389
CHAPTER ELEVEN
Nervous System II
Between them on either side in some regions is a protrusion
of gray matter called the
lateral horn.
Motor neurons with rel-
atively large cell bodies in the anterior horns (anterior horn
cells) give rise to axons that pass out through spinal nerves to
various skeletal muscles. However, the majority of neurons in
the gray matter are interneurons (see chapter 10, p. 358).
A horizontal bar of gray matter in the middle of the spi-
nal cord, the
gray commissure,
connects the wings of the
gray matter on the right and left sides. This bar surrounds
the
central canal,
which is continuous with the ventricles
of the brain and contains CSF. The central canal is promi-
nent during embryonic development, but it becomes almost
microscopic in adulthood.
The gray matter divides the white matter of the spinal
cord into three regions on each side—the
anterior, lateral,
and
posterior
funiculi
. Each column consists of longitudi-
nal bundles of myelinated nerve ± bers that comprise major
nerve pathways called
nerve tracts.
Functions of the Spinal Cord
The spinal cord has two main functions. First, it is a center
for spinal refl
exes. Second, it is a conduit for nerve impulses
to and from the brain.
Refl
ex Arcs
Nerve impulses follow nerve pathways as they travel through
the nervous system. The simplest of these pathways, includ-
ing only a few neurons, constitutes a
refl
ex
(re
fl eks)
arc.
Refl
ex arcs carry out the simplest responses—
refl
exes.
Recall that the nervous system receives sensory informa-
tion, processes it, and initiates appropriate responses by acti-
vating effector organs. For example, as you read this book,
your eyes send sensory information to your brain, where it
processes the information, interprets its meaning, and even
stores much of it in memory. For reading to continue, motor
commands to muscles point the eyes at what you are reading
and allow you to turn the pages. Some functions continue
without your awareness, such as breathing and heartbeat.
To begin to understand how the nervous system does
all of this, we will examine the simplest of the nervous sys-
tem functions that refl ect these processes—the refl exes. All
refl exes share the basic refl ex arc, as shown in
f gure 11.7
a
.
A refl
ex arc begins with a
sensory receptor
at the dendritic
end of a sensory neuron. Nerve impulses on these sensory
neurons enter the CNS and constitute a sensory or afferent
limb of the refl ex. The CNS is a processing center. Afferent
neurons may synapse with interneurons, which may in turn
connect with other parts of the CNS. Afferent neurons or
interneurons ultimately connect with motor neurons, whose
±
bers pass outward from the CNS to effectors. (It may help
to remember that
eff
erent neurons control
eff
ector organs.)
Refl
exes occur throughout the CNS. Those that involve
the spinal cord are called spinal refl
exes and refl ect the sim-
plest level of CNS function. Figure 11.7
b
shows the general
components of a spinal refl
ex.
lumbar enlargement, the spinal cord tapers to a structure called
the
conus medullaris.
From this tip, nervous tissue, including
axons of both motor and sensory neurons, extends downward
to become spinal nerves at the remaining lumbar and sacral
levels. Originating from among them, a thin cord of connective
tissue descends to the upper surface of the coccyx. This cord is
called the
F
lum terminale
(± g. 11.5
b
). The ± lum terminale and
the spinal nerves below the conus medullaris form a structure
that resembles a horse’s tail, the
cauda equina.
Two grooves, a deep
anterior median F
ssure
and a shal-
low
posterior median sulcus,
extend the length of the spinal
cord, dividing it into right and left halves. A cross section
of the cord
(f g. 11.6)
reveals that it consists of white mat-
ter surrounding a core of gray matter. The pattern the gray
matter produces roughly resembles a butterfl y with its wings
outspread. The upper and lower wings of gray matter are
called the
posterior horns
and the
anterior horns,
respectively.
Foramen
magnum
Brainstem
Cervical
enlargement
Cervical
enlargement
Spinal cord
Vertebral
canal
Lumbar
enlargement
Cauda
equina
Conus
medullaris
Conus
medullaris
Filum
terminale
Lumbar
enlargement
(a)
(b)
FIGURE 11.5
Spinal cord. (
a
) The spinal cord begins at the level of
the foramen magnum. (
b
) Posterior view of the spinal cord with the
spinal nerves removed.
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