The sympathetic division is also called the thoracolumbar division of the autonomic system because its preganglionic fibers exit the spinal cord from the first thoracic (T1) to the second lumbar (L2) levels. Most sympathetic nerve fibers, however, separate from the somatic motor fibers and synapse with postganglionic neurons within a double row of sympathetic ganglia, called paravertebral ganglia, located on either side of the spinal cord (fig. 9.2). Ganglia within each row are interconnected, forming a sympathetic chain of ganglia that parallels the spinal cord on each lateral side.
The myelinated preganglionic sympathetic axons exit the spinal cord in the ventral roots of spinal nerves, but they soon diverge from the spinal nerves within short pathways called white rami communicantes. The axons within each ramus enter the sympathetic chain of ganglia, where they can travel to ganglia at different levels and synapse with postganglionic sympathetic neurons. The axons of the postganglionic sympathetic neurons are unmyelinated and form the gray rami communicantes as they return to the spinal nerves and travel as part of the spinal nerves to their effector organs (fig. 9.3). Since sympathetic axons form a component of spinal nerves, they are widely distributed to the skeletal muscles and skin of the body, where they innervate blood vessels and other involuntary effectors.
Divergence occurs within the sympathetic chain of ganglia as preganglionic fibers branch to synapse with numerous post-ganglionic neurons located in ganglia at different levels in the chain. Convergence also occurs here when a postganglionic neuron receives synaptic input from a large number of preganglionic fibers. The divergence of impulses from the spinal cord to the ganglia and the convergence of impulses within the ganglia results in the mass activation of almost all of the post-ganglionic sympathetic neurons. This explains why the sympathetic system is usually activated as a unit, affecting all of its effector organs at the same time.
Many preganglionic fibers that exit the spinal cord below the level of the diaphragm pass through the sympathetic chain of ganglia without synapsing. Beyond the sympathetic chain, these preganglionic fibers form splanchnic nerves. Preganglionic fibers in the splanchnic nerves synapse in collateral, or pre-vertebral ganglia. These include the celiac, superior mesen-teric, and inferior mesenteric ganglia (fig. 9.4). Postganglionic
Sympathetic chain of paravertebral ganglion
Sympathetic chain of paravertebral ganglion
■ Figure 9.2 The sympathetic chain of paravertebral ganglia. This diagram shows the anatomical relationship between the sympathetic ganglia and the vertebral column and spinal cord.
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fibers that arise from the collateral ganglia innervate organs of the digestive, urinary, and reproductive systems.
The paired adrenal glands are located above each kidney. Each adrenal is composed of two parts: an outer cortex and an inner medulla. These two parts are really two functionally different glands with different embryonic origins, different hormones, and different regulatory mechanisms. The adrenal cortex secretes steroid hormones; the adrenal medulla secretes the hormone epi-nephrine (adrenaline) and, to a lesser degree, norepinephrine, when it is stimulated by the sympathetic system.
The adrenal medulla can be likened to a modified sympathetic ganglion; its cells are derived from the same embryonic tissue (the neural crest, chapter 8) that forms postganglionic sympathetic neurons. Like a sympathetic ganglion, the cells of the adrenal medulla are innervated by preganglionic sympathetic fibers. The adrenal medulla secretes epinephrine into the blood in response to this neural stimulation. The effects of epinephrine are complementary to those of the neurotransmitter norepineph-rine, which is released from postganglionic sympathetic nerve endings. For this reason, and because the adrenal medulla is stimulated as part of the mass activation of the sympathetic system, the two are often grouped together as a single sympathoadrenal system.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.