Babies are born with 33 separate vertebrae, some of which fuse over time. Despite being composed of bone, the spine or vertebral column is surprisingly flexible and resilient, as it provides a safe space for the spinal cord and control of many bodily functions we take for granted. Throughout a human life, a healthy spine's memory can help it endure many times the weight and pressure of its own body. It is a complex system whose overall objective is to protect and support.
There are five movable vertebrae in the lumbar region of the spine, at the lower back between the rib cage and the pelvis. Designated as L1 through L5, these vertebrae are wider and heavier than the others because they have to bear the weight of the upper body. They also provide protection and support for the tissue and nerves, which control communication between the brain and the legs. Spinal cord tissue only reaches L2, after which it bundles and tapers to form the conus medullairs.
Designated as T1 through T12, the thoracic region is a set of 12 vertebrae located above the lumbar region. The bones are angled downward and increase in size as they approach the lumbar region. These mid-back bones have limited motion -- their main function is to protect the heart and lungs. What makes them unique is that they are the only means of holding the ribs together. Additionally, the thoracic spine has overlapping spinous processes, bony projections on the back of the vertebrae, which provide extra support.
The cervical spine is the main attachment of the head, neck, and shoulder girdle. Within this region are the vertebrae and internal carotid arteries, which directly supply the brain. Seven vertebrae, C1 to C7, are classified as either typical or atypical. C3 to C6 are the typical cervical vertebrae that are smaller than other vertebrae in the spinal column because they bear less weight. The atypical cervical vertebrae are C1, Atlas, C2, Axis, and C7. The Atlas and Axis vertebrae specialize in facilitating head movement as well as skull attachment. C7 is the longest vertebrae of the cervical spine and is prominent when the head leans forward.
The sacral region is composed of five segments, S1 to S5, that fuse to form a triangular bone that sits between L5 and the tailbone. It forms the back of the pelvis and joints at the hip bones, known as the sacroiliac joints, which are major weight-bearing points. There are four openings on each side for nerves and blood vessels. The sacral nerves and vessels support muscles that control vital functions, such as the pelvic floor, bladder, and anal sphincter.
Composed of three to five small, fused bones, the coccyx or tailbone is the vestigial tail of the spine. It supports the body in the seated position to ensure an even distribution of weight. The tailbone connects the pelvic muscles, which aid in many movements including running and walking.
The spine has three main functions: protecting the spinal cord, maintaining proper posture, and enabling flexibility. The vertebrae that encase the cord help the body stay balanced and upright. Proper posture decreases stress on spine ligaments and abnormal wear on the joints. The neck vertebrae are the most flexible. In general, the joints deal with flexibility while muscles deal with mobility and, together with other components, provide the core of body movement and function.
The combination of muscles, ligaments, and tendons brace the spine to help protect it from injuries like a hyperextension. Ligaments are fibrous tissues that connect bones and stabilize joints. Tendons are more elastic and attach muscle to bone. This complex muscular system helps the spine flex, rotate, and extend, facilitating movement.
The spinal cord is a fragile cylindrical structure that transports electrical impulses between the brain and the spinal nerves. It extends from the brain stem to L1 or L2 and then branches off into a group of nerves that become the caudaequina. The neural structure within the spinal column includes a pair of nerves for each cervical, thoracic, lumbar and sacral vertebrae, forming the peripheral nervous system.
The four curves of the spine fall into two categories: kyphotic and lordotic. Kyphotic curves are the backward curves, the lower part of the 'S’ that naturally occur in the thoracic and sacral regions. The cervical and lumbar regions naturally curve in the opposite direction. This design helps distribute the stress incurred during activity and inactivity.
Intervertebral discs comprise approximately 25 percent of the spinal column length; these cushions of annulus fibrosus and nucleus pulposus act as shock absorbers for the spine. Annulus fibrosus is made of sheets of collagen fibers called lamellae. It encases the nucleus pulposus, which contains water and proteoglycans, a mix of proteins and amino sugars. Both work together to resist compression stress and provide rotational stability.
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