The sympathetic and parasympathetic nervous systems often have opposing control over many organs. They are sometimes referred to as the "fight or flight" and "rest and digest" systems, respectively. In response to a threat, the sympathetic nervous system is highly activated, leading to increased heart rate and blood pressure, blood redistribution to the muscles, decreased gastrointestinal activity, pupil dilation, and sweating. However, under normal conditions, the autonomic control system works together to regulate visceral function. Not all visceral structures receive innervation from both systems. For example, only a small portion of blood vessels have parasympathetic innervation, while the sympathetic system innervates smooth muscles, glands in the skin, and most blood vessels exclusively. The sympathetic preganglionic neurons are found in the thoracic and upper lumbar spinal cord segments, also known as the thoracolumbar division of the autonomic nervous system. Post-ganglionic neurons are typically located in the paravertebral ganglia and prevertebral ganglia. Paravertebral ganglia are situated alongside the vertebral column and are connected by longitudinally running axons forming a sympathetic trunk. Prevertebral ganglia are located in the abdominal cavity, including the Celiac and Superior and Inferior Mesenteric Ganglia. Preganglionic neurons have small myelinated nerve fibers called B fibers.
Control of the sympathetic and parasympathetic nervous systems of many organs is often antagonistic. To highlight this contrast, the sympathetic and parasympathetic systems are sometimes referred to as the “fight or flight” and the “rest and digest” systems, respectively. Indeed, the fight-orflight response to a threat to the organism reflects an intense activation of the sympathetic nervous system, which leads to a variety of responses, including increased heart rate and blood pressure, redistribution of blood to the muscles, decreased peristalsis and gastrointestinal secretions, pupil dilation, and sweating.
However, under most conditions, the two parts of the autonomic control system work in a coordinated manner—sometimes acting reciprocally and sometimes synergistically—to regulate visceral function. Furthermore, not all visceral structures are innervated by both systems. For example, the smooth muscles and glands in the skin and most of the blood vessels in the body receive sympathetic innervation exclusively; only a small fraction of the blood vessels have parasympathetic innervation. Indeed, the parasympathetic nervous system innervates not the body wall but only structures in the head and in the thoracic, abdominal, and pelvic cavities.
The Sympathetic Nervous System
The sympathetic preganglionic neurons are located in the thoracic and upper lumbar segments of the spinal cord. For this reason, the sympathetic nervous system is sometimes referred to as the thoracolumbar division of the autonomic nervous system. Specifically, sympathetic preganglionic neurons are concentrated in the intermediolateral cell column (lateral horn) in the thoracic and upper lumbar segments of the spinal cord (see
Fig. 11.2). Some neurons may also be found in the C8 segment. In addition to the intermediolateral cell column, groups of sympathetic preganglionic neurons are found in other locations, including the lateral funiculus, the intermediate gray matter, and the gray matter dorsal to the central canal. Sympathetic post-ganglionic neurons are generally found in the paravertebral •Fig. 11.2 Details of the Sympathetic Pathway at a Thoracic Spinal Segment. Autonomicsensoryfibersarerepresentedbyblue lines; sympatheticfibers,byred lines; preganglionicaxons,bysolid lines, andpostganglionicaxons,bydashed lines. (RedrawnfromParentA,CarpenterMB.Carpenter’s Human Neuroanatomy. 9thed.Philadelphia:Williams&Wilkins;1996:295.)White ramus Gray ramus Blood vessel Paravertebral ganglion Prevertebral ganglion Visceral sensory fiber Postganglionic fiber Postganglionic fibers Hair glands Skin Arrector pili muscle Preganglionic fibers Lateral horn Spinal ganglion Peripheral nerve Visceral afferent fiber Digestive tube Sympathetic trunk or prevertebral ganglia. The paravertebral ganglia form two sets of ganglia, each lateral to one side of the vertebral column. The individual ganglia on each side are linked by longitudinally running axons that form a sympathetic trunk (see
Figs. 11.1 11.2 ). Prevertebral ganglia are located in the abdominal cavity and include the Celiac and Superior and Inferior Mesenteric Ganglia (see
Fig. 11.1 ). Thus paravertebral and prevertebral ganglia are located at some distance from their target organs.
The axons of preganglionic neurons are often small, myelinated nerve fibers known as B fibers (see