MRCS PART A · APPLIED BASIC SCIENCES
Pudendal route & ischioanal relationships
Follow a nerve through its route, locate the space beside the anal canal, and distinguish a terminal branch from a parent nerve. Two selected pelvis/perineum objectives; four original SBAs. No injection, drainage or operative instructions.
2 topics · 4 original SBAs
1 · Out through greater, back through lesser
Your task: Trace the conventional pudendal route and separate it from neighbouring nerves.
Which roots contribute, and where does the nerve leave?
The usual pudendal root contribution is S2–S4. The nerve leaves the pelvic cavity through the greater sciatic foramen below piriformis; sharing sacral roots does not make it a pelvic splanchnic nerve.
How does it reach the perineum?
It turns near the ischial spine and sacrospinous ligament, then passes through the lesser sciatic foramen. Remember the sequence rather than memorising either opening in isolation.
What travels alongside it?
Internal pudendal vessels accompany much of its course. A nerve and an artery can share a route without having the same function.
Does the sciatic nerve follow the same return route?
No. It also leaves through the greater sciatic foramen but proceeds towards the lower limb rather than taking the pudendal return through the lesser foramen.
What changes if the finding is foot weakness instead of perineal sensory loss?
Reconsider a lower-limb nerve or more proximal lesion. The shared exit does not make distal territories interchangeable; this is an anatomical localisation exercise, not a diagnosis.
Where is the pudendal canal in the conventional model?
Along the lateral wall of the ischioanal fossa, associated with obturator internus fascia. It is commonly called Alcock's canal.
Can every branch be assumed to emerge at one fixed point?
No. Trunking, branch origins and fascial relationships vary. Inferior rectal branches may arise before the canal or follow a separate route. A textbook map is not a procedural safety guarantee.
2 · Fat-filled space, crossing branches and sphincter function
Your task: Identify lateral and medial relationships and distinguish somatic sphincter supply from autonomic control.
Where is the ischioanal fossa relative to the anal canal?
It is a fat-filled space lateral to the canal and below the pelvic diaphragm. Obturator internus and its fascia contribute to the lateral wall; levator ani and external anal sphincter are medial relationships.
Is it the same as the space above levator ani?
No. A supralevator space is above the pelvic diaphragm. First locate a process relative to levator ani before applying a compartment label.
Which branch crosses towards the external anal sphincter?
Inferior rectal branches ordinarily provide somatic motor supply to the external anal sphincter and sensation to the lower anal region. They pass medially across the fossa; branching and interconnections vary.
Does one branch explain all continence?
No. Continence also depends on other muscles, sensation, stool consistency and central control. Anatomical branch localisation is not a complete continence assessment.
Why distinguish external and internal anal sphincters?
External sphincter is striated muscle with somatic innervation; internal sphincter is smooth muscle with autonomic regulation. Similar names conceal different control systems.
Does a normal pudendal finding prove all pelvic-floor innervation is intact?
No. Levator ani has direct sacral nerve contributions; do not assign every pelvic-floor muscle exclusively to the pudendal nerve.