Human Physiology, 14th edition (2016)

246 Chapter 9

Sympathetic Division

The sympathetic division is also called the thoracolumbar

division of the autonomic system because its preganglionic

fibers exit the spinal cord, in the ventral roots of spinal nerves,

from the first thoracic (T1) to the second lumbar (L2) levels.

Most sympathetic nerve fibers separate from the somatic motor

fibers and synapse with postganglionic neurons within a dou-

ble 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 sympa-

thetic neurons are unmyelinated and form the gray rami com-

municantes as they return to the spinal nerves and travel as part

of the spinal nerves to their effector organs ( fig. 9.3 ). Because

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 gan-

glia as preganglionic fibers branch to synapse with numerous

postganglionic 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 pregan-

glionic fibers. The divergence of impulses from the spinal cord

9.2 DIVISIONS OF THE AUTONOMIC

NERVOUS SYSTEM

Preganglionic neurons of the sympathetic division origi-

nate in the thoracic and lumbar levels of the spinal cord

and send axons to sympathetic ganglia, which parallel the

spinal cord. Preganglionic neurons of the parasympathetic

division originate in the brain and in the sacral level of the

spinal cord, and send axons to ganglia located in or near

the effector organs.

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.

Spinal cord

Sympathetic chain of

paravertebral ganglia

Sympathetic ganglion

Vertebral body

Posterior (dorsal) root

Anterior (ventral) root

Rami communicantes

Spinal nerve

Rib

The sympathetic and parasympathetic divisions of the
autonomic system have some structural features in common.
Both consist of preganglionic neurons that originate in the
CNS and postganglionic neurons that originate outside of the
CNS in ganglia. However, the specific origin of the pregan-
glionic fibers and the location of the ganglia differ in the two
divisions of the autonomic system.

LEARNING OUTCOMES

After studying this section, you should be able to:

3. Describe the structure of the sympathetic nervous
   system, locating the ganglia and the preganglionic
   and postganglionic neurons.


4. Explain the relationship between the sympathetic
   nervous system and the adrenal medulla.


5. Describe the structure and innervation pathways
   of the parasympathetic division of the autonomic
   system.