Sympathetic Division of the Autonomic Nervous System (ANS) Anatomy and Physiology, Horner Syndrome

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What is the Autonomic Nervous System? What is the Sympathetic nervous system or the Sympathetic division of the Autonomic Nervous System?
Preganglionic neurons of the sympathetic division originate
in the intermediolateral horn of the thoracic (T1 to T12)
and upper lumbar (L1 to L3) spinal cord. The preganglionic
axons exit the spinal cord in the ventral nerve roots. Immediately
after the ventral and dorsal roots merge to form the
spinal nerve, the sympathetic axons leave the spinal nerve
via the white ramus and enter the paravertebral sympathetic
ganglia. The paravertebral ganglia form an
interconnected chain located on either side of the vertebral
column. These ganglia extend above and below the thoracic
and lumbar spinal levels, where preganglionic fibers emerge,
to provide postganglionic sympathetic axons to the cervical
and lumbosacral spinal nerves. The preganglionic
axons that ascend to the cervical levels arise from T1 to T5
and form three major ganglia: the superior, the middle, and
the inferior cervical ganglia. Preganglionic axons descend
below L3, forming two additional lumbar and at least four
sacral ganglia. Th e preganglionic axons may synapse with
postganglionic neurons in the paravertebral ganglion at the
same level, ascend or descend up to several spinal levels and
then synapse, or pass through the paravertebral ganglia en
route to a prevertebral ganglion.