The lumbosacral somatic motor center is located at the lumbosacral enlargement of the spinal cord. This center consists of a pair of anterior horn motor neuron columns extending from L2 to S3 spinal segments. The axons of these neurons exit through the ventral spinal surface of the cord. These axons form the ventral roots which travel for a long distance in the spinal canal before joining the dorsal roots just before the intervertebral foramina. The dorsal roots are composed of the central axons of the sensory neuron located in the dorsal ganglia.
The dorsal ganglia is located just before the union of the ventral and dorsal roots. The cauda equina is the structure created by the ventral and dorsal roots as they travel the distance from the spinal cord to their corresponding intervertebral foramina inside the spinal canal. The cauda equina extends from the twelfth thoracic vertebral body to the fifth sacral vertebral foramina. The spinal nerves are formed by the union of the dorsal and ventral roots. The spinal nerves exit through the intervertebral foramina and shortly after split into the dorsal and ventral rami (Figure 229.1). The dorsal rami innervate the skin and musculature of the lower trunk. The ventral rami form the lumbosacral plexus.

Figure 229.1.— Schematic representation of the formation of the lumbosacral plexus. The spinal cord, ventral and dorsal roots, and the dorsal ganglion form the cauda equina.