Overview of the Spine
Functions of the Spine
- Provide structural support and balance to maintain an upright posture.
- Protect the spinal cord, nerve roots and several of the body’s internal organs.
- Enable flexible motion.
Regions of the Spine
- Cervical (neck region)
- Thoracic (chest region)
- Lumbar (lower back)
- Sacral (tailbone)
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.
The ribs attach to the thoracic spine. These rib attachments add strength and stability the thoracic spine. The rib cage and ligaments limit range of motion and protect many vital organs.
The Skull and Pelvis
A kyphotic curve is a convex curve in the spine, i.e., a curve projecting outward. The curves in the healthy thoracic and sacral spine are kyphotic. However, sometimes a spine can be excessively kyphotic, creating a “humpback”.
A lordotic curve is concave, i.e. an inward curve. Lordotic curves are found in the healthy cervical and lumbar regions of the spine.
Body: This is the largest part of the vertebra. Viewed from above, it looks oval. Viewed from the side, the vetebral body is thicker at the ends and thinner in the middle.
Pedicles: Two short processes (finger-like projections) 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 at the midline.
Processes: Spinal processes are finger-like projections found on vertebral bodies. 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.
Two transverse processes (also called costal processes), project from either side of the point where the lamina meets the pedicle between the upper and lower articular processes. They help anchor the muscles and ligaments in that area.
Spinous processes extend from vertebrae toward the back, 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 anendplate. 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. This creates “tunnels” within the spine known as foraminal passageways. Foraminal passageways provide space for nerve roots to branch out from the spinal canal.
Each vertebra has two facet joints, one at the top (superior) and one at the bottom (inferior). Thesuperior 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 and protected by a specialized capsule. This capsule is made of connective tissue and produces synovial fluid to help lubricate and nourish the joint. The surfaces of the joint itself are coated with cartilage, which provides a smooth surface on which the parts of the joint can move (articulate) easily.
Each disc is made up of two parts: the annulus fibrosus and the nucleus pulposus.
The annulus is a layered structure consisting of water and rugged, elastic collagen fibers. Collagen, in turn, is made up of fibrous bundles of protein bound together by a proteoglycan gel. The fibers are oriented at different angles in the horizontal plane, similar to the construction of a radial tire.
Spinal Cord and Nerve Roots
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). The neural (nerve) structures of the body are listed below.
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 are involved with the head, neck, shoulders, arms, and hands.
- Thoracic nerves (12 pairs): These nerves are involved with portions of the upper abdomen and muscles in the back and chest areas.
- Lumbar nerves (5 pairs): These nerves are involved with the lower back and legs.
- Sacral nerves (5 pairs): These nerves are involved with 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
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).
Ligament name and function description:
- Anterior longitudinal ligament (ALL): A primary spine stabilizer About one-inch wide, the ALL runs the entire length of the spine from the base of the skull to the sacrum. It connects the front (anterior) of the vertebral body to the front of the annulus fibrosis.
- Posterior longitudinal ligament (PLL): A primary spine stabilizer of about one-inch wide, the PLL runs the entire length of the spine from the base of the skull to sacrum. It connects the back (posterior) of the vertebral body to the back of the annulus fibrosis.
- Supraspinous ligament: This ligament attaches the tip of one spinous process to the other.
- Interspinous ligament: This thin ligament attaches to another ligament called the ligamentum flavum that runs deep into the spinal column.
- Ligamentum flavum : This yellow ligament is the strongest ligament in the whole body. It runs from the base of the skull to the pelvis, in front of and between the lamina, and protects the spinal cord and nerves. The ligamentum flavum also runs in front of the facet joint capsules.
Tendons and Muscles
Muscles, either individually or in groups, are supported by fascia. Fascia is a type of 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 specific parts of the anatomy. For example, thesternocleidomastoid muscle (neck area) allows for movement of the head, while the psoas major muscle (low back area) allows bending or flexion of the thigh.