The lumbar spine consists of 5 vertebral segments and 5 lumbar nerve roots. L1 is identified as the 1st non rib bearing segment below T12. L5 is identified as the segment above S1. The spinal cord terminates between T12 and L2. The nerve roots below the spinal cord are collectively called the cauda equina or "horse tail". This helps to explain why lumbar punctures are typically performed below L2.
The cauda equina nerve roots have a specific orientation within the spinal canal. As in the thoracic spine the nerve root for a specific level exits below the corresponding vertebral body level. This "exiting" nerve root is found within the neural foramen. For example at the L2-L3 level, the exiting nerve root found in the neural foramen is L2 (see image below). The nerve root that is waiting to exit at the level below is referred to as the traversing nerve root and can be found within the lateral recess.
This diagram is an axial depiction of a lumbar level with a disk herniation. Notice how the disk herniationi in this diagram affects both the nerve in the neural foramen and within the lateral recess. Depending on the location of the disk herniation, patients may be asymptomatic, have a radiculopathy related to an affected nerve within the lateral recess and/or within the neural foramen.
1. Netter Diagram