Anatomy of the Spine
The human spine is made up of 5 main segments, the cervical, the thoracic, the lumbar, the sacral, and the coccyx. Each segment consists of individual bones known as vertebrae which are stacked upon each other.
The cervical spine consists of 7 vertebrae, the thoracic 12 vertebrae, and the lumbar consists of 5 vertebrae. The 5 vertebrae of the sacral region are fused together to form one bone. Similarly, the 4 vertebrae of the tailbone (coccyx) are also fused together to form one bone.
The shape and size of the vertebrae in different segments results in a characteristic double ‘’S’’ shape of the vertebral column in an upright position. The cervical spine is naturally bent with an inward curve, the thoracic is bent with an outward curve and the lumbar is bent with an inward curve.
X-ray of the normal Cervical and Lumbar Spine.
The cervical spine starts from the base of the skull and ends in the upper chest. The cervical spine helps in the movement of the head while providing stability. The thoracic spine connects the cervical spine with the lumbar spine, starting in the upper chest and ending in the mid-back.
The thoracic spine connects with the rib cage. The lumbar spine consists of large vertebrae as it supports the body weight, connecting the upper body with the pelvis.
MRI showing the lumbar spine in the axial and sagittal section.
Each vertebra creates a canal upon stacking on each other, creating a safe conduit for the spinal cord and the nerve roots. The spinal cord travels from the base of the brain through an opening in the lower skull and ends at the level of the 1st and 2nd lumbar vertebrae. After the spinal cord ends near L1 and L2 vertebrae it continues down as a bundle of nerves known as cauda equina.
The spinal cord gives branches which pass through the openings in the vertebrae known as the intervertebral foramen. These exiting nerves serve as a connection between the brain and the spinal cord with the tissues they supply.
The facet joints are responsible for the natural curves in the vertebral column. They also provide stability to the vertebral column and aids in its movement. Each vertebra has 2 pairs of facet joints, one pair facing upwards connecting with the vertebrae above and the other pair facing downwards, connecting with the vertebra below.
The facet joints stabilize the vertebral column during various movements such as twisting, bending forward, or backward. Like other joints in the body. Each facet joint is covered by a capsule. The ends of the bones forming the facet joints are covered with a protective tissue known as cartilage. The capsule of the joint secretes watery thin fluid to lubricate and nourish the joint.
The intervertebral disks connect each vertebral body with each other. The intervertebral disks are flat disc-shaped tissues that act as a cushion between the adjoining vertebrae. Each disk is made up of a thick fibrous outer ring known as the annulus and a soft inner center known as the nucleus pulposus. The nucleus pulposus has a gel-like consistency which gives the disk its flexibility and shock-absorbing property.
Ligaments are strong fibrous tissues connecting vertebrae. Various ligaments connect the corresponding vertebrae. Together, the spinal ligaments provide structural stability to the vertebral column at rest and during movement.
Spinal muscle and tendons around the vertebral column together help to move the column and provide stability. The muscles and tendons attached at the back and the sides help to bend the spine backward and the muscles in the front including the abdominal muscles help to bend the spine forward.
The above image shows a cervical spine vertebra with a prosthetic cervical disc. The prosthetic disc may be used to treat patients with diseased cervical intervertebral disc. The disc replacement has an advantage of maintaining the motion of the diseased segment as opposed to cervical spine fusion.