Neuromuscular disorders are those that affect the anterior horn (motor neuron diseases, such as amyotrophic lateral sclerosis [ALS]), roots (radiculopathies), plexuses (plexopathies), or peripheral nerves (polyneuropathy, mononeuropathy). These also include diseases of the neuromuscular junction (e.g., myasthenia gravis) and muscle fibers (myopathies) ( Fig. 1-1 ). Several of these disorders could involve other regions of the nervous system such as the spinal cord and its pathways or other organs in the body. The following is a review of the basic anatomy and physiology of muscle and nerve that is of importance in understanding these disorders.
The performance of movements requires the interaction of neuronal systems of the cerebral cortex and the motor neurons of the brainstem and anterior horns of the spinal cord. The fine modulation of these movements is regulated by several pathways that include the proprioceptive input for feedback, the interaction of the cortical neurons, limbic system, brainstem, and interneuronal systems.
The feedback is regulated by the interaction of receptors in muscle spindles and deep tendon organs . The muscle spindle has intrafusal muscle fibers that when stretched activate their 1 alpha nerve fibers that stimulate motor neurons of agonist muscles to contract. They also activate inhibitory neurons that go to motor neurons of antagonist muscles. The sensitivity of the spindles varies with their length, which is determined by the contraction of its intrafusal fibers that are innervated by gamma motor neurons. Another regulatory mechanism is the input of the deep tendon organs through their 1b afferent axons that are activated upon changes in muscle tension, causing the inhibition of agonist motor neurons, while facilitating antagonist muscles to contract.
The motor unit is the final pathway of the motor system. This is formed by the motor neurons of the spinal cord or brainstem, their myelinated axons, and the muscle fibers innervated by that neuron, which are intermixed with fibers from other motor units. The physiologic and biochemical characteristics of muscle fibers of a motor unit are determined by the rate of firing of their motor neurons.
There are two major types of muscle fibers, depending on their speed of contraction, their biochemical characteristics, and, thus, their histochemical staining, and all muscle fibers of a motor unit are of the same type. Type I fibers correspond to the red or dark meat in animals. Type II muscle fibers correspond to white meat ( Table 1-1 ). The characteristics of these fibers could be changed by cross-innervation from nerves of one type of muscle to the other or by prolonged stimulation of their axons at different rates. Type I muscle fibers are slow-contracting and stain pale with ATPase using alkaline pH, and dark with oxidative stains. These fibers also have subtypes that can be recognized by special histochemical stains, such as nonspecific esterase and menadione-mediated alpha glycerophosphate dehydrogenase.
| Type I | Type II | |
|---|---|---|
| Contraction time | Slow (tonic) | Fast (twitch) |
| Oxidative enzyme content (i.e., NADH-TR*) | High | Low |
| Capillary supply | Rich | Poor |
| Myofibrillar adenosine triphosphatase (pH 9.4) | Low | High |
| Myofibrillar adenosine triphosphatase (pH 4.3) | High | Low |
| Glycolytic activity | Low | High |
| Lipid content | High | Low |
Type II fibers stain dark with ATPase at alkaline pH and have two major subtypes. Type IIA fibers are fast-contracting, nonfatigable fibers that stain dark with alkaline ATPase and pale at acid pH of 4.6 and 4.3. Their function depends mainly on aerobic metabolism. The type IIB fibers stain intermediate with ATPase at pH 4.6 ( Fig. 1-2 ). These are fast-contracting, fast-fatiguing fibers that depend mainly on glycolytic, anaerobic metabolism.
In humans, muscle fibers of both fiber types and subtypes are intermixed with fibers of other motor units. They appear in muscle histology in an almost checkerboard pattern, as seen in Fig. 1-2 , with a predominance of one or the other in some muscles. The deltoid, for example, has mainly type I fibers, and the quadriceps has mainly type II.
Peripheral nerves carry axons from motor neurons and sensory afferents from the Golgi tendon organs and spindles. They also contain large myelinated fibers that carry proprioceptive sensation. Nerves also have unmyelinated and small myelinated axons that carry touch, pain, and temperature sensations. Their cell bodies are located in the dorsal root ganglia; peripheral nerves also have autonomic fibers with myelinated presynaptic and unmyelinated postsynaptic axons ( Table 1-2 ).
| Sensory and Motor Fibers | Sensory Fibers | Diameter (μm) | Velocity (m/s) | Function |
|---|---|---|---|---|
| A-α | Ia | 10–20 | 0–120 | Motor: alpha motor neurons Sensory: muscle spindle afferents |
| A-α | Ib | 10–20 | 50–120 | Sensory: Golgi tendon organs, touch, pressure |
| A-β | II | 4–12 | 25–70 | Motor: motoneurons to intra/extrafusal muscle fibers Sensory: secondary muscle spindle afferents, touch, pressure, vibration |
| A-γ | 2–8 | 10–50 | Motor: small gamma motoneurons to intrafusal muscle fibers | |
| A-σ | III | 1–5 | 3–30 | Sensory: small touch, pain, temperature fibers |
| B | 1–3 | 3–15 | Small unmyelinated preganglionic autonomic fibers | |
| C | IV | <1 | <2 | All postganglionic autonomic fibers, pain, and temperature fibers |
Anatomy of the Cranial and Peripheral Nerves
Human striated muscles are innervated by nerves that originate in the brainstem and spinal cord. These are summarized here. Motor cranial nerves include those to the extraocular muscles such as the oculomotor , abducens , and trochlear nerves; and the V cranial or trigeminal nerve that innervates muscles of mastication and provides sensation to the face. The facial or VII cranial nerve innervates muscles of facial expression, as well as the lacrimal and salivary glands, provides sensation and taste to the anterior part of the tongue, and relays sensation of the tympanic membrane, external auditory canal, and a small area of the skin behind the ear. The hypoglossal nerve supplies muscles of the tongue, while the glossopharyngeal , vagal , and cranial accessory nerves innervate the laryngeal muscles. The glossopharyngeal nerve also relays taste sensation of the posterior tongue. The spinal accessory nerve supplies the sternocleidomastoid and upper portion of the trapezius muscles. The sternocleidomastoid also receives innervation from C2 and C3 roots, while innervation of the upper trapezius also comes from C3 and C4 roots.
The anterior roots of the spinal cord carry motor axons from the anterior horn, and the posterior roots are composed of sensory axons, the cell bodies of which are in the posterior root ganglia. Both are formed by rootlets that exit or enter the cord. They then join to form the spinal nerves that exit the spinal canal.
There are 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal ( Fig. 1-3 ). After exiting the cord, these branch into two divisions: the posterior rami , which supply the posterior area of the skin in the neck and trunk and paraspinal muscles, and the anterior rami , which innervate the skin of the anterior lateral aspect of the trunk and form the different plexuses that give origin to the peripheral nerves for the arms and legs. The anterior rami of the thoracic spinal nerves form the intercostal nerves, supplying the intercostal and abdominal muscles, and provide sensation to these areas. The dermatomal distribution of the body is seen in Fig. 1-4 .
The cervical plexus is formed of the anterior rami of the cervical roots C1 to C4 , which innervate the anterior and lateral flexors of the neck, and form the phrenic nerve from C3 to C5 , which innervates the diaphragm . The trapezius and sternocleidomastoid muscles receive innervation from the accessory nerve and upper cervical roots, as described earlier.
The Brachial Plexus
The brachial plexus is formed by the anterior rami of C5 to T1 spinal nerves which give origin to the nerves of the upper extremities. The following is a summary of the anatomy of the brachial plexus ( Fig. 1-5 ):
The roots from C5 to C6 combine to form the upper trunk in the brachial plexus, the C7 roots form the middle trunk , and the C8 and T1 roots form the lower trunk . The three trunks have posterior and anterior divisions that form the cords .
The trunks pass by the region of the supraclavicular fossa forming their cords which travel through the thoracic outlet between the first rib and the clavicle.
The anterior divisions of the upper and middle trunk form the lateral cord which gives origin to the musculocutaneous nerve. The lateral cord also gives origin to the outer branch of the median nerve.
The posterior cord is formed by the union of the three posterior divisions and gives off the subscapular , thoracodorsal , and axillary nerves, then proceeds to form the radial nerve.
The anterior division of the lower trunk forms the medial cord that gives origin to the ulnar nerve, the medial branch of the median nerve, and the medial antebrachial cutaneous sensory nerve of the forearm.
The dorsal scapular nerve originates directly from the C4 and C5 roots and innervates the rhomboids and a portion of the levator scapulae . The rhomboid is innervated only by fibers from the C5 root. The long thoracic nerve also originates directly from C5 to C7 roots to supply the serratus anterior muscle. It is important to study the rhomboids and serratus anterior muscles electromyographically in suspected cervical radiculopathies, particularly for the diagnosis in those without evidence of denervation in paraspinal muscles, because the serratus and the rhomboid muscles are affected in radiculopathies, but not in brachial plexopathies.
Nerves of the Brachial Plexus
The suprascapular nerve arises from the C5 to C6 roots through the upper trunk supplying the supraspinatus and the infraspinatus muscles.
The subscapular nerve from C5 to C6 roots innervates the teres major muscle. The anterior thoracic nerve from the C5 to C8 roots innervates the pectoralis major and minor , and the thoracodorsal nerve from C6 to C8 roots innervates the latissimus dorsi muscle.
The axillary nerve is formed by fibers of the C5 to C6 roots which go through the posterior cord of the plexus and innervate the deltoid and teres minor muscles, as well as part of the skin of the lateral shoulder.
The musculocutaneous nerve originates in the lateral cord from fibers of the C5 to C6 roots and innervates the biceps , brachialis , and coracobrachialis muscles as well. Its sensory branch, the lateral antebrachial cutaneous nerve , innervates the lateral aspect of the forearm.
The radial nerve is formed of axons from C5 to C8 roots going through the posterior cord and innervates the triceps and the anconeus muscles in the arm; then, after passing the spiral groove of the humerus, it gives off a sensory branch, the posterior antebrachial cutaneous nerve which innervates the skin of the lateral and the dorsal forearm. After the groove, the nerve supplies the brachioradialis and the extensor carpi radialis longus . After entering the forearm, the radial nerve divides into the motor posterior interosseus nerve and its sensory branch, the superficial radial nerve, which innervates the skin of the lateral aspect of the dorsum of the hand. The posterior interosseus nerve innervates the supinator, extensor carpi radialis brevis, abductor pollicis longus, extensor carpi ulnaris, extensor digitorum communis, extensor pollicis longus and brevis, extensor digiti minimi , and extensor indicis .
The median nerve originates from axons of the C6 to T1 roots that travel through the lateral and medial cords of the brachial plexus. The nerve supplies the flexor carpi radialis, pronator teres, palmaris longus , and flexor digitorum superficialis . Before entering the carpal tunnel, the nerve gives off a sensory branch that innervates the skin of the thenar eminence. The nerve then passes through the carpal tunnel and supplies the lumbricals I and II and forms the recurrent thenar nerve that innervates the abductor pollicis brevis , the opponens pollicis , and the flexor pollicis . The latter is also partially innervated by the ulnar nerve.
In the forearm the median nerve also gives rise to a purely motor branch, the anterior interosseus nerve, which innervates the pronator quadratus, flexor pollicis longus , and flexor digitorum profundus I and II.
The sensory axons of the distal median nerve innervate the skin of the lateral aspect of the palm, the dorsal surface of the terminal phalanges, and the volar surface of the first three digits and the middle of the fourth digit. The axons innervating the skin of the thumb come from the C6 roots and part of C7; the index and the middle fingers, from the C7 root.
The most common anomalous cross-innervations in the arm are crossovers between the median and ulnar nerves. Their fibers, which originate from the median nerve, travel with the ulnar nerve to innervate muscles that are normally innervated by the ulnar nerve.
The ulnar nerve originates from the C8 and T1 roots through the medial cord of the plexus and travels in the arm next to the brachial artery and the median nerve. After entering the forearm, it supplies the flexor carpi ulnaris and flexor digitorum profundus III and IV , then also gives rise to the dorsal cutaneous branch , which innervates the skin of the medial dorsal aspect of the hand. It then passes the medial aspect of the wrist, then the hand, and gives origin to its superficial branch supplying the palmaris brevis muscle and the skin of the medial aspect of the hand, the hypothenar eminence, and the fifth digit and half of the fourth digit. The motor branch innervates the muscles of the hypothenar eminence and then innervates the dorsal interossei and lumbricals III and IV , the three volar interossei , and half of the flexor pollicis brevis .
The median antebrachial cutaneous nerve originates directly from the medial cord (from the lower trunk) and supplies the skin over the medial aspect of the forearm. This nerve is important in electrodiagnosis, because it could be affected in plexopathies, helping to differentiate them from ulnar neuropathies and C8–T1 radiculopathies, in which it is not involved.
The Lumbosacral Plexus
The spinal cord ends in the conus medullaris, and the roots of the lower segments form the cauda equina, which gives origin to the lumbosacral plexus ( Fig. 1-6 ). The anterior rami of the L1 to L3 roots and part of the L4 root form the lumbar plexus , which travels within the psoas muscle , giving origin to the following: distal branches to the quadratus lumborum and branches to the psoas ; the iliohypogastric nerve, which originates from the L1 root and innervates the skin of the upper buttocks and the hypogastric region; the ilioinguinal nerve, which arises from L1 to L2 roots and innervates the skin of the medial thigh, the root of the penis, part of the scrotum and labia majora, and internal oblique and transversalis muscles; the genitofemoral nerve from L1 to L2 roots, which forms the lumboinguinal nerve that supplies the skin of the femoral triangle; and the external spermatic nerve, which innervates the cremasteric muscle and the skin of the inner area of the upper thigh, scrotum, and labia.
The anterior divisions from the L2 to L4 roots give origin to the obturator nerve , which innervates the adductor muscles (the adductor magnus is also partially innervated by the sciatic nerve) and the skin of the medial and upper thigh.
The lateral femoral cutaneous nerve originates from the posterior division of the plexus from fibers of L2 to L3 roots and supplies the skin of the lateral thigh.
The femoral nerve originates from posterior divisions of the L2 to L4 roots to innervate the psoas and iliacus , then exits the pelvis under the inguinal ligament, and innervates the pectineus , sartorius , and quadriceps muscles . Its sensory branches supply the skin of the anterior thigh and the medial calf. The saphenous nerve is a sensory branch of the femoral nerve; it originates from the L2 and L4 roots and supplies the skin of the medial aspect of the knee, the medial leg, and medial side of the foot.
The sacral plexus originates from the L5 to S3 roots in front of the sacroiliac joint and the piriformis, giving off the superior gluteal nerve from the L4 to S1 roots, the inferior gluteal nerve from the L5 to S2 roots, and a branch to the piriformis muscle, all from the posterior divisions. Other posterior division fibers form the posterior femoral cutaneous and perforating cutaneous nerves. Fibers from the anterior divisions innervate the obturator internus, superior gemellus, quadratus femoris , and inferior gemellus . Branches from S2 to S5 anterior primary rami form the pelvic nerves such as the pudendal nerve, pelvic splanchnic , and branches to the levator ani, coccygeus , and external anal sphincter muscles.
The superior gluteal nerve innervates the gluteus medius and minimus and the tensor fascia latae . The inferior gluteal nerve innervates the gluteus maximus .
The sciatic nerve is formed by anterior and posterior divisions of the L4 to S2 roots and consists of the peroneal portion , which originates from the posterior divisions of the plexus, and the tibial portion , formed from the anterior divisions . The tibial portion innervates the skin of the outer thigh and the hamstring muscles, except for the short head of the biceps , which is innervated by the peroneal portion . The adductor magnus is also partially innervated by the obturator nerve.
The peroneal and tibial portions of the sciatic nerve separate to form the tibial and common peroneal nerves in the posterior aspect of the thigh.
The common peroneal nerve originates at the popliteal fossa from the lateral popliteal nerve of the peroneal portion of the sciatic , originating from axons of the L4 to S1 roots. The nerve travels around the fibular head and gives a sensory branch to the patella, then divides into the superficial and deep peroneal nerves. The superficial peroneal innervates the peroneus longus and brevis , and its sensory branch innervates the anterior lateral aspect of the lower half of the leg and the dorsum of the foot and toes. The deep peroneal nerve innervates the tibialis anterior, extensor hallucis longus, peroneus tertius , and extensor digitorum brevis and supplies the small area of the skin between the first and second toes.
The tibial nerve originates at the popliteal fossa from the medial popliteal nerve of the sciatic and innervates the gastrocnemius and soleus muscles, the tibialis posterior , the flexor digitorum longus , and the flexor hallucis longus . It then passes through the tarsal tunnel and into the foot, providing a sensory branch, the calcaneal nerve, and divides into the medial plantar and lateral plantar nerves. The medial plantar innervates the adductor hallucis , the flexor digitorum brevis , and the flexor hallucis brevis , and provides sensation to the skin of the medial anterior two-thirds of the sole of the foot and the plantar skin of the first three toes and part of the fourth toe. The lateral plantar innervates the abductor digiti minimi, flexor digiti minimi, adductor hallucis, quadratus plantae , and interossei muscles and supplies the skin of the fifth toe and the lateral aspect of the fourth toe.
The sural nerve originates below at the popliteal space and is formed purely of sensory axons from the peroneal communicating branch of the common peroneal nerve and the medial sural cutaneous branch of the tibial nerve. The sural supplies the skin of the posterolateral area of the distal leg and the lateral foot; its axons come mainly from the S1 root but also from the L5 and S2 roots.
Table 1-3 lists limbs, muscles, and their innervation; the anatomy of individual nerves and their muscles is discussed in various cases in this book.
| Muscles, Innervation | Nerve | Cord/Division | Trunk | Roots a | Activation |
|---|---|---|---|---|---|
| Upper Extremity | |||||
| Shoulder Joint | |||||
| Trapezius | Spinal accessory | Not from brachial plexus | Not from brachial plexus | C3, C4 (some branches) | Elevate shoulder |
| Rhomboids | Dorsal scapular | Directly from roots | Directly from roots | C4, C5 | Retract scapulae |
| Levator scapulae | Dorsal scapular + twigs from C3, C4 | Directly from roots | Directly from roots | C3, C4, C5 | Elevate scapulae |
| Serratus anterior | Long thoracic | Directly from roots | Directly from roots | C5, C6 , b C7 b | Push forward against resistance |
| Pectoralis major clavicular | Lateral pectoral | Lateral | Upper | C5, C6 , b (C7) c | Adduction of the shoulder |
| Sternal | Medial pectoral | Medial | Lower | C7, C8 , b (T1) c,d,e | Extend and rotate the shoulder |
| Pectoralis minor | Lateral and medial pectoralis | Lateral and medial | Upper, middle, and lower | C6, C7 , b C8 | Depress scapula |
| Supraspinatus and infraspinatus | Suprascapular | Upper | (C4), c C5 , C6 | Abduct arm and extend | |
| Latissimus dorsi | Thoracodorsal | Posterior | Upper, middle, and lower | C6 , b C7 , C8 | Internally rotate, adduct, and extend arm |
| Teres major | Subscapular | Posterior | Upper | C5, C6 , b (C7) e,f | Abduct, internally rotate arm |
| Teres minor | Axillary | Posterior | Upper | (C4), e C5 , b C6 | Abduct, flex, and extend arm (posterior deltoid) |
| Deltoid | Axillary | Posterior | Upper | (C4), C5 , C6 | Abduct for and extend the arm |
| Arm | |||||
| Biceps | Musculocutaneous | Lateral | Upper | C5, C6 b | Flex supinated forearm |
| Brachialis | Musculocutaneous | Lateral | Upper | C5, C6 , b (C7) c | Flex supinated forearm |
| Coracobrachialis | Musculocutaneous | Lateral | Upper and middle | (C5), c C6 , b C7 | Forward elevation of arm with elbow flexed |
| Triceps | Radial | Posterior | Middle and lower | C6, C7, C8 b | Extend forearm |
| Brachioradialis | Radial | Posterior | Upper | C5, C6 , (C7) e | Flex forearm at semipronation |
| Extensor carpi radialis longus | Radial | Posterior | Upper and middle | C5, C6 , (C7) c,e,f | Extend and abduct hand at wrist |
| Anconeus | Radial | Posterior | Middle and lower | (C5), c C7, C8 , (T1) e | Extend elbow |
| Supinator | Posterior interosseous | Posterior | Upper | C5, d,e,f C6 , b C7 | Supinate forearm with elbow extended |
| Extensor carpi ulnaris | Posterior interosseous | Posterior | Upper, middle, and lower | (C6) c,d,f C7, C8 b | Extend and adduct hand at wrist |
| Extensor digitorum communis | Posterior interosseous | Posterior | Middle and lower | (C6), c,f C7 , C8 | Extension of metacarpophalangeal joints |
| Abductor pollicis longus | Posterior interosseous | Posterior | Middle and lower | (C6), c,f C7, C8 b | Abduct thumb at carpometacarpal joint |
| Extensor pollicis longus | Posterior interosseous | Posterior | Middle and lower | C7, C8 b | Extend thumb at interphalangeal joint |
| Extensor pollicis brevis | Posterior interosseous | Posterior | Middle and lower | (C6), c,f C7, C8 b | Extend thumb at metacarpophalangeal joint |
| Extensor indicis proprius | Posterior interosseous | Posterior | Middle and lower | C7, C8 b | Extend index finger with flexion of others |
| Pronator teres | Median | Lateral | Upper and middle | C6, C7 b | Pronate forearm |
| Flexor carpi radialis | Median | Lateral and medial | Upper, middle, and lower | C6, C7 b | Flex and abduct hand at wrist |
| Flexor digitorum superficialis | Median | Lateral and medial | Middle and lower | C7, C8 , b T1 | Flex fingers at proximal interphalangeal joint |
| Palmaris longus | Median | Lateral and medial | Middle and lower | C7, C8 , b T1 | Cup palm of hand |
| Pronator quadratus | Anterior interosseous | Lateral and medial | Middle and lower | C7, C8 , b T1 | Pronate forearm |
| Flexor digitorum profundus | Anterior interosseous | Medial and lateral | Middle and lower | C7, C8 , (T1) c,e,f | Flex distal phalanx of index finger, third finger |
| Flexor pollicis longus | Anterior interosseous | Lateral and medial | Middle and lower | C7 , C8, (T1) c,e,f | Flex distal phalanx of thumb |
| Flexor carpi ulnaris | Ulnar | Medial | Lower | C7, C8 , T1 | Flex and adduct hand at wrist |
| Flexor digitorum profundus III and IV | Ulnar | Medial | Lower | C7, C8 , (T1) c,d,e,f | Flex distal interphalangeal joint |
| Hand | |||||
| Abductor pollicis brevis | Median | Medial | Lower | C8 , b T1 | Abduct thumb at right angle to palm |
| Flexor pollicis brevis | Median | Medial | Lower | C8 , b T1 | Flex thumb at metacarpophalangeal (S) joint |
| Opponens pollicis | Median | Medial | Lower | C8 , b T1 | Touch base of little finger with thumb |
| Lumbricals I and II | Median | Medial | Lower | C8, T1 | Extend finger at proximal interphalangeal joint |
| Abductor digiti minimi | Ulnar | Medial | Lower | C8, T1 | Abduct little finger |
| Flexor digiti minimi | Ulnar | Medial | Lower | C8, T1 | Flex little finger at metacarpophalangeal joint |
| Opponens digiti minimi | Ulnar | Medial | Lower | C8, T1 | Oppose little finger to thumb |
| Adductor pollicis | Ulnar | Medial | Lower | C8, T1 | Adduct thumb at right angle to palm |
| Flexor pollicis brevis | Ulnar | Medial | Lower | C8 , b T1 | Flex thumb at metacarpophalangeal joint |
| Palmar interossei | Ulnar | Medial | Lower | C8, T1 b | Adduct fingers |
| Dorsal interossei | Ulnar | Medial | Lower | C8, T1 | Adduct fingers |
| Lumbricals III and IV | Ulnar | Medial | Lower | C8, T1 b | Extend finger at proximal interphalangeal joint |
| Lower Extremity | |||||
| Hip and Thigh | |||||
| Iliopsoas | Femoral to psoas major, minor | Posterior | Lumbar | L1, L2, L3 , b L4 | Flexion of thigh |
| Quadriceps femoris | Femoral | Posterior | Lumbar | L2, L3, L4 | Extension of leg on the thigh |
| Pectineus | Femoral | Posterior | Lumbar | L2 , b L3, L4 | Adduct thigh |
| Sartorius | Femoral | Posterior | Lumbar | L2, L3 , L4 | Flexion, abduction of hip; flexion of knee |
| Adductor longus and brevis | Obturator | Anterior | Lumbar | L2, L3 , L4 | Adduction of thigh |
| Adductor magnus | Obturator; also from sciatic | Anterior | Lumbar | L2, L3, L4 b | Adduction of thigh |
| Gracilis | Obturator | Anterior | Lumbar | L2 , b L3, L4 | Adduct thigh and flex knee |
| Gluteus medius and minimus | Superior gluteal | Posterior | Sacral | L4, L5 , S1 | Adduct thigh |
| Tensor fasciae latae | Superior gluteal | Posterior | Sacral | L4, L5 , S1 | Flexion and internal rotation of thigh |
| Gluteus maximus | Inferior gluteal | Posterior | Sacral | L5, S1, S2 b | Extend at the hip |
| Piriformis | Nerve to piriformis | Posterior | Sacral | (L5), e S1 b , S2 | Externally rotate thigh |
| Adductor magnus | Sciatic, also from obturator | Anterior | Sacral | L2, L3, L4 | Adduction of thigh |
| Obturator internus and gemellus superior | Obturator internus | Anterior | Sacral | L5, S1 , b S2 | Externally rotate thigh with extended leg |
| Inferior gemellus | Quadratus femoris | Anterior | Sacral | L4, L5, S1 , S2 | Externally rotate thigh with extended leg |
| Quadratus femoris | Quadratus femoris | Anterior | Sacral | L4, L5, S1 | Externally rotate thigh |
| Semitendinosus | Sciatic (tibial portion) | Anterior | Sacral | L5 , b S1 , S2 | Flexion of knee |
| Semimembranosus | Sciatic (tibial portion) | Anterior | Sacral | (L4), f L5 , b S1 , S2 | Flexion of knee |
| Biceps (long head) | Sciatic (tibial portion) | Anterior | Sacral | L5, S1 , (S2) e | Flexion of knee |
| Biceps (short head) | Sciatic (common peroneal portion) | Posterior | Sacral | L5, S1 , S2 | Flexion of knee |
| Leg | |||||
| Gastrocnemius and soleus | Tibial | Anterior | Sacral | S1 , b S2 b | Plantar flexion and inversion of foot |
| Tibialis posterior | Tibial | Anterior | Sacral | L4, L5 , (S1) c,d,f | Plantar flexion and inversion of foot |
| Flexor digitorum longus | Tibial | Anterior | Sacral | L5, S1, S2 , (S3) e | Flexion of toes |
| Flexor hallucis longus | Tibial | Anterior | Sacral | L5, S1, S2 , b (S3) e | Flex big toe |
| Popliteus | Tibial | Anterior | Sacral | (L4), c,e L5 , S1 | Flex and internally rotate tibia |
| Tibialis anterior | Deep peroneal | Posterior | Sacral | L4 , L5, (S1) c,f | Dorsiflexion of foot |
| Extensor hallucis longus | Deep peroneal | Posterior | Sacral | (L4), c,f L5 , S1 | Dorsiflexion of distal phalanx of big toe |
| Extensor digitorum longus | Deep peroneal | Posterior | Sacral | (L4), c,f L5 , S1 | Dorsiflexion of toes |
| Peroneus tertius | Deep peroneal | Posterior | Sacral | L5 , S1 | Dorsiflexion and eversion of foot |
| Peroneus longus | Superficial peroneal | Posterior | Sacral | L5 , b S1 , b (S2) d,e | Eversion of foot |
| Peroneus brevis | Superficial peroneal | Posterior | Sacral | L5 , b S1 , b (S2) d,e | Eversion of foot |
| Abductor hallucis | Medial plantar | Anterior | Sacral | (L5), c,f S1, S2, S3 b | Plantar flexion abduction of first toe |
| Flexor digitorum brevis | Medial plantar | Anterior | Sacral | (L5), c,f S1, S2, S3 b | Flex middle phalanges of toes II–V |
| Flexor hallucis brevis | Medial plantar | Anterior | Sacral | (L5), f S1, S2, S3 b | Flex metatarsophalangeal joint of big toe |
| Flexor digiti minimi | Lateral plantar | Anterior | Sacral | S1, S2, S3 b | Flex metatarsophalangeal joint of little toe |
| Abductor digiti quinti | Lateral plantar | Anterior | Sacral | S1, S2, S3 b | Adduct little toe |
| Adductor hallucis | Lateral plantar | Anterior | Sacral | (L5), f S1, S2, S3 b | Adduct big toe |
| Interossei | Lateral plantar | Anterior | Sacral | S1, S2, S3 b | Spread toes |
| Quadratus plantae | Lateral plantar | Anterior | Sacral | S1, S2, S3 b | Flex toes |
| Extensor digitorum brevis | Deep peroneal | Posterior | Sacral | L5, S1 , (S2) e | Dorsiflexion of proximal phalanges of toes |
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