Lumbo sacral plexus

LUMBO SACRAL PLEXUS

  • The lumbosacral plexus supplies the muscles of the hind limb.
  • It is formed by the convergence of the ventral primary branches of the last three lumbar and first two or three sacral spinal nerves.
  • This plexus is in two parts-anterior and posterior parts connected by anastomotic branches.
  • The anterior part is formed by the fourth, fifth and sixth lumbar nerves and is situated between the psoas muscles while the posterior part is formed by the sixth lumbar, first and second sacral nerves and sometimes a branch of the third sacral and is on the pelvic face of the sacrosciatic ligament at the greater sciatic foramen.
  • The nerves from the anterior part of the plexus are the iliopsoas, femoral and obturator nerves.
  • The nerves from the posterior part are the anterior gluteal, sciatic and the posterior gluteal nerves. (View image)
  • Ilio psoas nerve
    • It derives its fibres from the fourth lumbar and supplies psoas major, psoas minor and iliopsoas muscles.
  • Femoral nerve
    • It derives its fibres from the fourth and fifth lumbar and is the largest nerve of the anterior part of the plexus.
    • It runs downward and backward between the psoas muscles, crosses the lateral face of the tendon of psoas minor, gets undercover of sartorius, lies on the termination of iliopsoas gives off a branch to the later, detaches the saphenous nerve and dips in between the rectus femoris and vastus medialis to supply the quadriceps femoris and articularis genu.
  • Saphenous nerve
    • The saphenous nerve is detached a little above the brim of the pelvis as the femoral nerve crosses the tendon of psoas minor, passes down at first between the sartorius and vastus medialis (femoral canal) and then gives off a long branch which passes backward into the head of the pectineus.
    • The nerve gains exit with the saphenous artery and saphenous vein at the space between the sartorius and gracilis at about the level of the stifle, divides into several branches to supply sartorius and gracilis.
    • Two sets of the terminal branches penetrate the deep fascia and pass subcutaneously to supply the skin on the medial aspect of the leg.
  • Obturator nerve
    • This nerve derives its fibres from all the three lumbar.
    • It passes downward and backward across the iliacus, then on the medial surface of the ilium covered by peritoneum then turns downward on the shaft of the ilium towards the obturator internus, passes through the obturator foramen, then through obturator externus supplying it and divides to supply pectineus, adductor and gracilis muscles.
  • Anterior gluteal nerve
    • This is the anterior of the nerves derived from the posterior part of the plexus and is in front of the sciatic nerve.
    • Its fibres are derived from the sixth lumbar and the first sacral nerves.
    • It divides into four or five branches that leave the cavity through the greater sciatic foramen in company with the anterior gluteal vessels and then supply the gluteus medius and deep gluteus or gluteus profundus.
    • A long branch passes between these two muscles to supply the tensor fasciae latae.
  • Posterior gluteal nerve
    • This nerve is behind the sciatic and its fibres are derived from the first and second sacrals.
    • It divides into branches that pass backward through the greater sciatic foramen over the gluteus medius and supply the upper part of the biceps femoris.
  •  Sciatic nerve
    • It is the largest nerve in the body. It is derived from the sixth lumbar and first and second sacral nerves.
    • It is a wide flat band, leaves the pelvic cavity through the greater sciatic foramen, passes downward and backward between the gluteal muscles, gains the hollow between the trochanter major and tuber ischii runs over the gemellus and quadratus femoris, bends downward, descends behind the thigh in a muscular canal bounded by the biceps femoris and semitendinosus laterally and adductor and semimembranosus medially, dips between the two heads of gastrocnemius and is continued as the tibial nerve.  ( View the nerve supply to the thigh )
  • Collateral branches
    • A slender muscular branch that passes along its posterior border through the lesser sciatic foramen to the obturator internus. This nerve is given off in the midway between the two sciatic foramina.
    • A small muscular branch from the deep face of the sciatic, passes backward between the nerve and gluteus profundus, supplies gemellus and passes through it to supply the quadratus femoris.
    • A large muscular branch to supply the biceps femoris, semitendinosus and semimembranosus.
    • The posterior cutaneous nerve of the leg is detached from the sciatic about the middle of the thigh. It descends over the lateral head of the gastrocnemius under the cover of biceps femoris. It runs downward in company with the recurrent tarsal vein in front of the outer edge of the gastrocnemius tendon to the lower third of the leg. It divides here into a number of branches that are distributed to the skin on the lateral aspect of the tarsal and metatarsal regions.
    • The peroneal nerve is a very large trunk detached about the middle of the thigh. It descends in front of the sciatic to the origin of the gastrocnemius, crosses over the lateral head of this muscle obliquely downward and forward, gains the level of the origin of the lateral digital extensor and divides into superficial and deep peroneal nerves. It also gives off branches to biceps femoris and the lateral cutaneous nerve of the leg, which perforates through the biceps femoris and supplies the skin of the stifle region.
      • The superficial peroneal nerve supplies branches to the lateral digital extensor and then crosses beneath the peroneus longus, passes down between it and the lateral digital extensor to the lower third of the leg. It there after runs down between the lateral and long digital extensors, gains the dorsolateral face of the lower part of the hock, runs down subcutaneously over the annular ligaments, runs down the metatarsal region on the extensor brevis, gains the company of the dorsal superficial metatarsal vein and divides below into three branches of which the lateral and medial are continued down as the lateral dorsal abaxial and medial dorsal abaxial digital nerves. The middle branch joins the dorsal branch of the deep peroneal to form the dorsal common digital nerve which divides into two dorsal axial digital nerves.
      • The deep peroneal nerve at first dips between the lateral digital extensor and peroneus longus and then between the lateral and long digital extensors. It gives branches to all the muscles of the dorsolateral group of the leg and is continued down on the lateral border of the tendon of the long digital extensor and reaches the hock, gives off branches to extensor brevis, passes under the annular ligament, runs down the metatarsal groove in company with the dorsal metatarsal vessels and divides above the fetlock into dorsal and plantar branches. The dorsal branch joins a branch of the superficial peroneal and the plantar branch joins the branch of the median plantar nerve to form the plantar common digital nerve that divides into two plantar axial digital nerves.
  • Tibial nerve
    • It is a continuation of the sciatic after it dips between the two heads of the gastrocnemius.
    • It passes along the posterior border of the superficial flexor of the digit and becomes subcutaneous by passing under the medial head of the gastrocnemius continues in front of the tendo-achilles on the medial aspect of the leg.
    • Its collateral branches supply all the muscles on the posterior aspect of the leg.
Last modified: Wednesday, 2 May 2012, 7:05 AM