The spinal cord is the part of the central nervous system, which extends from the brain to the lower back through the bony protection of the spinal canal. It acts as a conduit for sensory and motor information to travel to and from the brain. The spinal cord usually ends at the second lumbar level (L2) and then extends as a bundle of individual nerves, known as the cauda equina, which exit the spinal canal at each spinal level.
From the spinal cord, nerves exit the spinal canal between each vertebra on both sides. After exiting the spinal canal, spinal nerves then further entwine and extend to send signals between your brain and your organs, muscles, and other tissues. Cervical spinal nerves innervate (provide muscle activity and sensation functions to) your upper back, arms, and hands. Lumbar spinal nerves innervate your lower back, abdomen, and legs. If any of these nerves are pinched by a bulging disc or the position of your vertebrae, for example, you might experience back, groin, and/or leg pain or numbness.
Myotomes are muscle groups that are innervated by particular spinal nerve levels. By knowing which spinal nerves innervate specific muscles, we can monitor those muscles for changes in the nerves’ health. For example, we can monitor the following muscle groups for information about the corresponding spinal nerves:
• Quadriceps (front thigh muscles) L2, L3, L4
• Anterior Tibialis (shin muscle) L4, L5
• Hamstrings (back thigh muscles) L5, S1
• Gastrocnemius (calf muscle) S1, S2