Back Structure

Although many mammals have spines, the specific structure of the human back is what allows us to stand up, sit down, twist and move in the way we do. The spine is made up of many interconnected parts. The complexity of tissues, fibers, bones and nerves can be broken down into three general parts: the vertebral column (also called the spinal column), the neural aspect, and the soft tissues.

The first of the three parts of the spine is the bony structure, or the vertebral column. The vertebral column is made up of individual bones called the vertebra and each vertebra is then made up of three parts, the body, pedicle and the facet joints. These parts create a tube-like structure for the spinal cord and are the building blocks that keep the vertebrae connected.

The spinal column is divided into five sections that each play an individual role. The five sections from the top to the bottom are the cervical (neck), thoracic (ribs and upper back), lumbar (lower back), sacral and the coccyx. You might have heard people refer to their C4 or L5. This is a reference to a particular vertebra in a particular section.  Each section is numbered from top to bottom – starting again at 1 when getting to the next section.

• C1-C7 – The Cervical spine is the top section of the spinal column. The Cervical spine has seven vertebrae (C1-7), the first being the ring-shaped “Atlas” and the second being the toothed “Axis.” As Atlas had to carry the weight of the world on his shoulders, it is the top vertebra’s job to balance the weight of the head. The special construction of the Axis, with its tooth that fits into the ring of the Atlas (the tooth is called the “dens”), allows the neck to rotate.
• T1-T12 – The Thoracic spine is made up of 12 vertebrae (T1-12) and supports the middle part of the back and ribs. Although the ribs aren’t fused to the Thoracic vertebrae they are connected by soft tissues. This section primarily allows forward and backward motion and the bones are quite large, so nerve compression happens less often.
• L1 – L5 – The Lumbar spine has fewer vertebrae – just five (L1-5), but they take on most of the body’s weight and therefore most of the biomechanical stresses as well, which is why you often hear people talk about lower back pain. The lumbar vertebrae are the largest in size of the vertebrae.
• The Sacral spine (called the Sacrum) has five (or sometimes six) vertebrae that are fused together into a triangular shape. The Sacrum connects the spine to the pelvis and sits between the hipbones.
• The bottom three (sometimes four) bones of the spinal column are also fused together to form the Coccyx or tailbone. These bones are shaped differently than the rest of the vertebrae.

The second part of the spine is the neural aspect. This is made up of the spinal cord and the nerve roots (as well as spinal fluid around the whole area). Starting at the base of the brain and running two-thirds of the way down the back, the spinal cord ends in the thoracic spine and then splits off into a group of nerve roots. Because these nerve roots look like a horsetail, it is named the cauda equina (horse tail in Latin). All of the bones, fluids, muscles and other soft tissues all protect the spinal cord and the nerve roots. Nerve roots also spring out at each vertebral level to bring feeling to different parts of the body, e.g. nerve roots in the lumbar region bring feeling to the legs.

The third of the elements to the spine are the soft tissues. Ligaments, cartilage, muscles and invertebral disks support and hold up the spine and therefore the body. They provide cushioning, add lubrication to the joints, connect the vertebra to one another, and act as shock absorbers.

Ligaments connect the vertebrae and keep the spine in line. There are two main ligaments that provide stability, the anterior and posterior longitudinal ligaments that run all the way along the spine. The many muscles in the back keep vulnerable organs protected and provide additional stability. Invertebral disks provide cushioning between vertebrae. The disks are comprised of fibrous outer rings and a jelly-like nucleus and sit between most of the vertebrae and act as shock absorbers. The cartilage between and around the bones helps to lubricate the movements for easier motion of the spine.

The spine provides integral structural support for the whole body and protection from internal organs and the spinal cord.