Sympathetic Division

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.

Collateral Ganglia

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

Posterior (dorsal) root Anterior (ventral) root Rami communicantes Spinal nerve Rib

Spinal Nerve Sympathetic Division

Spinal cord

Sympathetic chain of paravertebral ganglion

Sympathetic ganglion Vertebral body

Spinal cord

Sympathetic chain of paravertebral ganglion

Sympathetic ganglion Vertebral body

■ 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.

The Autonomic Nervous System 223

fibers that arise from the collateral ganglia innervate organs of the digestive, urinary, and reproductive systems.

Adrenal Glands

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.

Essentials of Human Physiology

Essentials of Human Physiology

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Responses

  • Nora
    Where is the ventral rami root?
    7 years ago
  • ornella
    Why is the sympathetic division called the thoracolumbar division?
    7 years ago
  • peregrin
    What allows mass activation divergence?
    7 years ago
  • NORA
    How many sympathetic chain in the body?
    7 years ago
  • BILDAT
    What do nervous pathways in the white rami communicantes?
    7 years ago
  • Marcel Rothstein
    Do sympathetic ganglia and the adrenal cortex arise from the same empryonic tissue?
    7 years ago
  • ISABELLA
    What is the sypathetic chain ganglia composed of?
    7 years ago
  • Dudo
    Is rami located the vertebrae?
    7 years ago
  • columbus
    Where do we find the ganglia of each division?
    7 years ago
  • PIA BARESE
    Where is the sympathetic chain of ganglia?
    7 years ago
  • stephanie
    Why are sympathetic axons in nearly all nerves?
    7 years ago
  • neftalem
    Which rami communicantes go to sympathetic chain?
    7 years ago
  • mildred
    Is paravertebral sympathetic chain in spinal column?
    7 years ago
  • abi gray
    What sympathetic fibers form the splanchnic nerve?
    7 years ago
  • Johanna
    Which is a sympathetic collateral ganglion?
    7 years ago
  • elanor
    Which sympathetic fibers form the splanchic nerve?
    7 years ago
  • YEMANE
    Why is the sympathiec division called mass activation?
    6 years ago
  • Akseli
    Why is sympathetic division called thorocolumbar?
    6 years ago
  • Roman
    Why are the paravertebral ganglia important in divergence and convergence of the sympathetic system?
    6 years ago
  • Robert
    Which sympathetic fibers form a splanchnic nerves?
    6 years ago
  • girmay
    Where do the preganglionic fibers of the sympathetic division exit the spinal cord?
    6 years ago
  • Ilse
    Why is the andrenal medulla considered to be a modified sympathetic ganglion?
    6 years ago
  • Sarah
    Is collateral ganglia in the symphathetic division.edu?
    6 years ago
  • phillip searcy
    Why is the adrenal medulla important in the sympathetic branch?
    6 years ago
  • TEWELDE BRHANE
    Which part of the spinal cord is considered the sympathetic division?
    6 years ago
  • efrem tesfalem
    Why is the adrenal medulla innervated by the sympathetic division only?
    6 years ago
  • Toby Noakes
    Which part of the spinal cord is considered the sympathetic divison?
    6 years ago
  • orlando
    Where and how does the sympathetic nervous system have a high degree of divergence?
    6 years ago
  • ALANA JAMIESON
    Which part of the spinal cord is considered the parasympathetic division?
    6 years ago
  • Fulvio
    Why is the suprarenal medulla considered to be a modified sympathetic ganglion?
    6 years ago
  • Isto
    How does a preganglionic fiber travel to the adrenal medulla?
    6 years ago
  • awet ermias
    Where do preganglionic fibers headed for the adrenal medulla travel in?
    5 years ago
  • Carmen Koval
    Why is the sympathetic nervous system called thethoraco lumbar division?
    5 years ago
  • faramond
    How do the preganlionic axons in the sympathetic division travel to the ganglia?
    5 years ago
  • sara
    Which of the below responses is NOT caused by the sympathetic division.?
    5 years ago
  • feorie
    Why is the adrenal medulla innervated by the sympathetic division only and what is its roke?
    4 years ago
  • sabrina
    Where do postganglionic fibers exit?
    4 years ago
  • JENS
    What is a splanchnic nerve how it relates to the collateral ganglia?
    3 years ago
  • preston
    Why is adrenal medulla likened to a sympathetic chain ganglion?
    3 years ago
  • orlando
    What is the role of the collateral ganglia in the splanchnic route?
    3 years ago
  • Tewolde
    How is the adrenal medulla likened to a modified sympathetic ganglion?
    2 years ago
  • mikko sepp
    What is the splanchnic sympathetic system?
    2 years ago
  • TARQUINIO
    What are the division of the sympathetic chain ganglia?
    2 years ago
  • Aman
    What the collateral ganglia innervate?
    4 days ago

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