Learning Modules - Medical Gross Anatomy
Spinal Cord and Spinal Nerve - Page 2 of 14
The spinal cord is well-protected by the vertebral column, which extends from the skull to the coccyx, enclosing the entire cord. The
vertebral column comprises 33 vertebrae: 7 cervical vertebrae, 12 thoracic vertebrae, 5 lumbar vertebrae, 5 sacral vertebrae, and 4
coccygeal vertebrae. In adults, the 5 sacral vertebrae are fused to form the sacrum and the 4 coccygeal vertebrae are fused to form
the coccyx. The vertebrae are separated by tough fibrocartilage intervertebral discs that make up about one-fourth of the length of the
vertebral column. While the structure of the vertebrae varies from one region to another, the spinal cord runs through the vertebral
foramen in each vertebra. The vertebral foramen is bordered anteriorly by the vertebral body, laterally by the pedicles and posteriorly
by the laminae. Thus, the spinal cord is entirely protected by bone. Nerves enter and exit the spinal cord through intervertebral
foramina, which are enclosed by the inferior vertebral notch on the superior vertebra and the superior vertebral notch on the inferior
vertebra.
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Learning Modules - Medical Gross Anatomy
Spinal Cord and Spinal Nerve - Page 3 of 14
The spinal cord is further protected by three membranes,
collectively called the meninges. The outer membrane
surrounding the spinal cord is the dura mater (from Latin,
meaning durable mother). The dura, made of a dense
fibrous material, forms the dural sac, which surrounds the
spinal cord and cauda equina (to be discussed later) and
terminates at the level of the second sacral vertebra. The
dura is separated from the vertebrae by the epidural fat in
the epidural space (epi, meaning upon). The next layer is
the delicate arachnoid mater which is thin and has web-like
filaments connecting to the underlying pia mater (arachnoid
is Greek for spider). Beneath the arachnoid mater is the
subarachnoid space, which is filled with cerebrospinal fluid.
Abbreviated CSF, cerebrospinal fluid bathes the brain and spinal cord as well as the cauda equina,
providing protection, nourishment, and a medium for exchange of nutrients and waste. The innermost
membrane surrounding the spinal cord is the vascular pia mater (Latin, meaning tender or devoted mother),
which is very closely apposed to the spinal cord. The pia mater has paired specializations called denticulate
ligaments, which extend laterally from the surface of the spinal cord and pierce the arachnoid to attach to
the inner aspect of the dura mater at 21 pairs of denticulations (dentate means tooth-like - these are toothlike lateral projections). The denticulate ligaments run longitudinally between the dorsal and ventral roots of
the spinal cord and serve to suspend the spinal cord from side to side in the dural sac. The pia continues
inferiorly from the end of the spinal cord (at the level of the second lumbar vertebra) as the filum terminale
internum. As we said, the dural sac ends at the level of the second sacral vertebra (S2). Caudal to the end
of the dural sac is a specialization of meninges called the coccygeal ligament (or filum terminale externum)
that attaches the meninges, and consequently the spinal cord, to the coccyx. The coccygeal ligament is
composed of specializations of all three layers of meninges.
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Learning Modules - Medical Gross Anatomy
Spinal Cord and Spinal Nerve - Page 4 of 14
The spinal cord itself begins as a
continuation of the medulla
oblongata of the brain at the
level of the foramen magnum in
the skull. Along its course, the
spinal cord gives rise to 31 pairs
of spinal nerves: 8 cervical, 12
thoracic, 5 lumbar, 5 sacral, and
1 coccygeal. In general, the
spinal nerves emerge below the
pedicle of the vertebra for which
they are named. For example,
spinal nerve T3 exits through the
intervertebral foramen between
the 3rd and 4th thoracic
vertebrae. In the cervical region,
this rule does not apply. While
the cervical region of the spine
has 7 vertebrae, there are 8
cervical spinal nerves. Spinal
nerve C1 exits above vertebra
C1, spinal nerve C2 exits
through the intervertebral
foramen between the 1st and
2nd cervical vertebrae, etc. That
pattern holds until spinal nerve
C8 which exits between
vertebrae C7 and T1. The nerve
exiting below vertebra T1 is
spinal nerve T1. This
relationship is rather confusing in
words, but is fairly clear when
studied in a diagram. The spinal
cord terminates at the level of
the first or second lumbar
vertebrae as the conus
medullaris. Caudal to the conus
medullaris are the nerve roots of
the more caudal spinal nerves
which are collectively called,
because of their appearance,
the cauda equina (horse's tail).
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Learning Modules - Medical Gross Anatomy
Spinal Cord and Spinal Nerve - Page 5 of 14
While it seems somewhat counterintuitive that the
spinal cord would end at the level of the first or
second lumbar vertebra, (as is often the case in
anatomy) the explanation lies in the
embryological development of the spinal cord. In
embryos, the spinal cord occupies the entire
length of the vertebral canal, but during
development the vertebral column grows faster
than the spinal cord. Consequently, in an adult,
not all spinal cord segments are found at the level
of the vertebra of the same number. Because of
the discrepancy between the spinal cord segment
and the vertebral level, the spinal nerve roots of
the more caudal spinal cord segments are rather
long and descend for a significant distance before
reaching their respective intervertebral foramina.
It is these spinal nerve roots that compose the
cauda equina beyond L1/L2. The fact that the
spinal cord ends at L1/L2 is very useful in clinical
practice in that it allows for spinal taps to be
performed to sample CSF without the risk of
puncturing the spinal cord. Below L1/L2 but
above S2, there is still CSF contained in the dural
sac (in what is called the lumbar cistern), but
instead of a spinal cord there are only nerve
roots, which float away from the positive pressure
of a needle entering the space. Extreme caution
must be taken in performing a spinal tap on an
infant or a young child whose spinal cord may
extend as far as vertebra L4 because their
vertebral column has not reached its maximum
length.
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Learning Modules - Medical Gross Anatomy
Spinal Cord and Spinal Nerve - Page 6 of 14
Another important feature of the spinal cord,
which relates directly to its function, is its
external shape. The spinal cord is shaped
something like a glass soda-bottle. It has a
cervical enlargement which begins at roughly
C4 and extends to roughly T1 and a lumbar
enlargement that extends from roughly the
T11 vertebra through the L1 vertebral level.
The cervical enlargement is the site of the cell
bodies of the motor neurons that innervate
the upper limbs, as well as the site where the
sensory nerves from the upper limbs synapse.
The lumbar enlargement is the site of the cell
bodies of the motor neurons that innervate
the lower limbs and the site where the
sensory nerves from the lower limbs synapse.
These segments of spinal cord are enlarged
because of the extensive sensory input from
the limbs, especially from the hands and
fingers, as well as the complex and fine
musculature of the limbs which requires
significantly more innervation to control.
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Learning Modules - Medical Gross Anatomy
Spinal Cord and Spinal Nerve - Page 7 of 14
A cross section of the spinal cord reveals the
characteristic butterfly shape of the gray matter of
the spinal cord surrounded by white matter. The
gray matter changes shape throughout the spinal
cord depending on the neural requirements of a
given region. For example, the cervical and lumbar
enlargements that provide innervation to the limbs
have much more gray matter than a segment of
spinal cord taken from the thoracic region that
supplies a segment of the trunk. Regardless of
location, the cord consists of a pair of dorsal horns
and a pair of ventral horns of gray matter. The
dorsal horns are made of the cell bodies upon
which the axons carrying sensory information from
the periphery synapse. The ventral horns are
composed of the cell bodies of the motor neurons
innervating skeletal muscles. A useful mnemonic
for remembering the relationships in the spinal cord
is: SAME-DAVE (sensory-afferent, motor-efferent;
dorsal-afferent, ventral-efferent). In the thoracic
region, the upper lumbar region and in segments of
the sacral region of the spinal cord, there is also a
lateral horn of gray matter. In the thoracic and
lumbar regions of the spinal cord, the lateral horn
contains autonomic cell bodies for sympathetic
nervous system innervation to smooth muscles,
cardiac muscles and glands. In the sacral region of
the spinal cord, the lateral horn contains autonomic
cell bodies for parasympathetic innervation to
pelvic organs. The role of the lateral horn and
autonomic innervation will be discussed in another
module, however, it is helpful to recognize that the
lateral horns exist at this point.
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Learning Modules - Medical Gross Anatomy
Spinal Cord and Spinal Nerve - Page 8 of 14
We have mentioned that the spinal cord gives rise to 31
spinal nerves. The spinal nerves, along with the cranial
nerves, are the means by which the CNS communicates
with the periphery. Together, the spinal nerves and the
cranial nerves are referred to as peripheral nerves. The
cranial nerves take origin from the brain and will be
discussed in another module. The spinal nerves take
origin from the spinal cord (as we have discussed) and
will be the topic for the remainder of this module.
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Learning Modules - Medical Gross Anatomy
Spinal Cord and Spinal Nerve - Page 9 of 14
Spinal nerves are formed by the
union of rootlets that originate
from the dorsal and ventral
horns of the spinal cord. The
dorsal horn gives rise to several
dorsal rootlets per section of
spinal cord, and the ventral
horn gives rise to several
ventral rootlets per section of
spinal cord. These rootlets
converge to form the dorsal
roots and the ventral roots, one
from each spinal cord segment.
Keep in mind that the dorsal
horns are the location of the cell
bodies upon which the afferent
nerves from the periphery
synapse. Hence, the dorsal
roots and rootlets contain
afferent fibers carrying sensory
information from the periphery
to the CNS. The ventral horns,
on the other hand, are the
home of the cell bodies for the
motor neurons that carry motor
information from the CNS to the
periphery. Therefore, the
ventral rootlets and roots
contain efferent fibers.
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Learning Modules - Medical Gross Anatomy
Spinal Cord and Spinal Nerve - Page 10 of 14
Emerging from the dorsal and ventral horns of the
spinal cord are dorsal and ventral rootlets,
respectively. These rootlets carry nerve fibers both
toward and away from the spinal cord. The dorsal
rootlets carry sensory (afferent) fibers to the spinal
cord, and ventral rootlets carry motor (efferent) fibers
away from the spinal cord. Laterally, near each
intervertebral foramen, a number of dorsal rootlets
coalesce into a single dorsal root, and a number of
ventral rootlets coalesce into a single ventral root.
These roots then leave the vertebral canal to lie within
the intervertebral foramen. There, the dorsal and
ventral roots fuse to form the spinal nerve, which is a
mixed nerve carrying both sensory and motor fibers.
Along the dorsal roots, lateral to
the spinal cord and just medial
to fusing with ventral roots, are
the dorsal root ganglia, tucked
in the intervertebral foramina.
Recall that a ganglion is a
collection of neuron cell bodies
in the peripheral nervous
system. The dorsal root ganglia
(sometimes called the spinal
ganglia) contain the cell bodies
for the peripheral sensory nerve
fibers. The sensory peripheral
neurons have an unusual and
unique structure. Sensory
peripheral neurons are called
pseudounipolar neurons
because the cell body sits above the neuronal process, which extends in both directions (see diagram). It is very difficult
to define the nerve process as an axon or dendrite because the cell is not arranged as a typical neuron. Some texts
refer to the nerve process of a pseudounipolar neuron as an "axonodendrite," however usually you will hear it called
simply a nerve process. The receptors for the pseudounipolar neurons are located in the skin, subcutaneous tissue,
deep tissue or viscera and the nerve process synapses in the dorsal horn of the spinal cord. The cell bodies sit in the
dorsal root ganglia and support the neurons. It is very important to recognize that there are NO synapses in the dorsal
root ganglia! This is a very common point of confusion, but if you realize that the nerve process runs straight from the
receptor all the way into the dorsal horn and that the cell body is sitting in the dorsal root ganglia solely to support the
neuron, you should stay out of trouble. Again, there are no synapses in the dorsal root ganglia!
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Spinal Cord and Spinal Nerve - Page 11 of
14
The ventral roots are formed by the axons of motor (efferent) neurons found
in the ventral and lateral horns of the spinal cord. The lateral horns are
related to the autonomic nervous system and will be covered in that learning
module. The axons of the ventral horn cells (sometimes referred to as
anterior horn cells) exit the spinal cord as multiple ventral rootlets that
coalesce to form the ventral roots of the spinal nerves. The spinal nerves then
distribute regionally by way of dorsal and ventral primary rami. The ventral
horn cell axons traveling in the dorsal ramus innervate the skeletal muscle
forming the true (paraspinal) back muscles. Those axons traveling in the
ventral ramus innervate the skeletal muscle of the limbs and trunk.
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Learning Modules - Medical Gross Anatomy
Spinal Cord and Spinal Nerve - Page 12 of 14
Just distal to the dorsal root ganglia, the dorsal and ventral roots unite to form the 31 pairs of spinal nerves.
Therefore, spinal nerves are mixed nerves, carrying both afferent fibers and efferent fibers. This is the closest
mixed nerve to the spinal cord; the dorsal roots contain only afferent fibers and the ventral roots contain only
efferent fibers. The spinal nerves are extremely short. They project laterally for only about 1-2mm before
branching into dorsal and ventral primary rami as they exit the intervertebral foramina. The membranes
covering the spinal cord become continuous with the layers surrounding the spinal nerves. The dura extends
into the intervertebral foramina along the dorsal and ventral roots to form dural sleeves which eventually
blend with the epineurium of the spinal nerves. The spinal nerves are a critical component of the nervous
system, acting as the only path of communication between the peripheral nerves and the spinal cord.
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Spinal Cord and Spinal Nerve - Page 13 of 14
Shortly after exiting the intervertebral
foramina, the spinal nerves branch into
dorsal and ventral primary rami. Be
careful! Dorsal primary rami and ventral
primary rami are NOT the same as
dorsal roots and ventral roots! Think of
the spinal nerve as the trunk of a tree the word ramus means branch in Latin,
so the primary rami are the branches of
a spinal nerve, whereas the roots unite
to form the spinal nerve. These terms
are very easy to confuse. The primary
rami are mixed nerves carrying both
motor and sensory fibers. The primary
rami branch many, many times to
provide innervation to almost the entire
body. Nerves that branch from the
dorsal primary rami supply the deep
back muscles as well as the skin over
the back. Nerves from the ventral
primary rami supply the limbs and the
rest of the trunk.
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Learning Modules - Medical Gross Anatomy
Spinal Cord and Spinal Nerve - Page 14 of 14
In summary, the spinal cord acts principally as a highway for
information to travel between the brain and the periphery. The
spinal cord begins at the foramen magnum and ends at L1/L2 as
the conus medullaris. Caudal to the conus medullaris is the cauda
equina, which consists of spinal nerve roots from the more caudal
segments of the spinal cord. The spinal cord and most of the cauda
equina is enclosed by the dural sac which ends at S2 and is
connected to the coccyx by the coccygeal ligament. The 31 spinal
nerves, which exit the spinal cord through the intervertebral
foramina, are responsible for communicating between the spinal
cord and the rest of the body. Dorsal rootlets from the dorsal horns
and ventral rootlets from the ventral horns converge to form dorsal
and ventral roots. The dorsal roots carry only sensory information
and the ventral roots carry only motor information. Inside the
intervertebral foramina are the dorsal root ganglia, the cell bodies
for the peripheral sensory nerve fibers, which are pseudounipolar
neurons. Distal to the dorsal root ganglia, the dorsal and ventral
roots unite to form spinal nerves which are mixed nerves. The
spinal cord gives rise to 31 spinal nerves, named for the vertebral
segment where they exit the spinal cord. Spinal nerves are very
short, branching shortly after exiting the intervertebral foramina into
dorsal and ventral primary rami. The primary rami subsequently
branch to supply the body.
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