Spinal cord and nerve roots

The spinal cord comes off the base of the brain, runs throughout the cervical and thoracic spine, and ends at the lower part of the thoracic spine. Therefore, spinal cord damage may accompany trauma or diseases of the cervical or thoracic spine.

The spinal cord does not run through the lumbar spine. After the spinal cord stops in the lower thoracic spine, the verve roots come off the bottom of the cord like a "horse’s tail" (cauda equina)

Therefore, because the lumbar spine has no spinal cord and comprises a large amount of space for the nerve roots, even serious conditions (such as a large disc herniation) are unlikely to cause paraplegia (loss of motor function in the legs).

The nerve roots run through the bony canal, and at each level a pair of nerve roots exits from the spine. In the cervical spine, the nerve root is named for the lower segment that it runs between (e.g. C6 at C5-C6 segment). In the lumbar spine, the nerve is named for the upper segment that it runs between (e.g. L4 at L4-L5 segment)

The nerve passing to the next level runs over a weak spot in the disc space, which is the reason discs tend to herniate (extrude) right under the nerve root and can cause leg pain (radiculopathy).

Cervical disc herniations tend to irritate the nerve exiting at a particular level (e.g. C6 at C5-C6) Lumbar disc herniations tend to irritate the nerve that lies across a particular level (e.g. L5 at L4-L5) 2) Thoracic disc herniations are very rare

Sometimes, a herniated disc will cause only leg/arm pain and not low back/neck pain, and may initially be thought to be a problem with the leg/arm.

Arm pain from a cervical disc herniation is usually accompanied by numbness/tingling and runs to the fingers Leg pain from a lumbar disc herniation will usually below the knee, and possibly to the foot, and is may be accompanied by numbness

The two nerves most commonly pinched are L5 (lumbar 5) and S1 (sacral 1). The L5 nerve supplies the nerves to the muscles that raise the foot and big toe, and consequently, impingement of this nerve may lead to weakness in these muscles. Likewise, S1 impingement can lead to weakness with the large gastronemius muscle in the back of the calf, causing difficulty with foot push off

Numbness for L5 runs over the top of the foot and for S1 it runs on the outside of the foot. The S1 nerve root also supplies innervation for the ankle jerk (tap on the achilles tendon and the foot goes down), and a loss of this reflex indicates S1 impingement, although it does not create loss of function.