Sacral plexus

SACRAL PLEXUS

• Sacral plexus (L4-S4) – forms anterior to piriformis muscle.
• Lies immediately inferior to the lumbar plexus.
• Formed by the ventral (anterior) rami of spinal nerves L4, L5, and S1 through S4.

STRUCTURE:

• Formed from ventral rami of L4-S4; Contribution of L4-L5 is from Lumbosacral trunk.
•  The nerves forming the sacral plexus converge towards the lower part of the greater sciatic foramen and unite to form a flattened band.
• The branches of the sacral plexus arise from anterior and posterior surfaces of this flattened band.
• Branches innervating the pelvis and perineum remain in the pelvis.
• Branches innervating the lower limb exit through greater sciatic foramen.

 S1,S2: Nerve to Piriformis

1. S2, S3: Perforating cutaneous nerve – to medial part of buttock
2. S1,S2,S3: Posterior femoral cutaenous nerve – To buttock and uppermost medial and posterior surfaces of thigh
3. S2,S3,S4:

• Pudendal nerve – leaves the pelvis through greater sciatic foramen but re-enters through lesser sciatic foramen and enters the pudendal or alcock’s canal (a fascial canal formed by splitting of obturator fascia on lateral wall of ischiorectal fossa).
• Inferior rectal nerve: Perianal skin and External anal sphincter
• Dorsal nerve of penis or clitoris: Accompanies dorsal artery of penis and clitoris and supplies skin of penis or clitoris and labia majora.
• Perineal nerve: Superficial and Deep perineal muscles and External urethral sphincter
• Parasympathetic pelvic splanchnic nerves (nervi erigentes) – Ascend to join inferior hypogastric plexus and together supply pelvic viscera.

1. S4: Perineal branch of Pudendal nerve

Exam Question

• Nerves passing through lesser sciatic foramen are obturator internus & pudenal nerve
• Superior gluteal nerve supplies Gluteus medius, Gluteus minimus & Tensor fasia lata 
• Pudendal nerve supplying motor part to external sphincter is derived from S2-S4 roots
• Gluteus medius is supplied by superior gluteal nerve
• Pudenal nerve is both sensory and motor 
• Sacral plexus,  Nervi erigentes (S2,3,4) & pudenal nerve are involved in process of erection of penis
• Pudendal nerve leaves pelvis and enters gluteal region through greater sciatic foramen 
• Pudendal nerve leaves the gluteal region by passing through the lesser sciatic foramen and enters the pudendal canal
• Anal canal Supplied by pudendal nerve
• Ischial spine is related to pudenal nerve 
• Structures passing through lesser sciatic foramen:
• Pudendal nerve( leaves gluteal region to enter pudenal canal) lies in relation to Ischial spine
• Internal pudendal vessels
• Nerve to obturator internus
• Tendon of obturator internus