Anatomy of the Nerve Roots of the Lower Extremity

At the thoracic, lumbar, and sacral levels, roots are numbered by the vertebral level below which they exit: The T1 root exits below the T1 vertebra (between the T1 and T2 vertebrae), the L1 root exits below the L1 vertebra (between the L1 and L2 vertebrae), and the S1 root exits below the S1 vertebra (between the S1 and S2 vertebrae). The spinal cord ends at the L1-L2 vertebral level and the lumbar and sacral roots must therefore descend to reach the vertebral levels at which they exit. These descending roots are referred to as the cauda equina (See Fig. 15-1).

The sensory supply to the anterior thigh is covered by L1, L2, and L3 in three diagonal stripes running from proximal/lateral to distal/medial. L4, L5, and S1 cover the anterior shin in vertical stripes from medial to lateral: L4 covers the medial knee, medial shin, and instep; L5 covers the anterior and lateral shin and dorsum of the foot; and S1 the covers the distal lateral calf and lateral aspect and plantar surface of the foot (Fig. 17–1). A mnemonic way to remember the dermatomes of the lower extremity is to place your hands on your hips pointing inward/downward and then pat the thighs three times moving distally toward the knee (L1, L2, L3). From the knee, point the hands directly downward toward the feet and pat the shins three times from medial to lateral (L4, L5, S1). This medial-to-lateral pattern continues on the foot with the medial foot (instep) supplied by L4, the lateral foot supplied by S1, and the majority of the dorsum of the foot supplied by L5 between the medial L4 and lateral S1 dermatomes.

The posterior middle thigh and calf are supplied by S1 (laterally) and S2 (medially), and the S1 and S2 dermatomes are bounded by L3-L4 medially and L5 laterally. See also Fig. 15-2.

To learn the motor actions controlled by each root, practice moving through the sequence of tested muscle groups from proximal to distal in front, then from proximal to distal in back, naming the roots as you move the associated muscles (Fig. 17–2): L2-L3 (hip flexion), L3-L4 (knee extension), L4-L5 (dorsiflexion of the foot), L5-S1 (hip extension), L5-S1 (knee flexion), S1-S2 (plantarflexion of the foot). All root pairs in the movement sequence are overlapping and in sequence, but note that L5-S1 is repeated for both hip extension and knee flexion.

Anatomy of the Lumbosacral Plexus and the Nerves of the Lower Extremity

The lower extremity is supplied by nerve roots L1 through S3. These nerve roots converge to form the lumbosacral plexus, which is divided into the lumbar plexus and the sacral plexus. Although diagrams of the lumbosacral plexus look equally as complex as those of the brachial plexus, localization is more straightforward since the foot is not as intricately controlled as the hand. Before going into the details, note that in general the lumbar plexus (L1-L4) only supplies muscles of the hip and thigh (though not all of them), and the sacral plexus (L4-S4) supplies all muscles distal to the knee (as well as muscles of the posterior and lateral hip and thigh that are not supplied by the lumbar plexus).

The L1-L4 roots supply the lumbar plexus, which innervates the muscles of the anterior and medial hip and thigh and provides sensory innervation to the anterior, medial, and lateral thigh, as well as the medial foot and shin. The sensory innervation of the medial foot and shin is supplied by the saphenous nerve, which is the only below-the-knee function of the lumbar plexus.

The functions of the lumbar plexus include (Table 17–1):

• 
  

  Motor

  
  
  
  
  • 
    

    Femoral nerve: iliopsoas (hip flexion) and quadriceps (knee extension and patellar reflex)

    
    
  • 
    

    Obturator nerve: adductor muscles (hip/thigh adduction)

  
  
• 
  

  Sensation

  
  
  
  
  • 
    

    Anterior thigh: femoral nerve

    
    
  • 
    

    Medial thigh: branches of the femoral and obturator nerves

    
    
  • 
    

    Lateral thigh: lateral femoral cutaneous nerve

    
    
  • 
    

    Medial shin and foot: saphenous nerve, a branch of the femoral nerve (Mnemonic to remember the origin of the saphenous nerve: saphenous nerve arises from femoral nerve)

The lumbar plexus also supplies the less clinically relevant ilioinguinal, iliohypogastric, and genitofemoral nerves, which supply the muscles of the lower abdominal wall and sensation in the inguinal region.

The L4-S4 roots supply the sacral plexus, which innervates all muscles below the knee as well as the muscles of the lateral and posterior thigh. The sensory supply of the sacral plexus covers the posterior thigh, and the shin, calf, and foot except for the medial shin/foot (supplied by the saphenous nerve from the lumbar plexus). The functions of the sacral plexus include (Table 17–1):

• 
  

  Motor:

  
  
  
  
  • 
    

    Superior and inferior gluteal nerves: gluteal muscles (gluteus maximus, minimus, and medius: thigh extension and abduction)

    
    
  • 
    

    Sciatic nerve: hamstrings (biceps femoris, semitendinosus, semimembranosis: knee flexion). The sciatic nerve is composed of two component nerves that diverge at the level of the knee:

    
    
  • 
    

    Peroneal nerve: muscles of the lateral and anterior compartment of the shin/calf (tibialis anterior, peroneus longus and brevis: ankle dorsiflexion and eversion; extensors of the toes)

    
    
  • 
    

    Tibial nerve: muscles of the medial and posterior compartment of the shin/calf and intrinsic muscles of the foot (gastrocnemius, soleus, tibialis posterior: ankle plantar flexion and inversion; flexors of the toes)

  
  
• 
  

  Sensation:

  
  
  
  
  • 
    

    Posterior thigh and calf: posterior femoral cutaneous nerve

    
    
  • 
    

    Anterior and lateral shin and foot: peroneal nerve

    
    
  • 
    

    Plantar surface of the foot: tibial nerve branches

The overlap of roots and nerves for the main clinically tested lower extremity muscles is shown in Table 17–2. Muscle names in bold also have associated reflexes. The muscles are listed across from the nerve that supplies them and under the most prominent root supply (most muscles receive root supply from 1-3 adjacent nerve roots). This chart can aid in differentiating between nerve and root lesions based on the pattern of weak muscles.

The muscles, nerves, and nerve roots associated with the commonly tested actions of the lower extremity are listed in Table 17–3 (categorized by action tested) and Table 17–4 (categorized by nerve tested).