Anatomy and Function
A Patient's Guide to Anatomy and Function of the Spine
Introduction
The spine is one of the most important parts of your body. Without it, you
could not keep yourself upright or even stand up. It gives your body structure
and support. It allows you to move about freely and to bend with flexibility.
The spine is also designed to protect your spinal cord. The spinal cord is a
column of nerves that connects your brain with the rest of your body, allowing
you to control your movements. Without a spinal cord, you could not move any
part of your body, and your organs could not function. This is why keeping your
spine healthy is vital if you want to live an active life.
Anatomy
What exactly is the spine? Your spine is made up of 24 small bones (vertebrae)
that are stacked on top of each other to create the spinal column. Between each
vertebra is a soft, gel-like cushion called a disc that helps absorb pressure
and keeps the bones from rubbing against each other. Each vertebra is held to
the others by groups of ligaments. Ligaments connect bones to bones; tendons
connect muscles to bones. There are also tendons that fasten muscles to the
vertebrae. The spinal column also has real joints (just like the knee or elbow
or any other joints) called facet joints. The facet joints link the vertebrae
together and give them the flexibility to move against each other.
Each vertebra has a hole in the center, so when they stack on top of each other
they form a hollow tube that holds and protects the entire spinal cord and its
nerve roots. The spinal cord itself is a large collection of nerve tissue that
carries messages from your brain to the rest of your body. In order for your
body to function, you need your nerves. The spine branches off into thirty-one
pairs of nerve roots. These roots exit the spine on both sides through spaces
(neural foramina) between each vertebra.
The spine itself has three main segments: the cervical
spine, the thoracic
spine, and the lumbar
spine. The cervical is the upper part of the spine, made up of seven vertebrae
(bones). The thoracic is the center portion of the spine, consisting of 12 vertebrae.
The lower portion of the spine is called the lumbar spine. It is usually made
up of five vertebrae, however, some people may have six lumbar vertebrae. Having
six vertebrae does not seem to cause a problem. Below the lumbar spine is the
sacrum. The sacrum
is actually a group of specialized vertebrae that connects the spine to the
pelvis. During development (those nine months before birth), these vertebrae
grow together or fuse creating one large "specialized" vertebral bone that forms
the base of your spine and center of your pelvis. The nerves that leave the
spine in the sacral region control the bowel and bladder functions and give
sensation (feeling) to the crotch area.
The normal spine has an "S"-like curve when looking at it from the side. This
allows for an even distribution of weight. The "S" curve helps a healthy spine
withstand all kinds of stress. The cervical spine curves slightly inward, the
thoracic curves outward, and the lumbar curves inward. Even though the lower
portion of your spine holds most of the body's weight, each segment relies upon
the strength of the others to function properly.
Now let us look at the specific parts that make up your spine.
Vertebrae
The individual bones of the spine are the vertebrae. These are the building
blocks of the spinal column. The vertebrae protect and support the spinal cord.
They also bear the majority of the weight put upon your spine. The body of each
vertebra is the large, round portion of bone. The body of each vertebra is attached
to a bony ring. When the vertebrae are stacked
one on top of the other, this ring creates a hollow tube where the spinal
cord passes through.
The bony ring attached to the vertebral body consists of several parts. First,
the laminae extend from the body to cover the spinal canal, which is the hole
in the center of the vertebrae. Second, the spinous process is the bony portion
opposite the body of the vertebra. You feel this part if you run your hand down
a person's back. Then there are two transverse processes (little bony bumps),
where the back muscles attach to the vertebrae. Finally, the pedicle is a bony
projection that connects to both sides of the lamina.
The vertebra, like all bones, has an outer shell called cortical bone that
is hard and strong. The inside is made of a soft, spongy type of bone that is
called cancellous bone.
Intervertebral Disc
The intervertebral
discs are flat, round "cushions" that act as shock absorbers between each
vertebra in your spine. There is one disc between each vertebra. Each disc has
a strong outer ring of fibers called the annulus, and a soft, jelly-like center
called the nucleus pulposus.
The annulus is the disc's outer layer and the strongest area of the disc. It
also helps keep the disc's center intact. The annulus is actually a strong ligament
that connects each vertebra together.
The mushy nucleus of the disc serves as the main shock absorber. The nucleus
is made up of tissue that is very moist because it has high water content. The
water content helps the disc act like a shock
absorber - somewhat like a waterbed mattress.
Facet Joint
The facets are the "bony knobs" that meet between each vertebra to form the
facet joints that join your vertebrae together. There are two facet joints between
each pair of vertebra, one on each side. They extend and overlap each other
to form a joint between the neighboring vertebra facet joints. Without the facet
joints, you would not have flexibility in your spine, and you could only move
in very straight and stiff motions.
The facet
joints are what are known as synovial joints. A synovial joint, such as
the knee or elbow, is a structure that allows movement between two bones. In
a synovial joint, the ends of the bones are covered with a material called articular
cartilage. This material is a slick spongy material that allows the bones to
glide against one another without much friction.
Surrounding the facet joint is a watertight sack made of soft tissue and ligaments.
This sack creates what is called the "joint capsule". The ligaments are soft
tissue structures that hold the two sides of the facet joint together. The ligaments
around the facet joint combine with the synovium to form the joint capsule that
is filled with fluid (synovial fluid). This fluid lubricates the joint to decrease
the friction, just like oil lubricates the moving parts of a machine.
Neural Foramen
The neural foramen is the opening between every two vertebrae where the nerve
roots exit the spine. The nerve roots travel through the foramen to reach the
rest of your body. There are two neural foramina between each pair of vertebrae
- one on each side. Without the foramen, nerve signals could not travel to and
from the brain to the rest of your body. Without nerve signals, your body would
not be able to function.
Spinal Cord and Nerve Roots
The spinal cord is a column of millions of nerve fibers that run through your
spinal canal. It extends from the brain to the area between the end of your
first lumbar vertebra and top of your second lumbar vertebra. At the second
lumbar vertebra, the spinal cord divides into several different groups of fibers
that form the nerves that will go to the lower half of the body. For a small
distance, the nerves actually travel through the spinal canal before exiting
out the neural foramen. This collection of nerves is called the cauda equina
while it is still inside the spinal canal.
A protective membrane called the dura
mater covers the spinal cord. The dura mater forms a watertight sack around
the spinal cord and the spinal nerves. Inside this sack, the spinal cord is
surrounded by spinal fluid.
The nerve fibers in your spinal cord branch off to form pairs of nerve roots
that travel through the small openings (foramina) between your vertebrae. The
nerves in each area of the spinal cord connect to specific parts of your body.
This is why damage to the spinal cord can cause paralysis in certain areas and
not others -- it depends on which spinal nerves are affected. The nerves of
the cervical spine go to the upper
chest and arms. The nerves in your thoracic spine go to your chest
and abdomen. The nerves of the lumbar spine then reach to your
legs, bowel, and bladder. These nerves coordinate and control all the body's
organs and parts, and let you control your muscles.
The nerves also carry electrical signals back to the brain that allow you to
feel sensations. If your body is being hurt in some way, your nerves signal
the brain that you have been hurt. Damage to the nerves themselves can cause
pain, tingling, or numbness in the area where the nerve travels.
Paraspinal Muscles
The paraspinal
muscles refer to the muscles next to the spine. They support the spine and
are the motor for movement of the spine. Your joints allow flexibility and your
muscles allow mobility. There are many small muscles in the back - each controlling
some part of the total movement
between all the vertebrae and the rest of the skeleton. These muscles can
be injured directly, such as when you have a pulled muscle or muscle strain
of the back muscles. The muscles can also cause problems indirectly, such as
when the muscles are in spasm after injury to other parts of the spine.
When you experience a muscle spasm, it is because your muscle tightens up and
will not relax. These spasms usually occur as a reflex - meaning that you cannot
control the contraction of these muscles. When any part of the spine is injured
including: a disc, ligaments, bones, or muscles, the muscles automatically go
into spasm to reduce the motion around the area. This protective mechanism is
designed to protect the injured area.
When muscles are in spasm they produce too much of the chemical, lactic acid.
Lactic acid is a waste product produced by the chemical reaction inside muscle
cells that must occur to allow the muscle to contract. If the muscle cell cannot
relax, too much lactic acid builds up inside the muscles. The buildup of lactic
acid causes a painful burning sensation.
The main reason that lactic acid builds up inside the muscle cells is that
when the muscles contract, the small blood vessels traveling through the muscles
are pinched off (just like a tube pinched between your thumb and finger). When
the muscle relaxes, the lactic acid is eventually washed away by fresh blood
flowing into the muscle as the blood vessels open up.
Spinal Segment
Back specialists sometimes look at a spinal segment to understand and explain
how the whole spine works. A spinal segment is made up of two vertebrae attached
together by ligaments, with a soft disc separating them. The facet joints fit
between the two vertebrae, allowing for movement, and the neural foramen between
the vertebrae allow space for the nerve roots to travel freely from the spinal
cord to the body.
The spinal segment allows us to focus on the repeating parts of the spinal
column to better understand what can go wrong with the various parts of the
spine. Sometimes problems in the spine involve only one spinal segment, while
other times the problems involve multiple segments.
Each spinal segment is like a well-tuned part of a machine. All of the parts
should work together to allow weight bearing, movement, and support. When all
the parts are functioning properly, all spinal segments join to make up a remarkably
strong structure called the spinal column. When one segment deteriorates to
the point of instability, it can lead to problems at that segment causing pain
and other difficulties.
Now that you know the parts of the spine, let us look at the spine itself, which
has three main segments - the lumbar, thoracic, and cervical spines.
Lumbar Spine
The lowest part of the spine is called the lumbar
spine. This area has five vertebrae. However, sometimes people are born
with a sixth vertebra in the lumbar region. The base of your spine (sacrum)
is a fusion of many bones, and when one of them forms as a vertebra rather than
part of the sacrum, it is called a transitional (or sixth) vertebra. This occurrence
is not dangerous and does not appear to have any serious side effects.
The lumbar spine's shape has what is called a lordotic curve. The lordotic
shape is like a backwards "C". If you think of the spine as having an "S"-like
shape, the lumbar region would be the bottom of the "S". The vertebrae in the
lumbar spine area are the largest of the entire spine, so the lumbar spinal
canal is larger than in the cervical or thoracic parts of the spine. Because
of its size, the lumbar spine has more space for the nerves to move about.
Low back pain is a very common complaint for a simple reason. Since the lumbar
spine is connected to your pelvis, this is where most of your weight bearing
and body movement takes place. Typically, this is where people tend to place
too much pressure, such as: lifting up a heavy box, twisting to move a heavy
load, or carrying a heavy object. Such repetitive injuries can lead to damage
to the parts of the lumbar spine.
Thoracic Spine
The thoracic
spine is made up of the middle 12 vertebra of the spine. These vertebrae
connect to your ribs and form part of the back wall of the thorax (the ribcage
area between the neck and the diaphragm). This part of the spine has very narrow,
thin intervertebral discs, so there is much less movement allowed between vertebrae
than in the lumbar or cervical parts of the spine. It also has less space in
the spinal canal for the nerves. The thoracic spine's curve is called kyphotic
because of its shape, which is a regular "C"-shaped curve with the opening of
the "C" in the front.
Cervical Spine
The cervical
spine is made up of the first seven vertebrae in the spine. It starts just
below the skull and ends just above the thoracic spine. The cervical spine has
a lordotic curve (a backward "C"-shape) - just like the lumbar spine. The cervical
spine is much more mobile than both of the other spinal regions - think about
all the directions and angles you can turn your neck.
Unlike the rest of the spine, there are special openings in each vertebra in
the cervical spine for the arteries (blood vessels that carry blood away from
the heart), as well as the spinal canal that carries the spinal cord. The arteries
that run through these openings bring blood to the brain.
Two vertebrae in the cervical spine, the atlas and the axis, differ from the
other vertebrae because they are designed specifically for rotation. These two
vertebrae are what allow your neck to rotate in so many directions, including
looking to the side.
The atlas is the first cervical vertebra - the one that sits between the skull
and the rest of spine. The atlas does not have a vertebral body, but does have
a thick forward (anterior) arch and a thin back (posterior) arch, with two prominent
sideways masses.
The atlas sits on top of the second cervical vertebra - the axis. The axis
has a bony knob called the odontoid process that sticks up through the hole
in the atlas. It is this special arrangement that allows the head to turn from
side to side as far as it can. Special ligaments between these two vertebrae
allow a great deal of rotation to occur between the two bones.
Though the cervical spine is very flexible, it is also very much at risk for
injury from strong, sudden movements, such as whiplash-type injuries. This high
risk of harm is due to: the limited muscle support that exists in the cervical
area, and because this part of the spine has to support the weight of the head.
This is a lot of weight for a small, thin set of bones and soft tissues to bear.
Therefore, sudden, strong head movement can cause damage.
Copyright © 2003 DePuy Acromed.
This page was last updated: June 17, 2013