Chapter 6 The Nervous System 219
All of the rami are mixed nerves, carrying
both afferent and efferent signals.
• The small dorsal rami transmit motor impulses
to the posterior trunk muscles and relay
sensory impulses from the skin of the back.
• The ventral rami in the thoracic region of
the spine (T
1
–T
12
) become the intercostal
nerves (running between the ribs). They
communicate with the muscles and skin of
the anterior and lateral trunk.
• The ventral rami in the cervical and lumbar
regions branch out to form complex
interconnections of nerves called plexuses.
Most of the major efferent nerves in the neck,
arms, and legs originate in the plexuses.
The four plexuses in the body are
summarized in Figure 6.20 on the next page.
To see how the major nerves branch out from
the lower three plexuses, refer to Figure 6.21.
Figure 6.21 shows that other large nerves branch
out from the plexuses listed in Figure 6.20.
Autonomic Nervous System
As described in the fi rst lesson of this
chapter, the peripheral nervous system has two
divisions—the somatic nervous system and the
autonomic, or involuntary, nervous system. The
somatic nervous system sends impulses to activate
the skeletal muscles, whereas the autonomic
Dorsal
ramus
Ventral
ramus
Ventral root
Dorsal root
Spinal
nerve
Spinal
cord
Figure 6.19 The spinal nerves, formed from dorsal
and ventral roots, immediately branch into dorsal and
ventral rami. What motor impulses do the dorsal and
ventral rami transmit? What sensory nerve impulses
do they transmit?
nervous system is programmed by the CNS to
activate the heart, smooth muscles, and glands.
Within the autonomic system, two nerves
connect the CNS to the organs supplied. The
cell body of the fi rst nerve originates in the gray
matter of the brain or spinal cord.
The autonomic cell bodies that originate in
the spinal cord reside in the lateral horn. The
axons of these nerves terminate with a synapse
to a second neuron in an enlarged junction called
a ganglion (GAYNG-glee-ahn). The second
neuron then courses from the ganglion to the
cardiac muscle, smooth muscle, or gland.
As you might suspect, the fi rst neuron
in the sequence just described is called the
preganglionic (PREE-gayng-glee-AHN-ik)
neuron. The second is called the postganglionic
(POHST-gayng-glee-AHN-ik) neuron. Now
let’s look more closely at the two divisions of
the autonomic nervous system. These are the
sympathetic and parasympathetic divisions.
Sympathetic Nerves
The sympathetic nerves activate the fi ght-or-
fl ight response by stimulating the adrenal gland
to release epinephrine, also known as adrenaline.
Supposedly in primitive times, when a person
was confronted by a predator, the fi ght-or-fl ight
response—characterized by increased heart and
breathing rates and sweating—prepared the
individual either to fi ght or run. In modern times
the sympathetic response is physiologically
the same, but it can be triggered by any type of
situation that is perceived to be stressful. You
will learn more about the fi ght-or-fl ight response
in Chapter 8, The Endocrine System.
The preganglionic neurons in the sympathetic
system arise from the spinal segments extending
from T
1
–L
2
. For this reason, the sympathetic system
is also called the thoracolumbar (THOH-rah-
koh-LUM-bar) division. These neurons secrete
acetylcholine to stimulate the postganglionic
neurons in the paravertebral ganglia (pair-a-VER-
teh-bral GAYNG-glee-a). The paravertebral ganglia
are named after their location; they lie parallel to
the spinal cord. The postganglionic neurons release
the neurotransmitter norepinephrine (NOR-ehp-
i-NEHF-rin).