Microanatomy of the Lumbar Plexus

Nerve roots, as well as related motor, sensory, and sympathetic axons, were examined at their origins in the subarachnoid space and along their exits through the intervertebral foramina. Four cross-sectional cuts were obtained at each level. The first consisted of a complete transverse cross-section of the dural sac and its contents, focusing the image exclusively on the anterolateral region of the subarachnoid space called the subarachnoid angle, where nerve root cuffs and their contents can be found. The second cut consisted of a full transverse cross-section of the nerve root cuff at a strategic point between the dural sac and the dorsal root ganglion. The third cut provided a full transverse cross-section of the nerve root cuff at the level of the dorsal root ganglion including the anterior nerve root at that same level. A final fourth cut consisted of a complete transverse cross-section distal to the dorsal root ganglion.


The term “nerve root” can be confusing if used to refer to structures found at different levels as they might not share the same morphology.


Nerve rootlets leave the spinal cord at the ventrolateral and dorsolateral sulci. Anterior rootlets contain predominately efferent fibers from the ventral horn and carry motor signals to voluntary muscles. In the thoracic and upper lumbar regions, they also carry preganglionic sympathetic fibers from the lateral horns. Posterior rootlets are elongations of pseudounipolar nerve cells located in dorsal root ganglions (DRGs).


Different rootlets join together and form the anterior and posterior roots: about 6 to 8 anterior rootlets and 8 to 10 posterior ones. Therefore, the rootlets are formed by successive additions of axonal groups either arriving at or leaving the spinal cord. Cross-sections of nerve roots within the subarachnoid space such as the cauda equina below L2 revealed that each nerve root is formed by multiple units. Each unit appears to be separated from neighboring units by a thin layer of collagen fibers. Similarly, each nerve root of the cauda equina is covered by pia mater. The cerebrospinal fluid is located on the outer aspect of these nerve roots.


Outside the pia mater, the nerve roots are surrounded by arachnoid sleeves. These are formed by arachnoid cells and collagen fibers of the trabeculated arachnoid. At the subarachnoid angle, the arachnoid sleeves delimit and separate the anterior and posterior rootlets, as (▶ Fig. 25.1, ▶ Fig. 25.2, ▶ Fig. 25.3).



Human dural sac, nerve root cuffs, roots of lumbar plexus, sacral plexus, and sympathetic chain at lumbar level. The dural sac appears flattened owing to loss of cerebrospinal fluid after dissection o


Fig. 25.1 Human dural sac, nerve root cuffs, roots of lumbar plexus, sacral plexus, and sympathetic chain at lumbar level. The dural sac appears flattened owing to loss of cerebrospinal fluid after dissection of the sample.



Human dural sac at the 12th thoracic vertebral level. The lines point to the cross-sections and their relationships to the numbers of the succeeding figures.


Fig. 25.2 Human dural sac at the 12th thoracic vertebral level. The lines point to the cross-sections and their relationships to the numbers of the succeeding figures.



Transverse cross-section of nerve root cuff at 12th thoracic vertebral level. The image shows the 12th thoracic anterior and posterior nerve roots within the subarachnoid space, precisely at the level


Fig. 25.3 Transverse cross-section of nerve root cuff at 12th thoracic vertebral level. The image shows the 12th thoracic anterior and posterior nerve roots within the subarachnoid space, precisely at the level of the subarachnoid angle. anterior NR, anterior nerve root; CSF, cerebrospinal fluid; posterior NR, posterior nerve root; Trabec, arachnoid trabeculae.



Although whether these nerve roots in the cauda equina should be called rootlets or nerve roots can be questioned, the term “subarachnoid nerve root” seems accurate. Subarachnoid nerve roots join together in numbers of 1 to 2 anterior nerve roots and 2 to 5 posterior ones to exit through the anterolateral subarachnoid space toward the nerve root cuffs. Inside the cuffs, these nerves are known as anterior and posterior nerve roots, respectively (▶ Fig. 25.4). In more distal successive cross-sectional cuts of the nerve root cuffs at the level of the DRG, it was observed that the bodies of these ganglia are similar to those of pseudounipolar nerve cells and sensory axons (▶ Fig. 25.5, ▶ Fig. 25.6). At the same cross-sectional level, motor nerves known as anterior nerve roots were present. However, their structural microanatomy does not resemble that of previous and more proximal cross-sectional images. Adjacent to the anterior and posterior nerve roots, structures similar to the pseudo-fascicular type can be found, as the axons contained are not enveloped by epineurium but surrounded by a transitional arachnoid layer with different properties. At the level of the DRG, cross-sectional images demonstrate that the multifascicular appearance of pseudo-fascicles is no longer evident. Whereas axons forming multiple pseudo-fascicles in anterior nerve roots appear in groups of mainly one or two motor units, the axons located in posterior nerve roots are joined either in a single ganglion, one bilobed, or two dorsal nerve ganglia parallel to each other.



Transverse cross-section of the nerve root cuff between the dural sac and the dorsal root ganglion at the 12th thoracic vertebral level. The image shows the anterior and posterior nerve roots separate


Fig. 25.4 Transverse cross-section of the nerve root cuff between the dural sac and the dorsal root ganglion at the 12th thoracic vertebral level. The image shows the anterior and posterior nerve roots separately enveloped by a transitional arachnoid layer and by dura mater on the outer aspect. In the area of dural cuffs, adipose tissue appears sparsely distributed among the dural laminas that form the dura mater. The dura mater at this level can be thinner than in the dural sac, although the dura covering nerve roots is thicker. Anterior NR, anterior nerve root; Posterior NR, posterior nerve root.



Transverse cross-section of a nerve root cuff at the level of the 12th thoracic dorsal root ganglion. Image includes the anterior nerve root at the same level. Anterior NR, anterior nerve root.


Fig. 25.5 Transverse cross-section of a nerve root cuff at the level of the 12th thoracic dorsal root ganglion. Image includes the anterior nerve root at the same level. Anterior NR, anterior nerve root.



Transverse cross-section of the 12th thoracic nerve root located distal to the dorsal root ganglion, immediately after leaving the spinal cord toward the external orifice of the foramen.


Fig. 25.6 Transverse cross-section of the 12th thoracic nerve root located distal to the dorsal root ganglion, immediately after leaving the spinal cord toward the external orifice of the foramen.



In the transverse cross-section obtained beyond and distal to the DRG, fascicles were found. This is the site of origin of fascicles in the peripheral nerve. Successive distal cross-sections at 1 to 2 mm intervals showed different intrafascicular topograms.


Adipose tissue appears sparsely distributed among the dural laminas of the dura mater around the area of nerve root cuffs. The dura mater at this level can be thinner than in the dural sac, although overall the nerve root coverings are thicker.


Details of the microanatomy of the femoral and obturator nerves were included. The microanatomy of the sympathetic chains at the thoracolumbar level was studied, showing the differences between the histology of nerve root and sympathetic ganglia.


25.2 Lumbar Plexus and Associated Structures


The following figures are images of successive cross-sections obtained from same cadaver, where we can see nervous structures of lumbar plexus from subarachnoid space at T12 to L4 level to distal nerves as femoral and obturator nerves, also including sympathetic nervous structures (Fig. 25.1 – Fig. 25.45).



Human dural sac at the first lumbar vertebral level. Lines point toward the cross-sections indicating the relationship to the numbers of the succeeding figures.


Fig. 25.7 Human dural sac at the first lumbar vertebral level. Lines point toward the cross-sections indicating the relationship to the numbers of the succeeding figures.



Transverse cross-section of nerve root cuff at first lumbar vertebral level. The anterior and posterior first lumbar nerve roots within the subarachnoid space precisely around the subarachnoid angle.


Fig. 25.8 Transverse cross-section of nerve root cuff at first lumbar vertebral level. The anterior and posterior first lumbar nerve roots within the subarachnoid space precisely around the subarachnoid angle. Anterior NR, anterior nerve root; CSF, cerebrospinal fluid; Posterior NR, posterior nerve root.



Transverse cross-section of nerve root cuff between the dural sac and dorsal root ganglion at first lumbar vertebral level. The image shows the anterior and posterior nerve roots, each enveloped by a


Fig. 25.9 Transverse cross-section of nerve root cuff between the dural sac and dorsal root ganglion at first lumbar vertebral level. The image shows the anterior and posterior nerve roots, each enveloped by a transitional arachnoid layer and outwardly by dura mater. Anterior NR, anterior nerve root; Posterior NR, posterior nerve root.

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May 21, 2019 | Posted by in NEUROSURGERY | Comments Off on Microanatomy of the Lumbar Plexus
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