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Fig 1: Spine
Fig 2: Spinal Cord and Nerve Roots
Fig 3: Distribution of Disc Herniations
Fig 4: Distribution of Skin Innervation

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 nerve roots come off the bottom of the cord like a "horseís 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.

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

Disc Herniations

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).

bulletCervical disc herniations tend to irritate the nerve exiting at a particular level (e.g. C6 at C5-C6)
bulletLumbar disc herniations tend to irritate the nerve that lies across a particular level (e.g. L5 at L4-L5) (Figure 2)
bulletThoracic 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.

bulletArm pain from a cervical disc herniation is usually accompanied by numbness/tingling and runs to the fingers
bulletLeg pain from a lumbar disc herniation will usually run below the knee, and possibly to the foot, and 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.

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

bulletC5 - shoulder pain, deltoid weakness, and possibly a small area of numbness in the shoulder. On physical exam, a patientís biceps reflex may be diminished.
bulletC6 - 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.
bulletC7 - pain/numbness that runs down the arm to the middle finger. On physical exam, the triceps reflex may be diminished.
bulletC8 - 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.

The nerve consists of one long cell from the back/neck down to the foot/hand, so the nerves tend to heal slowly. They heal from the top down, and depending on how much damage is done at the time the nerve becomes impinged, it may take weeks to months to heal.

Treatment of nerve impingement is directed at relieving the pain and then allowing the nerve to heal on its own. Your nerves need both inflammation and pressure to be painful, so either relieving the inflammation or the pressure has the potential to relieve your pain.



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