Let’s start with the big picture. The nervous system is a mechanism of
delivering information to and from the brain via the brainstem, the spinal
cord, and the nerves that run out to the peripheries, the organs, the glands,
and the muscles. The central nervous
system is the brain and spinal cord. It
is also a system of homeostasis that works to maintain a certain balance.
Motor neurons tell the body what to do. This happens in two ways. Firstly, there is the somatic nervous system,
which is what one uses to pick up a pencil and write a sentence. Secondly, and more importantly for our
purposes, is the autonomic nervous system.
Whereas picking up a pencil and writing is a self-initiated activity,
the activity of the autonomic nervous system seems to happen automatically, or
without conscious control. The ANS
controls whether or not one breaks into a sweat or has a racy heart when
getting unexpected news.
The autonomic nervous system (ANS) is a way of regulating
the body’s functions in light of external circumstances. The system is actually two mutually exclusive
“drives” that act as a sort of gas pedal and a break. Just as in a car, the driver’s foot
alternates between the gas and the break, the human body also alternates
between sympathetic nervous system as the dominant drive or the parasympathetic
nervous system as the dominant drive.
The sympathetic nervous system kicks in with the perception
of danger. Via the “fear” reaction of
the amygdala, in the limbic system, the hypothalamus then carries out four
functions, which if you're interested in the neurochemistry... look it up!
The
release of these hormones into the blood provides a burst of energy to deal
with a stressful situation. These
hormones increase blood glucose levels (to prepare to run, for instance). They also increase oxygen available to cells
by increasing the heart rate and dilating the bronchioles. In addition, they increase blood supply to
essential organs such as the heart, brain, and skeletal muscles and they divert
blood away from nonessential organs such as the digestive system.
Short term stress is designed to help a human escape a
threatening situation. It is designed to
be short term, and followed by a “fight or flight” reaction. The stress reaction helps a human to deal
with a stressful situation like getting chased by a bear quite
efficiently. However, the system is not
meant to deal with long-term stresses like losing a job while trying to pay a
mortgage, which can stretch months into years in duration.
Chronic stress can contribute to or cause cardiovascular
disease. The blood volume increases, its
force and speed increase. Vasopressin
causes the kidneys to absorb less water.
There is considerable wear and tear on the system. Atherosclerosis, the accumulation of plaque,
is also a risk.
Robert Sapolsky notes in Why Zebras Don't Get Ulcers that the inhale is more associated with
sympathetic activation (the heart rate speeds slightly on inhale), while the
exhale is more associated with parasympathetic activation (the heart rate slows
slightly on exhale). People with
hypertension sometimes cannot “slow down” naturally with the exhale. This can be a marker for trouble.
Stress (sympathetic activation due to a psychological or
physical stimulus) is particularly bad for metabolic issues like Type-2
Diabetes. The stress response dumps
energy resources out into the blood stream.
It liquidates the stores thinking a person needs to run from a
lion. Not only does it dump the stores,
but it stops any future storage projects in anticipation of a physical
event. However, sitting on a stressful
conference call is not exactly charging across the savannah. This dipping into the bank account can be
quite disruptive to feeling adequately nourished and rested.
The immune system has an interesting reaction to sympathetic
activation. Upon short term activation,
the immune system’s function is actually heightened. However, that comes at a cost. And after a short term gain in immunity, the
body slips to a lowered state of immunity, and stays there. This second state is lower than the starting
state. This means that long-term
sympathetic nervous system activation
(long-term stress, like a mortgage) may lead to more incidents of
getting sick. There are many caveats and
this is not a blanket statement. For
instance, cancer has not shown to be affected by stress. But many other types of sickness or disease
process are affected. The common cold
has been shown to be more common for subordinate, stressed mice.
There is a strong link in extreme psychological stress and
depression. Stress brings about some of
the typical endocrine changes of depression.
Genes that predispose to depression only do so in a stressful
environment. Glucocorticoids, the
central hormone of the stress response, can bring about depression-like states
in an animal, and can cause depression in humans.
All of this has come together to make me realize how
important it is to recognize and live as though every day is NOT an emergency.
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