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Lumbar and sacral plexuses
ANATOMY
Lumbar and sacral plexuses
Lumbar plexus
T12
John Craven
Iliohypogastric nerve
L1 L2
Ilioinguinal nerve Genitofemoral nerve
Lumbar plexus The lumbar plexus (Figure 1) is formed by the anterior primary rami of L1–4 nerves. It lies within psoas major on the posterior abdominal wall. All nerves receive grey rami communicantes from the sympathetic trunk. From the plexus emerge: • the obturator nerve (L2–4), supplying the thigh adductors • the femoral nerve (L2–4), supplying iliacus and knee extensors • the lumbosacral trunk (L4–5), which descends over the sacrum to contribute to the sacral plexus • the ilioinguinal (L1) and the iliohypogastric nerves (L1), which obliquely traverse the abdominal wall muscles, supplying them and the inguinal and suprapubic skin • the lateral femoral cutaneous nerve (L2–3), entering the thigh just medial to the anterior superior iliac spine to supply the skin of the anterolateral surface of the thigh • the genitofemoral nerve (L1–2), supplying the skin of the genitalia and the femoral triangle.
Lateral cutaneous nerve of thigh
L4
To psoas and iliacus
L5
Femoral nerve Obturator nerve Lumbosacral trunk The yellow branches are derived from the anterior and the green branches from the posterior divisions of the anterior primary rami of the lumbar nerves 1
Sacral plexus The sacral plexus is formed by the lumbosacral trunk, a conjunction of the anterior divisions of the L4 and L5 nerves and the upper four sacral nerves. It lies in front of the sacrum on piriformis, deep to the pelvic fascia (Figure 2). Its branches can be divided into pelvic branches and those leaving the pelvis. In the pelvis there are muscular branches to piriformis, levator ani, coccygeus and the external anal sphincter. Pelvic splanchnic nerves arise from the anterior primary rami of S2–4 to supply the pelvic viscera. These have a vasodilator function when acting on the erectile tissue (nervi erigentes), an inhibitor function on the internal anal and vesical sphincters and a motor function on the smooth muscle of the rectum and bladder. Most of these branches leaving the pelvis pass through the greater sciatic foramen above or below piriformis. Above the piriformis the superior gluteal nerve supplies the gluteus medius and minimus. Below the piriformis the sciatic nerve (L 4–5, S1–3) descends in the posterior compartment of the thigh to end by dividing into the tibial and peroneal nerves just above the popliteal fossa. It supplies the hip and knee joints and the hamstrings, semimembranosus, semitendinosus and biceps femoris. In the buttock, the sciatic nerve lies midway between the ischial tuberosity and the greater trochanter and may be injured by a carelessly placed intramuscular injection. The only safe area of the buttock in which to place injections is the upper outer quadrant. The pudendal nerve (S2–3) enters the perineum via the lesser sciatic foramen and runs forwards in the pudendal canal on the medial surface of obturator internus. It supplies levator ani, the perineal muscles (including the external anal sphincter) and the perianal and perineal skin. It also
Lateral wall of pelvis showing sacral plexus Internal iliac artery Lumbosacral trunk External iliac vein and artery
S1 nerve S2 nerve Piriformis S3 nerve
Sciatic nerve Obturator nerve
Obturator internus and overlying fascia Dorsal nerve of penis or clitoris Perineal nerve
S4 nerve Pudendal nerve
Ischial spine
Nerves in pudendal canal
2
carries parasympathetic fibres to the corpora of the penis/clitoris. A pudendal nerve block can be readily achieved by guiding the needle to the ischial spine with a finger in the vagina and injecting local anaesthetic around the ischial spine. An effective nerve block results in relaxation of levator ani, anaesthesia over the vulva and loss of the anal reflex. Muscular branches supply gluteus maximus, obturator internus and quadratus femoris. Cutaneous branches supply the back of the thigh and the gluteal region. The nerves of the thigh, leg and foot are described on page 112.
John Craven was formerly Consultant Surgeon at York District Hospital, York, UK. He is past chairman of the primary examiners of the Royal College of Surgeons of England.
ANAESTHESIA AND INTENSIVE CARE MEDICINE
L3
108
© 2004 The Medicine Publishing Company Ltd
Lumbar and sacral plexuses
Lumbar plexus
T12
John Craven
Iliohypogastric nerve
L1 L2
Ilioinguinal nerve Genitofemoral nerve
Lumbar plexus The lumbar plexus (Figure 1) is formed by the anterior primary rami of L1–4 nerves. It lies within psoas major on the posterior abdominal wall. All nerves receive grey rami communicantes from the sympathetic trunk. From the plexus emerge: • the obturator nerve (L2–4), supplying the thigh adductors • the femoral nerve (L2–4), supplying iliacus and knee extensors • the lumbosacral trunk (L4–5), which descends over the sacrum to contribute to the sacral plexus • the ilioinguinal (L1) and the iliohypogastric nerves (L1), which obliquely traverse the abdominal wall muscles, supplying them and the inguinal and suprapubic skin • the lateral femoral cutaneous nerve (L2–3), entering the thigh just medial to the anterior superior iliac spine to supply the skin of the anterolateral surface of the thigh • the genitofemoral nerve (L1–2), supplying the skin of the genitalia and the femoral triangle.
Lateral cutaneous nerve of thigh
L4
To psoas and iliacus
L5
Femoral nerve Obturator nerve Lumbosacral trunk The yellow branches are derived from the anterior and the green branches from the posterior divisions of the anterior primary rami of the lumbar nerves 1
Sacral plexus The sacral plexus is formed by the lumbosacral trunk, a conjunction of the anterior divisions of the L4 and L5 nerves and the upper four sacral nerves. It lies in front of the sacrum on piriformis, deep to the pelvic fascia (Figure 2). Its branches can be divided into pelvic branches and those leaving the pelvis. In the pelvis there are muscular branches to piriformis, levator ani, coccygeus and the external anal sphincter. Pelvic splanchnic nerves arise from the anterior primary rami of S2–4 to supply the pelvic viscera. These have a vasodilator function when acting on the erectile tissue (nervi erigentes), an inhibitor function on the internal anal and vesical sphincters and a motor function on the smooth muscle of the rectum and bladder. Most of these branches leaving the pelvis pass through the greater sciatic foramen above or below piriformis. Above the piriformis the superior gluteal nerve supplies the gluteus medius and minimus. Below the piriformis the sciatic nerve (L 4–5, S1–3) descends in the posterior compartment of the thigh to end by dividing into the tibial and peroneal nerves just above the popliteal fossa. It supplies the hip and knee joints and the hamstrings, semimembranosus, semitendinosus and biceps femoris. In the buttock, the sciatic nerve lies midway between the ischial tuberosity and the greater trochanter and may be injured by a carelessly placed intramuscular injection. The only safe area of the buttock in which to place injections is the upper outer quadrant. The pudendal nerve (S2–3) enters the perineum via the lesser sciatic foramen and runs forwards in the pudendal canal on the medial surface of obturator internus. It supplies levator ani, the perineal muscles (including the external anal sphincter) and the perianal and perineal skin. It also
Lateral wall of pelvis showing sacral plexus Internal iliac artery Lumbosacral trunk External iliac vein and artery
S1 nerve S2 nerve Piriformis S3 nerve
Sciatic nerve Obturator nerve
Obturator internus and overlying fascia Dorsal nerve of penis or clitoris Perineal nerve
S4 nerve Pudendal nerve
Ischial spine
Nerves in pudendal canal
2
carries parasympathetic fibres to the corpora of the penis/clitoris. A pudendal nerve block can be readily achieved by guiding the needle to the ischial spine with a finger in the vagina and injecting local anaesthetic around the ischial spine. An effective nerve block results in relaxation of levator ani, anaesthesia over the vulva and loss of the anal reflex. Muscular branches supply gluteus maximus, obturator internus and quadratus femoris. Cutaneous branches supply the back of the thigh and the gluteal region. The nerves of the thigh, leg and foot are described on page 112.
John Craven was formerly Consultant Surgeon at York District Hospital, York, UK. He is past chairman of the primary examiners of the Royal College of Surgeons of England.
ANAESTHESIA AND INTENSIVE CARE MEDICINE
L3
108
© 2004 The Medicine Publishing Company Ltd