Spinal cord and spinal 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 injury or damage may accompany trauma or diseases of the cervical spine or thoracic spine.

The spinal cord does not run through the lumbar spine (lower back). After the spinal cord stops in the lower thoracic spine, the nerve roots from the lumbar and sacral levels come off the bottom of the cord like a "horse’s tail" (cauda equina) and exit the spine (see Figure 1). 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 spinal cord can be divided into segments according to the nerve roots that branch off of it. Nerves along the cord consists of 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal nerves. The nerve roots run through the bony canal, and at each level a pair of nerve roots exits from the spine.

Cervical spine nerve roots. In the cervical spine (neck), the nerve root is named for the lower segment that it runs between (e.g. C6 nerve root at C5-C6 segment).
Lumbar spine nerve roots. In the lumbar spine (lower back), the nerve is named for the upper segment that it runs between (e.g. L4 nerve root at L4-L5 segment)

Nerve irritation from a herniated disc
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 or sciatica).

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

Sometimes, a herniated disc will cause only leg pain or arm pain and not low back or neck pain, and may initially be thought to be a problem with the patient’s leg or 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 run below the knee, and possibly to the foot, and may be accompanied by numbness

Pinched nerve pathology
The two nerves most commonly pinched in the lower back are L5 (lumbar 5) and S1 (sacral 1).

Pinched nerve at L5. 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. Numbness for L5 runs over the top of the foot
Pinched nerve at S1. Impingement of the S1 nerve can lead to weakness with the large gastronemius muscle in the back of the calf, causing difficulty with foot push off (see Figure 3).
Numbness for the S1 nerve 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.

Most cervical pathology will lead to pinching of either C6 or C7 nerve roots in the neck, although sometimes the C5 or C8 nerves may be pinched. Depending on which nerve root is pinched, the following symptoms are likely:

Pinched nerve at C5. This can cause shoulder pain, deltoid weakness, and possibly a small area of numbness in the shoulder. On physical exam, a patient’s biceps reflex may be diminished.
Pinched nerve at C6. This can cause weakness in the biceps and wrist extensors, and pain/numbness that runs down the arm to the thumb. On physical exam, the brachioradialis reflex (mid-forearm) may be diminished.
Pinched nerve at C7. This can cause pain/numbness that runs down the arm to the middle finger. On physical exam, the triceps reflex may be diminished.
Pinched nerve at C8. This can cause hand dysfunction (this nerve supplies innervation to the small muscles of the hand). Pain/numbness can run to the outside of the hand (little finger) and impair its reflex.

Healing from a pinched nerve
The nerve consists of one long cell from the low back or neck down to the foot or hand, so the nerves tend to heal slowly. The nerves heal from the top down, and depending on how much damage is done at the time the nerve becomes impinged (pinched), it may take weeks to months for the nerve to fully to heal.

Treatment of neural impingement is directed at relieving the pain and then allowing the nerve to heal on its own. Nerves need both inflammation and pressure to be painful, so either relieving the inflammation or the pressure can relieve the pain.