Anatomy of the muscles and nerves of the posterior abdominal wall

Chapters:

Watch video only

Introduction 0:00–0:33

The posterior abdominal wall is a complex area of the body made up of muscles, vessels and nerves that work together to hold abdominal organs in place, adjust our posture, stabilize the hip joint and even flex our thighs on leg day!
The main paired muscles of the posterior abdominal wall include psoas major, iliacus and quadratus lumborum; and naturally, for those muscles to work properly, they receive innervation from various nerves.Okay, now let’s take a closer look at these muscles!

Psoas major0:33–2:03

First up is the psoas major, which is a long, thick muscle that lies lateral to the lumbar vertebrae. It originates superiorly on the transverse processes of the bodies, and adjacent intervertebral discs of the T12 through L5 vertebrae.
The psoas major passes inferolaterally, deep to the inguinal ligament in order to insert on the lesser trochanter of the femur.
Finally, approximately half the population has a psoas minor muscle that sits on the anterior aspect of each psoas major muscle.Now, the psoas major muscle is covered by the psoas fascia or the psoas sheath, which is attached medially to the lumbar vertebrae and pelvic brim.
The psoas sheath is thickened superiorly forming the medial arcuate ligament. Inferior to the iliac crest, the psoas fascia is continuous with the part of the iliac fascia that covers the iliacus muscle.
When acting inferiorly with the iliacus muscle, the psoas muscle flexes and laterally rotates the thigh and when it acts superiorly it causes lateral flexion of the vertebral column.
When sitting, the psoas muscle acts with the iliacus muscle to flex the trunk. The innervation of the psoas major muscle comes from the anterior rami of the L1, L2 and L3 spinal nerves.
Next is the iliacus muscle, which is a large triangular muscle that lies lateral to the inferior portion of the psoas major muscle.

Iliacus2:03–2:47

It originates superiorly on the superior two thirds of the iliac fossa, ala of sacrum, and anterior sacroiliac ligaments.
It also inserts at the lesser trochanter of femur. In fact, most of its fibers join the tendon of the psoas major muscle to form the iliopsoas muscle, which is the main flexor of the thigh and a stabilizer of the hip joint, and it also helps maintain erect posture.
The innervation of iliacus comes from the femoral nerve, which is derived from the anterior rami of spinal nerves L2 through L4.
Now let’s look at the quadratus lumborum muscle, which forms a thick muscular sheet in the posterior abdominal wall, along the transverse processes of the lumbar vertebrae.

Quadratus lumborum2:47–3:48

It originates inferiorly on the iliac crest and iliolumbar ligament and inserts superiorly on the medial half of the inferior border of the 12th rib and the tips of the lumbar transverse processes.
The quadratus lumborum muscle is enclosed within two layers of the thoracolumbar fascia. The anterior layer of the thoracolumbar fascia - also called the quadratus lumborum fascia - thickens in the superior aspect to form the lateral arcuate ligament.
Bilaterally, the quadratus lumborum muscles extend the trunk and also fix the 12th pair of ribs during inspiration. Unilaterally, it causes lateral flexion of the vertebral column.
Its innervation comes from the subcostal nerve and anterior rami of the L1–L4 nerves.So far so good! Time for a quick quiz.

Quiz3:48–4:04

What are the main actions of the composite iliopsoas muscle and the quadratus lumborum muscle?Now let’s have a quick look at the nerves of the posterior abdominal wall, which provide somatic innervation through the subcostal nerves and the lumbar plexus; and autonomic innervation through the sympathetic trunks and associated ganglia.

Nerves of posterior abdominal wall4:04–4:21

First up, the subcostal nerves are paired nerves that represent the anterior rami of T12. They arise in the thorax and pass posterior to the lateral arcuate ligaments into the abdomen.

Subcostal nerves4:21–4:51

Then they run inferolaterally on the anterior surface of the quadratus lumborum to eventually pass through the transversus abdominis, internal oblique, and external oblique muscles innervating them and the skin of the anterolateral abdominal wall.

Lumbar plexus4:51–5:22

Next up is the lumbar plexus, which forms within the psoas major muscle. This is a network of nerves that is composed of the anterior rami of the L1 through L4 spinal nerves.
The nerves that arise from the lumbar plexus include: the iliohypogastric nerve; the ilioinguinal nerve; the genitofemoral nerve; the lateral cutaneous nerve of the thigh; the femoral nerve; the obturator nerve; and finally, the lumbosacral trunk.First, let’s look at the ilioinguinal and the iliohypogastric nerves, both of which arise from the anterior ramus of L1.

Ilioinguinal nerve5:22–6:06

They enter the abdomen posterior to the medial arcuate ligament and then pass inferolaterally, anterior to the quadratus lumborum.
Then they run superior and parallel to the iliac crest with the iliohypogastric nerve travelling more superiorly than the ilioinguinal nerve.
Then they pierce the transversus abdominis and internal oblique muscles near the anterior superior iliac spine supplying them with motor innervation.
Finally, they pass through the external oblique muscle in order to supply this muscle and the skin of the inguinal and pubic regions.
Next up is the genitofemoral nerve, which arises from the anterior rami of L1 and L2. It travels within the psoas major, and eventually pierces through it to run inferiorly on its anterior surface.

Genitofemoral nerve6:06–6:48

Lateral to the common and external iliac arteries, it divides into femoral and genital branches. The femoral branch of the genitofemoral nerve provides sensation to the skin of the upper, anterior thigh.
The genital branch of the genitofemoral nerve supplies motor innervation to the cremaster muscle and sensation to the scrotal skin in males and the skin of mons pubis and labia majora in females.
Next is the lateral cutaneous nerve of the thigh, also called the lateral femoral cutaneous nerve, which arises from the anterior rami of L2 and L3.

Lateral cutaneous nerve of thigh6:48–7:14

This nerve runs inferolaterally on the iliacus muscle and enters the thigh deep to the inguinal ligament, medial to the anterior superior iliac spine.
Its role is to provide sensation to the skin on the anterolateral surface of the thigh. Moving on, next is the femoral nerve, which arises from the anterior rami of L2 through L4.

Femoral nerve7:14–7:47

It emerges from the lateral border of the psoas major muscle, gives motor innervation to the iliacus muscle and then it passes deep to the inguinal ligament into the anterior thigh.
In the thigh it gives motor innervation to the flexors of the hip and extensors of the knee, and gives rise to cutaneous branches that provide sensation to the antero-medial thigh, and the medial leg and foot.

Obturator nerve7:47–8:11

Next up is the obturator nerve, which also arises from the anterior rami of L2 through L4 and emerges from the medial border of the psoas major muscle.
It passes into the lesser pelvis, and then through the obturator foramen, to enter the medial thigh, where it gives motor innervation to the obturator externus and adductor muscles and sensation to the medial thigh.
Now, in about 10% of people, there’s an accessory obturator nerve, which arises from the anterior rami of L3 and L4. It parallels the medial border of the psoas major muscle, anterior to the obturator nerve, and crosses superior to the superior pubic ramus in close proximity with the femoral vein.

Accessory obturator nerve8:11–8:33

Finally, the lumbosacral trunk, which is formed from the anterior rami of L4 and L5, passes over the ala of the sacrum and runs inferiorly into the pelvis to contribute to the formation of the sacral plexus, along with the anterior rami of the S1 through S4 nerves.

Lumbosacral trunk8:33–8:54

And last but not least, there’s the abdominal portion of the sympathetic trunk. There are typically four lumbar paravertebral sympathetic ganglia and the inter-ganglionic branches that connect them.

Sympathetic trunk8:54–9:31

They are continuous with the thoracic portion of the sympathetic trunk deep to the medial arcuate ligaments of the diaphragm.
Now, the abdominal sympathetic trunk descends on the anterolateral aspects of the bodies of the lumbar vertebrae, just adjacent to the psoas major muscle.
Inferiorly, they cross the sacral promontory and then continue down into the pelvis as the sacral portion of the sympathetic trunk.
Alright, as a quick recap. The psoas major muscle originates from the T12 to L5 vertebrae and intervening intervertebral discs and inserts on the lesser trochanter of the femur.

Review9:31–10:55

The iliacus muscle originates on the iliac fossa and inserts on the psoas major tendon and lesser trochanter of the femur.
The iliacus muscle and the psoas muscle form the iliopsoas muscle, which is the main hip flexor. The quadratus lumborum originates on the iliac crest and inserts on the lumbar transverse processes and inferior border of the 12th rib.
It extends and laterally flexes the vertebral column and fixes the 12th rib during inspiration. The subcostal nerve arises in the thorax and its main role is to supply the anterolateral abdominal wall muscles and a portion of the skin of the anterolateral abdominal wall.
The lumbar plexus is a network of nerves that’s composed of the anterior rami of L1 through L4. It’s branches include: the iliohypogastric nerve and the ilioinguinal nerve the genitofemoral nerve, the lateral cutaneous nerve of the thigh, the femoral nerve, the obturator nerve and sometimes an accessory obturator nerve and finally, the lumbosacral trunk.
Lastly, are the abdominal and sacral portions of the sympathetic