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The human spine is made up of 24 spinal bones, called vertebrae. Vertebrae are stacked on top of one another to create the spinal column. The spinal column gives the body its form. It is the body’s main upright support. From the side, the spine forms three curves. The neck, called the cervical spine, curves slightly inward. The mid back, or thoracic spine, curves outward. The outward curve of the thoracic spine is called kyphosis. The low back, also called the lumbar spine, curves slightly inward. An inward curve of the spine is called lordosis.
The lumbar spine is made up of the lower five vertebrae. Doctors often refer to these vertebrae as L1 to L5. The lowest vertebra of the lumbar spine, L5, connects to the top of the sacrum, a triangular bone at the base of the spine that fits between the two pelvic bones. Some people have an extra, or sixth, lumbar vertebra. This condition doesn’t usually cause any particular problems.
Each vertebra is formed by a round block of bone, called a vertebral body. The lumbar vertebral bodies are taller and bulkier compared to the rest of the spine. This is partly because the low back has to withstand pressure from body weight and from daily actions like lifting, carrying, and twisting. Also, large and powerful muscles attaching on or near the lumbar spine place extra force on the lumbar vertebral bodies.
A bony ring attaches to the back of each vertebral body. When the vertebrae are stacked on top of each other, these rings form a hollow tube. This bony tube surrounds the spinal cord as it passes through the spine. Just as the skull protects the brain, the bones of the spinal column protect the spinal cord.
The spinal cord extends down to the L2 vertebra. Below this level, the spinal canal encloses a bundle of nerves that goes to the lower limbs and pelvic organs. The Latin term for this bundle of nerves is cauda equina, meaning horse’s tail.
As the spinal cord travels from the brain down through the spine, it sends out nerves on the sides of each vertebra called nerve roots. These nerve roots join together to form the nerves that travel throughout the body and form the body’s electrical system. The nerve roots that come out of the lumbar spine form the nerves that go to the lower limbs and pelvis. The thoracic spine nerves go to the abdomen and chest. The nerves coming out of the cervical spine go to the neck, shoulders, arms, and hands.
It is sometimes easier to understand what happens in the spine by looking at a spinal segment. A spinal segment includes two vertebrae separated by an intervertebral disc, the nerves that leave the spinal cord at that level, and the small facet joints that link each level of the spinal column.
The intervertebral disc normally works like a shock absorber. It protects the spine against the daily pull of gravity. It also protects the spine during heavy activities that put strong force on the spine, such as jumping, running, and lifting.
An intervertebral disc is made up of two parts. The center, called the nucleus, is spongy. It provides most of the disc’s ability to absorb shock. The nucleus is held in place by the annulus, a series of strong ligament rings surrounding it. Ligaments are strong connective tissues that attach bones to other bones.
Between the vertebrae of each spinal segment are two facet joints. The facet joints are located on the back of the spinal column. There are two facet joints between each pair of vertebrae, one on each side of the spine. A facet joint is made of small, bony knobs that line up along the back of the spine. Where these knobs meet, they form a joint that connects the two vertebrae. The alignment of the facet joints of the lumbar spine allows freedom of movement as you bend forward and back.
The surfaces of the facet joints are covered by articular cartilage. Articular cartilage is a smooth, rubbery material that covers the ends of most joints. It allows the bone ends to move against each other smoothly, without pain.
Two spinal nerves exit the sides of each spinal segment, one on the left and one on the right. As the nerves leave the spinal cord, they pass through a small bony tunnel on each side of the vertebra, called a neural foramen. (The term used to describe more than one opening is neural foramina.)
The lumbar spine is supported by ligaments and muscles. The ligaments are arranged in various layers and run in multiple directions. Thick ligaments connect the bones of the lumbar spine to the sacrum (the bone below L5) and pelvis.
The muscles of the low back are also arranged in layers. Those closest to the surface are covered by a thick tissue called fascia.
The middle layer, called the erector spinae, has strap-shaped muscles that run up and down over the lower ribs, chest, and low back. They blend in the lumbar spine to form a thick tendon that binds the bones of the low back, pelvis, and sacrum.
The deepest layer of muscles connects along the back surface of the spine bones. The muscles also connect the low back, pelvis, and sacrum. These deepest muscles coordinate their actions with the muscles of the abdomen to help hold the spine steady during activity.
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