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SCIATICA: ANATOMY
To better understand all the terms related to the spine and to sciatica, this section expands on the basic
anatomy and gives you more in depth details.
The spine is made up of a combination of bones and associated joints, discs, ligaments, the spinal cord, and
spinal nerves and their associated coverings.
Bones
The spine is made up of 26 bones; 24 vertebrae, a sacrum and a coccyx. (Image 1) The upper most level is the
cervical region and has 7 vertebrae. The mid back is known as the
thoracic spine and is comprised of 12 vertebrae. The low back, is
known as the lumbar spine and is comprised of 5 vertebrae. The
sacrum is comprised of 5 vertebrae that are fused together, S1
through S5. The normal abbreviation for each vertebra corresponds
to the location and the order. For instance, the cervical spine is made
up of C1 thru C7. The thoracic spine is T1 thru T12. And the lumbar
spine is L1 thru L5.
The vertebrae interlock to form the spinal column. The primary role
of the cervical spine is to support the weight of the head. The primary
role of the thoracic spine is to hold the rib cage which in turn protects
the heart and the lungs and to a lesser degree the other major
organs. The lumbar spine’s primary function is to support the weight
of the body, which is why the vertebrae are much larger in size to
absorb the stress of lifting and carrying large objects.
Image 1
The vertebrae have 3 main parts (image 2):
• body,
• vertebral arch, and
• processes.
The body of the vertebrae is designed to bear weight and withstand compressive forces whereas the vertebral
arch surrounds the spinal canal and is designed to protect the spinal cord. The processes, including the spinous,
transverse, and superior articular are where muscles attach to the bone and where each vertebral bone joins the
next level (AKA-facet joints).
Each vertebrae has 2 facet joints that connect the vertebrae above called the superior facets, and 2 facet joints
that connect the vertebrae below called the inferior facets. Additional facet joints are for the articulations
(joints) between the vertebrae and the ribs. (Image 3)
Image 2
Image 3
Discs
Between each vertebral body of the spine there is an intervertebral disc, which is essentially a liquid or gel filled
cushion which keeps the bones from rubbing together and provides space between the bones through which
the spinal nerves pass to innervate different parts of the body.
There are two primary components of the disc, the outer layers called the annulus and
the inner gel-filled center called the nucleus. (Images 4 & 5) The annulus is made up of
15-25 layers of a fibrous bands that alternate in a crisscross like pattern to hold the
vertebral bodies together and prevent the gel like
nucleus from escaping.
The nucleus, as mentioned previously is primarily fluid
Image 4
and gel like, and is designed to move around as you
move to protect the vertebral structures including the joints and nerve roots.
During the ages of birth to 20, the majority of the weight through the spine is
borne through the posterior or back 1/3 of the disk. As we age beyond that
point, weight bearing is shifted even further posteriorly (backward) to the facet
joints which connect each vertebrae or a neighboring structure together.
Image 5
Ligaments
There are 3 primary ligaments (Image 6) which are strong fibrous bands that are the major structures that hold
the vertebrae together as well as offer stability and protection to the discs. They are the:
•
•
•
anterior longitudinal ligament (ALL)
posterior longitudinal ligament and the (PLL)
ligamentum flavum.
Both the ALL and PLL are continuous bands that run the length of the entire spinal
column along the front (anterior) and back (posterior) of the vertebral bodies. The
ligamentum flavum is not continuous, but instead attaches between the lamina of each
vertebra. All 3 ligaments work together to prevent excessive movement of the bones. A
4th ligament runs between each spinous process and is called the interspinous ligament.
Image 6
Another interesting fact regarding the size and shape of the ALL and the PLL concerns the evolution of man from
being on all fours to upright in stature. (Image 7) It is because of our
start on all fours, that the ALL is much broader to support the
downward forces of gravity on what would have been considered the
bottom of the spine versus the front or anterior portion of the spinal
column today. The common opinion is that the broadness of the ALL
compared to the narrower PLL, creates a weak point on either side of
the PLL and is the major reason why herniations typically occur
Image 7
posteriorly rather than anteriorly.
Spinal Cord
Averaging about 18 inches long in men and 17 inches in women, the spinal cord is about ½ inch thick. It runs
inside the spinal canal from the brainstem to the 1st of the 5 lumbar vertebra. At the lower most end of the cord,
the fibers separate into the Cauda equina (which means horses tail) and continues down to the tail bone
(coccyx). The primary function of the spinal cord is to relay information from the brain to different areas of the
body so that movement can occur. In turn, our arms, legs, and body, send signals to the brain through the spinal
cord as to what we touch and feel. Our spinal reflexes also occur at the spinal cord but they do so without
sending signals to the brain. The spinal reflexes are designed to protect our body from harm.
The spinal cord is made up of nerve cells called upper motor neurons. The lower motor neurons branch off of
the spinal cord to each side of the spine between the vertebrae and go to different parts of the body dependent
upon what level of the spine they exit from. Any damage to the cord or upper motor neurons can cause loss of
either sensory or motor function below the level of the damage or injury.
Spinal nerves
There are 31 pairs of spinal nerves that branch off of the spinal cord. A “pair”
is made up of a left which controls sensation and motor function on the left
side of the body and one on the right side which controls sensation and
motor function on the right side. Each spinal nerve has two “nerve roots”.
One which carries impulses to the cord and brain called the dorsal root, and
one which carries impulses away from the brain and cord called the ventral
root. The dorsal and ventral roots fuse together and form the spinal nerve.
(Image 8)
The spinal nerves exit through the intervertebral foramen. (Image 9) At that point, the
nerve branches out to include the anterior and posterior primary rami each of which
includes motor (muscle function) and sensory (sensation) fibers. The smaller is the
posterior primary ramus and supplies the skin and muscles of the back of the body.
The larger anterior primary ramus supplies the skin and muscles of the front of the
body as well as forms most of the major nerves of the body.
Image 10
Image 8
Image 9
Because each spinal nerve “innervates” a specific part of
the body, a system of identification is necessary to keep
track of the nerves and where they go. (Image 10) Each
spinal nerve is labeled according to the vertebrae either
above or below where they emerge dependent upon
which section they are in. In the case of the cervical spine,
they are named for the vertebra below, with exception to
C8 which emerges below C7 and above T1. So even
though there are only 7 cervical vertebrae, there are 8
pairs of cervical spinal nerves. For the thoracic and
lumbar spine they are labeled according to the vertebrae
above. There are therefore 12 pairs of thoracic spinal
nerves, 5 pairs of lumbar spinal nerves and 5 pairs of
sacral nerves. The final, or 31st spinal nerve is the
coccygeal nerve which only supplies the skin over the
back of the coccyx (tail bone).
The area of the skin supplied by a single spinal nerve is called a
dermatome, (Images 11 & 12) whereas the group of muscles that are
controlled by a single spinal nerve is called a myotome. (Chart 1) By using
the chart of the dermatomes and myotomes, one can often times identify
the location of the spinal problem based upon the area of pain or muscle
weakness. That is why it is so important to pay close attention to where
your symptoms are and convey that information to the medical
practitioner. Where you feel the pain is not necessarily the source of the
pain. For instance, pain in your lower leg may not indicate an injury to
your leg, but an injury to the lower part of your spine (L4-S2).
Myotomes of upper and lower
extremities
C1/C2: neck flexion/extention
C3: neck lateral flexion
C4: shoulder elevation
C5: shoulder abduction
C6: elbow flexion/wrist extension
C7: elbow extension/wrist flexion
C8: finger flexion
T1: finger abduction
L2: hip flexion
L3: knee extension
L4: ankle dorsi-flexion
L5: great toe extension
S1: ankle plantar-flexion/ankle
eversion/hip extension
S2: knee flexion
S3-S4: anal wink
Chart 1
Image 11
Image 12
Coverings and spaces
The meninges are membranes that cover the spinal cord (as well as the brain). (Image 13) There are 3 distinct
layers. The inner, the mid, and the outer. The inner membrane, called the pia mater, is actually attached to the
cord. Between the pia mater the arachnoid mater (mid membrane) is the subarachnoid space which surrounds
the spinal cord and contains the cerebrospinal fluid. The tough outer membrane is called the dura mater. The
space between the dura mater and the bone is the epidural space. It is the epidural space that is most often
accessed to deliver an epidural numbing agent (anesthetic) or steroid injection (to help fight the disease process
such as swelling and pain), whereas the subarachnoid space is most often accessed to perform a lumbar
puncture or myelogram.
Image 13