The Nerve Root Supply of the Upper Extremity

In the cervical spine, nerve roots are numbered by the cervical level above which they exit: The C1 root exits above the C1 vertebra, the C2 root above the C2 vertebra (between C1 and C2), the C7 root above the C7 vertebra (between C6 and C7). The root exiting between C7 and T1 is the C8 root (there is no C8 vertebra). This is different from the thoracic, lumbar, and sacral levels, where roots are numbered by the vertebral level below which they exit: the T1 root exits below T1 (between T1 and T2), the L1 root exits below L1 (between L1 and L2), the S1 root exits below S1 (between S1 and S2) (see Fig. 15–1).

The lateral upper arm is supplied by C5, the lateral forearm and lateral hand (including the thumb) is supplied by C6, the middle of the hand (including the middle finger) is supplied by C7, the medial hand (including the ring finger and the fifth finger) and medial forearm are supplied by C8, the medial upper arm is supplied by T1, and the axilla is supplied by T2; the index finger may be supplied by C6 or C7. (Fig. 16–1). On the dorsum of the arm/hand this same pattern is maintained. To remember this, trace around your own arm naming the dermatomes: On the arm with the palm facing upward, trace around the arm from lateral upper arm (C5) to lateral forearm (C6), around the hand from the thumb (C6) to the index and middle fingers (C7) to the ring and fifth fingers (C8), around to the medial forearm (C8) to the medial upper arm (T1) to the axilla (T2).

The Brachial Plexus

The nerve roots supplying the upper extremity (C5-T1) join to form the brachial plexus. The terminal branches of the brachial plexus are the nerves to the upper arm, forearm, and hand: the axillary, musculocutaneous, radial, ulnar, and median nerves. Nerves that arise from the plexus proximal to the terminal branches supply the shoulder muscles (long thoracic, suprascapular, subscapular nerves), pectoral muscles (medial and lateral pectoral nerves), and sensation to the medial upper arm and forearm (medial brachial cutaneous and medial antebrachial cutaneous nerves).

The best way to learn the brachial plexus is to draw it over and over again (Fig. 16–2). A mnemonic theme in drawing it is the theme of threes. Figure 16-2 is a schematic of the left brachial plexus, oriented with the left arm held out with the left hand on the right side of the page and the thumb up. In the arm, the radius/thumb side is considered lateral and the ulna/fifth finger side is considered medial.

1. 
   

   Draw three headless, armless, short-legged, long-bodied stick figures with the first and third having their small legs pointing to the left, and the middle one shorter and with the small legs pointing to the right (Fig. 16–2A).

   
   
2. 
   

   Draw three lines in the shape of another headless, armless stick figure on its side with the legs connecting to the upper and lower lines and the “body” running parallel to those lines (Fig. 16–2B).

   
   
3. 
   

   Draw three more lines: an X connecting the upper two horizontal lines, and a diagonal line connecting the bottom two (Fig. 16–2C).

   
   
4. 
   

   Label the diagram from proximal (left of the diagram) to distal (right of the diagram): roots, trunks, divisions, cords, branches (nerves) (Fig. 16–2D):

   
   
   
   
   • 
     

     The roots that make up the brachial plexus are C5, C6, C7, C8, and T1. These correspond to the five starting points on the left side of the diagram from top to bottom.

     
     
   • 
     

     The segments before the X and diagonal line are the upper, middle, and lower trunks.

     
     
   • 
     

     The X and the diagonal line are the anterior and posterior divisions, which join to form the three cords.

     
     
     
     
     • 
       

       The three posterior divisions join to form the posterior cord.

       
       
     • 
       

       The anterior divisions of the upper and middle trunks form the lateral cord.

       
       
     • 
       

       The anterior division of the lower trunk forms the medial cord.

       
       

       The posterior, medial, and lateral in the names of the cords are based on their anatomic relationship to the axillary artery.

     
     
   • 
     

     The terminal branches are the five main nerves to the upper arm, forearm, and hand.

     
     
     
     
     • 
       

       The terminal branches of the posterior cord are the radial nerve and the axillary nerve.

       
       
     • 
       

       From top to bottom, the three terminal nerves originating from the sideways “M” shape are the musculocutaneous nerve, median nerve, and ulnar nerve.

Being able to reproduce this much from memory and knowing the muscles supplied by the terminal nerves will get you quite far in terms of localization to nerve(s), cord(s), trunk(s), and/or root(s). The next level of detail is the domain of neuromuscular experts.

Eleven additional nerves come off of the brachial plexus—nine of these can be remembered by several more rules of threes (Fig. 16–2E).

• 
  

  Three nerves arise from roots:

  
  
  
  
  1. 
     

     Long thoracic (C5-C7): innervates serratus anterior (weakness leads to scapular winging)

     
     
  2. 
     

     Nerve to the subclavius (C5-C6): innervates subclavius (depresses the shoulder)

     
     
  3. 
     

     Dorsal scapular nerve (C4-C5): innervates rhomboids (bring the scapula toward the back)

  
  
• 
  

  Three nerves arise from the posterior cord:

  
  
  
  
  1. 
     

     Upper subscapular: innervates subscapularis (internal rotation of the upper arm)

     
     
  2. 
     

     Lower subscapular: innervates subscapularis and teres major (adducts and internally rotates the upper arm)

     
     
  3. 
     

     Thoracodorsal: innervates latissimus dorsi (various movements of the shoulder)

  
  
• 
  

  Three nerves arise from the medial cord, all of which have medial in their names:

  
  
  
  
  1. 
     

     Medial pectoral: innervates pectoralis major and minor (various movements of upper arm)

     
     
  2. 
     

     Medial brachial cutaneous: sensation to medial upper arm

     
     
  3. 
     

     Medial antebrachial cutaneous: sensation to medial forearm

  
  
• 
  

  The last two nerves can be remembered by their relation to two other previously mentioned nerves:

  
  
  
  
  1. 
     

     There is a medial pectoral nerve from the medial cord, so there must be a lateral pectoral nerve, logically arising from the lateral cord (innervates pectoralis major).

     
     
  2. 
     

     There are subscapular nerves, so there must be a supra scapular nerve (innervates supraspinatus and infraspinatus)—this nerve arises from the upper trunk, the only nerve to arise from a trunk. No nerves arise from divisions.

Drawings of the brachial plexus usually focus disproportionately on the complex web of trunks, divisions, and cords as is done here. This “web” is actually much shorter in length than the terminal nerves. The roots and trunks are located between the cervical spine and clavicle, the divisions are just posterior to the clavicle, the cords travel from just below the clavicle to the axilla, and the terminal nerves arise in the axilla. Therefore, pathology affecting the neck and upper chest can cause a plexopathy, whereas pathology in or distal to the axilla will usually affect one or more individual nerves.

The Anatomy of the Nerves of the Upper Extremity

The upper extremity can be divided into the upper arm, forearm, and hand. The thumb and fingers are moved by both intrinsic hand muscles and muscles in the forearm with tendon insertions in the fingers and thumb. To summarize the big picture (see Table 16–1 at the end of this chapter):

• 
  

  The musculocutaneous nerve and axillary nerve only supply upper arm muscles (musculocutaneous: biceps, brachialis; axillary nerve: deltoid, teres minor).

  
  
• 
  

  The radial nerve supplies muscles of the upper arm (triceps, brachioradialis) and forearm. Mnemonic: Radial nerve is the BEST: brachioradialis, extensors, supinator, and triceps.

  
  

  All radial nerve–innervated forearm muscles control the wrist, fingers, and thumb except the supinator (which supinates the forearm). All radial nerve-innervated muscles except triceps, brachioradialis, and the long head of extensor carpi radialis are innervated by the posterior interosseous nerve branch of the radial nerve.

  
  
• 
  

  All intrinsic hand muscles are innervated by the median and ulnar nerves. These two nerves also innervate forearm muscles, but do not innervate any upper arm muscles. All of the forearm muscles innervated by the median and ulnar nerves are involved in wrist, thumb, or finger movements except the median nerve–innervated pronator muscles (which pronate the forearm)

With respect to the wrist, fingers, and thumb, some important anatomic points are as follows (see Table 16–2 at the end of this chapter):

• 
  

  All wrist, finger, and thumb extensors are radial nerve–innervated

  
  
• 
  

  All flexor digitorum superficialis muscles are median nerve-innervated

  
  
• 
  

  All interossei are ulnar nerve-innervated.

  
  
• 
  

  Where there is a division of labor between median and ulnar nerves, the median nerve innervates lateral/radius/thumb–side muscles and the ulnar nerve innervates medial/ulna/fifth finger–side muscles:

  
  
  
  
  • 
    

    Flexor carpi radialis (median) vs flexor carpi ulnaris (ulnar)

    
    
  • 
    

    Flexor digitorum profundus 1 and 2 (median) vs flexor digitorum profundus 3 and 4 (ulnar)

    
    
  • 
    

    Lumbricals 1 and 2 (median) vs lumbricals 3 and 4 (ulnar)

  
  
• 
  

  All nonextensor muscles of the fifth finger are ulnar nerve-innervated (abductor digiti minimi, opponens digiti minimi, flexor digiti minimi, flexor digitorum profundus, lumbrical 4) except the flexor digitorum superficialis (which is median nerve-innervated)

Each muscle of the upper extremity is innervated by a single nerve except the flexor pollicis brevis, which is innervated by both the median and ulnar nerves. However, each nerve is made up of multiple roots, most roots supply multiple nerves, and each muscle is generally supplied by more than one root. Table 16–3 (at the end of this chapter) shows the overlap of roots and nerves for the main clinically tested upper extremity muscles. This table can aid in differentiating between nerve and root lesions based on the pattern of weak muscles.

For example, axillary nerve pathology will cause weakened shoulder abduction (deltoid), but will spare shoulder external rotation (infraspinatus; suprascapular nerve) and internal rotation (subscapularis; subscapular nerve), whereas a C5 root lesion will weaken all of these actions (C5 input lost to axillary, suprascapular, and subscapular nerves). In a radial nerve palsy, the brachioradialis will be weak, but the biceps will remain strong (musculocutaneous nerve), whereas a C6 root lesion will affect both the biceps and the brachioradialis (C6 input lost to musculocutaneous and radial nerves). A C7 root lesion would mimic a radial nerve lesion except that the brachioradialis (muscle and reflex) would be spared and shoulder adduction (latissimus dorsi) would be involved.

The muscles, nerves, and nerve roots associated with the commonly tested actions of the upper extremity are listed in Table 16–4 (categorized by action tested) and Table 16–5 (categorized by nerve tested) at the end of this chapter.