In this note, we are going to discuss the anatomy of the sacral plexus (plexus sacralis), which innervates various structures of the pelvis and lower limb.
We will also briefly refer to the anatomy of the coccygeal spinal nerve and the plexus it forms (plexus coccygeus).
The sacral plexus is formed by the anterior branches (rami anteriores) of the sacral spinal nerves (S1-S4),
as well as the lumbosacral trunk (truncus lumbosacralis), which receives contributions from L4 and L5.
The sacral plexus is located anteriorly to the piriformis muscle and resembles a triangle, the base of which is located at the pelvic sacral foramina, and the apex is at the lower border of the greater sciatic foramen.
The branches of the sacral plexus are divided into three groups: anterior, posterior, and terminal.
Let’s discuss in greater detail the nerves of each group, starting from the posterior group.
Superior gluteal nerve (nervus gluteus superior)
It is formed by fibers from L4, L5, and S1. It exits the pelvic cavity through the greater sciatic foramen (foramen ischiadicum majus),
in particular through its suprapiriform part (foramen suprapiriforme).
The nerve passes between the gluteus minumus and medius muscles, innervating them. It also gives off a branch to the tensor fasciae latae.
Inferior gluteal nerve (nervus gluteus inferior)
It is formed by fibers from L5, S1, and S2 spinal nerves. The nerve also passes through the greater sciatic foramen, but in this case, through its infrapiriform part (foramen infrapiriforme).
It innervates the gluteus maximus and the capsule of the hip joint.
Perforating cutaneous nerve (nervus cutaneus perforans)
It is formed by fibers from S2 and S3. The nerve passes through the greater sciatic foramen, pierces the sacrotuberous ligament and innervates a small inferior medial area of the skin in the gluteal region.
Pelvic splanchnic nerves (nn. splanchnici pelvici)
These nerves are formed by preganglionic parasympathetic and motor fibers from S2, S3 and S4. They exit through the anterior sacral foramina (foramina sacralia anteriora)
and pass to the parasympathetic ganglia of the inferior hypogastric plexus (plexus hypogastricus inferior) and plexuses located near internal organs or deep within the organs of the pelvis. For the most part, they innervate the urinary bladder and distal parts of the large intestine.
The pelvic splanchnic nerves also participate in clitoral (or penile) erection.
Nerve to piriformis (nervus piriformis)
This nerve is formed by fibers from S1 and S2, sometimes only S2. It innervates the piriformis muscle.
Nerve to the quadratus femoris muscle (nervus musculi quadrati femoris)
It receives contributions from L4, L5, and S1. The nerve supplies the quadratus femoris muscle, but it also innervates the inferior gemellus muscle and gives off a small articular branch to the hip joint.
Internal obturator nerve (nervus obturatorius internus)
It is formed by fibers from L5, S1 and S2. The nerve passes through the greater sciatic foramen, where it innervates the gemellus superior muscle.
Then it circumflexes the ischial spine and re-enters the pelvis, but through the lesser sciatic foramen, and innervates the obturator internus muscle
Posterior femoral cutaneous nerve (nervus cutaneus femoris posterior)
It receives fibers from the posterior divisions S1 and S2 as well as the anterior divisions of S2 and S3, so this nerve can be attributed to both the groups of anterior and posterior branches of the plexus. It leaves the pelvic cavity through the infrapiriform foramen, passes down next to the sciatic nerve, and innervates the skin of the posteromedial surface of the thigh down to the popliteal fossa.
Along its course, it gives off the inferior cluneal nerves (nervi clunium inferiores), which innervate the skin of the gluteal region, and the perineal nerves (nervi perineales), which innervate the skin of the perineum.
Pudendal nerve (nervus pudendus)
It is formed by fibers of S2, S3, and S4.
It exits the pelvis through the greater sciatic foramen, through its infrapiriform part (foramen infrapiriforme), circumflexes the ischium,
and enters the ischiorectal fossa through the lesser sciatic foramen.
The nerve gives off the following branches:
- Inferior rectal nerves (nervi rectales inferiores), which innervate the external anal sphincter and the skin of the perianal area.
- Perineal nerves (nn. perineales), which innervate the muscles and skin of the perineum, as well as the mucous membrane of the urethra and vagina in women.
- The dorsal nerve of the penis (clitoris) (nervus dorsalis penis / clitoridis), which is the terminal branch of the pudendal nerve
In men, it innervates the skin and cavernous bodies of the penis, and in women, it innervates the labia majora and minora, as well as the clitoris. It also gives off branches to the deep transverse perineal muscle and the external urethral sphincter.
The terminal group includes the sciatic nerve (nervus ischiadicus) and its branches.
It is formed by fibers of L4–S3.
The nerve exits the pelvic cavity through the infrapiriform foramen, passes under the gluteus maximus to the posterior surface of the thigh, and descends between the adductor magnus muscle and the long head of the biceps femoris.
In the pelvic region and on the thigh, it gives off muscular branches to the adjacent muscles.
In the superior part of the popliteal fossa, the sciatic nerve divides into its two terminal branches: the tibial and common fibular nerves.
The tibial nerve (nervus tibialis) is a continuation of the sciatic nerve in the leg. It descends through the popliteal fossa, then under the tendinous arch of the soleus muscle,
and goes into the tarsal tunnel (a canal on the posteromedial aspect of the ankle).
The medial sural cutaneous nerve (nervus cutaneus surae medialis), which passes inferiorly, arises from the tibial nerve in the area of the popliteal fossa.
At the level of the lower third of the leg, a branch of the common fibular nerve, the lateral sural cutaneous nerve (nervus cutaneus surae lateralis),
joins it, and together they form the sural nerve(nervus suralis).
The medial sural cutaneous nerve innervates the skin of the posterior surface of the leg.
The sural nerve passes posteriorly to the lateral malleolus and innervates the skin of the lateral part of the heel region and the lateral border of the dorsum of the foot.
In the ankle region, the tibial nerve gives off medial calcaneal branches (rami calcanei mediales), which supply the skin on the medial surface and sole of the heel.
Along its course, the tibial nerve also gives off muscular branches to numerous muscles of the posterior parts of the leg and foot:
- Gastrocnemius muscle
- Soleus muscle
- Plantaris muscle
- Popliteus muscle
- Flexor digitorum longus muscle
- Flexor digitorum longus muscle
- Tibialis posterior muscle
The sensory branches of the tibial nerve also innervate the knee joint capsule, the interosseous membrane of the leg, the capsule of the ankle joint, and the bones of the leg.
Emerging from the tibial canal posterior to the medial ankle, the tibial nerve divides into its terminal branches: the medial plantar nerve and the lateral plantar nerve.
The medial plantar nerve (nervus plantaris medialis) innervates the first and the second lumbrical muscles, the flexor digitorum brevis, the flexor hallucis brevis, and the abductor hallucis. The cutaneous branches of the nerve provide sensory innervation of the medial part of the sole and three and one-half toes.
The lateral plantar nerve (nervus plantaris lateralis) innervates the quadratus plantae, the flexor digiti minimi brevis, the interosseous, three lumbrical muscles, the abductor digiti minimi, and the adductor hallucis.
The cutaneous branches of the nerve provide sensory innervation of the lateral part of the sole, as well as the little toe and the lateral part of the fourth toe.
Common fibular nerve
Let’s discuss the second terminal branch of the sciatic nerve — the common fibular nerve (nervus fibularis communis).
It passes inferiorly and laterally, and in its distal part circumflexes the head of the fibula, passing to the anterior surface of the tibia.
Along its course, the common fibular nerve and its terminal branches innervate numerous muscles of the anterior and lateral parts of the leg:
- Tibialis anterior muscle
- Fibularis longus muscle
- Fibularis brevis muscle
- Fibularis tertius muscle
- Extensor hallucis longus muscle
- Extensor digitorum longus muscle
In the popliteal fossa, the common fibular nerve gives off the lateral sural cutaneous nerve (nervus cutaneus surae lateralis), which merges with the branch of the tibial nerve, the medial sural cutaneous nerve, forming the sural nerve, which we’ve already discussed earlier.
The lateral sural cutaneous nerve directly innervates the skin of the lateral surface of the superior part of the leg.
Deep within the peroneus longus muscle, the common fibular nerve is divided into two terminal branches: the superficial and deep fibular nerves.
The superficial fibular nerve (nervus fibularis superficialis). Its cutaneous branches innervate the skin of the lateral and anterior surfaces of the middle and inferior parts of the leg.
Then it passes to the dorsum of the foot, where it divides into its terminal branches: the medial and intermediate dorsal cutaneous nerves of the foot.
The medial dorsal cutaneous nerve (nervus cutaneus dorsalis medialis) innervates the skin of the medial border of the foot, the medial side of the hallucis and the skin of the surfaces of the second and the third toes facing each other.
The intermediate dorsal cutaneous nerve (nervus cutaneus dorsalis intermedius) innervates the skin of the sides of the middle toe, the fourth toe, and the little toe facing each other.
The deep fibular nerve (nervus fibularis profundus) passes through the anterior surface of the interosseous membrane of the leg, subsequently reaching the dorsum of the foot.
Its cutaneous branches innervate the skin of the sides of the hallux and the second toe, facing each other.
The coccygeal nerve is the last spinal nerve. Right after exiting the sacral hiatus (hiatus sacralis),
the anterior branch arises from it, forming the coccygeal plexus (plexus coccygeus) together with S4 and S5.
The anococcygeal nerves (nervi anococcygei), which innervate the sacrotuberous ligament, as well as the skin of the coccygeal region and around the anus, arise from the plexus.
Sacral plexus. Coccygeal plexus
- Sacral plexus
- plexus sacralis
- Coccygeal plexus
- plexus coccygeus
- Anterior ramus
- r. anterior
- Lumbosacral trunk
- truncus lumbosacralis
- Superior gluteal nerve
- nervus gluteus superior
- Greater sciatic foramen
- foramen ischiadicum majus
- Suprapiriform foramen
- foramen suprapiriforme
- Inferior gluteal nerve
- nervus gluteus inferior
- Infrapiriform foramen
- foramen infrapiriforme
- Pelvic splanchnic nerves
- nn. splanchnici pelvici
- Anterior sacral foramina
- foramina sacralia anteriora
- Inferior hypogastric plexus
- plexus hypogastricus inferior
- Pelvic splanchnic nerves
- nn. splanchnici pelvici
- Piriformis muscle nerve
- nervus piriformis
- Nerve of the quadratus femoris muscle
- nervus musculi quadrati femoris
- Internal obturator nerve
- nervus obturatorius internus
- Posterior femoral cutaneous nerve
- nervus cutaneus femoris posterior
- Lower clunial nerves
- nervi clunium inferiores
- Pudendal nerve
- nervus pudendus
- Lower rectal nerves
- nervi rectales inferiores
- Perineal nerves
- nervi perineales
- Dorsal nerve of the penis
- nervus dorsalis penis
- Dorsal nerve of the clitoris
- nervus dorsalis clitoridis
- Sciatic nerve
- nervus ischiadicus
- Tibial nerve
- nervus tibialis
- Medial sural cutaneous nerve
- nervus cutaneus surae medialis
- Lateral sural cutaneous nerve
- nervus cutaneus surae lateralis
- Sural nerve
- nervus suralis
- Medial calcaneal branches
- rami calcanei mediales
- Medial plantar nerve
- nervus plantaris medialis
- Lateral plantar nerve
- nervus plantaris lateralis
- Common fibular nerve
- nervus fibularis communis
- Superficial fibular nerve
- nervus fibularis superficialis
- Medial dorsal cutaneous nerve
- nervus cutaneus dorsalis medialis
- Intermediate dorsal cutaneous nerve
- nervus cutaneus dorsalis intermedius
- Deep fibular nerve
- nervus fibularis profundus
- Coccygeal nerve
- n. coccygeus
- Sacral hiatus
- hiatus sacralis
- Anococcygeal nerves
- nervi anococcygei