The lumbosacral trunk is a thick nervous band that arises from the merger of the anterior/ventral rami of the last two lumbar spinal nerves (L4 and L5). Upon originating, the trunk descends over the wing (ala) of the sacrum and joins the sacral spinal nerves to form the sacral plexus.
The main function of the lumbosacral trunk is to provide nerves for motor and sensory innervation of the skin and muscles of the sacral region, posterior thigh, leg and foot. The lumbosacral trunk is also the connection between two major nervous plexuses of the abdominopelvic cavity, the lumbar and sacral plexuses.
This article will discuss the anatomy and function of the lumbosacral trunk.
- Origin and course
- Branches and innervation
|Origin||Anterior divisions of spinal nerves L4, L5|
|Branches||Sciatic nerve (L4-S3)
Superior gluteal nerve (L4-S1)
Inferior gluteal nerve (L5-S2)
The nerve to obturator internus and superior gemellus muscles (L4-S1)
The nerve to quadratus femoris and inferior gemellus muscles (L5-S2)
|Function||The lumbosacral trunk provides branches that supply the motor and sensory innervation for the skin and muscles of the gluteal region, thigh, leg and foot.|
Origin and course
The lumbosacral trunk is a cord-like nervous structure that arises from the merger of part of the anterior ramus of L4 and the entire anterior ramus of L5, which are the most inferior lumbar spinal nerves. These two divisions unite immediately superior to the pelvic brim, at the medial margin of psoas major muscle.
Upon uniting, the nerve trunk descends along the anterior aspect of the ala of the sacrum and anterior to the sacroiliac joint to join the first anterior sacral ramus. This way, the lumbosacral trunk participates in the formation of the sacral plexus.
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Branches and innervation
Being a part of the sacral plexus, the lumbosacral trunk contributes to the formation of several nerves of the lower extremity. These nerves include the:
- Sciatic nerve (L4-S3)
- Superior gluteal nerve (L4-S1)
- Inferior gluteal nerve (L5-S2)
- Nerve to quadratus femoris and inferior gemellus muscles (L4-S1)
- Nerve to obturator internus and superior gemellus muscles (L5-S2)
The sciatic nerve is the largest nerve in the human body. It arises from the anterior rami of spinal nerves L4-S3, on the anterior surface of the piriformis muscle. This nerve exits the pelvic cavity via the greater sciatic foramen (inferior to the piriformis muscle) into the gluteal region. From here, it runs through the posterior aspect of the thigh to reach the popliteal fossa where it terminates into two large nerve branches: the tibial nerve and the common peroneal nerve.
The sciatic nerve is responsible for motor and sensory innervation of the thigh, leg, and foot. More specifically, this nerve provides motor supply for the muscles of the posterior thigh, a portion of the adductor magnus, muscles of the posterior, anterior, and lateral compartments of the leg and foot muscles. In addition, it provides sensory innervation for the posterior thigh, lateral and posterior leg, as well as the dorsum and sole of the foot.
Superior gluteal nerve
The superior gluteal nerve is formed by the dorsal fibers of the spinal nerves L4, L5 and S1. The nerve leaves the pelvis and enters the gluteal region through the greater sciatic foramen (superior to the piriformis muscle). The superior gluteal nerve is a motor nerve that provides innervation for the gluteus medius and
gluteus minimus muscles, as well as the tensor fasciae latae.
Inferior gluteal nerve
The inferior gluteal nerve originates from the dorsal divisions of the spinal nerves L5, S1 and S2. Similar to the superior gluteal nerve, the inferior gluteal nerve also travels to the gluteal region through the greater sciatic foramen (inferior to the piriformis muscle) to innervate the gluteus maximus muscle.
The nerve to obturator internus and superior gemellus muscles
This nerve is formed by fibers from the anterior divisions of spinal nerves L5, S1 and S2. Similar to other branches of the lumbosacral trunk, this nerve also passes through the greater sciatic foramen (inferior to the piriformis muscle) to innervate the muscles listed in its name: obturator internus muscle and superior gemellus muscle.
The nerve to quadratus femoris and inferior gemellus muscles
This nerve is formed by the fibers of the anterior divisions of spinal nerves L4, L5 and S1. From its origin in the pelvis, the nerve also enters the gluteal region via the greater sciatic foramen (inferior to the piriformis muscle). It descends along the ischium and terminates by giving off branches to supply the quadratus femoris muscle and the inferior gemellus muscle.
Lumbosacral trunk: want to learn more about it?
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