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Sympathetic and parasympathetic divisions typically function in opposition to each other. But this opposition is better understood as complementary in nature rather than antagonistic. For an analogy, one may think of the sympathetic division as the accelerator and the parasympathetic division as the brake. The sympathetic division typically functions in actions requiring quick responses. The parasympathetic division functions with actions that do not require immediate reaction. A useful acronym to summarize the functions of the parasympathetic nervous system is SLUDD (salivation, lacrimation, urination, digestion and defecation).
The parasympathetic nerves (PSN) are visceral, autonomic branches of the peripheral nervous system (PNS). The autonomic nervous system (ANS), which includes sympathetic and parasympathetic divisions, regulates the body's visceral organs via the innervation of three kinds of tissues: smooth muscle, cardiac muscle, and glands. The sympathetic and parasympathetic system work in tandem to create a synergistic stimulation that is not merely on or off, but has been described as a continuum depending upon how vigorously each division is attempting to carry out its actions. The regions of the body associated with the parasympathetic division of the ANS are in the cranial and sacral regions of the spinal cord. Because of its location the parasympathetic system is commonly referred to as having craniosacral outflow whereas the sympathetic system is referred to as thoracolumbar outflow (T1-L2 spinal nerves). In the cranium the PSN originate from cranial nerves CN III (oculomotor nerve), CN VII (facial nerve), CN IX (glossopharyngeal nerve) and CN X (vagus n.) In the sacral region of the body the PSN is derived from spinal nerves S2, S3 and S4, commonly referred to as the pelvic splanchnics.
Sympathetic nerves originate inside the vertebral column, toward the middle of the spinal cord in the intermediolateral cell column (or lateral horn), beginning at the first thoracic segment of the spinal cord and are thought to extend to the second or third lumbar segments. Because its cells begin in the thoracic and lumbar regions of the spinal cord, the SNS is said to have a thoracolumbar outflow. Axons of these nerves leave the spinal cord through the anterior rootlet/root. They pass near the spinal (sensory) ganglion, where they enter the anterior rami of the spinal nerves. However, unlike somatic innervation, they quickly separate out through white rami connectors (so called from the shiny white sheaths of myelin around each axon) which connect to the either the paravertebral (which lie near the vertebral column) or prevertebral (which lie near the aortic bifurcation) ganglia extending alongside the spinal column.
In order to reach the target organs and glands, the axons must travel long distances in the body, and, to accomplish this, many axons relay their message to a second cell through synaptic transmission. The ends of the axons link across a space, the synapse, to the dendrites of the second cell. The first cell (the presynaptic cell) sends a neurotransmitter across the synaptic cleft where it activates the second cell (the postsynaptic cell). The message is then carried to the final destination.
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