The spine is a very complex
mechanical structure that is highly flexible yet
very strong and stable. In the normal spine, regardless
of your position or activity, including sleeping,
there is always some type of physical demand being
placed on it.
The primary functions
of the spine include:
- Protect the spinal
cord, nerve roots, and internal organs
- Provide flexibility
- Provide structural
support and balance for upright posture.
- The spine bears the
load of the head, shoulders and arms, and upper
body. The upper body weight is then distributed
to the hips and legs. The spine attempts to
keep the body's weight balanced evenly over
the pelvis. This reduces the amount of work
required by the spinal muscles and can eliminate
muscle fatigue and back pain.
The normal adult spine
is balanced over the pelvis, requiring minimal
workload on the muscles to maintain and upright
Loss of spinal balance
can result in strain to the spinal muscles and
deformity of the spine as it attempts to maintain
an upright posture.
are 33 vertebrae (bones) in the spine. Anatomically,
the spine is divided into four regions:
- The top 7 vertebrae
that form the neck are called the cervical spine
and are labeled C1-C7.
- The upper back, or
thoracic spine, has 12 vertebrae, labeled T1-T12.
- The lower back, or
lumbar spine has 5 vertebrae, labeled L1-L5.
- The sacrum and coccyx
(tailbone) are made up of 9 vertebrae that are
fused together to form a solid bone. The sacrum
is labeled S1.
Curves of the Spine
When viewed from
the front or back, the normal spine is in a straight
line, with each vertebra sitting directly on top
of the other. A side-to-side curve in the spine
is called a scoliosis.
When viewed from the
side, the normal spine has three gradual curves:
- The neck has a lordosis;
it curves towards the front.
- The thoracic spine
has a kyphosis; it curves towards the
- The lumbar spine also
has a lordosis.
These curves help the
spine to support the load of the head and upper
body, and maintain balance in the upright position.
Although the vertebrae
have slightly different appearances as they range
from the cervical spine to the lumbar spine, they
all have the same basic structures, and the structures
have the same names. Only the first and second
cervical vertebrae are structurally different
in order to support the skull.
Each vertebra has an
anterior arch and a posterior arch, which form
a hole, called a foramen. The spinal cord passes
through foramen of each vertebra.
The anterior arch
is called the vertebral body. Discs connect one
vertebral body to another to allow motion of the
spine and cushion it against heavy loads. Together,
the vertebral bodies and discs bear about 80 percent
of the load to the spine.
The posterior arch consists
of the pedicles, laminae, and processes.
The pedicles are
two short cylinders of bone that extend from the
vertebral body. Nerve roots branch off the spinal
cord and exit to the body between the pedicles
of two vertebrae. If the spine becomes unstable,
the pedicles may compress the nerve root, cause
pain or numbness.
Laminae are two
flattened plates of bone that form the walls of
the posterior arch. Over time, the laminae may
thicken, a process called stenosis. This
thickening compresses the spinal cord and/or nerves
causing pain or numbness.
transverse, and spinous processes
project off the laminae. Ligaments and tendons
attach to the processes. The articular processes
join one vertebra to another posteriorly.
The transverse processes
extend out on either side of the laminae. The
spinous process is the bony projection
that can be felt through the back of someone's
are located between each vertebra from C2-C3 to
L5-S1. Combined, they make up one fourth the height
of the spinal column. The discs act as shock absorbers
to the loads placed on the spine and allow movement
of the spine. Movement at a single disc level
is limited, but all of the vertebrae and discs
combined allow for a significant range of motion.
The intervertebral disc
is made up of two components: the annulus fibrosus
and the nucleus pulposus. The annulus fibrosus
is the outer portion of the disc. It is composed
of layers of collagen and proteins, called lamellae.
The fibers of the lamellae slant at 30-degree
angles, and the fibers of each lamella run in
a direction opposite the adjacent layers. This
creates a structure that is exceptionally strong,
yet extremely flexible.
The nucleus pulposus
is the inner gel material surrounded by the annulus
fibrosus. It makes up about 40 percent of the
disc. This ball-like gel is contained within the
lamellae. The nucleus is composed primarily of
loose collagen fibers, water, and proteins. The
water content of the nucleus is about 90 percent
at birth and decreases to about 70 percent by
the fifth decade.
Injury or aging of the
annulus fibrosus may allow the nucleus pulposus
to be squeezed through the annulus fibers either
partially, causing the disc to bulge, or completely,
allowing the disc material to escape the disc.
The bulging disc or nucleus material may compress
the nerves or spinal cord, causing pain.
In the early years of
life, the discs have a blood supply that nourishes
them. In the second and third decades, discs gradually
lose this blood supply, until they are avascular.
At this point, the disc begins to degenerate,
or age. By the age of 50, over 95 percent of all
people will have disc degeneration. The disc begins
to lose water content and shrinks. The spine's
range of motion and shock-absorbing ability are
decreased. This may result in injury to the nerves
and vertebrae, and the aging disc itself may generate
brain and spinal cord together make up the central
nervous system. The spinal cord is located
immediately below the brain stem. It extends through
the foramen magnum, a hole at the base
of the skull.
The spinal cord functions
as a sophisticated network that carries information
from the outer elements of the body (skin, muscles,
ligaments, joints) through the sensory tracts,
to the central "computer," the brain. Data are
processed there, and new information such as muscle
control is sent out through the motor tracts of
the spinal cord.
The spinal cord ends
as the conus medullaris at the L1 vertebral
level, where it branches into the cauda equina,
a collection of nerves that extend from the conus
medularis to the sacrum. The conus medularis nerves
float freely in spinal fluid, making it possible
to pass a needle safely into the area to draw
a sample of spinal fluid or inject drugs, anesthetics,
or radiologic substances for x-ray, MRI or CT
As your spine ages, the
gelatin-like centers of your discs begin to dry
out, thereby reducing their effectiveness as "shock
absorbers." As this protection is lost, the simple
"wear and tear" of everyday activity can cause
the bone matter of your vertebrae to develop jagged
edges, called bone spurs. As these spurs develop
and extend outward, they can cause both the spinal
canal and the foramen to become narrowed. The
result is often the pinching (compression) of
the spinal cord and/or a spinal nerve root.
As discs dry out, your
vertebrae begin to "settle." This "settling" causes
the window-like openings of the foramen and the
spinal canal to become smaller and smaller. Eventually,
these openings can become so small that a spinal
nerve(s) becomes "pinched" against a vertebra.
It's similar to slowly closing a window on your
hand. There will be a point at which you begin
to feel the pressure. The more the window is closed,
the greater the pressure and the greater the pain.