With slight variation, the basic pattern of the spinal nerve repeats itself thirty-one times along the entire length of the spinal cord. With the exception of the fi rst spinal nerve, each spinal nerve level emerges from within the vertebral column to pass

peripherally between successive vertebrae. Because of the developmental differences in the growth rate of the vertebral column and associated spinal cord, the lower roots of the spinal nerves are dragged downward by the lengthening vertebral column. With each succeeding spinal nerve level the roots become longer and more oblique in their course, eventually extending beyond the end of the spinal cord as the vertically oriented cauda equina.

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Dissection exposing cauda equina

Posterior view

Dissection revealing spinal cord and brain

Posterior view

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The dorsal rami of the spinal nerves arise at all spinal levels and pursue a posterior course into the muscles, connective tissue, and skin of the back. They innervate all

the epaxial muscles comprising the extensors of the vertebral column. The cutaneous distribution of the dorsal rami spans from the top of the head, down the posterior trunk, to the superior half of the gluteal region. With the exception of levels C1, S4, S5, and the coccygeal, the dorsal rami split into lateral and medial branches as they course posteriorly into the back.

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Deep dissection exposing dorsal rami

Posterior view

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Erector spinae muscle removed to expose dorsal rami

Posterior view

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The brachial plexus arises from the last four cervical ventral rami and the fi rst thoracic ventral ramus. The four cervical ventral rami pass later-

ally between the middle and internal layers of the lateral cervical body wall, the middle and anterior scalene muscles, respectively. As they emerge through the scalenes, they connection with with one another as well as with the ascending branch of the fi rst thoracic ventral ramus. This is the beginning of the nerve plexus that will innervate almost all the muscles and associated skin of the upper limb.

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Dissection of brachial plexus

Anterior view

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The lumbar plexus arises from the ventral rami of the first four lumbar spinal nerves. The plexus emerges laterally through the intervertebral foramina to pass anterolateral between the two

heads of the psoas major muscle. The more superior branches of the plexus enter the abdominal body wall to innervate the abdominal muscles. The lower nerves of the plexus course into the lower limb as the lateral femoral cutaneous, femoral, and obturator nerves. The lumbar plexus is a transitory plexus that begins as a series of body wall nerves and eventually transitions into limb innervation. The first ventral ramus of the plexus is basically a segmental nerve that follows the basic segmental nerve pattern in the ventral body wall. The second lumbar ventral ramus forms segmental branches in the body wall and other branches that contribute to limb innervation. The third and fourth ventral rami contribute solely to innervation of the lower limb anatomy.

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Pelvic dissection exposing lumbar and sacral plexus

Anterior view

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The sacral plexus forms from the ventral rami of the last two lumbar and the fi rst four sacral spinal nerves. The fourth and fi fth lumbar spinal nerves form a descending communication,

the lumbosacral trunk, that joins with the upper sacral spinal nerves as they exit the anterior foramina of the sacrum. On the anterior surface of the sacrum the large roots of the plexus are noticeable before they exit through the greater sciatic notch on their course into the pelvic wall and lower limb. This plexus forms the total nerve supply to the pelvic body wall, and, along with the limb branches from the lumbar plexus, is the nerve supply for the lower limb.

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Dissection of sacral plexus nerves

Posterior view

Dissection of pudendal nerves and vessels

Lateral view

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