Posterior Abdominal Wall Anatomy

Overview of the Posterior Abdominal Wall

• The session focuses on the muscles of the posterior abdominal wall, vertebrae, and fascia.
• Aims to understand the structure and function of these muscles.
• Notes may include additional observations beyond the primary focus.
• The diaphragm is a key divider between the thoracic and abdominal cavities and contributes to the posterior abdominal wall.
• Lumbar vertebrae are larger than thoracic vertebrae due to the increased weight they bear in a bipedal stance.
• The size of the lumbar vertebrae affects the structure of the posterior abdominal wall.

Anatomical Structure

• The diaphragm curves around, contributing to both the anterior and posterior abdominal walls.
• The inferior vena cava and aorta are anterior to the vertebral bodies, creating gutters on either side.
• These "gutters" form spaces in the posterior abdominal wall where kidneys and other organs reside.

Muscles of the Posterior Abdominal Wall

• The diaphragm helps form the abdominal wall.
• Psoas major is a large, prominent muscle of the posterior abdominal wall.
• Origin: Transverse processes of lumbar vertebrae and T12.
• Insertion: Lesser trochanter of the femur.
• Function: hip flexor, flexing the femur at the hip joint.
• Involvement in lateral flexion of the vertebrae or trunk.
• Psoas minor is a thin muscle on the anterior surface of psoas major and is not present in all individuals.
• Origin: Runs to the superior ramus of the pubis bone.
• Nervous innervation of psoas major comes directly from spinal nerves at levels L1, L2, and L3 due to its proximity to the vertebrae.
• The nerves do not form specific named nerves before innervating the muscle.
• Nerve fibers run directly from the spinal nerves to the psoas major muscle.
• Iliacus attaches to the wing of the ilium and the anterior superior iliac spine.
• The iliacus also runs down to the lesser trochanter of the femur, enabling femur flexion at the hip joint.
• It cannot contribute to trunk flexion or lateral flexion.
• The iliacus and psoas major muscles blend together to form the iliopsoas.
• When the hip is flexed, iliacus can laterally rotate the femur.
• The femoral nerve innervates iliacus and comes from levels L2, L3, and L4 of the lumbar plexus.
• Quadratus lumborum is a rectangular-shaped muscle in the lower back.
• Origin: Iliac crest and ilio lumbar ligament.
• Insertion: Transverse processes of L1 to L4 and the 12th rib.
• Function: Plays a role in anchoring the 12th rib and the rib cage.
• Contraction Results: Lateral flexion of the lumbar vertebrae or trunk to one side.
• Bilateral Contraction Results: Assists with extension if the vertebral column is flexed.
• Spinal Nerves: Receives innervation from the anterior rami of spinal nerves T12, L1, L2, L3, and L4.

Fascia of the Posterior Abdominal Wall

• Each muscle is held together by connective tissue that bundles muscle cells together.
• The outer layer of connective tissue around a muscle can merge with other fascias.
• Thoracic lumbar fascia is found deep in the posterior abdominal wall.
• The tissue connects fibers from the latissimus dorsi.
• The anterior part of the thoracic lumbar fascia forms a sheath around the quadratus lumborum.
• Thoracic lumbar fascia is formed by the blending of connective tissues.
• Holds the muscles and components of the posterior portion together.
• Peritoneum lines the abdominal cavity and covers the viscera and is separate from the fascia.
• The peritoneum is a serous membrane that holds organs in place.

Neural Structures

• Lumbar plexus (also called the lumbosacral plexus) is located within the posterior abdominal wall.
• It links with the sacral plexus.
• Nerves from the lumbar and sacral levels form nerves like the femoral and sciatic nerves.