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The Lumbosacral Plexus
by
Prof. Dr. Muhammad Imran Qureshi
Psoas Major Muscle
The Psoas Major lies in the gutter between the
bodies and transverse processes of the lumbar
vertebrae. In addition, the muscle is attached to
the medial ends of Transverse processes of the
lumbar vertebrae. The muscle passes
downwards along the pelvic brim, and then
beneath the inguinal ligament into the thigh,
where its tendon is attached to the lesser
trochanter of the femur.
The Lumbar Plexus
The Lumbar plexus is embedded within these
two masses of origin of this muscle, and thus
in line with the intervertebral foramina.
It is formed by the first three and most of the
fourth lumbar ventral rami.
The First lumbar ventral ramus gets a twig
from the last thoracic ventral ramus (T12, the
subcostal nerve).
Having done that, it immediately bifurcates
into an upper larger branch and a lower
smaller branch.
The upper larger branch divides again into the
iliohypogastric and the ilioinguinal nerves
The smaller lower branch unites with a branch
from the second lumbar ventral ramus to form
the genitofemoral nerve.
The remaining part of the second lumbar ventral ramus, the whole of the third
lumbar ventral ramus, and part of the fourth lumbar ventral ramus that join the
plexus divide into ventral and dorsal branches.
The ventral branches of second to fourth ventral rami join to form the obturator
nerve.
The dorsal branches of the ventral rami of the second and third lumbar nerves divide
again into smaller and larger divisions.
The smaller divisions of the two join to form the lateral femoral cutaneous nerve.
Th larger divisions of the two join with the dorsal branch of the fourth ventral ramus
to form the Femoral nerve.
The branches of the Lumbar plexus are:
Muscular
Iliohypogastric
Ilioinguinal
Genitofemoral
T12, L1-4
L1
L1
L1, L2
Lateral Femoral Cutaneous
Femoral
Obturator
Accessory Obturator
L2, L3
L2, L3, L4 (Dorsal Divisions)
L2, L3, L4 (Ventral Divisions)
L2, L3
The Sacral Plexus
The Sacral Plexus is formed by the lumbosacral
trunk, the first, second, third and part of the
fourth sacral ventral Rami.
Having given off its branches to the lumbar
plexus, the ventral ramus of L4 emerges from the
medial border of Psoas major and joins the
ventral ramus of the L5 to form the Lumbosacral
Trunk.
This large nerve passes over the ala of the
sacrum and crosses the pelvic brim anterior to
the sacroiliac joint, separated from the obturator
nerve by the iliolumbar artery and veins. It
descends to join the anterior rami of the upper
four sacral nerves in the formation of the
Sacral Plexus.
The sacral nerves give off certain branches
and then divide into anterior and posterior
divisions, which thereupon branch and
reunite to form the nerves that supply the
Flexor and Extensor compartments of the
lower limb respectively.
The branches of the Sacral plexus can be
considered under THREE headings:
I.
Branches from the Sacral Nerves:
All of them come from the Sacral
segments, and all have the initial “P”.
Three of them arise from behind and
Three from in front. They are:
From Behind:
Nerve to Piriformis – S(1), 2
Perforating Cutaneous Nerves – S2, 3
Posterior Femoral Cutaneous Nerve – S1, 2
From In Front:
Pelvic Splanchnics – S, 2, 3, (4)
Pudendal Nerve – S2, 3, 4
Perineal branch of S4
The Posterior femoral cutaneous nerve also gets contribution from in front.
The segments are S2, 3
II. Branches from Anterior divisions:
These nerves are destined to the Flexor compartment of the Lower limb. They
are:
Tibial part of the Sciatic nerve – L4,5,S1,2,3
Nerve to Obturator Internus and Superior Gemellus – L5,S1,2
Nerve to Quadratus Femoris and Inferior Gemellus – L4,5, S1
III. Branches from Posterior divisions:
These are the nerves of the Extensor compartment of the Lower limb. They
are:
Common Peroneal part of the Sciatic nerve – L4,5,S1,2,
Superior Gluteal Nerve – L4,5,S1
Inferior Gluteal Nerve – L5,S1,
Relations:
This broad triangular plexus forms lateral to
the anterior sacral foramina. It rests upon
Piriformis and is covered anteriorly by strong
membrane of parietal pelvic fascia, which
covers the muscles.
Anterior to the fascia, the lateral sacral arteries
and veins lie in front of the sacral nerves.
At a higher level, the common iliac vessels lie
over the lumbosacral trunk.
The superior gluteal vessels lie between the
lumbosacral trunk and first sacral ventral
ramus or between the first and second sacral ventral rami.
The
Inferior
gluteal
vessels lie
between
the first
and
second or
second
and third
sacral
ventral
rami.
The Ureter, in front of the internal iliac vessels lies well anterior to the upper part of
the plexus and,
In front of all are the parietal pelvic peritoneum and pelvic viscera – the sigmoid
colon on the left and terminal iliac coils on the right side.
FOUR branches of the internal iliac artery pierce the sacral plexus:
The Iliolumbar artery (Between Obturator nerve and Lumbosacral trunk),
The Superior Gluteal artery,
The Inferior Gluteal artery,
Internal Pudendal arteries (Between Sciatic and Pudendal nerves)