The lower back is home to the lumbar vertebrae, the sacrum, and the coccyx. These are some of the bones responsible for posture, movement, and protection of the spinal cord, which is a long tube of nervous tissue connected to the brain. The brain and spinal cord make up the central nervous system of the human body.
As the spinal cord extends down the back from the brain, it has numerous branches that extend all over the body — to the tip of each finger and toe. The spinal cord consists of two tracts: the ascending tract and the descending tract. The ascending tract receives sensory information from nerves and sends it to the brain. The brain sends messages to the body through the descending tract. These messages tell muscles what they should do.
In addition to the vertebral column, membranes called meninges also protect the spinal cord by enveloping them and holding a protective fluid called cerebrospinal fluid, commonly known as spinal fluid. This fluid helps protect the spine from shocks and other types of damage. It also acts as a transport system for nutrients to keep the spinal cord healthy.
The spinal cord has three meningeal membranes, or layers:
- Dura mater, the outermost layer
- Arachnoid mater, the middle layer
- Pia mater, the innermost layer that is closest to the brain and spinal cord
The spinal cord ends near the first or second lumbar vertebrae, or in the small of the back. From there, it tapers into a dangling structure of nerves called the cauda equina.
A collection of nerves branch out from the sacrum, the triangle-shaped bone at the end of the spinal column. These serve the pelvic area, genitals, buttocks, and parts of the legs and feet. The collection is called the sacral plexus.
The sacral plexus also gives rise to the sciatic nerve, a large nerve fiber that runs into the pelvis, travels through the buttock beneath the gluteus maximus muscle, and extends down the back of the thigh. There is one sciatic nerve in each leg. It is about one inch wide in the pelvis and narrows as it travels down the leg.