Nerve distribution across superior surface of right pelvic floor in a cadaver with an inferior rectal nerve variant (IRN variant). a Sagittal view of the right hemipelvis with bladder reflected anterior out of view. Photograph of the levator ani nerve (LAN) and IRN variant. b This is an outline drawing of the relevant structures shown in a. Nerves from the S3 root extend branches that form the LAN, IRN variant, and a nerve to the coccygeus muscle. The LAN courses superior to the coccygeus muscle (CM)–sacrospinous ligament (SSL) complex and sends branches that enter the iliococcygeus muscle (ICM). The IRN variant extends branches that enter the superior surface of the CM and the ICM and then pierces the CM–SSL complex. L5, S1, S2, S3, S4 Nerve roots originating from lumbar and sacral foraminas
In seven cadavers (41.2%), an anatomically variant IRN, independent
from the pudendal nerve, originated directly from the S3 and/or S4
roots of the sacral plexus (Figs. 1 b, 3 a, b): from the S3
alone in two cadavers, from the S4 alone in three cadavers, and
from both the S3 and S4 in two cadavers.
The variant IRN passed superior to the coccygeus muscle in a
lateral and caudal direction and extended nerve branches that
entered the superior surface of the ICM in three cadavers (17.6%;
Figs. 1 b, 3 a, b).
Next, the variant IRN pierced the mid-portion of the sacrospinous
ligament–coccygeus muscle complex (Figs. 1 b, 3 a, b).
In 12 cadavers (70.6%), nerve branches originating from the sacral
roots S3 and/or S4 entered the pelvis, merged, and coursed over the
superior–medial surface of sacrospinous ligament–coccygeus muscle
complex (Figs. 1 a, b, 3 a, b, 5 a).
In three cadavers, the nerves originated from the sacral nerve root
S3 (Fig. 3 a, b), in three cadavers, the nerves originated
from the sacral nerve root S4, and in five cadavers, the nerves
originated from both the S3 and S4.
The S3 and/or S4 nerves passed over the superior surface of the ICM
and sent branches that entered directly into the superior surface
of the LAM, as described below (Table 1 , Figs. 1 a, b, 3 a,
b, 5 a).
Variant IRN entered the ICM in three (17.6%) cadavers (Figs.
1 b, 3 a, b).
In 11 cadavers (64.7%), the ICM received innervation from the
direct sacral nerves S3 and/or S4 (Figs. 3 a, b, 5 a).
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