Nervous System: Lumbar and Sacral Plexus

NERVOUS SYSTEM: LUMBAR PLEXUS AND SACRAL PLEXUS

Dr. Jay Ferguson, DC

OBJECTIVES

  • Describe the attachment of a spinal nerve to the spinal cord: Understanding the anatomical relationship and termination of spinal nerves at the spinal cord.

  • Identify and describe the anatomy of the five plexuses of spinal nerves: Detailed overview of the structure and function specific to each plexus.

  • Identify major nerves and their functions: Listing significant nerves arising from each plexus and their respective roles.

  • Explain the relationship between dermatomes to the spinal nerve: Discussing the skin areas innervated by specific spinal nerves.

THE LUMBAR PLEXUS

  • Location: Formed by anterior rami of spinal nerves L1-L4.

SPINAL NERVE ROOTS AND BRANCHES

Spinal Nerve Roots

  • L1: Anterior ramus

  • L2: Anterior ramus

  • L3: Anterior ramus

  • L4: Anterior ramus

  • L5: Anterior ramus

Major Nerves of the Lumbar Plexus

  • Iliohypogastric nerve (L1)

    • Pathway: Passes over the anterior surface of the quadratus lumborum, pierces the transversus abdominis, and lies between the internal abdominal oblique and transversus abdominis muscles.

    • Innervation:

    • Internal abdominal oblique muscle

    • Transversus abdominis muscle

    • Cutaneous branches:

    • Lateral cutaneous branch: Innervates the skin of the posterolateral gluteal region.

    • Anterior cutaneous branch: Innervates the skin in the pubic region.

  • Ilio-inguinal nerve (L1)

    • Pathway: Inferior to iliohypogastric nerve as it crosses the quadratus lumborum, pierces transversus abdominis, then internal oblique, and enters the inguinal canal.

    • Innervation:

    • Internal abdominal oblique muscle

    • Transversus abdominis muscle

    • Skin of the upper medial aspect of the thigh

    • Skin over the root of the penis and anterior scrotum in males; labia majora and mons pubis in females.

    • Clinical Implications: Damage may result from surgical procedures or blunt trauma, leading to loss of sensation or pain in the groin and genital area.

  • Genitofemoral nerve (L1-L2)

    • Pathway: Pierces the psoas major and descends on its anterior surface, dividing into genital and femoral branches.

    • Genital branch: Innervates the cremaster muscle in males and both the anterior scrotum and the labia majora in females.

    • Femoral branch: Innervates a portion of the skin of the upper anterior thigh.

  • Lateral cutaneous nerve of thigh (L2-L3)

    • Pathway: Emerges lateral to the psoas major, crosses the iliacus, passes posterior to the inguinal ligament, and enters the thigh.

    • Innervation: Skin over the anterolateral thigh.

  • Femoral nerve (L2-L4)

    • Pathway: Lies between the lateral border of the psoas major and the anterior surface of the iliacus muscle, passes posterior to the inguinal ligament into the thigh, where it divides into multiple branches.

    • Innervation:

    • Muscles: Iliacus, pectineus, sartorius, quadriceps femoris.

    • Skin: Anteromedial thigh (via medial and intermediate cutaneous nerves).

    • Articular branches: Supply hip and knee joints.

    • Saphenous nerve: Extension of femoral nerve, innervating the skin of the medial leg and foot.

  • Obturator nerve (L2-L4)

    • Pathway: Emerges medial to the psoas major, dividing into anterior and posterior branches (separated by the adductor brevis).

    • Innervation:

    • Muscles: Obturator externus, pectineus, adductor longus, adductor brevis, gracilis, adductor magnus.

    • Skin: Patch on the medial thigh.

    • Articular branches: Supply hip and knee joints.

Additional Lumbar Plexus Components

  • Lumbosacral trunk (L4-L5): Collection of neural fibers that connect the lumbar plexus to the sacral plexus.

THE SACRAL PLEXUS

  • Location: Extends from L4 to S4, contributing to several important nerves.

Major Nerves of the Sacral Plexus

  • Superior gluteal nerve (L4-S1)

    • Pathway: Exits pelvic cavity through the greater sciatic foramen (superior to piriformis muscle).

    • Innervation: Gluteus medius, gluteus minimus, tensor fasciae latae muscles. No cutaneous sensory branch.

  • Inferior gluteal nerve (L5-S2)

    • Pathway: Exits pelvic cavity through the greater sciatic foramen (inferior to piriformis muscle).

    • Innervation: Gluteus maximus. No cutaneous sensory branch.

  • Nerve to piriformis (S1-S2)

    • Innervation: Piriformis muscle. Remains within the pelvic cavity; no cutaneous sensory branch.

  • Posterior femoral cutaneous nerve (S1-S3)

    • Innervation: Skin of the posterior thigh. Compression may mimic sciatica.

  • Sciatic nerve (L4-S3)

    • Pathway: Leaves pelvic cavity through the greater sciatic foramen inferior to the piriformis muscle. Contains two nerves within a single neural sheath:

    • Tibial nerve: Ventral divisions of L4-S3.

    • Common peroneal (fibular) nerve: Dorsal divisions of L4-S2.

    • Clinical Note: Generally splits into divisions just proximal to the knee area.

BRANCHES OF THE COMMON PERONEAL (FIBULAR) NERVE

  • Common peroneal nerve (L4-S2)

    • Innervation: Short head of biceps femoris muscle, skin of proximal anterolateral leg, and parts of the foot.

    • Divisions:

    • Superficial branch: Innervates peroneus longus and peroneus brevis muscles; provides sensation to the distal half of the anterior-lateral leg and dorsal foot.

    • Deep branch: Innervates tibialis anterior, extensor hallucis longus, extensor digitorum longus, peroneus tertius, extensor digitorum brevis, and extensor hallucis brevis muscles, and affords sensation to a small area of skin on the dorsal aspect of the foot between the first two digits.

PUDENDAL NERVE

  • Pudendal nerve (S2-S4)

    • Pathway: Leaves the pelvic cavity through the greater sciatic foramen (inferior to piriformis muscle) to enter the gluteal region.

    • Innervation: Muscles of the perineum, including levator ani, external anal sphincter, and external urethral sphincter.

    • Cutaneous innervation: Skin of the perineum, exterior genitalia in both males and females (penis, scrotum, clitoris, labia).

    • Clinical Application: Blocked during childbirth to alleviate pain (episiotomy).

HILTON’S LAW

  • Definition: The principle that the nerve supplying a joint also innervates the muscles acting on the joint and the skin covering the area.

    • Alternate Explanation: The nerve trunk serving a joint innervates both the overlying skin and the muscles that move the joint.

SOMATIC NERVOUS SYSTEM

  • Dermatomes: The skin area supplied by a single spinal cord level or pair of spinal nerves.

  • Myotomes: The portion of a skeletal muscle innervated by a single spinal cord level or a pair of spinal nerves.