Understanding the Stress Response and Its Effects
The Stress Response
The stress response allows us to get ready for vigorous activity and is perfectly normal. All the physiological responses involved in stress are catabolic; they help mobilize the body’s energy stores. The sympathetic branch of the autonomic nervous system (ANS) becomes active, causing the adrenal medulla to secrete epinephrine (adrenaline), resulting in:
- Increased glucose availability and metabolism (energy stores become available)
- Increased cardiovascular output and blood flow to the muscles
- Increased lung capacity (dilation of bronchi)
Glucocorticoids
The release of cortisol begins in the paraventricular nucleus of the hypothalamus (PVN), which releases corticotropin-releasing hormone (CRH) into the pituitary blood supply. CRH stimulates the anterior pituitary to secrete adrenocorticotropic hormone (ACTH). This results in the release of cortisol, a glucocorticoid hormone produced in the adrenal cortex (located atop the kidney). In rodents, this is called corticosterone and affects glucose metabolism.
Hypothalamus (CRH) → Anterior Pituitary Gland → ACTH → Adrenal cortex → CORT → Feedback to brain
Effects of Long-Term Stress
The harmful effects of chronic stress are thought to be caused by the chronic release of glucocorticoids, and include:
- Long-term high blood pressure
- Damage to muscle tissue
- Inhibition of inflammatory and immune responses
- Damage to the brain
Chronic Stress and the Brain
Certain areas of the brain appear to be vulnerable to chronic stress. In animal studies, long-term exposure to glucocorticoids leads to atrophy of dendrites in the CA1/CA3 subfields of the hippocampus. If stress continues, hippocampal neurons will die. Glucocorticoids are thought to kill neurons by decreasing the reuptake of glutamate. This excessive glutamate in the synapse increases cell excitability and Ca2+ entry, which is toxic in large numbers.
Stress on Hippocampal LTP and Memory
Chronic stress impairs hippocampal synaptic plasticity and memory. Chronically stressed animals have impaired Long-Term Potentiation (LTP). Stress impairs performance on a variety of hippocampal-dependent memory tasks.
Stress: Hippocampal vs. Amygdala
Stress affects the amygdala differently. Recent work has shown that chronic inescapable stress leads to an increase in the number of dendrites in lateral amygdala pyramidal cells. This altered morphology of amygdala neurons may be one reason why individuals exposed to chronic stressors are more likely to develop anxiety disorders.
Elevated Plus Maze as a Measure of Anxiety
In the Elevated Plus Maze test:
- The rat begins in the middle.
- A normal rat will spend some amount of time exploring both arms.
- An anxious rat will spend more time in enclosed arms.
Variations in Maternal Care and Emotional Reactivity
Naturally occurring variations in maternal care in rats have been shown to affect stress reactivity. This appears to occur within a critical period of the first 10 days of life. Rat mothers show individual differences in how they respond to pups:
- High licking/grooming and arched-back nursing (High LG-ABN) is associated with lower stress reactivity.
- Low licking/grooming and arched-back nursing (Low LG-ABN) is associated with higher stress reactivity.
Summary of Emotional Development
Emotional and stress reactivity are profoundly affected by maternal care, at least in rodent models. Even relatively minor maternal influences can influence the emotion system. If present in humans, such maternal influences may also help to explain differences in emotional reactivity, emotion regulation, and affective style.