General branching plan of spinal nerves
The lumbar, sacral, and coccygeal nerves follow the general laws of spinal nerve anatomy. After exiting the intervertebral foramen, each such trunk divides into four mandatory groups of branches:
- Meningeal branches (rr. meningei) — return back into the vertebral canal.
- Communicating branches (rr. communicantes) — connect the somatic nervous system with the autonomic nervous system.
- Dorsal branches (rr. dorsales) — travel to the posterior surface of the trunk (to the back).
- Ventral branches (rr. ventrales) — run anteriorly and are the largest, as they form the nerve plexuses.
The combination of all ventral rami of the segments in question forms a single and extensive lumbosacral plexus (plexus lumbosacralis).
Anatomy of lumbar nerves (L₁–L₅)
The lumbar spinal nerves (nn. lumbales) are represented by five pairs. Their branches have strict specialization.
Dorsal branches of the lumbar nerves (rr. dorsales nn. lumbalium) are traditionally further divided into two bundles:
- Medial branches (rr. mediales)
- Lateral branches (rr. laterales)
Of particular interest are the lateral cutaneous branches arising from the upper three lumbar nerves (L₁–L₃). They pierce the muscle thickness, penetrate the thoracolumbar fascia, and descend below the iliac crest. Their ultimate goal is the innervation of the skin of the gluteal region. In clinical anatomy, these structures are named superior clunial nerves (nn. clunium superiores).
Important nuance: the dorsal branches of the lower two lumbar segments (L₄ and L₅) have no cutaneous representation at all. They supply branches exclusively to the muscular structures of the back and lower back.
Formation of the lumbar plexus
While the dorsal rami go to the back, the ventral branches of the lumbar nerves (rr. ventrales nn. lumbalium) take on the formation of plexuses.
The ventral rami of the first four lumbar nerves (L₁–L₄), which are also joined by a small portion of fibers from the twelfth thoracic nerve (Th₁₂), unite with each other. This structure is called the lumbar plexus (plexus lumbalis), which is the upper "level" of the common lumbosacral plexus.
To remember the origin of the superior clunial nerves, use the rule "Upper buttocks are the first three lumbar segments": nn. clunium superiores are formed strictly from the dorsal rami of L1, L2, and L3.
Class and exam questions
Which branches of the spinal nerves form the lumbosacral plexus?
The plexus is formed exclusively by the ventral rami (rr. ventrales) of the corresponding spinal nerves.
Do the L4 and L5 nerves participate in the sensory innervation of the back skin?
No, the dorsal branches of the fourth and fifth lumbar nerves (L4–L5) do not innervate the skin; they provide motor fibers only to the muscles.
From which segments is the lumbar plexus (plexus lumbalis) formed?
The lumbar plexus is formed by the ventral rami of the first through fourth lumbar nerves (L1–L4) with the obligatory participation of fibers from the twelfth thoracic nerve (Th12).
What else to study on this topic
- Topography and branches of the lumbar plexus
- Muscles innervated by the medial dorsal branches of the lumbar nerves
- Anatomy of the thoracolumbar fascia (at the site of cutaneous branch passage)
- Course and functions of meningeal branches in the vertebral canal
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