OVERVIEW
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. This is why keeping your spine healthy is vital if you want to live an active life.
BASIC STRUCTURE
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 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.
VERTEBRAE
Spine anatomy is divided into 4 major sections, typically defined by the number of vertebrae (the
round bones that make up the structure of one’s back bone) in each section. Vertebrae are also
sometimes called vertebral bodies.
Cervical spine (neck)
comprised of 7 cervical vertebrae (termed C1 to C7),
starting with C1 at the top of the spine and ending with C7 at the bottom of the
cervical portion of the spine. Neck problems can cause neck pain and/or pain that
radiates down the arms to the hands and fingers.
Thoracic spine (upper back)
made up of 12 thoracic vertebrae (known as T1
to T12), which are attached to the rib bones and sternum (breast bone). Because
this part of the spine is firmly attached to the ribs and sternum, it is very stable and
has fewer problems associated with motion.
Lumbar spine (lower back)
typically including 5 vertebrae (known as L1 to L5),
which have a great deal of motion and flexibility. Because this section of the spine
bears most of the body’s weight and allows for the most motion (which stresses
the anatomical structures), this is the area associated with most back problems.
Problems in the low back can cause pain that radiates down the legs to the feet.
Sacral region (bottom of the spine)
located below the lumbar spine,
the sacrum is a series of 5 bony segments fused together (known as S1 to S5)
that create a triangular-shaped bone that serves as the base of the spine and
makes up part of the pelvis.The segment where the lumbar spine meets the
sacral region, L5-S1, is an area that is prone to degenerate and create back
problems. Four small bones that extend down from the sacrum make up
the coccyx (the tailbone at the very bottom of the spine).
INTERVERTEBRAL DISC
The intervertebral discs make up one fourth of the spinal column’s length. Discs are
not vascular and therefore depend on the end plates to diffuse needed nutrients. The
cartilaginous layers of the end plates anchor the discs in place..The intervertebral
discs are cushions serving as the spine’s shock absorbing system, which protect the
vertebrae, brain, and other structures (i.e. nerves). The discs allow some vertebral
motion: extension and flexion. Individual disc movement is very limited – however
considerable motion is possible when several discs combine forces.
Annulus Fibrosus and Nucleus Pulposus
Intervertebral discs are composed of an annulus fibrosus and a nucleus pulposus.
The annulus fibrosus is a strong radial tire–like structure made up of lamellae;
concentric sheets of collagen fibers connected to the vertebral end plates. The
sheets are orientated at various angles. The annulus fibrosus encloses the nucleus
pulposus.
The nucleus pulposus contains a hydrated gel–like matter that resists compression.
The amount of water in the nucleus varies throughout the day depending on activity.
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.
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 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.
Spinal curves
Curves are a normal part of the spine’s structure. Looking at the spine from the side
(lateral), several curves can be seen. From this angle, the spine almost resembles a
soft ‘S’ shape. When viewing the spinal column from the rear (posterior), the spine
appears vertically straight — from the neck to the tailbone. The normal curves are
termed lordosis and kyphosis. However, the natural curves should not be mistaken
for spinal disorders also called Lordosis and Kyphosis. We will talk about abnormal
curves later in this article.
Normal lordosis is the two forward curves seen in the neck (cervical spine) and low
back (lumbar spine). Normal kyphosis is the two backward curves seen in the chest
(thoracic spine) and hip areas (sacral spine). Each of the naturally occurring and
normal soft curves serves to distribute mechanical stress incurred as the body is at
rest and during movement.
Abnormal Curves
As mentioned earlier, Lordosis and Kyphosis are terms that are used to refer to
specific spinal disorders. Scoliosis is another type of disorder identified by abnormal
curvature of the spine. Each of these disorders is defined below.
- Lordosis is defined by an excessive inward curve of the spine. Although it primary affects the lumbar spine, it does occur in the neck too. Patients with excessive lumbar lordosis may appear swayback, the buttocks more prominent, and in general their posture appears exaggerated. Lordosis affects people of all ages and when it affects the low back, it can cause back pain making movement difficult and painful. Spondylolisthesis, osteoporosis, and even obesity may lead to abnormal lordosis.
- Kyphosis is defined by an excessive outward curve of the spine and may cause a deformity such as a humpback or hunchback. Abnormal kyphosis is more commonly found in the thoracic or thoracolumbar (chest area/low back), but can affect the neck too. Patients with excessive kyphosis may appear with a pitched-forward appearance. There are two types of abnormal kyphosis; that caused by poor posture and structural kyphosis.
- Scoliosis is the abnormal curving of the spine to the left or right side. Scoliosis most often affects the thoracic spine and children, although it is found in adults too.