Central integration occurs at the highest level in the midbrain
and medulla. The two divisions of the autonomic nervous system and
the endocrine system are integrated with each other, with sensory
input, and with information from higher central nervous system centers.
Within the various organ systems of the body, the interactions between
the sympathetic and parasympathetic divisions are often in opposition
to each other (Table 4–4). Thus, the parasympathetic system
is often referred to as a trophotropic system,
leading to growth, and the sympathetic system is referred to as
an ergotropic system, leading to
energy expenditure. For example, slowing of the heart and stimulation
of digestive activity are typical energy-conserving (“rest
and digest”) actions of the parasympathetic system. In
contrast, cardiac stimulation, increased blood sugar, and cutaneous
vasoconstriction are responses produced by sympathetic discharge
that are suited to fighting or surviving an attack (“fight
or flight”). At a more subtle level of interactions in
the brain stem, medulla, and spinal cord, there are important cooperative
interactions between the parasympathetic and sympathetic systems.
For some organs, sensory fibers associated with the parasympathetic
system exert reflex control over motor outflow in the sympathetic
system. Thus, the sensory carotid sinus baroreceptor fibers have
a major influence on sympathetic outflow from the vasomotor center.
Similarly, parasympathetic sensory fibers in the wall of the urinary
bladder significantly influence sympathetic inhibitory outflow to
that organ.