VH Dissector: Lower Limb: Lumbosacral Plexus

Lumbar Plexus:

The Lumbar Plexus is the upper portion of the lumbosacral plexus. It is formed by the ventral rami of L1, L2, L3, L4. Nerves of the lumbar plexus only run as far as the knee except for a cutaneous branch which runs to the foot.

Each of the L1-L4 nerve roots splits to form an anterior and posterior division. The branches of the lumbosacral plexus are like those of the brachial plexus. They receive contribution from either the anterior or posterior divisions depending on what structures they supply.

Begin with the right pelvis and femur.

STEP 1

Add the L1 nerve root which gives rise to the iliohypogastric nerve and the ilioinguinal nerve, both primarily cutaneous (they do have some motor innervations to the abdominal muscles that we will discuss later). The iliohypogastric nerve runs over the iliac spine and is susceptible to injury by a direct blow.

STEP 2

Add the iliohypogastric nerve, a branch off of L1

STEP 3

Add the ilioinguinal nerve, a branch off of L1 supplies superomedial thigh.

STEP 4

Add the genitofemoral nerve, a branch off of L1 which travels inferior to unite with a small part of the anterior division of L2 to form the genitofemoral nerve. Iliohypogastric, ilioinguinal, and genitofemoral provide sensation to the lower abdominal and hip area and the genital areas.

STEP 5

Add the L2 Nerve Root. The Anterior Division contributes to the formation of the Obturator Nerve (gives motor supply to the medial thigh muscles). The Posterior Division which contributes to both the lateral femoral cutaneous and Femoral Nerves (supplies motor to the anterior thigh muscles).

STEP 6

Add the Lateral Cutaneous Nerve of the thigh (a.k.a., lateral femoral cutaneous)

STEP 7

Add the L3 and L4 Nerves. The Anterior Divisions contribute to the formation of the Obturator Nerve. The Posterior Divisions contribute to the formation of the Femoral Nerve.

STEP 8

Add the Obturator Nerve.

STEP 9

Add the Femoral Nerve.

STEP 10

Note: The Anterior Cutaneous branch of the Femoral Nerve supplies the skin of the anterior and medial thigh. (NOT IDENTIFIED IN THIS CADAVER).

Sacral Plexus

The Sacral Plexus joins the lumbar plexus giving contributions from L4 and L5 via the lumbosacral trunk

Add the L5 nerve root.

STEP 12

Add the S1 nerve root.

STEP 13

Add the S2 nerve root

STEP 14

Add the S3 nerve root

STEP 15

Add the Sciatic Nerve which is the largest branch of the sacral plexus. The sciatic nerve exits the pelvis via the Greater Sciatic Foramen. The Greater Sciatic Foramen is defined by the opening created by the ilium, sacrum, the sacrotuberous and sacrospinous ligaments. The sacrospinous ligament is (NOT IDENTIFIED IN THIS CADAVER).

NOTE that the model has been rotated slightly so that you can appreciate that the sciatic nerve exits posteriorly to the gluteal region (whereas the branches of the lumbar plexus viewed previously exited anteriorly and medially).

STEP 16

Add the Sacrotuberous Ligament.

STEP 17

Rotate the cadaver further for a posterior view.

STEP 18

Add the Piriformis muscle.

STEP 19

Remember that the sciatic nerve is actually two separate nerves that are wrapped in a common sheath until it passes the piriformis muscle. Once it passes the piriformis muscle the two nerves leave the common sheath to take their own separate paths to the leg. The two nerves that run in this common sheath are the:

Common Fibular (Peroneal) Nerve:
Tibial Nerve

Additional branches of the lumbo-sacral plexus:

Add the Superior Gluteal Nerve (L4-S1).

STEP 20

Add the Inferior Gluteal Nerve (L5-S2).

STEP 21

Add the Posterior Femoral Cutaneous (S1-S2).

STEP 22

Two small nerves also come from the anterior division (not shown):

nerve to the quadratus femoris and inferior gemellus (L4-S1).
nerve to the obturator internus and superior gemellus from L5-S2.

Continue to Build-It: Gluteal Region

Credits:

VH Dissector steps modified for Drexel Dissector by Haviva Goldman, PhD, Noel Goodstadt, DPT and Sophie Geagan (MD student) from original website activity created by Jeffrey Fahl, MD, Kyle Petersen, PhD, Richard Drake, PhD, Alesha Petitt, MA, Claira Ralston, MS and Kim Price, MA and modified by Jeffrey Fahl, MD, Michael Smith, PhD, Albany Medical College.

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