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Lumbar Spine (Lower Back) Anatomy and Function


Lumbar Spine (Lower Back) Anatomy and Function

The Spinal Column

The spinal column is made up of 26 bones: 24 unique vertebrae plus the sacrum and coccyx (tail bone) at the end of the backbone. The Vertebrae seem to be chained together). The vertebrae include:
• 7 cervical vertebrae which makes up the neck
• 12 thoracic vertebrae of the chest
• 5 lumbar vertebra or the “lower back”—L1, L2, L3, L4 and L5.

Bones and disks of lumbar spine
Bones, disks and facet joints of lumbar spine

The back can move in many different directions, it can stiffen as well as be supple. When looked at from the back, the spine appears to be straight, but looked at from the side you can see 2 curves which cause the back to have an “S” shape—it curves forward at the neck (cervical spine) and lower back (lumbar spine) and slightly backwards at the thoracic spine and sacral region. These curves help support the head and provides strength, flexibility and provides super shock absorbing abilities. Many problems with the back are associated with the normal curvature of the back.


Between each vertebra is a cushion called an intervertebral disk. On the anterior side of each vertebra is an oval shaped disk called the vertebral body. On the posterior side of each vertebra is the vertebral foramen, which is an opening through which the spinal cord passes. A crucial job of the back is to protect and support the vital spinal cord and spinal nerves.

A Spinal Segment

Spinal Segment
Segment is made of two vertebrae, the intervertebral disk, and two spinal nerves

A spinal segment forms a functional unit and is made up of two adjacent vertebrae, the intervertebral disk between them, the two spinal nerves that exit from each side of the spinal cord, ligaments and muscles.

The Sacrum

Side view of the sacrum and tailbone
Side view of the sacrum and tailbone, the body of the sacrum forms a joint with the 5th lumbar vertebra.

The sacrum is the last segment of the spine. At birth, it is made of several vertebrae. By the time you’re an adult these vertebrae have fused together to form the sacrum. The Sacrum is a large, triangular bone, in the lower part of the vertebral column and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones; its upper part or base joins with the 5th lumbar vertebra by intervertebral fibrocartilage and at the bottom it joins with the coccyx or tailbone.

The Lumbar Spine

The Lumbar spine consists of the vertebral body, posterior elements, intervertebral disks, and ligaments. The lumbar spine is made up of the five lumbar vertebrae located between the thoracic spine and the sacrum. This area is commonly called the “lower back”. The lumbar vertebrae are the largest of the vertebrae because of their weight-bearing function supporting the torso and head.

L-5, the 5th lumbar vertebra.
L-5, the 5th lumbar vertebra.
Labeled lumbar vertebra
Labeled lumbar vertebra

The function of the structures of the lumbar spine are to protect and support the spinal cord and spinal nerves. The spinal nerves pass through a large hole (foramen) in the center of each vertebrae, which when lined up is called the spinal canal. The lumbar spinal nerves branch off the spinal cord at each level between the vertebrae. The joints—a joint is where two or more bones meet—between the vertebrae contain a disk (intervertebral disk) that acts as a shock absorber.

The vertebrae of the back are “linked” together by pedicles (lamina, transverse process, and spinous process) to form facet joints.

Ligaments of the Back

The function of ligaments is to attach bones to bones and give strength and stability to the back. Ligaments are strong, tough bands that are not very flexible. The vertebral bodies of the back are connected to each other by multiple ligaments which include:
• posterior longitudinal ligaments
• anterior longitudinal ligaments
• intertransverse ligaments
• interspinous ligaments
• supraspinous ligaments

Tendons of the Back

Tendons are elastic tissues that connect muscles to bones.

Muscles of the Back

Muscles support and move the spine.

Nerves of the Back
Lumbar segment, spine, nerves
Lumbar segment, spine, nerves
Vascular structures of the Back

Arteries supply the vertebrae, ligaments, and muscles with nourishment.

Problems, Treatments and Surgery for the Lumbar Spine

• Osteoporosis
• Osteomalacia
• Arthritis
• Ankylosing Spondylitis
• Sacroiliitis
• Lumbosacral sprain and strain
• Acute Cauda Equina Syndrome
• Intervertebral Disk Disease
• Scoliosis
• Spinal Instability
Herniated Disk
Spinal Stenosis

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