Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 17

The Autonomic Nervous System 263

The cell bodies of the preganglionic neurons are located in the
intermediolateral column (IML) of the spinal cord and in mo-
tor nuclei of some cranial nerves. In contrast to the large diam-
eter and rapidly conducting
α
-motor neurons, preganglionic
axons are small-diameter, myelinated, relatively slowly con-
ducting B fibers. A preganglionic axon diverges to an average
of eight or nine postganglionic neurons. In this way, auto-
nomic output is diffused. The axons of the postganglionic
neurons are mostly unmyelinated C fibers and terminate on
the visceral effectors.

SYMPATHETIC DIVISION

In contrast to
α
-motor neurons, which are located at all spinal
segments, sympathetic preganglionic neurons are located in
the IML of only the first thoracic to the third or fourth lumbar
segments. This is why the sympathetic nervous system is
sometimes called the thoracolumbar division of the ANS. The
axons of the sympathetic preganglionic neurons leave the spi-
nal cord at the level at which their cell bodies are located and
exit via the ventral root along with axons of
α

• and
  γ
  -motor

neurons (Figure 17–2). They then separate from the ventral

root via the

white rami communicans

and project to the ad-

jacent

sympathetic paravertebral ganglion,

where some of

them end on the cell bodies of the postganglionic neurons.

Paravertebral ganglia are located adjacent to each thoracic and

upper lumbar spinal segments; in addition, there are a few

ganglia adjacent to the cervical and sacral spinal segments.

These ganglia form the

sympathetic chain

bilaterally. The

ganglia are connected to each other via the axons of pregangli-

onic neurons that travel rostrally or caudally to terminate on

postganglionic neurons located at some distance. This ar-

rangement is seen in Figures 17–2 and 17–3.

Some preganglionic neurons pass through the paravertebral

ganglion chain and end on postganglionic neurons located in

prevertebral

(or

collateral

)

ganglia

close to the viscera,

including the celiac, superior mesenteric, and inferior mesen-

teric ganglia (Figure 17–3). There are also preganglionic neu-

rons whose axons terminate directly on the effector organ, the

adrenal gland.

The axons of some of the postganglionic neurons leave the

chain ganglia and reenter the spinal nerves via the

gray rami

FIGURE 17–2
Projection of sympathetic preganglionic and postganglionic fibers.
The drawing shows the thoracic spinal cord, paraver-
tebral, and prevertebral ganglia. Preganglionic neurons are shown in red, postganglionic neurons in dark blue, afferent sensory pathways in blue,
and interneurons in black.
(Reproduced with permission from Boron WF, Boulpaep EL:
Medical Physiology.
Elsevier, 2005.)