Understanding Spinal Anatomy: Overview of the Spine

A basic understanding of the spine’s anatomy and its functions is extremely important to patients with spinal disorders. This information provides a straightforward overview of the spine’s remarkable and complex anatomy.

Functions of the Spine

The three main functions of the spine are to:

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

Regions of the Spine

The regions of the spine consist of the cervical, thoracic, lumbar, and sacral.

Cervical Spine

The neck region is 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 supports the skull. C2 is called the Axis. It is circular in shape with a blunt tooth-like structure (called the Odontoid Process or dens) that projects upward into the Atlas. Together, the Atlas and Axis enable the head to rotate and turn. The other cervical vertebrae (C3-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 12 Thoracic vertebrae abbreviated T1-T12 (top to bottom). T1 is the smallest and T12 is the largest thoracic. The thoracic vertebrae are larger than the cervical bones and have longer spinous processes.

Rib attachments add to strength and stability the thoracic spine. The rib cage and ligaments limit range of motion and protect many vital organs.

Lumbar Spine

The Lumbar Spine consists of 5 vertebrae abbreviated L1-L5. The lumbar vertebrae are the largest and carry most of the body’s weight. This region allows more range of motion than the thoracic spine, but less than the cervical. Lumbar facet joints enable significant flexion and extension movement, but limits rotation.

Sacral Spine

The Sacrum is located behind the pelvis. Five bones, abbreviated S1-S5, fused into a triangular shape, form the sacrum. The sacrum fits between the two hip bones 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 Skull and Pelvis

Although not typically viewed as part of the spine, the skull and pelvis inter-relate with the spinal column and impact balance.

Anatomical Planes

To spatially describe spinal anatomy, spine specialists use terms to define body planes. A body plane is an imaginary flat, two-dimensional surface that defines a particular area of anatomy.

Term	Meaning
Frontal or Coronal Plane	The frontal plane divides the front and back halves of the entire body.
Median or Midsagittal Plane	The median plane divides the left and right sides of the entire body.
Transverse or Horizontal Plane	The transverse plane divides the body at the waist (top and bottom halves of the body).

Spinal Curves

Viewed from the front, a healthy spine is straight up and down. Viewed from the side, a spine has four distinct curves. Curves are described as being either kyphotic or lordotic.

• A kyphotic curve is a convex curve in the spine (i.e. convexity towards the back of the spine). The curves in the thoracic and sacral spine are kyphotic.
• A lordotic curve is concave (i.e. concavity towards the back of the spine), and is found in the cervical and lumbar levels of the spine.

Vertebral Structures

The outer shell of a vertebra consists of cortical bone. Cortical bone is dense, solid and strong. Inside each vertebra is cancellous bone, which is weaker than cortical bone and consists of loosely knit structures that resemble honeycomb. Bone marrow, which forms red blood cells and some types of white blood cells, is found within the cavities of cancellous bone. Vertebrae consist of the following common elements:

• Body: The body is the largest part of a vertebra. Viewed from overhead, it appears oval. From the side, it appears slightly hour-glass shaped; thicker at the ends and thinner in the middle.
  
  
• Pedicles: Two short processes made up of strong cortical bone, that protrude from the back of the vertebral body.
  
  
• Laminae: Two relatively flat plates of bone that extend from the pedicles on either side and join in the midline.
  
  
• Processes: There are three types of processes: articular, transverse and spinous. The processes serve as connection points for ligaments and tendons. Four articular processes join with the articular processes of adjacent vertebrae to form the facet joints. The facet joints, combined with the intervertebral discs, allow for motion in the spine. Spinous processes extend posteriorly from vertebrae where the two laminae join, and act as a lever to effect vertebral motion.
  
  
• Endplates: The top (superior) and bottom (inferior) of each vertebral body is coated with an endplate. Endplates are complex cartilaginous structures that blend into the intervertebral disc and help support the disc.
  
  
• Intervertebral Foramen: The pedicles have a small notch on their upper surface and a deep notch on their bottom surface. These notches form a hollow passageway between vertebrae. The foraminal passageways allow a place for nerve roots to branch out from the spinal canal.
  
  
• Facet Joints These joints are formed at the back (posterior) of each vertebral body. Facet joints help the spine to bend, twist, and extend in different directions. The facet joints restrict excessive movement such as hyperextension and hyperflexion (i.e. whiplash). Each vertebra has two facet joints. The superior articular facet faces upward and works like a hinge with the inferior articular facet (below). Like other joints in the body, each facet joint is surrounded by a capsule of connective tissue and produces synovial fluid to nourish and lubricate the joint. The surfaces of the joint are coated with cartilage that helps each joint to move (articulate) smoothly.

Intervertebral Discs

Between each vertebral body is a cushion; the intervertebral disc. Discs absorb stresses the body incurs during movement and prevents vertebrae from grinding against one another. The intervertebral discs are the largest structures in the body without a vascular supply. By means of osmosis, each disc absorbs needed nutrients. Each disc is made up of two parts: the annulus fibrosis and the nucleus pulposus.

Annulus Fibrosus

The annulus fibrosus is a sturdy tire-like structure that encases a gel-like center, the nucleus pulposus. The annulus enhances the spine’s rotational stability and helps to resist compressive stress.

The annulus is a layered structure consisting of water and sturdy elastic collagen fibers. The fibers are oriented at different angles horizontally similar to the construction of a radial tire. Collagen consists of fibrous bundles made of protein bound together by proteoglycan gel.

Nucleus Pulposus

At the center of each disc is a gel-like elastic substance. Together with the annulus fibrosis, the nucleus pulposus distributes stress and weight from vertebra to vertebra. Structurally, the nucleus pulposus is similar to the annulus fibrosis; water, collagen and proteoglycans. However, the concentration of these substances differs, as the nucleus contains more water than the annulus.

Endplates

The top (superior) and bottom (inferior) of each vertebral body is coated with an endplate. Endplates are cartilaginous structures that blend into the intervertebral disc to hold it in place.

Spinal Cord and Nerve Roots

The spinal cord is a slender cylindrical structure about the diameter of the little finger. The spinal cord is contained and protected with the spinal canal. 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 a horse’s tail. Nerve roots exit the spinal canal through the intervertebral foramen.

The brain and the spinal cord make up the Central Nervous System (CNS). Nerve roots branch out through the foramen into the body to form the Peripheral Nervous System (PNS).

Type of Neural Structure	Role/Function
Brain Stem	Connects the spinal cord to other parts of the brain.
Spinal Cord	Carries nerve impulses between the brain and spinal nerves.
Cervical Nerves (8 pairs)	These nerves supply the head, neck, shoulders, arms, and hands.
Thoracic Nerves (12 pairs)	Connects portions of the upper abdomen and muscles in the back and chest areas.
Lumbar Nerves (5 pairs)	Feeds the lower back and legs.
Sacral Nerves (5 pairs)	Supplies the buttocks, legs, feet, anal and genital areas of the body.
Dermatomes	Areas on the skin surface supplied by nerve fibers from one spinal root.

Ligaments, Tendons and Muscles

Ligaments and tendons are fibrous bands of connective tissue that attach to bone. Ligaments connect two or more bones together and help stabilize joints. Tendons attach muscle to bone. Tendons vary in size and are somewhat elastic.

The system of ligaments in the vertebral column, combined with the tendons and muscles, provides a natural brace to help protect the spine from injury. Ligaments aid in joint stability during rest and movement and help prevent injury from hyperextension and hyperflexion (excessive movements).

Tendons and Muscles

Tendons are similar to ligaments, except these tension-withstanding fibrous tissues attach muscle to bone. Tendons consist of densely packed collagen fibers. Muscles, either individually or in groups, are supported by fascia. Fascia is strong sheath-like connective tissue. The tendon that attaches muscle to bone is part of the fascia.

The muscular system of the spine is complex, with several different muscles playing important roles. The muscles in the vertebral column provide spinal support and stability and serve to flex, rotate, or extend the spine. Specific muscles are associated with movement of parts of the anatomy. For example, the Sternocleidomastoid muscle (neck area) assists with movement of the head, while the Psoas Major muscle (low back area) is associated with flexion of the thigh.