The spinal cord is composed of millions of nerve fibers, which form a tube-like structure that extends from the brain to the area between the L1 and L2 vertebrae in the upper lumbar region. The nerve fibers branch off from the spinal cord and form the nerve roots, which are paired at each spinal level for a total of 32 pairs. These nerve fibers are directed to various parts of the body. The cervical nerve roots connect to the upper body and arms and hands, and the nerve roots in the thoracic spine go to the chest and abdomen. The nerve roots in the lumbar spine innervate the legs, bowel and bladder. The dorsal root ganglion is a nodule on the dorsal (posterior) root portion of the spinal nerve that contains cell bodies of outgoing spinal nerve neurons.

The end of the spinal cord is tapered and is called the conus medullaris. There is a thread that continues from the conus called the filum terminale. These structures are surrounded by the roots of the caudal spinal nerves (nerves at the lower end) and, all together, are termed the cauda equina. The nerve roots each leave the vertebral canal through an intervertebral foramen, so the roots in the cauda equina get fewer as it gets farther down the spinal canal. The nerve roots in the cauda equina go to the lower extremities and the bowel and bladder.

The thecal sac is a protective membrane that covers the spinal cord and cauda equina and contains cerebrospinal fluid that provides nutrition to the spinal cord. The membrane is composed of several layers: the outermost is the dura mater, the middle layer is the arachnoid mater and the inner layer is called the pia mater. The thecal sac is separated from the wall of the vertebral canal by the epidural space, which contains epidural fat.