Spinal Motion Segment

The spinal motion segment is also known as a functional spinal unit. The human spine is a complex structure that consists of individual components, that all work together to allow for movements of the torso while still retaining enough structural rigidity in order to protect the spinal cord. A spinal motion segments includes two vertebral bones, the intervertebral disc between them, as well as their connection to one another at the facet joints. The spinal motion segment also includes the spinal ligaments that support the structure and prevent against excessive motion away from one another. The abdominal and back muscles that connect to the vertebral bones in the functional spinal unit are essential for its movement but aren't technically part of the structure. Because there are three main structures, this complex of vertebral bones and soft tissue structure (the disc) is also known as the "articular triad." (an articulation is the joint or joint space between two moving skeletal bones).

For us to be able to maintain a full range of motion as we move our necks, torso, and extremities, while still protecting the nerves, all of the functional spinal units should be strong and healthy. Healthy ligaments, discs, and facet joints allow for movement, while still maintaining enough rigidity to prevent against movements that would hurt the spinal nerves within the spinal cord, and those that project through the intervertebral foramina. Injuries or degenerative changes to the structures of the functional spinal unit, or those that support it, may result in one of three things:
  • Compression or irritation of the nerves that travel exit the spine near these structures
  • Hypomobility (loss of full range of movement) between the moving parts of the spine
  • Hypermobility (over flexibility) between the moving parts of the spine


Hypermobility or hypomobility between the joints and structures of the spinal motion segments may cause strain on the soft tissues or compression or irritation of the spinal nerves, resulting in back pain.

Spinal Motion: The two bones of this structural unit, the vertebral bones (spinal bones), are indirectly connected to one another. At the front of the spine, the vertebral bodies are separated by a joint known as the intervertebral disc. The jellylike nucleus of this disc enable it to be squeezed downwards and outwards when we put more weight on our spine. When we lie down at night, these discs re-inflate themselves back to their tallest height as they reabsorb water and nutrients. This accounts for the reason why we are slightly taller in the morning than we are when we lie back down at night. Located on opposite sides of the vertebral bodies are the anterior longitudinal ligament, located in the front of the vertebral bodies, and the anterior longitudinal ligament, located behind the vertebral bodies. Both of these ligaments travel the length of the spine.

Behind the vertebral bodies, there is a vertebral arch and several processes project off of this oval-shaped ring of bone. The vertebral arch has a cavity in its center, through which the spinal canal travels. The spinal canal includes a three layered structure called the Dura, which protects the spinal cord and nourishes it with nutrients and oxygen. The spinal cord is found within the innermost layer of the Dura.

Four processes project upwards and downwards off of the vertebral arch to provide for another source of stability to the spinal motion segment. On each spinal bone, two superior articular processes project upwards and two project downwards. Each process joint up with one above or below, and held together by a structure known as the facet joint. Two facet joints are found in each spinal motion segment.