Anatomy of the Spine

Introduction

Your spine is one of the most important parts of your body. It gives your body structure and support. Without it you could not stand up or keep yourself upright. Your spine allows you to move about freely and to bend with flexibility. Your spine also protects your spinal cord. The spinal cord is a column of nerves that connects your brain to 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.

Keeping your spine healthy is vital if you want to live an active life.

The Spinal Column

The spine is made up of 24 bones, called vertebrae. Ligaments and muscles connect these bones together and form what is called the spinal column. The spinal column has three main segments. From top to bottom they are the cervical spine, the thoracic spine, and the lumbar spine. Each of these segments is discussed in detail below.

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 spine curves slightly outward, and the lumbar spine curves slightly 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.

Cervical Spine (Neck)

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 backward "C"-shape-just like the lumbar spine. This type of curve is called a lordotic curve. The cervical spine is much more mobile than both of the other segments of the spine. Just think about all the directions and angles you can turn your neck and head!

Unlike the rest of the spine, there are special openings in each vertebra in the cervical spine for arteries (blood vessels that carry blood away from the heart). The arteries that run through these openings bring blood to the brain.

The two upper vertebrae of the cervical spine are called the atlas and the axis. They differ from the other vertebrae because they are designed specifically for rotation. These two vertebrae are the reason your neck and head can move in so many directions.

The atlas is the first cervical vertebra--the one that sits between the skull and the rest of the spine. The atlas does not have a vertebral body, but it 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, which 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 attached to the atlas and the axis allow for a great deal of rotation.

The cervical spine is very flexible, but 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 the fact that this part of the spine has to support the weight of the head--an average of about 15 pounds. This is a lot of weight for a small, thin set of bones and soft tissues to bear. Sudden, strong head movements can cause damage to the bones, ligaments, or even the arteries that carry blood to the brain.

The Thoracic Spine (Mid Back)

The thoracic spine is made up of 12 vertebrae. It is located below the top 7 vertebrae of the cervical spine and above the lumbar 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). The curve of the thoracic spine is a "C"-shape with the opening of the "C" in the front. This type of curve is called a kyphotic curve.

The thoracic spine has very narrow, thin intervertebral discs. In addition, the ribs are connected to the spine in the thoracic segment. These rib connections, combined with the smaller intervertebral discs, limit the amount of movement in the thoracic spine compared to the lumbar or cervical segments of the spine. There is also less space inside the spinal canal.

The Lumbar Spine (Low Back)

The lowest part of the spine is called the lumbar spine. This area usually has 5 vertebrae. However, sometimes people are born with 6 vertebrae in the lumbar region. The base of your spine (called the sacrum) is a group of specialized vertebrae that connects the spine to the pelvis. When one of the bones 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.  Occasionally, the lowest lumbar (5^th) vertebra can also be (partially) fused to the sacrum. This can lead to abnormal force transmission and degeneration of the above disc at L4/5.

The lumbar spine is shaped like a backward "C", just like the cervical spine. This shape is called a lordotic curve. If you think of the entire spine as having an "S"-like shape, the lumbar region would be the bottom of the "S". The vertebrae in the lumbar spine are the largest of the entire spine. The lumbar spinal canal is also larger than in the cervical or thoracic segments of the spine. The size of the lumbar spine allows for more space for nerves to move about.  The thickest disc is typically at L4/5.

When people experience pain in their spine it often occurs in the lumbar region. This is because the lumbar spine is connected to your pelvis, which is where most weight bearing and body movement takes place. Typically, this is where people tend to place too much pressure, such as when lifting up a heavy box, twisting to move a heavy load, or carrying a heavy object. These activities can cause repetitive injuries that can lead to damage to the parts of the lumbar spine.

The Sacroiliac Region (Joint)

There are two SI joints in your pelvis that connect the sacrum (tailbone) and the ilium (large pelvic bone). The SI joints connect your spine to the pelvis, and thus, to the entire lower half of your skeleton.

Like any other joints, there is articular cartilage on both sides of the SI joint surfaces. But unlike most other joints, the SI joints are covered by two different kinds of cartilage. The articular surfaces have a hyaline (glassy, slick) cartilage cover, but in the more posterior (extra-articular part, there is fibrocartilage (spongy) surfaces that rub against each other, and this is where multiple strong ligaments attach. The articular joints also have many large ridges (bumps) and depressions (dips in the surface that fit together like a puzzle).

The Si joints are also unique in that they are not designed for much motion. It is common for the SI joint to become stiff and actually "lock" as people age. The SI joint has only a small amount of motion occurring in the joint, and is described as a "gliding" type of motion. Due to the small amount of movement and the complexity, finding out about the SI joints' motion is very difficult during a physical exam.

The SI joints are viscoelastic joints, meaning that the major movement comes from giving or stretching. This motion is quite different than the hinge motion of your knee or the ball and socket motion of your hip. The main function of the SI joints is to provide shock absorption for your spine through stretching in various directions. The SI joints may also provide a "self-locking" mechanism that helps you to walk. The joints lock on one side as weight is transferred from one leg to the other. The majority of the load forces are transmitted through the very strong ligament complex around the joint.

Important Structures of the Spine

There are a number of important structures of the spine. Knowing about these structures will help you understand how the spine works and what's happening when a problem develops in the spine. These structures include vertebrae, intervertebral discs, facet joints, neural foraminae, the spinal cord, nerve roots, paraspinal muscles, and spinal segments.

Vertebrae

Your spine is made up of 24 small bones, called vertebrae. The vertebrae protect and support the spinal cord. They also bear the majority of the weight put upon your spine. Vertebrae, like all bones, have an outer shell, called cortical bone, which is hard and strong. The inside is made of a soft, spongy type of bone, called cancellous bone.

The vertebral body is the large, round portion of bone. Each vertebra is attached to a bony ring. When the vertebrae are stacked one on top of the other, the rings create a hollow tube for the spinal cord to pass through. 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 bony ring attached to the vertebral body consists of several parts. The laminae extend from the body to cover the spinal canal, which is the hole in the centre of the vertebra. 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. There are two transverse processes (little bony bumps), where the back muscles attach to the vertebrae. The pedicle is a bony projection that connects the laminae to the vertebral body.

Intervertebral Disc

Between each vertebra is a soft, gel-like cushion, called an intervertebral disc. These flat, round "cushions" act like shock absorbers by helping absorb pressure put on the spine. The discs prevent the bones from rubbing against each other.

Each disc has a strong outer ring of fibres called the annulus fibrosus, and a soft, jelly-like centre called the nucleus pulposus. The annulus is the strongest area of the disc. It helps keep the disc's centre 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 is what helps the disc act like a shock absorber-somewhat like a waterbed mattress.  Disc dehydration and degeneration begins as early as 21 years of age.

Facet Joints

Although you may not think so, the spinal column has real joints just like the knee or elbow. In the spine, these joints are called facet joints. The facet joints link the vertebrae together and give them the flexibility to move against each other. The facets are the "bony knobs" that meet between each vertebra. There are two facet joints between each pair of vertebrae, one on each side. They extend and overlap each other to form a joint between the neighbouring vertebra facet joint. The facet joints give the spine its flexibility.

The facet joints are synovial joints, which means they allow movement between two bones. The ends of the bones that make up a synovial joint are covered with articular (hyaline) cartilage, a slick spongy material that allows the bones to glide against one another without much friction. Synovial fluid inside the joint keeps the joint surfaces lubricated, like oil lubricates the parts of a machine. This fluid is contained inside the joint by the joint capsule, a watertight sac of soft tissue and ligaments that fully surrounds and encloses the joint

Neural Foraminae

The spinal cord branches off into 31 pairs of nerve roots, which exit the spine through small openings on each side of the vertebra called neural foraminae. The two nerve roots in each pair go in opposite directions when traveling through the foraminae. One goes out through the left foramina; the other goes out through the right foramina. The nerve root allows nerve signals to travel to and from your brain to the rest of your body.

Spinal Cord

The spinal cord is a column of millions of nerve fibres that carries messages from your brain to the rest of your body. It extends from the brain to the area between the end of the first and second lumbar vertebrae. The spinal cord lies within the hollow canal called the spinal canal that is formed when the spinal vertebrae stack on top of each other. The spinal canal holds and protects the entire spinal cord and its nerve roots.

The spinal cord only goes down to the second lumbar vertebra. Below this level, the spinal canal contains a group of nerve fibres, called the cauda equina (derived from Latin meaning “horse’s tail”). This group of nerves goes to the pelvis and lower limbs.

A protective membrane, called the dura mater covers the spinal cord. The dura mater forms a watertight sac around the spinal cord and the spinal nerves. Inside this sac, the spinal cord is surrounded by spinal fluid, which also protects the spinal cord.

Nerve Roots

The nerve fibres in your spinal cord branch off to form pairs of nerve roots that travel through the small openings 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 of the thoracic spine go to the chest and abdomen. The nerves of the lumbar spine go to the legs, pelvis, bowel, and bladder. These nerves coordinate and control all your body's organs and parts, and allow you to control your muscles.

The nerve roots 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. Damage to the nerves themselves can cause pain, tingling, or numbness in the area where the nerve travels. Without nerve signals, your body would not be able to function.

Paraspinal Muscles

The muscles next to the spine are called the paraspinal muscles. They support the spine and act as motors to move the spine. Joints allow flexibility, and muscles allow mobility. There are many small muscles in your back. Each controls some part of the total movement between the vertebrae and the rest of the skeleton. These muscles can be directly injured, such as when you have a pulled muscle or muscle strain. They can also cause problems indirectly, such as when they are in spasm after injury to other parts of the spine.

A muscle spasm is experienced when your muscle tightens up and will not relax. Spasms usually occur as a reflex (meaning that you cannot control the contraction). When any part of the spine is injured-including a disc, ligament, bone, or muscle-the muscles automatically go into spasm to reduce the motion around the area. This mechanism is designed to protect the injured area.

Spinal Segments

Doctors 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 foraminae between the vertebrae allow space for the nerve roots to travel freely from the spinal cord to the body. The spinal segment allows doctors to examine the repeating parts of the spinal column to understand what can go wrong with the various parts of the spine. 

It also allows interpretation of referred pain, as in sciatica, to help confirm the most likely level of the spinal problem or cause.