Dermatomes

Each of the spinal nerves exits the spinal canal between two of the vertebra.  Each then goes to a particular area of the body.  The area of skin served by each of these nerves is called its dermatome.

The nerves from the upper cervical spine serve the skin of the neck.  The nerves from C5 to T1 (the mid-cervical spine to the upper thoracic spine) go to the arms.  The nerves from T2 to L2 (thoracic to upper lumbar) go to the chest and abdomen.  Those from L3 to S1 (lumbar and the first sacral) go to the skin of the legs.  The remainder of the sacral nerves, and the coccygeal nerves, go to the groin.

For example, when the C6 nerve is pinched, there is pain and numbness in the thumb and index finger.  When the L5 nerve is pinched, there is pain and numbness in the big or “great” toe.

List of Dermatomes of Commonly Injured Nerve Roots

C5 – The area over the shoulder.

C6 – The thumb and part of the forearm.

C7 – The middle finger.

C8 – The smallest fingers and part of the forearm.

L4 – The thigh.

L5 – The medial part of the calf and foot, the big toe.

S1 – The lateral part of the calf and foot, the smaller toes.

Myotomes

Each of the spinal nerves controls certain muscles.  The muscles (or muscles) controlled by a particular nerve root are called its myotome. 

Nerves from the neck levels control the muscles in the arms.  Those from the thoracic spine control the chest and abdominal muscles.  The nerves from the low back control the muscles in the legs.

For example, when the C6 nerve is pinched, there is weakness in the biceps muscle.  When the biceps is weak, bending the arm at the elbow is difficult.  When the L5 nerve is pinched, there is weakness in the tibialis anterior muscle in the shin.  With L5 weakness, extending the ankle (to walk on the heels) is difficult. 

List of Myotomes of Commonly Injured Nerve Roots

C5 – The deltoid muscle (abduction of the arm at the shoulder).

C6 – The biceps (flexion of the arm at the elbow).

C7 – The triceps (extension of the arm at the elbow).

C8 – The small muscles of the hand.

L4 – The quadriceps (extension of the leg at the knee).

L5 – The tibialis anterior (upward flexion of the foot at the ankle).

S1 – The gastrocnemius muscle (downward flexion of the foot at the ankle).

Reflexes

Some, but not all, of the nerve roots have a reflex.  C5, C6 and C7 have reflexes.  L4 and S1 have reflexes.

For example, when the C6 nerve is pinched, there is loss of the pronator reflex in the forearm.  When the L5 nerve is pinched, there is no reflex loss.  Not all nerves have a reflex which can be tested.

List of Reflexes of Commonly Injured Nerve Roots

C5 – Flexion at the elbow, biceps.

C6 – Flexion at the elbow, brachioradialis.

C7 – Extension at the elbow, triceps.

C8 – Finger flexion.

L4 – The knee reflex, quadriceps.

L5 – No reflex.

S1 – The ankle reflex, gastrocnemius.

The Neurological Examination

The neurologic examination is a series of physical examination tests used to identify the exact location of a problem.  The doctor asks about the location and the character of one’s pain.  He or she then checks the sensation, strength, and reflexes. 

By determining the location of a deficit, one can identify the probable location of an abnormality.  For example, if there is numbness of the lateral part (outside) of the foot, weakness in plantar flexion at the ankle (pushing the foot down) and absence of the ankle reflex, the S1 nerve is likely damaged.  The problem is almost always between the L5 bone and the sacrum.  It is almost always due to a ruptured disc.  Similarly, many problems are identified by careful examination alone.

Go to the next page on special tests.

Go to the next chapter on symptoms.