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The sympathetic nervous system is a subdivision of the autonomic nervous system, which is
the subdivision of the peripheral nervous system that controls automatic processes in
the body like digestion, heart rate, and respiration.
The sympathetic nervous system is typically associated with expending energy and the “fight-or-flight”
response, which is a general response to alarming or threatening situations that involves increasing
the ability of the body to act.
The idea that the sympathetic nervous system is only active during “fight-or-flight”
responses is an oversimplification, however, as it is also active in non-emergency responses
and during times of rest.
The nerves that make up the sympathetic nervous system originate in the thoracic and lumbar
spinal cord.
Like other neurons of the autonomic nervous system, they do not typically travel directly
from the spinal cord to their targets but Instead extend from the spinal cord to clusters
of neurons in the peripheral nervous system known as sympathetic ganglia.
The sympathetic neurons that travel to the adrenal medulla are one exception to this
rule as they extend from the spinal cord directly to the adrenal medulla.
Some of the ganglia of the sympathetic nervous system form a network of interconnected ganglia
called the sympathetic chain ganglia.
The sympathetic chain ganglia are found close to the spinal cord, while other ganglia known
as prevertebral ganglia are closer to the organs they supply.
The neurons that travel from the spinal cord to the ganglia are called preganglionic neurons,
and they typically synapse with neurons in ganglia called postganglionic neurons.
The postganglionic neurons then extend to the targets of the sympathetic nervous system,
which are diverse and found throughout the body.
The preganglionic neurons of the sympathetic nervous system primarily release the neurotransmitter
acetylcholine, while the postganglionic cells primarily release norepinephrine---although
there are some postganglionic neurons that use acetylcholine as their primary neurotransmitter.