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THE SYMPATHETIC SYSTEM
The Autonomic Nervous System
The autonomic nervous system (ANS) regulates the functions of our
internal organs (the viscera) such as the heart, stomach and intestines.
The ANS is part of the peripheral nervous system and it also controls some
of the muscles within the body. We are often unaware of the ANS because
it functions involuntarily and reflexively. However, some people can be
trained to control some functions of the ANS such as heart rate or blood
pressure.
The ANS is most important in two situations:
• In emergencies that cause stress and require us to "fight" or take
"flight" (run away)
• In non-emergencies that allow us to "rest" and "digest."
The ANS regulates:
• Muscles
• - in the skin (around hair follicles; smooth muscle)
- around blood vessels (smooth muscle)
- in the eye (the iris; smooth muscle)
- in the stomach, intestines and bladder (smooth muscle)
- of the heart (cardiac muscle)
• Glands
• The ANS is divided into three parts:
a) The sympathetic nervous system
b) The parasympathetic nervous system
c) The enteric nervous system
As we notice in the picture, the sympathetic nervous system
originates in the spinal cord. Specifically, the cell bodies of the first
neuron (the preganglionic neuron) are located in the thoracic and
lumbar spinal cord. Axons from these neurons project to a chain of
ganglia located near the spinal cord. In most cases, this neuron
makes a synapse with another neuron (post-ganglionic neuron) in
the ganglion. A few preganglionic neurons go to other ganglia
outside of the sympathetic chain and synapse there. The postganglionic neuron then projects to the "target" - either a muscle or a
gland.
The fight or flight response
Situation: It is a nice, sunny day...I’m going on a nice walk in the
woods. Suddenly, a fierce boar appears in front of me.
What shall I do? Stay and fight OR run away?
These are "Fight or Flight" responses. In these types of situations,
the sympathetic nervous system is called into action - it uses
energy - so blood pressure increases, heart beats get faster, and
digestion slows down.
Neurotransmitters
The synapse in the sympathetic ganglion uses acetylcholine as a
neurotransmitter; the synapse of the post-ganglionic neuron with
the target organ uses the neurotransmitter called norepinephrine.
Acetylcholine
Acetylcholine (often abbreviated ACh) is the most common
neurotransmitter. It is located in both the central nervous system
(CNS) and the peripheral nervous system (PNS).
• Acetylcholine was the first neurotransmitter to be identified. It was
discovered by Henry Hallett Dale in 1914 and its existence was
later confirmed by Otto Loewi. Both individuals were awarded the
Nobel Prize in Physiology in 1936 for their discovery.
Norepinephrine
Norepinephrine is both a neurotransmitter and a hormone. It is a
catecholamine (a) produced by the (b) adrenal glands using (c)
dopamine as a starting point.
• (a) Catecholamines are a type of hormone known to strongly
affect blood pressure by acting on the blood vessels and the heart.
They are made from the amino acid tyrosine. Illness and certain
types of cancers can increase the level of catecholamines in the
body, leading to elevations in blood pressure and heart rate.
• (b) The adrenal glands are small, triangle- or pyramid-shaped
organs found near the top of each kidney. There are two adrenal
glands - one on each side of the body. The adrenal glands produce
a variety of hormones involved in stress reactions, blood pressure
regulation, and the handling of vitamins and minerals. Certain types
of adrenal gland problems can cause an increase in hormone
release, leading to elevated blood pressure, rapid heart beat, or
high/low levels of minerals and/or nutrients.
• (c) Dopamine is a body chemical that plays important roles at
several locations in the body.
Dopamine levels are high in the brain, where it is necessary for:
- Learning
- Mood regulation
- Information processing/integration
Dopamine can also be given intravenously as a medication. In this
capacity, it acts as a stimulant on the cardiovascular system and
results in increased heart rate, increased blood pressure, and
increased blood flow. Dopamine given this way does not affect the
brain. Dopamine is the root chemical the body uses to synthesize
catecholamines.
The parasympathetic system
In the picture the cell bodies of the parasympathetic nervous
system are located in the spinal cord (sacral region) and in the
medulla. In the medulla, the cranial nerves III, VII, IX and X form
the preganglionic parasympathetic fibers. The preganglionic fiber
from the medulla or spinal cord projects to ganglia very close to the
target organ and makes a synapse. This synapse uses the
neurotransmitter called acetylcholine. From this ganglion, the postganglionic neuron projects to the target organ and uses
acetylcholine again at its terminal.
The rest and digest response
Situation: It is a bright, summer day...John is playing on the sand
with Richard, his son, while Margie, his wife, is sitting on a beach
towel in total relax. After some time, John decides to sit on a chair
to relax, too.
This calls for "Rest and Digest" responses. Now it is the time for the
parasympathetic nervous to work to save energy. This is when
blood pressure can decrease, pulse rate can slow, and digestion
can start.
The restless activity of ANS
It should be noted that the autonomic nervous system is always
working. It is NOT only active during "fight or flight" or "rest and
digest" situations. Rather, the autonomic nervous system acts to
maintain normal internal functions and works with the somatic
nervous system.
The enteric nervous system is a third division of the autonomic
nervous system. It is a meshwork of nerve fibers that innervate the
viscera (gastrointestinal tract, pancreas, and gall bladder.
The Enteric Nervous System
Summing up: the effects of sympathetic
and parasympathetic stimulation
Structure
Sympathetic Stimulation
Iris (eye muscle)
Pupil dilation
Salivary Glands
Saliva production reduced
Oral/Nasal Mucosa
Mucus production reduced
Heart
Heart rate and force increased
Lung
Bronchial muscle relaxed
Stomach
Peristalsis reduced
Small Intestine
Motility reduced
Large Intestine
Motility reduced
Liver
Increased
conversion
glycogen to glucose
Kidney
Decreased urine secretion
Adrenal medulla
Norepinephrine
epinephrine secreted
Bladder
Wall
Sphincter closed
of
and
relaxed
Parasympathetic Stimulation
Pupil constriction
Saliva production increased
Mucus production increased
Heart rate and force decreased
Bronchial muscle contracted
Gastric juice secreted; motility increased
Digestion increased
Secretions and motility increased
Increased urine secretion
Wall contracted
Sphincter relaxed