Chapter 2: Glands, Gooseflesh, and Hormones

The brain sends messages to your body through the nerves that branch from your brain down your spine and out to the periphery of your body.

• The voluntary nervous system: you decide to move  a muscle and it happens – conscious
• The autonomic nervous system: The  set of nerve projections to places like sweat glands carry messages that are relatively involuntary

One half of the autonomic nervous system is activated in response to stress, one half is suppressed

• Sympathetic NS*: kicks into action during emergencies, or what you think are emergencies (fight or flight) – epinephrine and norepinephrine are secreted
• Parasympathetic NS: mediates calm, vegetative activities – promotes growth and energy storage

There is another way, besides the sympathetic system, that can mobilize activity in response to a stressor: the secretion  of hormones

• When the sympathetic nerve endings in your heart secrete norepinephrine, which causes the heart muscle to work differently, norepinephrine is playing a neurotransmitter role. If a neuron secrets a messenger that percolates into the bloodstream, that messenger is a hormone.
• All sorts of glands secrete hormones; the secretion of some  of them is turned on during stress, and the secretion of others is turned off.

The hypothalamus contains a huge array of releasing and inhibiting hormones which instruct the pituitary, which in turn regulates the secretions of the peripheral glands

• The brain can trigger the release of pituitary hormone X through the action of a single releasing hormone
• Sometimes it can halt the release of pituitary hormone Y by releasing a single inhibiting hormone
• In some cases, a pituitary hormone is controlled by the coordination of both a releasing and an inhibiting hormone from the brain (dual control)

As the  master gland,  the brain can experience or think of something stressful and activate components of the stress-response hormonally

• The hormones epinephrine, norepinephrine and glucocorticoids are important for stress-response.
• Glucocorticoids: steroid hormones secreted by the adrenal gland. They often act in ways similar to epinephrine. While epinephrine acts within seconds; glucocorticoids back this activity up over the course of minutes or hours.

Glucocorticoid secretion:

• A stressor is sensed or anticipated in the brain, triggering the release of CRH (corticotropin releasing hormone) in the hypothalamus.
• These  hormones enter the private circulatory system linking the hypothalamus and the anterior pituitary, causing the release of ACTH (corticotropin) by the anterior pituitary.
• ACTH enters the general circulation and triggers the release of glucocorticoids by the adrenal gland.

Other hormones:

• In times of stress your pancreas is stimulated to release a hormone called glucagon. It raises the glucose (sugar) levels along with glucocorticoids and the sympathetic nervous system.
• The pituitary secretes prolactin, which, among other effects, plays a role in suppressing reproduction during stress.
• The pituitary and the brain secrete a class of endogenous morphine-like substances called endorphins and enkephalins, which help blunt pain perception.
• The pituitary also secretes vasopressin, which plays a role in the cardiovascular stress-response.
• During stress, the secretion of some hormones, such as estrogen, progesterone, testosterone (reproductive hormones) and insulin (pancreatic hormone) are inhibited.  

Cannon: the first to recognize the role of epinephrine, norepinephrine, and the sympathetic nervous system

A few complications:

• There are some striking gender differences in stress management styles: it has been suggested that females stress-response is not about fight-or-flight, but about “tend and befriend”—taking care of the young and seeking social affiliation.
• Stress response works differently in different species. For example, while stress causes a decline in the secretion of growth hormone in rats, it causes a transient increase in growth hormone in humans.
• Since glucocorticoids works slower, it sometimes doesn’t mediate the stress response itself, but rather the recovery from the stress response.
• It turns out that not all stressors produce the exact same response. The speed and magnitudes of the sympathetic and glucocorticoid branches can vary depending on the stressor and not all endocrine components are activated for all stressors.
• Two identical  stressors can cause very different stress signatures, depending on the psychological context of the stressors.

*nervous system

Resources: Sapolsky, R. Why zebras don’t get ulcers: The acclaimed guide to stress, stress-related diseases, and coping. New York (NY): Henry Holt and Company. 2004 3rd edition