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Spine Anatomy

Why is the spine so important?

The human spine is the central part of our body. Everything else about us is structured around it. The spine is also referred to as the ‘vertebral column’, which means that it consists of individual vertebrae which are stacked on top of each other, separated by cartilage between them. This cartilage consists of spinal discs in the front of the spine and joints in the back.

The spine has two main functions:

  1. It protects the spinal cord and the spinal nerves against damage.

The vertebrae form a wall of bone around the spinal cord which protects it against trauma.

  1. It allows us to walk upright and bend in all directions.

The spine is rigid enough to allow us to stand and walk straight, yet flexible enough to permit the muscles to bend us in different directions.

Why is our spine curved?

The spine has natural curves. When you look at the spine from the side, it looks like an ‘S’. This S-shaped curve is like a spring which can coil with downward force on the spine and recoil when it is relieved. The spinal discs make up the coils of the spring and are the main shock absorbers. The reason for the curves is the fact that each part of our spine has different roles to play depending on which section of the human body it supports.

The neck (cervical spine) and low back (lumbar spine) are bent backward into a ‘reverse C’ called a lordosis, while the mid-back (thoracic spine) and tailbone (sacral spine) are bent forward into a ‘C’, called a kyphosis.

The neck has to have a lordosis (backward bent) to support the weight of the head, while the low back has to support the weight of the chest and abdomen. Without this lordosis, the head, chest and abdomen would be able to pull us forward and we would be crawling rather than standing and walking.

The forward bent (kyphosis) in the mid-back and tailbone areas are there to compensate for the backwards bend (lordosis) of the neck and low back. If we did not have this compensation in curves, we would just follow the neck and low back and be bent backwards all the time.

So, the spinal curves compensate for each other to support each other’s function. If we did not have these spinal curves, we would have very little mobility in the spine and no ability to compensate for bending in any direction.

Looking at the spine from the front and back, we do not have curvatures. We can therefore only side- bent to a small degree when compared to our ability to bend forward and arch backward. It is abnormal for us to have a curvature when looking at the spine from the back. This is called a scoliosis.

What are the different parts of the spine and what do they do?

Our vertebrae are essentially blocks of bone which fit on top of each other like pieces of a three-dimensional puzzle. These stacks of vertebrae are separated from each other by a shock-absorbing disc in the front and supported in the back by two joints. So, there are three places where the vertebrae meet each other, the disc in the front and two joints in the back.

We have a total of 33 vertebrae spread out over the four sections of our spine. These four sections are:

  1. Neck (cervical spine),
  2. Mid-Back (thoracic spine),
  3. Low-Back (lumbar spine)
  4. Tail-Bone (sacral spine)

Of the 33 vertebrae, 24 are mobile and the remaining 9 are fused together.

The vertebrae for each section look slightly different since they play different roles in each segment.

The vertebrae are held together by many ligaments which are stiff enough to provide strength, yet flexible enough to permit movement. Ligaments are essentially made up of many tough strands of fibers held together as one.

While the vertebrae are pieces of solid bone, they still have different sections, each with a different purpose:

  1. Vertebral Body

The vertebral body is the main part of the vertebrae, i.e. the largest and strongest portion. It is also the part which carries the most weight (approximately 80%). Due to that, it is vulnerable to breaking (fracture) when our bones soften (e.g. osteoporosis).

The vertebral body has a strong outer layer of tough bone (cortical bone), and a softer inner layer (cancellous bone) which contains the bone marrow and blood vessels.

The top and bottom of the vertebral body are called the ‘endplates’. These endplates are important as they allow for nutrition to reach our shock-absorbers, the discs.

As we age, much of the bone spurring in the spine happens at these endplates. This process happens when the discs get harder and degenerate. As these discs shrink and harden, the endplates become subject to more force and wear and tear. Eventually the bone at the endplate will show signs of that in the form of bone spurring. In general, bone spurring often happens when bone wears on another bone or other hard materials.

  1. Pedicle

The pedicle connects the front of the vertebrae, i.e. the vertebral body, to the back of the vertebrae where we have facet joints, lamina, spinous and transverse processes, explained below. Located between the front and the back parts of our spine is a large bony canal which contains the spinal cord and its fluid filled sack as well as the beginning of the spinal nerves.

The pedicles then are essentially small bone bridges connecting the front and the back of the spine, creating the space for our spinal canal.

       2. Transverse Processes

 Transverse processes are stubby wings which are located on the back part of the vertebrae, pointing sideways. If you think of the whole vertebrae like the fuselage of an airplane, the transverse processes are the wings. These wings do not bear weight, but rather serve as attachment points for muscles and ligaments.

     3. Lamina

The lamina are small bone shelves which face the spinal canal and provide protection for it. They overlap with each other, like shingles on a roof.

     4. Facet Joints

The facet joints are located on the back of the vertebrae, one on each side. They are one area where the vertebrae communicate with each other. They carry approximately 20% of the body’s weight, as opposed to the 80% carried by the disc. Each joint is made up of a portion of bone from two adjacent vertebrae with a small cartilage in between.

They are surrounded by a tight capsule of touch fibers. The facet joints allow us to bend forward and backwards, while keeping us from going too far either way. Just as knee and hip joints do, facet joints can also become arthritic and show signs of wear and tear in the form of arthritis. As part of the arthritis these joints often enlarge and grow (hypertrophy) into the spinal canal. This can cause a significant narrowing of the spinal canal (spinal stenosis) and compress the spinal nerves. This is the most common reason for the development of spinal stenosis.

5. Spinous Process

The spinous process is a spike of bone which points backwards toward the skin. In dinosaurs they are the large spikes on top of their backs. In humans they serve as further protection against an injury to the spinal cord or nerves and also help anchor muscles, tendons and ligaments to the spine.

6. Pars Interarticularis

The ‘pars’ is a small section of the back of the vertebrae which connects the pedicle and the lamina. The significance of the ‘pars’ is the fact that it is the weakest part of the vertebrae and often breaks (fractures) when we are younger and try to lift something very heavy or bend beyond our limits. This often results in a chronic fracture (spondylolysis) and makes the spine less stable. In some patients it allows one segment of the spine to shift forward on another called a sponylolisthesis.

      7. The Disc

The human disc, also called the ‘intervertebral disc’ (disc between vertebrae) is essentially a cushion which separates the vertebrae and can absorb weight and force. Just like a balloon will bulge outward when pushed on from the top, the disc can bulge as well when weight is placed on it. Similarly, when we lie down and the weight of gravity is taken off the disc, it will expand again, just like the balloon.

The disc is made up of a tough outer shell and a softer inner core. The outer shell is called the ‘Annulus Fibrosus’ and consists of tough “ligament-like” fibers which criss-cross each other and then attach to the endplate of the vertebrae above and below. They consist mostly of tough collagen fibers. Even though they are very strong and attach the vertebrae to each other, they still allow for motion between the vertebrae. The Annulus Fibrosus also protect the core of the disc from leaking out, which is a disc herniation.

That core of the disc which is called the ‘Nucleus Pulposus’ is made up of many soft fibers which look like crab meat and have a similar consistency. These fibers remain soft and moist by attracting water through the endplates of the vertebrae. Together they form a ‘mattress’ on which the top and bottom vertebrae rest. This ‘mattress’ is soft enough to be a good cushion, yet tough enough to keep the vertebrae from touching each other which would cause pain as well as wear and tear on the vertebrae.

When the outer shell (annulus fibrosus) of the disc weakens, a disc can either bulge outward (disc bulge), protrude or push further outward (disc protrusion), or rupture (disc herniation).

A disc may also lose its water content over time, which is due to a change in the chemicals in the disc and a hardening of the endplates of the vertebrae (top and bottom sides of the vertebral body). When this happens to a significant degree, the disc as a whole will degenerate and often get thinner. It will eventually lose its function as a shock absorber. This is called degenerative disc disease. Since the discs make up 1/4th to 1/3rd of the length of our spine, degenerative disc disease is one of the reasons why we shrink as we age. Even on any given day, we are taller in the morning than at night. This is because gravity and weight bearing on the spine will push some of the fluid back into the adjacent vertebrae, which shrinks the disc. So, as the day goes on, we lose some height. We regain it overnight, as the fluid is absorbed back into the disc.

The discs are numbered by the vertebrae above and below. For instance, the space between the 4th and 5th lumbar (low back) vertebrae contains the L4/5 disc. The numbering gets more difficult when one area of the spine meets another. For instance, the disc between the lowest cervical vertebrae (C-7) and the first thoracic vertebrae (T-1) is called the C7/T1 disc. Similarly, the disc between the lowest lumbar (L-5) and first sacral (S-1) vertebrae is called the L5/S1 disc.

Our discs have very little blood supply, only on their outside (periphery). They receive their nutrition by a process called diffusion through the endplates of the vertebrae above and below. Diffusion allows chemicals to flow from an area of abundance to an area with need, through a gradient.

     8. Muscles

While the vertebral column allows us to be upright and straight, the spinal muscles do the work. They attach to many different parts of the spine and provide the force to make the individual vertebrae move.

The two main groups of spine muscles are the flexors (forward benders) and the extensors (backward benders). They have to be at a constant balance to permit coordinated movement. The spinal cord and the spinal nerves communicate with these muscles and determine their function.

A frequent reason for severe muscle spasms in the spine is a disc herniation. The herniated disc material irritates and inflames a spinal nerve, which will then send a signal to the muscle. The muscle will then tighten up and contract as a protective measure against further injury to the spine. Unfortunately this often results in spasms which can be painful and sometimes difficult to treat.

The main flexor (forward bending) muscle of the spine is the rectus abdominis muscle which is a large abdominal muscle. It becomes the proverbial six-pack when exercised rigorously.

The main extensor (backward bending) muscles of the spine are a group called the erector spinae muscles. They vary in size and length and are made up of the iliocostalis, spinalis and longissimus muscles. The importance of these muscles is that they assist us to large degree in maintaining an upright posture.

      9. Ligaments

The vertebral column and its individual vertebrae are connected through a number of different ligaments. Ligaments connect bone to bone and provide additional stability and strength. When located around joints they do allow motion but often prevent the joint from moving beyond its normal range of motion, thereby preventing injury. Ligaments are also prone to injury and wear. Our spine is very reliant on the normal function of these ligaments. In severe spinal injuries, the spine becomes unstable when a certain number of these ligaments are torn or disrupted. Here are the main spinal ligaments:

a. Supraspinous Ligament

“Supraspinous” means ‘at the top (tip) of the spinous process’. Therefore this ligament connects the tops of the spinous processes together. The spinous processes are the small stubby bones which we can feel when we touch the center of our backs. This ligament resists the spine from flexing (bending) too far forward.

      b. Interspinous Ligament:

‘Interspinous’ means ‘between the spinous processes’. This ligament connects the spinous processes in their mid-portion rather than the tops. It has a similar function as the supraspinous ligament in resisting too much flexion of the spine.

      c. Ligamentum Flavum:

‘Ligamentum Flavum’ means ‘Yellow Ligament’ in Latin. This is due to its yellow coloration when seen during surgery. This ligament connects the lamina which are the small bony shelves making up the back portion of the spinal canal. This ligament also resists too much flexion in the spine. It often thickens with overuse and age and eventually starts to take up space in the spinal canal, compressing the spinal nerves. That often contributes to the condition of spinal stenosis (narrowing of the spinal canal). During disc surgery and surgery for spinal stenosis, a portion or the entire ligament is removed.

      d. Anterior Longitudinal Ligament:

‘Anterior Longitudinal Ligament’ means ‘a ligament which runs up and down in front of the spine’. In fact this ligament connects the bodies of the vertebrae in front in their midline. Thus it resists excessive backward bending (extension) and limits it to stay within a range our spine can tolerate.

      e. Posterior Longitudinal Ligament:

‘Posterior Longitudinal Ligament’ means ‘a ligament which runs up and down on the back of the spine’. This ligament is the counterpart of the anterior longitudinal ligament and connects the backs of the vertebral bodies in the midline. It resists excessive forward bending (flexion) of the spine.

      10. Spinal Canal

The spinal canal is located in the center of the spinal bones. These bones form a circle in their midst, which makes up the spinal canal.

The front (anterior) of the spinal canal is made up of the bodies of the vertebrae and the posterior longitudinal ligament which connects them. The sides (lateral) of the spinal canal are made up of the pedicles which are the small bony bridges which connect the front and the back of the spinal bones.

The back part (posterior) of the spinal canal is made up of the ligamentum flavum and the lamina.

On the back but off to the side (posterolateral) are the two facet joints which project into the spinal canal.

The back of the spine is often referred to as the “vertebral arch”. This makes sense when looking at the spine from a cross-section view. The cross-section of the spine shows a vertebral body in the front and an arch in the back. The arch consists of the pedicles and the lamina.

      11. Spinal Nerve Supply

      a. Spinal Cord

The spinal cord starts at the base of our brain (brainstem) and runs down inside the spinal canal all the way to the upper part of our lumbar spine (low back). It often ends at the spinal level of L-1 or L-2 but this can vary. In an adult it is about as thick as a thumb.

The spinal cord contains many tracts of nerves which send signals up and down to and from the brain.

An example of this flow of nerve information is touching a hot stove. The information is relayed from the small nerves in our hand to the larger spinal nerves, to the spinal cord, and up to the brain. The brain interprets the heat and the fact that it could harm us. It sends signals back down through the spinal cord, to the spinal nerves. The spinal nerves initiate the movement by sending the signal to the muscles of the hand.

As the spinal cord ends, it sends some of the spinal nerves further downward the spinal canal. These nerves run together in a bundle, called the ‘cauda equina’, Latin for a ‘horse’s tail’. That is in fact what they look like when seen in surgery or on an MRI scan.

The spinal cord and the cauda equina are bathed in spinal fluid (Cerebrospinal Fluid or CSF), all of which are contained inside a very delicate sac, called the dura mater. ‘Dura mater’ means ‘outer layer’. This spinal fluid continues all the way to the brain and is the same fluid which bathes the brain. It provides nutrition to the spinal cord and brain and also creates a buffer against injury.

      b. Spinal Nerves

The spinal nerves are the final ends of the spinal cord and travel from the spinal cord outwards through the side holes of the spinal canal (foramen) and from there to their final destinations in the arms and legs.

In the low back (lumbar spine) for instance, the lumbar spinal nerves travel down our legs and give us strength, sensation and reflexes.

There are 31 pairs of spinal nerves, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.

A spinal nerve originates from the spinal cord as the union of two branches, the dorsal (back) and ventral (front) branches. The dorsal branch carries sensory information (sensation) while the ventral branch carries motor (motion, movement) information.

      12. Intervertebral Foramen

‘Intervertebral foramen’ means ‘an opening between the vertebrae’. In fact this is the opening on the outer sides (lateral) of the vertebral column between two vertebrae. Since the pedicles which connect the front and back parts of the vertebrae are thin, there is a space remaining above and below each pedicle. This space is the intervertebral foramen. It provides an exit point for the spinal nerves to leave the spinal canal and travel to their respective endpoints in the arms and legs. Several blood vessels also enter and exit the spinal canal through the foramen.

        13. Blood Supply

The largest artery in the body, the aorta sends small arteries to the spine which provide the needed oxygen and nutrients. These arteries typically run in front and on the back of the spinal canal. Typically there is one artery in the front (anterior) and two in the back (posterior).

There is a particularly important artery, called the ‘artery of Adamkiewicz’, according to the doctor who first found it. This artery supplies the majority of the mid-section of the spinal cord with blood. An injury to this single artery could cause a significant problem for the spinal cord.

 

What is a spinal segment?

A spinal segment is one functional unit of the spine. In other words it is a segment of the spine which shows us how the spine functions overall. A spinal segment consists of two vertebrae, separated by a disc in the front and joined by two facet joints in the back. The front (anterior) part of the segment is primarily responsible for weight bearing, while the back (posterior) part is primarily responsible for motion. Looking at how each segment is structured allows us to understand the function of the spine as a whole.

What are the spinal regions?

The spinal regions are divided from top to bottom into the neck (cervical spine), mid-back (thoracic spine), low back (lumbar spine) and tailbone (sacral spine).

A. Cervical Spine (Neck)

The main function of the cervical spine is to support the weight of our head which often weighs around 10 to 15 pounds in adults. Since the weight of the head is small in comparison to our chest and abdomen, the 7 vertebrae of the cervical spine (C-1 through C-7) are smaller in size than the thoracic (mid-back) and lumbar (low back) spine.

Compared to the other segments of the spine, the cervical spine has the most range of motion. This is due to the two highest cervical vertebrae called the atlas (C-1) and the axis (C-2).

‘Atlas’ was a Greek mythological figure which held up the world on its shoulders. Similarly, the vertebra ‘atlas’ (C-1) carries the weight of the skull. The atlas is located just under the skull and allows the skull to bend forward (flexion) and backwards (extension). It does not have a vertebral body, but rather looks like a ring with a thicker front, thinner back, and two wider areas on the sides on which the skull rests. This area accounts for approximately half of the flexion and extension in the cervical spine. There is no disc between the atlas and the axis.

The vertebrae below the atlas (C-1) is called the ‘axis’ (C-2). ‘Axis’ means ‘to spin something on its surface’. Similarly, the axis which has a small piece of bone pushing straight up under the atlas, called the Odontoid, can ‘spin’ or rotate the atlas. This accounts for approximately half of the cervical spine’s rotation. Special ligaments connect the atlas and the axis to permit the motion. They also provide stability and limit the motion between these two vertebrae to avoid injury.

So, the first two cervical vertebrae are unique, while the remaining five cervical vertebrae are quite similar. In total then there are seven cervical vertebrae, numbered C-1 through C-7. The ‘C’ stands for cervical.

One of the unique features of the cervical spine are the small holes in each transverse process (wing of the vertebrae). These openings which are called Transverse Foramen, allow the ‘vertebral artery’, to run upward through the openings bringing oxygenated blood to the brain. This artery is essentially protected by the vertebrae on its way to the brain.

The cervical spine is surrounded by a great deal of musculature to allow for the complex motion. While these muscles are quite strong they are not adequate to protect us from injury such as whiplash.

The cervical spine has a spinal canal just alike all parts of the spine, however smaller in size. Unlike the lumbar spine, the cervical spine has the spinal cord running through the entire length of its spinal canal. Subsequently, a significant injury to any part of the cervical spine could result in a spinal cord injury.     

On the contrary, in the lumbar spine, the spinal cord often ends at L-1 or L-2, which is the highest part of the lumbar spine. An injury below that level of the lumbar spine cannot directly injure the spinal cord, but rather the bundle of spinal nerves (cauda equina), perhaps resulting in a less significant injury. Similarly, a very large disc herniation in the cervical spine can push directly on the spinal cord, whereas in the lumbar spine below L-1 or L-2 that would be unlikely.

B. Thoracic Spine (Mid-Back)

The main function of the thoracic spine is to protect the organs in our chest, namely the heart and lungs, against injury. The thoracic spine (mid-back) has 12 vertebrae and is the longest part of our spine. The thoracic vertebrae are intermediate in size between that of the cervical and lumbar spine. It runs from the base of our neck (cervical spine) to the diaphragm which is the breathing muscle at the base of our lungs, where it connects to the low back (lumbar spine).

The ribs attach to each thoracic vertebra, which anchors the rib cage and provides it with stability. The ribs attach to two points on each side of the thoracic vertebrae. On point is at the Transverse Process, while the other is at the back of the Vertebral Body. Small joints (costo-vertebral joints) are present at each of these attachment points. Strong ligaments support the rib attachments to the vertebrae. This provides stability to the spine, but limits its mobility significantly.

The thoracic spinal canal contains spinal cord through the entire length of the canal. However, when compared to the cervical and lumbar spine, the spinal canal in the thoracic spine leaves less room around the spinal cord. Compared to the cervical spine, the thoracic spine is not quite as injury prone due to the stability provided by the rib attachment. The thoracic vertebrae are also larger and therefore stronger than those of the cervical spine.

The discs between the thoracic vertebrae are much thinner than in the cervical or lumbar regions. They are also much less likely to be injured or degenerate due to the minimal movement in this part of the spine. Similarly, the facet joints on the back of the spine face much less degeneration than in other parts of the spine.

C. The Lumbar Spine (Low Back)           

The main function of the lumbar spine is to support the weight of our body. Due to that, the 5 lumbar vertebrae (L-1 through L-5) are large and bulky.

It is not unusual to have a missing or an extra lumbar vertebra. This is called ‘transitional anatomy’. The reason for the word ‘transitional’ is the fact that the extra or missing vertebrae always occur at the bottom of the lumbar spine where it ‘transitions’ to the sacral spine.

The lumbar spine has more mobility than the thoracic spine, especially in the upper levels, L1-L3. However, most of the wear and tear such as disc herniations occur in the lower levels, L4-S1 due to the ever increasing weight placed on the lower lumbar vertebrae. This is the reason why disc herniations in the lumbar spine are so common at the low levels such as L4/5 (disc between the 4th and 5th lumbar vertebrae), and L5/S1 (disc between the 5th lumbar and 1st sacral vertebrae).

Just like the discs of the lumbar spine, the Facet Joints are also subject to much weight bearing and mobility. They often show signs of degeneration long before the other parts of the spine do.

The bottom of the lumbar spine connects to the tailbone (sacrum).

The spinal cord often ends at around the L-1 or L-2 level, in the uppermost part of the lumbar spine. After the spinal cord ends, it sends a bundle of spinal nerves called the cauda equina (horse’s tail) further downward into the spinal canal.

D. The Sacral Spine (Tailbone)

The function of the sacral spine is to transmit the weight from the lumbar spine to the pelvis. The sacrum has 5 vertebrae (S-1 through S-5) which are typically fused together and therefore are one large bone which is wedge- shaped. The sacrum does have openings (foramen) for nerves to travel from the cauda equina to their destinations.

The sac containing the spinal fluid (dura) typically ends at the S-2 segment where it is anchored to the bone by a ligament called the filum terminale, which is Latin for ‘terminal thread’.

At the end of the sacral spine we have a coccyx (ends of the tailbone). The two are connected by ligaments. It is this area which is commonly injured by a fall on the butt. The coccyx is the very end of the spinal column. It consists of 4 fused vertebrae and serves as an attachment point for muscles.

The joint between the sacrum and the pelvis is called the Sacroiliac Joint. This joint serves the purpose of transmitting the weight of the body from the spine to the hip joint and legs. The joint has very minimal motion (micro-motion) except in pregnancy and childbirth where it can open significantly to allow the baby’s head and body to pass through the pelvis.

Why is the Spine so easily injured?

The two parts of our spine where we have most of our problems are in the neck (cervical) and low back (lumbar). These are the two most mobile parts of our spine which makes them more susceptible to injury.

  1. The Neck (Cervical Spine)

a. Injury from Trauma:

The neck is easily inured by trauma. The neck carries the entire weight of our head. So, when the head is suddenly thrown forward (e.g. car accident) its weight resists that force. At some point, resisting this force overwhelms the stability of the neck and causes an injury. The most devastating ones results from broken vertebrae which then push into the spinal cord and cause paralysis.

b. Injury from Degeneration:

Degeneration typically affects the lower parts of the cervical spine from C-5 to C7, since more weight is carried by the lower cervical vertebrae than the upper ones. This is where disc herniation commonly occur.

The discs are most vulnerable in the area on the back and sides of the disc (posterior-lateral). This is the part where the outer ring of fibers (annulus fibrosus) is the thinnest. Repetitive injury to the annulus fibrosus causes it to become brittle and thinner. Eventually the annulus fibrosus can rupture allowing the softer material of the nucleus pulposus (inner core of the disc) to push out through its fibers, causing a disc herniation.

      2. The Low Back (lumbar spine)

      a. Injury from Trauma:

The low back is not as prone to injury from high speed trauma, but more likely from repetitive trauma, such as heavy lifting and running on hard surfaces. Similar to the neck, most problems occur in the lowest spinal segments of the lumbar spine, at L4/5 and L5/S1. Again this is due to the fact that these segments bear more of the weight than the upper ones do.

      b. Injury from Degeneration:

Similar to the cervical spine, the outer ring of the lumbar discs is thinner on the posterior-lateral corners (back and sides). In addition, a ligament called the ‘posterior longitudinal ligament’ supports the mid-section of the posterior (back) part of the disc, but does not reach the vulnerable posterior-lateral corners. This is where the majority of disc herniations occur. Repetitive injury to the annulus fibrosus causes it to become brittle and thinner. Eventually the annulus fibrosus can rupture allowing the softer material of the nucleus pulposus(inner core of the disc) to push out through its fibers, causing a disc herniation.

Why does our low back hurt so much?

The low back (lumbar spine) carries much of our body’s weight. Every time we are upright, we place weight on that part of our spine. If we have a low back injury, lying down is the only time there is not stress on the injured part of the spine. In our busy lives it is difficult to be lying down for long periods of time to rest our spine.

The low back muscles are also prone to spasms. Since the muscles in the low back are large and powerful, they will often cause more pain than other muscles when they go into spasm. In addition these are the same muscles which keep our skeleton upright, so they are used extensively. Typically back spasms are due to severe inflammation in the spine. One of the common reasons is the presence of a disc herniation. When material herniates from the disc it often makes contact and inflames a spinal nerve causing sciatica. The same nerve that gives us the sciatica (leg pain) has a branch which travels to our back muscles. It sends a signal of nerve inflammation to the muscle which then contracts and spasms in a futile attempt to lock the back up and prevent further injury. Back spasms typically cause severe pain and create a cycle which can be difficult to break.

The low back has a very dense supply of nerves, ranging from the large spinal nerves to many smaller branches which can sense inflammation of the joints, ligaments, bones and muscles of our spine. All of this contributes to the severity of low back pain.