membranes and across the blood brain barrier. Cortisol receptors are widespread in the brain and studies show that long term exposure to cortisol can be damaging. It is produced in the zona fasciculata of the adrenal cortex
Describe the negative feedback mechanism of cortisol release
Cortisol binds to intracellular glucocorticoid receptors in the PVN of the hypothalamus and anterior pituitary to inhibit release of CRH and ACTH.
What are the 2 different cortisol receptors and describe their different stress responses.
Mineralocorticoids have a high affinity for cortisol so can be activated even at low cortisol levels. They feedback to the stress response and amplify excitatory glutamatergic signalling in the hippocampus.
Glucocorticoids have a lower affinity for cortisol so are only activated at low cortisol levels. They act to terminate the stress response aswell as regulating gene expression to activate many stress related responses
Describe the difference between acute and chronic stress
Acute stress is a short term, immediate response to a stressor. It activates the sympathetic nervous system and the HPA
Chronic stress is a longer term prolonged response. It leads to persistent HPA activation.
Describe how chronic stress can be damaging
In chronic stress, the prolonged activation of the HPA, leads to the secretion of glucocorticoids like cortisol at high levels. This can cause the chronic release of proinflammatory cytokines that can promote systemic inflammation and the development of many disorders like depression, cardiovascular disease or immune dysfunction. This can come in the form of activated astrocytes and microglia that release the cytokines and can cause neurotoxicity.
