A basic understanding of the spine’s anatomy and its functions is extremely important for patients with spinal disorders. This article provides a straightforward overview of the spine’s remarkable and complex anatomy. It begins by providing a “big picture” of the functions of the spine, its regions, and major curves. This is followed by detailed information on specific anatomic elements such as vertebral structures, intervertebral discs, the spinal cord and nerve roots, joints, muscles and ligaments.

Functions of the Spine
 
The three main functions of the spine are to:

 - Protect the spinal cord, nerve roots and several of the body’s internal organs.
 - Provide structural support and balance to maintain an upright posture.
 - Enable flexible motion.

Regions of the Spine

Typically, the spine is divided into four main regions: cervical, thoracic, lumbar and sacral. Each region has specific characteristics and functions.

Cervical Spine

The neck region of the spine is known as the Cervical Spine. This region consists of seven vertebrae, which are abbreviated C1 through C7 (top to bottom). These vertebrae protect the brain stem and the spinal cord, support the skull, and allow for a wide range of head movement.

The first cervical vertebra (C1) is called the Atlas. The Atlas is ring-shaped and it supports the skull. C2 is called the Axis. It is circular in shape with a blunt peg-like structure (called the Odontoid Process or “dens”) that projects upward into the ring of the Atlas. Together, the Atlas and Axis enable the head to rotate and turn. The other cervical vertebrae (C3 through C7) are shaped like boxes with small spinous processes (finger-like projections) that extend from the back of the vertebrae.

Thoracic Spine

Beneath the last cervical vertebra are the 12 vertebrae of the Thoracic Spine. These are abbreviated T1 through T12 (top to bottom). T1 is the smallest and T12 is the largest thoracic vertebra. The thoracic vertebrae are larger than the cervical bones and have longer spinous processes. In addition to longer spinous processes, rib attachments add to the thoracic spine’s strength. These structures make the thoracic spine more stable than the cervical or lumbar regions. In addition, the rib cage and ligament systems limit the thoracic spine’s range of motion and protect many vital organs.

Lumbar Spine

The Lumbar Spine has 5 vertebrae abbreviated L1 through L5 (largest). The size and shape of each lumbar vertebra is designed to carry most of the body’s weight. Each structural element of a lumbar vertebra is bigger, wider and broader than similar components in the cervical and thoracic regions. The lumbar spine has more range of motion than the thoracic spine, but less than the cervical spine. The lumbar facet joints allow for significant flexion and extension movement but limit rotation.

Sacral Spine

The Sacrum is located behind the pelvis. Five bones (abbreviated S1 through S5) fused into a triangular shape, form the sacrum. The sacrum fits between the two hipbones connecting the spine to the pelvis. The last lumbar vertebra (L5) articulates (moves) with the sacrum. Immediately below the sacrum are five additional bones, fused together to form the Coccyx (tailbone).

The Pelvis and the Skull

Although not typically viewed as part of the spine, the pelvis and the skull are anatomic structures that closely inter-relate with the spine, and have a significant impact on the patient’s balance.

Intervertebral Discs

Between each vertebral body is a "cushion" called an intervertebral disc. Each disc absorbs the stress and shock the body incurs during movement and prevents the vertebrae from grinding against one another. The intervertebral discs are the largest structures in the body without a vascular supply. Through osmosis, each disc absorbs needed nutrients.

The Spinal Cord and Nerve Roots

The spinal cord is a slender cylindrical structure about the width of the little finger. The spinal cord begins immediately below the brain stem and extends to the first lumbar vertebra (L1). Thereafter, the cord blends with the conus medullaris that becomes the cauda equina, a group of nerves resembling the tail of a horse. The spinal nerve roots are responsible for stimulating movement and feeling. The nerve roots exit the spinal canal through the intervertebral foramen, small openings between each vertebra.

The brain and the spinal cord make up the Central Nervous System (CNS). The nerve roots that exit the spinal cord/spinal canal branch out into the body to form the Peripheral Nervous System (PNS).

Between the front and back portions of the vertebra (i.e. the mid-region) is the spinal canal that houses the spinal cord and the intervertebral foramen. The foramen are small openings formed between each vertebra. These “holes” provide space for the nerve roots to exit the spinal canal and to further branch out to form the peripheral nervous system. 

                              Type of 
                              Neural Structure         Role/Function