The spine is the column of bones that encase and protect the spinal cord, composed of 33 individual vertebrae known as vertebrae, also referred to as back vertebrae or spine vertebrae.
Between the vertebrae are oval-shaped discs that act as shock absorbers. Each vertebra has four facet joints to connect its members together.
Vertebrae
The spinal column consists of vertebrae, irregular bones composed primarily of tough, strong cortex bone (cortical) and soft, spongy cell bone (cancellous). Vertebral shapes vary greatly among species.
Each vertebra consists of a body that is surmounted by a curved arch formed of pedicles and laminae that connects them, which allows passage for spinal nerves. Each vertebra also includes a spinous process extending downward as well as transverse processes which articulate with other vertebrae in its vicinity.
Primitive chordates feature a rodlike structure called the notochord to stiffen their bodies and provide the first form of vertebral arches. As higher vertebrates evolve into vertebrates with more complex bodies and centrums of vertebrae are gradually replaced. Each vertebra also contains an intervertebral disc for padding purposes and to allow movement between adjacent vertebrae bodies.
Intervertebral Discs
Intervertebral discs provide structural support and flexibility to the spine, transmitting loads from muscle activity and body weight through to allow bending and flexion of the spinal column. An avascular, aneural disc (AF) typically comprises 15-25 sheets of cartilaginous tissue known as lamellae that are layered upon one another in different planes forming a radial-ply structure which increases strength while resisting shear forces.
The AF is composed of an annulus fibrosus with a thick outer tire-like structure called nucleus pulposus that houses its gelatinous core – known as nucleus pulposus or “NP”. This gel-like core acts like a shock absorber by carrying loads from upper and lower vertebrae without grinding into each other and acting as a shock absorber; mutations involving genes involved with production of aggrecan and type II collagen cause premature degeneration of this area of cartilage.
Facet Joints
Facet joints (zygapophyseal joints) are synovial joints located between adjacent vertebrae. They function to restrict vertebral movement while providing load transmission. Each one is covered with a tough fibrous capsule.
Facet joint pain can be difficult to diagnose, since its symptoms overlap with many other spine conditions. A detailed history and physical exam are crucial. Your physician may also order imaging tests such as an X-ray, CT scan or MRI in order to pinpoint its source.
While facets facilitate spinal motion and provide stability, they are susceptible to degeneration which can lead to pain and disability. New management strategies continue to be explored such as steroid joint injections or facet replacement systems.
Ligaments
Ligaments are strong bands of connective tissue that connect bones together at their joints. Ligaments also stabilize these joints to prevent excessive movement or dislocation from taking place, helping ensure they remain steady over time.
Your body contains over 900 ligaments, most of which are found in your arms and legs. Ligaments serve as strong, tightly attached straps or ropes that connect bones, joints and organs together and support them as necessary.
Ligaments don’t always connect directly to bones, however. Instead, they serve to ensure internal organs stay in their proper places in the abdomen – for instance, gastrosplenic ligament which secures stomach and spleen or round ligament of uterus (which helps the womb remain positioned during gestation). They may also connect organs together such as liver intestine and stomach ligament connecting these structures in the abdominal cavity.
Muscles
Muscles are flexible tissues that contract to generate force. When our brain sends a signal for a muscle to contract, its fibers shorten and pull on nearby bone structure causing tension on that particular bone. Muscles work in pairs – when one shortens, its partner lengthens.
Each muscle is enclosed by a sheath of connective tissue known as the epimysium and features bundles of muscle fibers called fasciculi within this sheath, each fascicle being comprised of myofibrils; myofibrils being groups of muscle fibers bundled in an irregular fashion creating what’s known as striated appearance. Actin and myosin proteins play an essential role in muscle contraction within sarcomeres as basic units of contraction.
Different muscles can be distinguished based on their shape, size and direction. For instance, the deltoids have a triangular or delta shape; serratus muscles have saw-tooth edges; while rectus abdominis muscles possess straight, linear orientations.