Posterior Abdominal Wall and Lumbar Plexus

• Lecture 3 covers the muscles of the posterior abdominal wall, the lumbar plexus, joints of the pelvis, and muscles of the back.
• The intended learning outcomes include describing the posterior abdominal wall muscles (origin, insertion, actions, and nerve supply), describing the lumbar plexus (location, formation, and branches), identifying joints and ligaments of the pelvis, and identifying deep muscles of the back.

Posterior Abdominal Wall Muscles

• Includes: Psoas major, Psoas minor, Quadratus lumborum, Iliacus

Psoas Major

• Origin: bodies and transverse processes of T12-L5 vertebrae.
• Insertion: Lesser trochanter of the femur via the iliopsoas tendon.

Iliacus

• Origin: iliac fossa and ala of the sacrum
• Insertion: lesser trochanter of the femur (iliopsoas tendon).
• Actions of both Iliacus and Psoas Major (iliopsoas): Main flexor of the hip joint and lateral rotation of the hip joint.

Psoas Minor

• Lies anterior to the psoas major.
• Origin: bodies of T12 and L1, and the disc between them.
• Insertion: superior pubic ramus.
• Action: weak flexor of the trunk.

Quadratus Lumborum

• Origin: posterior part of the iliac crest & iliolumbar ligament
• Insertion: 12th rib and transverse processes of the upper 4 lumbar vertebrae
• Action: Unilateral: lateral flexion of trunk. Bilateral: Extension of trunk (lumbar vertebrae). It fixes the last rib during inspiration.

Nerve Supply To Posterior Abdominal Wall Muscles

• Iliacus: Femoral nerve
• Psoas major: Ventral rami of lumbar nerves
• Psoas minor: Ventral rami of lumbar nerves
• Quadratus lumborum: Ventral rami of lumbar nerves

Spinal Nerves

• Spinal nerves leave the vertebral canal through intervertebral foramina and then divide into: Posterior ramus (small) supplies deep muscles of the back (intrinsic) and Anterior ramus (large) shares in plexuses formation (except thoracic nerves as it form intercostal nerves) and supplies most of the body.

Lumbar Plexus

• Anterior rami of L1-L4 form the lumbar plexus.
• Anterior rami of lumbar nerves give off branches to psoas and quadratus muscles.
• Anterior rami of lumbar nerves enter the substance of the psoas major muscle to form the lumbar plexus.
• Concerned with innervation of lower part of abdominal wall, anterior and medial compartments of the thigh.

Main Branches of Lumbar Plexus

• Iliohypogastric (L1)
• Ilio-inguinal (L1)
• Genitofemoral (L1,2)
• Lateral cutaneous nerve of the thigh (L2,3)
• Femoral nerve (L2,3,4)
• Obturator nerve (L2,3,4)
• Iliohypogastric nerve (L1) and Ilioinguinal nerve (L1) motor innervation: supply the lower part of the internal oblique and transversus abdominis muscles.
• Ilioinguinal nerve sensory innervation: the upper part of anteromedial aspect of the thigh (small area)
• Iliohypogastric nerve sensory innervation: the lateral side of the gluteal region.
• Femoral nerve (L2,3,4): largest branch emerges from the lower part of lateral border of psoas major, descends in the groove between the psoas major and iliacus, enters the thigh behind the inguinal ligament then runs in the femoral triangle.
• Femoral nerve motor innervation: iliacus & pectineus & muscles of anterior compartment of thigh.
• Femoral nerve sensory innervation: anterior and medial aspects of the thigh, gives rise to saphenous nerve. Joints: It supplies both the hip joint and knee joints.
• Obturator nerve (L2,3,4) motor innervation: Obturator externus and adductors of the thigh.
• Obturator nerve (L2,3,4) sensory innervation: medial side of thigh. Joints: supplies both the knee and hip joints.
• Genitofemoral nerve (L1,2) sensory innervation: genitalia & upper segment of the anterior of thigh.
• Lateral cutaneous nerve of the thigh (L2,3): emerges from the lateral border of the psoas major, crosses the iliacus obliquely to enter the thigh behind the lateral end of the inguinal ligament
• Lateral cutaneous nerve of the thigh sensory innervation: skin of the thigh (anterolateral till the knee)
• Lateral cutaneous nerve of the thigh motor innervation: No motor innervation

Joints of The Pelvis

• Sacro-iliac: between the auricular surfaces of both the sacrum and ilium. Its is a plane synovial joint with strong anterior and posterior sacroiliac ligaments with a limited degree of gliding and rotation.
• Symphysis pubis: between the 2 pubic bones. It is a 2ry cartilagineous joint with limited movement.
• Sacro-coccygeal: between apex of the sacrum and base of the coccyx. It is a 2ry cartilagineous joint limited to minor flexion and extension which occurs passively, for example during defecation and labor.

Ligaments of The Pelvis

• From L5 transverse process: Ilio-lumbar ligament (from the tip of L5 transverse process to the iliac crest) and Lumbo-sacral ligament from inferior aspect of L5 transverse process to the ala of the sacrum.
• Both of these ligaments support the 5th lumbar vertebra preventing its displacement antero-inferiorly by the effect of the body weight.
• From sacrum and coccyx: Sacro-tuberous ligament from side of both sacrum and coccyx to the ischial tuberosity and Sacro-spinous ligament from side of lower part of sacrum + upper part of coccyx to the ischial spine.
• These ligaments support the sacrum helping it to transmit the body weight.

Back Muscles

• Extrinsic back muscles (superficial group): Muscles functionally belong to the upper limbs but are situated on the posterior aspect of the trunk. Includes:Trapezius, Latissimus dorsi, Rhomboids and Levator scapulae.
• Intrinsic back muscles (deep group): Muscles that act specifically on the vertebral column. Includes: Superficial - Splenius & Erector spinae; Deep - Transversospinales, Interspinales, & Intertranversarii.

Intrinsic Back Muscles

• Superficial Group: Splenius (Capitis & Cervicis) and Erector spinae (Spinalis, Longissimus & Iliocostalis).
• Dorsal rami of spinal nerves provide the Innervation.
• These muscles maintain posture and move the vertebral column.
• Erector spinae muscles: Includes 3 columns from medial to lateral: Spinalis, Longissimus & Iliocostalis. They lie on each side of vertebral spines.
• Origin of the erector spinae: Back of sacrum, Posterior aspect of iliac crest, Lumbar and lower thoracic vertebrae

Insertion of The Erector Spinae

1- Spinalis: the most medial and the smallest column (prominent only in the thoracic region): - Capitis: Back of the skull - Cervicis: Cervical spines - Thoracis: Thoracic spines 2- Longissimus muscle: the middle column and the longest of the back muscles: - Capitis: skull - Cervicis: cervical transverse processes - Thoracis: thoracic transverse processes 3- Iliocostalis muscle: the lateral column : - Cervicis: lower cervical transverse processes - Thoracis: angles of the ribs - Lumborum: lumbar vertebrae transverse processes

• Action of erector spinae: Bilateral: extend the vertebral column and head and unilateral: lateral (side) flexion of the vertebral column.

Deep Intrinsic Back Muscles

• Transversospinales: small muscles between the vertebral transverse processes and spines
• Interspinales: small muscles between the vertebral spines
• Intertranversarii: small muscles between the vertebral transverse processes.
• Innervation: All innervated by Posterior rami of the spinal nerves.
• Transversospinales: Deep to erector spinae , a group of short muscles, occupy area between the transverse and spinous processes of the vertebral column and originate from the transverse processes and insert in the spinous processes of superior vertebra.
• 3 major muscles in this group: the semispinalis, multifidus and rotatores.
• Semispinalis (thoracis, cervicis and capitis): the most superficial of the deep intrinsic muscles Action: Extends and contralaterally rotates the head and vertebral column.
• Multifidus: Deep to semispinalis muscle and it is best developed in the lumbar area.
• Rotatores: They are most prominent in the thoracic region.
• Multifidus and Rotatores both Stabilize the vertebral column.
• Minor Deep Intrinsic Muscles: Interspinales: between adjacent spinous processes (Stabilize the vertebral column.) and Intertranversarii between adjacent transverse processes (Stabilize the vertebral column.)