The spine is one of the most remarkable parts of the human body. A basic understanding of the spine’s anatomy and its functions is extremely important to patients with spinal disorders.
Functions of the Spine
The three main functions of the spine are to:
- 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
The spine consists of three main regions: cervical, thoracic, lumbar, and sacral.
Cervical Spine
The cervical spine is the spine in the neck. 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 vertebrae. The thoracic vertebrae are larger than the cervical bones and have longer spinous processes.
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.
Lumbar Spine
The lumbar spine consists of five vertebrae abbreviated L1 to L5. The lumbar vertebrae are the largest vertebrae and carry most of the body’s weight. This region allows more range of motion than the thoracic spine, but less than the cervical. The lumbar vertebrae “interlock” with each other forming facet joints. Lumbar facet joints enable significant flexion and extension movement, but limit rotation (twisting).
Sacral Spine
The sacrum is located behind the pelvis. Five bones, abbreviated S1 to 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 connect to the spinal column and impact balance.
Anatomical Planes
Physicians who need to describe specific locations of the body often use body planes. A body plane is an imaginary flat, two-dimensional surface that defines a particular area of anatomy.
| Body Plane | In Plain Language |
| Frontal or coronal plane | A plane that divides the front and back halves of the entire body, head to toe |
| Median or sagittal plane | A plane that divides the left and right sides of the entire body, head to toe |
| Transverse or axial plane | A plane that 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., a curve projecting outward. The curves in the healthy thoracic and sacral spine are kyphotic. However, sometimes a spine can be excessively kyphotic, such as 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.
Vertebral Structures
The vertebrae are themselves complex 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. Important parts of the vertebrae include the following.
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 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. This creates “tunnels” within the spine known as foraminal passageways. Foraminal passageways provide space for nerve roots to branch out from the spinal canal.
Facet Joints
At the back of each vetebral body is a specialized type of connection called a facet joint. Facet joints connect one vetebral body to the next.Facet joints help the spine to bend, twist, and stretch in different directions. The facet joints restrict excessive movement such as hyperextension and hyperflexion, i.e. whiplash.
Each vertebra has two facet joints, one at the top (superior) and one at the bottom (inferior). 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 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.
Intervertebral Discs
Between each pair of vertebral bodies is an intervertebral disc (sometimes just called the disc). The intervertebral discs cushion the vertebral bodies by absorbing stress as the body moves. These discs also prevent the vertebral discs from grinding against each other. They are the largest structures in the body without a their own system of blood vessels. For that reason, discs have to rely on osmosis to absorb the nutrients they need from the blood supply.
Each disc is made up of two parts: the annulus fibrosus and the nucleus pulposus.
Annulus Fibrosis
The annulus is a ring-shaped structure that surrounds and 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 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.
Nucleus Pulposus
At the center of each disc, surrounded by the annulus fibrosis, is a gel-like elastic substance known as the nucleus pulposus. Working 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 endplates on the top and bottom of each vertebral body are made of cartilage and blend into the intervertebral disc in order to anchor it firmly in place.
Spinal Cord and Nerve Roots
The spinal cord is contained and protected within the spinal canal (at the center of the vertebrae). The spinal cord is a slender cylindrical structure about the diameter of the little finger. It 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). 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
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).
| Ligament Name and Function | Description |
|
Anterior longitudinal ligament (ALL) |
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) |
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 The strongest ligament |
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
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 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, the sternocleidomastoid muscle (neck area) allows for movement of the head, while the psoas major muscle (low back area) allows bending or flexion of the thigh.