Lumbar Plexus and Sacral Plexus

Chapter 20 Lumbar Plexus and Sacral Plexus




Overview of the Principal Nerves of the Lower Limb (Figs 20.1, 20.2)










Dermatomes


Our knowledge of the extent of individual dermatomes, especially in the limbs, is based largely on clinical evidence. The dermatomes of the lower limb arise from spinal nerves T12 to S3 (Figs 20.320.5).





The preaxial border starts near the midpoint of the thigh and descends to the knee. It then curves medially, descending to the medial malleolus and the medial side of the foot and hallux. The postaxial border starts in the gluteal region and descends to the centre of the popliteal fossa, then deviates laterally to the lateral malleolus and the lateral side of the foot. The ventral and dorsal axial lines exhibit corresponding obliquity. The ventral axial line starts proximally at the medial end of the inguinal ligament and descends along the posteromedial aspect of the thigh and leg to end proximal to the heel. The dorsal axial line begins in the lateral gluteal region and descends posterolaterally in the thigh to the knee; it inclines medially and ends proximal to the ankle. Considerable overlap exists between adjacent dermatomes innervated by nerves derived from consecutive spinal cord segments.



Myotomes


Tables 20.1 to 20.4 summarize the predominant segmental origin of the nerve supply for each of the lower limb muscles and for movements that take place at the joints of the lower limb. Damage to these segments or to their motor roots results in maximal paralysis.


Table 20.1 Movements, muscles and segmental innervation in the lower limb



Table 20.2 Segmental innervation of muscles of the lower limb



























L1 Psoas major, psoas minor
L2 Psoas major, iliacus, sartorius, gracilis, pectineus, adductor longus, adductor brevis
L3 Quadriceps, adductors (magnus, longus, brevis)
L4 Quadriceps, tensor fasciae latae, adductor magnus, obturator externus, tibialis anterior, tibialis posterior
L5 Gluteus medius, gluteus minimus, obturator internus, semimembranosus, semitendinosus, extensor hallucis longus, extensor digitorum longus, peroneus tertius, popliteus
S1 Gluteus maximus, obturator internus, piriformis, biceps femoris, semitendinosus, popliteus, gastrocnemius, soleus, peronei (longus and brevis), extensor digitorum brevis
S2 Piriformis, biceps femoris, gastrocnemius, soleus, flexor digitorum longus, flexor hallucis longus, some intrinsic foot muscles
S3 Some intrinsic foot muscles (except abductor hallucis, flexor hallucis brevis, flexor digitorum brevis, extensor digitorum brevis)

Table 20.3 Segmental innervation of joint movements of the lower limb







































Hip Flexors, adductors, medial rotators L1–3
  Extensors, abductors, lateral rotators L5, S1
Knee Extensors L3, L4
  Flexors L5, S1
Ankle Dorsiflexors L4, L5
  Plantar flexors S1, S2
Foot Inverters L4, L5
  Everters L5, S1
  Intrinsic muscles S2, S3




Lumbar Plexus


The posterior abdominal wall contains the origin of the lumbar plexus (Fig. 20.6) and numerous autonomic plexuses and ganglia that lie close to the abdominal aorta and its branches.



Lumbar ventral rami increase in size from first to last and are joined, near their origins, by grey rami communicantes from the four lumbar sympathetic ganglia. These rami, long and slender, accompany the lumbar arteries around the sides of the vertebral bodies, behind psoas major. Their arrangement is irregular: one ganglion may give rami to two lumbar nerves, or one lumbar nerve may receive rami from two ganglia. Rami often leave the sympathetic trunk between ganglia. The first, second and sometimes third lumbar ventral rami are each connected with the lumbar sympathetic trunk by a white ramus communicans. The lumbar ventral rami descend laterally into psoas major. The first three and most of the fourth form the lumbar plexus; the smaller moiety of the fourth joins the fifth as a lumbosacral trunk, which joins the sacral plexus. The fourth is often termed the nervus furcalis, being divided between the two plexuses; however, the third is occasionally the nervus furcalis. Alternatively, both the third and fourth may be furcal nerves, in which case the plexus is termed ‘prefixed.’ More frequently, the fifth nerve is furcal, and the plexus is then termed ‘postfixed.’ These variations modify the sacral plexus.


The lumbar plexus lies within the substance of the posterior part of psoas major, anterior to the transverse processes of the lumbar vertebrae. It is formed by the first three and most of the fourth lumbar ventral rami. The first lumbar ramus receives a branch from the last thoracic ventral ramus. The paravertebral part of psoas major consists of posterior and anterior masses, which arise from different attachments. The lumbar plexus lies between these masses and hence is ‘in line’ with the intervertebral foramina. Although there may be minor variations, the most common arrangement of the plexus is described here.


The first lumbar ventral ramus, joined by a branch from the twelfth thoracic ventral ramus, bifurcates, and the upper and larger part divides again into the iliohypogastric and ilioinguinal nerves. The smaller lower part unites with a branch from the second lumbar ventral ramus to form the genitofemoral nerve. The remainder of the second, third and parts of the fourth lumbar ventral rami join the plexus and divide into ventral and dorsal branches. Ventral branches of the second to fourth rami join to form the obturator nerve. The main dorsal branches of the second to fourth rami join to form the femoral nerve. Small branches from the dorsal branches of the second and third rami join to form the lateral femoral cutaneous nerve. The accessory obturator nerve, when it exists, arises from the third and fourth ventral branches. The lumbar plexus is supplied by branches from the lumbar vessels, which supply the psoas major.


The branches of the lumbar plexus are as follows:




























Muscular T12, L1–4
Iliohypogastric L1
Ilioinguinal L1
Genitofemoral L1, L2
Lateral femoral cutaneous L2, L3
Femoral L2–4 dorsal divisions
Obturator L2–4 ventral divisions
Accessory obturator L2, L3

Division of constituent ventral rami into ventral and dorsal branches is not as clear in the lumbar and lumbosacral plexuses as it is in the brachial plexus. Anatomically, the obturator and tibial nerves (via the sciatic) arise from ventral divisions, and the femoral and peroneal nerves (via the sciatic) arise from dorsal divisions. Lateral branches of the twelfth thoracic and first lumbar ventral rami are drawn into the gluteal skin, but otherwise, these nerves are typical. The second lumbar ramus is difficult to interpret. It not only contributes substantially to the femoral and obturator nerves but also has an anterior terminal branch (genital branch of the genitofemoral) and a lateral cutaneous branch (lateral femoral cutaneous nerve and femoral branch of the genitofemoral). Anterior terminal branches of the third to fifth lumbar and first sacral rami are suppressed, but the corresponding parts of the second and third sacral rami supply the skin of the perineum.


Inflammatory processes may occur in the posterior abdominal wall in the tissues anterior to psoas major, such as retrocaecal appendicitis on the right and diverticular abscess on the left. This may cause irritation of one or more of the branches of the lumbar plexus and lead to presenting symptoms of pain or dysaesthesia in the distribution of the affected nerves in the thigh, hip or buttock.




Muscular Branches


Small branches are derived from all five lumbar roots.



Iliohypogastric Nerve






Ilioinguinal Nerve






Genitofemoral Nerve






Lateral Femoral Cutaneous Nerve of the Thigh


The lateral (femoral) cutaneous nerve of the thigh arises from the dorsal branches of the second and third lumbar ventral rami and emerges from the lateral border of psoas major, crossing the iliacus obliquely toward the anterior superior iliac spine. It supplies the parietal peritoneum in the iliac fossa. The right nerve passes posterolateral to the caecum, separated from it by the fascia iliaca and peritoneum; the left passes behind the lower part of the descending colon. Both pass behind or through the inguinal ligament, variably medial to the anterior superior iliac spine (commonly about 1 cm) and anterior to or through sartorius into the thigh, where they divide into anterior and posterior branches. The anterior branch becomes superficial approximately 10 cm distal to the anterior superior iliac spine and supplies the skin of the anterior and lateral thigh as far as the knee. It connects terminally with the cutaneous branches of the anterior division of the femoral nerve and the infrapatellar branch of the saphenous nerve, forming the peripatellar plexus. The posterior branch pierces the fascia lata higher than the anterior, and it divides to supply the skin on the lateral surface from the greater trochanter to about mid thigh. It may also supply the gluteal skin (see Case 2).


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Aug 14, 2016 | Posted by in NEUROLOGY | Comments Off on Lumbar Plexus and Sacral Plexus

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