ANATOMY OF THE SPINE

The spine consists of up to 24 bones, called vertebrae. Ligaments and muscles connect these bones together to form the spinal column. Also, the spinal column gives the body form and function and protects the spinal cord.

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Anatomy of the Spine

Anatomy of the Spine, illustrationYour spine constitutes one of the most important parts of your body because it gives structure and support.  Furthermore, without it, you could not stand-up, freely move about, and maintain a flexible body.  Also, the spine protects your spinal cord consisting of nerves connecting the brain to the body.  Thus, 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 provides the ability to live an active life.  Learn about the anatomy of the spine including: 

  • How the spine functions
  • The differentiates the cervical, thoracic and lumbar spinal segments
  • The important structures that make up the spine

The Anatomy of the Spine

The anatomy of the spine consists of 24 bones, called vertebrae. Ligaments and muscles connect these bones together to form the spinal column. The spinal column gives the body form and function. Also, the spinal column holds and protects the spinal cord, which consists of a bundle of nerves that sends signals to other parts of the body. The many muscles that connect to the spine help support the upright posture of the spine and move the spine.

To further clarify, the spinal column has three main sections — the cervical spine, the thoracic spine, and the lumbar spine. The first seven vertebrae form the cervical spine. The mid-back, called the thoracic spine, consists of 12 vertebrae. Also, the lower portion of the spine, called the lumbar spine, consists of five vertebrae. However, some people have a sixth lumbar 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 slightly outward, and the lumbar slightly inward. Therefore, 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.

Spine Anatomy

The spine serves as a highly complex mechanical structure that is both flexible and strong. There is always some type of physical demand placed on the normal spine, regardless of your position or activity, including sleeping. For example, the spine’s primary functions include:

  • Providing protection for the spinal cord, nerve roots, and internal organs.
  • Allowing for greater mobility.
  • Structural support and balance for upright posture. The spine supports the weight of the head, shoulders, arms, and upper body. The weight of the upper body is then distributed to the hips and legs. The spine attempts to distribute the body’s weight evenly over the pelvis This reduces the amount of work that the spinal muscles have to do and can eliminate muscle fatigue and back pain.

The normal adult spine appears over the pelvis, requiring little muscular effort to maintain an upright posture.  Loss of spinal balance can cause strain on the spinal muscles and spine deformity as it attempts to maintain an upright posture.

A Healthy Spine

The healthy spine appears straight when viewed from the front (coronal plane). Scoliosis appears as a sideways curve in the spine. The mature spine has four distinct curves when viewed from the side (sagittal plane). These curves are classified as either kyphotic or lordotic. A convex curve in the spine is referred to as a kyphotic curve (i.e., convexity toward the back of the spine). The thoracic and sacral spine curves are kyphotic. A concave (i.e., concavity towards the back of the spine) lordotic curve is found in the cervical and lumbar levels of the spine.

Regions of the Spine

The spine consists of up to 33 vertebrae, or bones. These bones are classified into four groups. As such, the cervical, thoracic, lumbar, sacrum, and coccyx vertebrae are located from the neck down the spine.

  1. Cervical (neck)- C1-C7 refers to the top seven vertebrae/bones.
  2. T1-T12 are the next 12 vertebrae/bones in the thoracic (upper back) region.
  3. Lumbar (lower back)- the following five vertebrae/bones are referred to as L1-L5.
  4. The sacrum and coccyx (tailbone) are made up of nine fused vertebrae/bones.

Cervical Spine (Neck )

The anatomy of the spine includes the cervical spine, which consists of the first seven vertebrae in the spine. It starts just below the skull and ends just above the thoracic spine. The cervical spine possesses a lordotic curve, a backward “C” shaped just like the lumbar spine. The cervical spine’s mobility helps both of the other spinal regions. Think about all the directions and angles you can turn your neck.

Anatomy of the Spine Cervical spine, illustrationUnlike the rest of the spine, special openings in each vertebra in the cervical spine contain arteries (blood vessels that carry blood away from the heart). The arteries that run through these openings bring blood to the brain.

Vertebrae

Two vertebrae in the cervical spine, the atlas, and the axis differ from the other vertebrae because they facilitate rotation. These two vertebrae are the reason your neck can move in so many directions.

The atlas is the first cervical vertebra and its location is between the skull and the rest of the spine. Additionally, 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.

In addition, 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. Special ligaments between the atlas and the axis allow for a great deal of rotation. Also, this unique situation allows the head to turn from side to side as far as it can.

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 occurs due to the limited muscle support that exists in the cervical area, and the fact that this part of the spine supports the weight of the head, which weighs an average of 15 pounds. This is a lot of weight for a small, thin set of bones and soft tissues to bear. Accordingly, sudden, strong head movements can cause damage.

Anatomy of the Spine – Thoracic Spine (Mid Back) 

Thoracic spine, illustrationThe thoracic spine consists of the middle 12 vertebrae. These vertebrae connect to your ribs and form part of the back wall of the thorax (the rib cage area between the neck and the diaphragm).   The thoracic spine appears unique in relation to other segments of the spine because pairs of rib bones extend from the spaces between its vertebrae. The ribs curved shapes create a cage-like structure that houses and protects many vital organs, including the heart and lungs.

Because the thoracic spine moves so much in daily life, it can get injured for many reasons, from improper posture to a compression fracture. Some injuries can put pressure on the spinal nerves, creating even stronger pain and other symptoms.

The thoracic spine’s curve is kyphotic, a “C”-shaped curve with the opening of the “C” in the front. This part of the spine has very narrow, thin intervertebral discs. Rib connections and smaller discs in the thoracic spine limit the amount of spinal movement in the mid-back compared to the lumbar or cervical parts of the spine. 

Anatomy of the Spine lumber Spinal Fusion SurgeryLumbar Spine (Low Back )

Another part of the anatomy of the spine,  the lowest part of the spine, is called the lumbar spine. Additionally, this area usually has five vertebrae. However, sometimes people are born with a sixth vertebra in the lumbar region. Also, the base of your spine (called the sacrum) consists of 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, doctors refer to it as a transitional (or sixth) vertebra. Nevertheless, this occurrence does not appear to have any serious side effects.

The lumbar spine’s shape has a lordotic curve-shaped like a backward “C”. If you think of the spine as having an “S” like shape, the lumbar region’s location can be found at the bottom of the “S”. The vertebrae in the lumbar spine area are the largest of the entire spine. Conversely, the lumbar spinal canal is also larger than in the cervical or thoracic parts of the spine, and as such, the size of the lumbar spine allows for more space for nerves to move about.

Low back pain occurs in lots of people because the lumbar spine connects to the pelvis where most of your weight-bearing and body movement takes place. Typically, people tend to place too much pressure in this area, such as when lifting a heavy box, twisting to move a heavy load, or carrying a heavy object. Consequently, these activities can cause repetitive injuries that can lead to damage to the parts of the lumbar spine.

Anatomy of the Spine

Vertebrae

Your spine consists 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. Normally, vertebrae, like all bones, have a hard and strong outer shell, called cortical bone. Basically, the inside consists of a soft, spongy type of bone, called cancellous bone.

The vertebral body consists of a large, round portion of the bone. Each vertebra attaches 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 attaches to the others by groups of ligaments. Ligaments connect bones to bones like tendons connect muscles to bones. Equally, there are also tendons that fasten muscles to the vertebrae.

The bony ring attached to the vertebral body consists of several parts. One such part, the laminae extends from the body to cover the spinal canal, which consists of a hole in the center of the vertebra. The spinous process consists of 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 constitutes 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. The discs prevent the bones from rubbing against each other. Altogether, 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 strongest area of the disc. It 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. Furthermore, the nucleus consists of moist tissue because it has high water content. The water content helps the disc act like a shock absorber-somewhat like a waterbed mattress.

Facet Joints

A person’s spinal column has real joints (just like the knee, elbow, etc.) 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 neighboring vertebra facet joints. Lastly, the facet joints give the spine flexibility.

Facet joints are synovial joints, structures that allow movement between two bones. In this case, the ends of the bones that make up a synovial joint are covered with articular cartilage, a slick spongy material that allows the bones to glide against one another without much friction. The 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 Foraminal

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 foraminal. The two nerve roots in each pair go in opposite directions when traveling through the foraminal. One goes out the left foramina and the other goes out through the right foramina. Then the nerve root allows nerve signals to travel to and from your brain to the rest of your body.

Spinal Cord

The spinal cord contains a column of millions of nerve fibers that carry messages from your brain to the rest of your body. Hence, it extends from the brain to the area between the end of your first lumbar vertebra and the top of your second lumbar vertebra. Each vertebra has a hole in the center, so when they stack on top of each other they form a hollow tube (spinal canal) that 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 fibers, called the caude equina. 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.

Nerve Roots

The nerve fibers 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. And the nerves of the lumbar spine reach the legs, pelvis, bowel, and bladder. Finally, these nerves coordinate and control all the body’s organs and parts, and allow you to control your muscles.

The nerves carry electrical signals back to the brain that allows 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 function.

Paraspinal Muscles

The muscles next to the spine are called the paraspinal muscles. They support the spine and provide the ability for the spine to move. Joints allow flexibility, and muscles allow mobility. There are many small muscles in the back. Each controls some part of the total movement between the vertebrae and the rest of the skeleton. These muscles can sustain injuries, 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 occurs 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. Chiefly, this happens to protect the injured area.

Muscles that spasm produce excess lactic acid, a waste product from the chemical reaction inside muscle cells. When muscles contract, the small blood vessels traveling through the muscles are pinched off (like a tube pinched between your thumb and finger), which causes a build-up of lactic acid. If the muscle cells cannot relax and too much lactic acid builds up, it causes a painful burning sensation. Afterwards, the muscle relaxes as the blood vessels open up, and the lactic acid is eventually washed away by fresh blood flowing into the muscle.

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 allows space for the nerve roots to travel freely from the spinal cord to the body. The spinal segment allows physicians to examine the repeating parts of the spinal column and to understand what can go wrong by studying the anatomy of the spine.

Vertebral column

This is the longest segment of a vertebra. It has an oval shape when viewed from above. The vertebral body is shaped like an hourglass when viewed from the side: thicker at the ends and thinner in the middle. Basically, the body is made up of cancellous bone and is covered in strong cortical bone.

Pedicles

Two short processes protruding from the back of the vertebral body consist of strong cortical bone.

Laminae

Two relatively flat bone plates extend from the pedicles on either side and join in the midline.

Processes

Processes are classified into three types: articular, transverse, and spinous. The processes act as points of contact for ligaments and tendons. To form the facet joints, the four articular processes connect with the articular processes of adjacent vertebrae. Facet joints and intervertebral discs allow for motion in the spine. Meanwhile, the spinous process extends posteriorly from the point where the two laminae meet, acting as a lever to cause vertebral motion. The transverse processes are the bony swellings from the right and left sides of each vertebra. As such, these processes act as attachment points for muscles and ligaments, as well as points of articulation for the thoracic ribs.

Endplates

An endplate covers the top (superior) and bottom (inferior) of each vertebral body. Endplates are incredibly complex structures that blend into the intervertebral disc and support it.

Intervertebral foramen

The pedicles have a small notch on the top and a deep notch on the bottom. When the vertebrae are stacked on top of one another, the intervertebral foramen is formed by the pedicle notches. This area is critical because nerve roots exit the spinal cord through this area and travel to the rest of the body.

Fibrosis Annulus

The annulus is a strong, tire-like structure that surrounds a gel-like nucleus pulposus. The annulus improves the rotational stability of the spine and helps it resist compressive stress. An annulus is made up of water and layers of strong elastic collagen fibers. The fibers are oriented at various horizontal angles, similar to the construction of a radial tire. Collagen is made up of strong fibrous bundles of protein that are linked together.

Nucleus Pulposus

The center of each intervertebral disc is filled with a gel-like elastic substance. The nucleus pulposus, in conjunction with the annulus fibrosus, transmits stress and weight from vertebra to vertebra. A nucleus pulposus, like the annulus fibrosus, is composed of water, collagen, and proteoglycans. However, the proportion of these substances in the nucleus pulposus varies. The nucleus contains more water than the annulus.

Spinal cord

The spinal cord is about 18 inches long and the thickness of your thumb. It runs through the spinal canal from the brainstem to the first lumbar vertebra. The cord fibers separate into the cauda equina at the end of the spinal cord and continue down through the spinal canal to your tailbone before branching off to your legs and feet. The spinal cord acts as a superhighway for information, relaying messages between the brain and the body. The brain sends motor messages to the limbs and body via the spinal cord, which allows movement.

Through the spinal cord, the limbs and body send sensory messages to the brain about what we feel and touch. The spinal cord can sometimes react without sending information to the brain. These special pathways, known as spinal reflexes, are designed to protect our bodies from harm right away. Any spinal cord injury can cause a loss of sensory and motor function below the level of injury. For example, a thoracic or lumbar injury may result in motor and sensory loss in the legs and trunk (called paraplegia). A cervical (neck) injury can result in sensory and motor loss in the arms and legs (called tetraplegia, formerly known as quadriplegia).

Spinal nerves

The spinal cord is divided into thirty-one pairs of spinal nerves. The spinal nerves function as “telephone lines,” relaying messages between your body and the spinal cord to control sensation and movement. Each spinal nerve is made up of two roots. The ventral (front) root transports motor impulses from the brain, while the dorsal (back) root carries sensory impulses to the brain. The ventral and dorsal roots fuse to form a spinal nerve that travels down the spinal canal alongside the cord until it reaches its exit hole, the intervertebral foramen.

The nerve branches after passing through the intervertebral foramen; each branch contains both motor and sensory fibers. The smaller branch (known as the posterior primary ramus) turns posteriorly, supplying the skin and muscles of the back of the body. Then the larger branch (known as the anterior primary ramus) turns anteriorly to supply the skin and muscles of the front of the body and gives rise to the majority of the major nerves.

Coverings & spaces

The spinal cord is surrounded by the same three membranes that cover the brain, known as meninges. The pia mater is the inner membrane that is intimately connected to the cord. The arachnoid mater is the next membrane. The dura mater is the tough outer membrane. There are spaces between these membranes that are used in diagnostic and treatment procedures. The wide subarachnoid space, which surrounds the spinal cord and contains cerebrospinal fluid, is located between the pia and the arachnoid mater (CSF). This space is typically accessed during a lumbar puncture to sample and test CSF or a myelogram to inject the contrast dye. The epidural space is the space between the dura mater and the bone. This area is most commonly used to deliver anesthetic numbing agents, also known as epidurals, and to inject steroid medication (see Epidural Steroid Injections).

If you or a loved one suffers from spinal pain, you owe it to yourself to call Southwest Scoliosis and Spine Institute at 214-556-0555 to make an appointment.