466
UNIT 3
Regulation and Integration of the Body
12
puncture
or
tap
(Figure 12.27)
. Because the spinal cord is ab-
sent there and the delicate nerve roots drif away From the point
oF needle insertion, there is little or no danger oF damaging the
cord (or spinal roots) beyond L
3
.
InFeriorly, the spinal cord terminates in a tapering cone-shaped
structure called the
conus medullaris
(ko
9
nus me
˘
0
dul-ar
9
is). Te
filum terminale
(fi
9
lum ter
0
mĭ-nah
9
le; “terminal filament”), a
fibrous extension oF the conus covered by pia mater, extends
inFeriorly From the conus medullaris to the coccyx, where it
anchors the spinal cord so it is not jostled by body movements
(±igure 12.26a, d). ±urthermore, saw-toothed shelves oF pia
mater called
denticulate ligaments
(den-tik
9
u-lāt; “toothed”)
secure the spinal cord to the tough dura mater meninx through-
out its length (±igure 12.26c).
In humans, 31 pairs oF
spinal nerves
—part oF the peripheral
nervous system—attach to the cord by paired roots. Each nerve
exits From the vertebral column by passing superior to its cor-
responding vertebra via the intervertebral Foramen, and travels
to the body region it serves. While each nerve pair defines a seg-
ment oF the cord, the spinal cord is, in Fact, continuous through-
out its length and its internal structure changes gradually.
Te spinal cord is about the width oF a thumb For most oF its
length, but it has obvious enlargements in the cervical and lum-
bosacral regions, where the nerves serving the upper and lower
limbs arise. Tese enlargements are the
cervical
and
lumbar
enlargements
, respectively (±igure 12.26a).
Because the cord does not reach the end oF the vertebral col-
umn, the lumbar and sacral spinal nerve roots angle sharply
downward and travel inFeriorly through the vertebral canal For
some distance beFore reaching their intervertebral Foramina.
Ligamentum
flavum
Supra-
spinous
ligament
Lumbar puncture
needle entering
subarachnoid
space
Filum
terminale
Inter-
vertebral
disc
T
12
L
5
Cauda equina
in subarachnoid
space
Dura
mater
L
5
L
4
L
5
L
4
S
1
Arachnoid
mater
Figure 12.27
Diagram of a lumbar tap.
This procedure removes
CSF for testing.
Te collection oF nerve roots at the inFerior end oF the vertebral
canal is named the
cauda equina
(kaw
9
da e-kwi
9
nuh) because
it resembles a horse’s tail (±igure 12.26a, d). Tis strange ar-
rangement reflects the Fact that during Fetal development, the
vertebral column grows Faster than the spinal cord, Forcing the
lower spinal nerve roots to “chase” their exit points inFeriorly
through the vertebral canal.
We will discuss the spinal nerves in more detail in Chapter 13.
Spinal Cord Cross-Sectional Anatomy
Te spinal cord is somewhat flattened From Front to back and
two grooves mark its surFace: the
ventral (anterior) median
fissure
and the shallower
dorsal (posterior) median sulcus
(Figure 12.28b)
. Tese grooves run the length oF the cord and
partially divide it into right and lef halves. Te gray matter oF
the cord is located in its core, the white matter outside.
Gray Matter and Spinal Roots
In cross section the gray matter oF the cord looks like the let-
ter H or like a butterfly (±igure 12.28b). It consists oF mirror-
image lateral gray masses connected by a crossbar oF gray mat-
ter, the
gray commissure
, that encloses the central canal. Te
two dorsal projections oF the gray matter are the
dorsal (pos-
terior) horns
, and the ventral pair are the
ventral (anterior)
horns
. In 3-D, these horns Form columns oF gray matter that
run the entire length oF the spinal cord. Te thoracic and supe-
rior lumbar segments oF the cord have an additional pair oF gray
matter columns, the small
lateral horns
.
All neurons whose cell bodies are in the spinal cord gray
matter are multipolar. Te dorsal horns consist entirely oF in-
terneurons. Te ventral horns have some interneurons but
mainly house cell bodies oF somatic motor neurons. Tese mo-
tor neurons send their axons out to the skeletal muscles (their
effector organs) via the
ventral rootlets
that Fuse together to be-
come the
ventral roots
oF the spinal cord (±igure 12.28b).
Te amount oF ventral gray matter present at a given level
oF the spinal cord reflects the amount oF skeletal muscle inner-
vated at that level. As a result, the ventral horns are largest in the
limb-innervating cervical and lumbar regions oF the cord and
are responsible For the cord enlargements seen in those regions.
Te lateral horns consist mostly oF the cell bodies oF autonomic
(sympathetic division) motor neurons that serve visceral organs.
Teir axons leave the cord via the ventral root along with those
oF the somatic motor neurons. Because the ventral roots contain
both somatic and autonomic efferent fibers, they serve both motor
divisions oF the peripheral nervous system
(Figure 12.29)
.
Afferent fibers carrying impulses From peripheral sensory re-
ceptors Form the
dorsal roots
oF the spinal cord that Fan out as
the
dorsal rootlets
beFore they enter the spinal cord (±igure 12.28).
Te cell bodies oF the associated sensory neurons are Found in an
enlarged region oF the dorsal root called the
dorsal root ganglion
or
spinal ganglion
. Afer entering the cord, the axons oF these
neurons may take a number oF routes. Some enter the dorsal
white matter oF the cord directly and travel to synapse at higher
cord or brain levels. Others synapse with interneurons in the dor-
sal horns oF the spinal cord gray matter at their entry level. Te
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