What is it?

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.

Nerves control the body’s functions including the vital organs, sensation, and movement. The nervous system receives information and initiates an appropriate response. It is affected by internal and external factors (i.e. stimulus).

Nerves follow tracts and cross over junctions called synapses. Simplified, it is a complex communicative process between nerves conducted by chemical and/or electrical changes. For more detailed information regarding nerves go here.

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 "horse’s tail". This is known as the cauda equina. (see figure to left). For more detailed infomration regarding the spinal cord go here.

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. The nerve roots pass out of the spinal canal through the intervertebral foramen, where they feed the body either anteriorly (motor) or posteriorly (sensory). The anterior divisions supply the front of the spine including the limbs. The posterior divisions are distributed to the muscles behind the spine.

This image is a schematic that shows the divisions, distribution and major functions of the nervous system.
 

Other Spinal Cord and Nerve Structures

Cerebrospinal Fluid (CSF)
Cerebrospinal fluid is a clear fluid found in the brain chambers (Ventricles), spinal canal, and spinal cord. This fluid is secreted from the Choroids Plexus, a vascular part in the ventricles of the brain. CSF bathes and circulates among these tissues and acts as a shock absorber to protect against injury. The fluid contains different electrolytes, proteins, and glucose. In an average adult the total volume of CSF is about 150 milliliters.

Meninges
Meninges are membranes that cover and protect the brain and spinal cord. There are three primary types: (1) Dura Mater, (2) Arachnoid Mater, and (3) Pia Mater.

(1) The dura mater, or dura, is the gray outer layer of the spinal cord and nerve roots. It is made of strong connective tissue.
(2) The arachnoid mater resembles a loosely woven fabric of arteries and veins. This layer is thinner than the dura mater. The subarachnoid space is filled with cerebrospinal fluid.
(3) The pia mater is the innermost layer and is a delicate and highly vascular membrane providing blood to the neural structures.

Dermatomes

A dermatome is an area of skin supplied by fibers from a single spinal nerve root.

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) (Figure 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 gastrocnemius muscle in the back of the calf, causing difficulty with foot push off (see Figure 3).

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:

• C5 - 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.
• C6 - 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.
• C7 - pain/numbness that runs down the arm to the middle finger. On physical exam, the triceps reflex may be diminished.
• C8 - 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 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.

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