interconnected whole—an organism, the structure, function, and
evolution of which can ultimately be understood only as a holistic
system.Neural fibers of the sympathetic nervous system emerge from the
spinal cord and form connections with clusters of nerve cells just
outside the spinal cord all along its length. These clusters of cells
are called the sympathetic ganglia. The word ganglion (plural ganglia)
refers to a cluster of nerve cells. Cells in the sympathetic ganglia send
fibers out to the various target organs and other structures. Structures
in the upper body that make connections with the sympathetic ner-
vous system have ganglia along the upper part of the spinal cord, and
structures in the lower body that make connections with the sympa-
thetic system have ganglia along the lower part of the spinal cord.
For the upper and middle body, neural fibers of the parasympa-
thetic nervous system connect with the CNS via cranial nerves 3, 7,
and 10. For the lower body (pelvic region), the connections are via
the lower end of the spinal cord. These fibers connect with clusters
of neurons called the parasympathetic ganglia, anatomically located
farther from the CNS than are the sympathetic ganglia. Cells in the
parasympathetic ganglia send fibers out to the various target organs.
The regulatory actions of the sympathetic and parasympathetic
systems have opposite effects; for example, heart rate is speeded by
sympathetic input and slowed by parasympathetic input. Both the
sympathetic and the parasympathetic systems are active all the time,
and the balance of activity between the two systems determines the
ultimate effects on the target organ. Thus, in times of stress, when
the body may benefit from marshaling resources to respond rapidly
to a situation, the sympathetic system is more active, resulting, for