SYNOPSIS OF
THE NERVES ASSOCIATED WITH
THE PELVIS
by O.W. Henson, Jr., Ph.D.
University of North Carolina, Chapel Hill
It is important to
understand the origin, course and distribution of all nerves associated with
the pelvis. The following basic points
are important to understand.
Abbreviations:
SA – somatic afferent
SE – somatic efferent
VA – visceral afferent
VE – visceral efferent
NERVES PASSING THROUGH THE PELVIS OR TO ITS WALLS
1. General
concepts about autonomic cell body locations
a) All preganglionic autonomic efferent fibers associated with the pelvis (and lower limbs) have cell bodies in the
lower thoracic (T9-12), upper lumbar (L1-2), and sacral (S2-4) segments of the
spinal cord. L3-5 and S1 represent
spinal cord segments which lack autonomic cell bodies (Netter 310).
b) The cell
bodies in the thoracolumbar segments represent sympathetic neurons while those
at the S2-4 levels represent parasympathetic neurons.
c) The T9-L2 thoracolumbar spinal cord segments
reside at T7-10 lumbar vertebral levels (see Clemente Fig 564).
d) The S2-4
spinal cord segments lie at vertebral
levels T12 and L1 (see Clemente Fig 564).
e) The
parasympathetic fibers are distributed to the pelvic and external genitalia
while the sympathetic fibers are distributed throughout the pelvis and lower
limb.
f) Points to ponder:
* If the sacral segments of the spinal cord
were damaged by trauma would the pelvic viscera have autonomic input? If so, what type (preganglionic vs
postganglionic; sympathetic vs parasympathetic)?
* If the lower
thoracic and upper lumbar portion of the spinal cord were lesioned would the
pelvic viscera have autonomic input? If
so what type?
* If the intervening segments of the spinal
cord (L4 and L5) were lesioned how would the autonomic input to the pelvis be
affected? (see Netter 310)
* Can you localize the area of the body
(vertebral levels) on a living person where the autonomic spinal cord segments
are located?
* Damage to
which segments could affect the pelvic viscera and external genitalia
only? The lower limb only? Both the lower limb and external genitalia?
2. General concepts about afferent fibers
accompanying autonomic fibers
a) Autonomic
fibers are accompanied by visceral afferent (visceral afferent) fibers.
b) Visceral
afferents cell bodies are in the dorsal root ganglia
c) Dorsal
root ganglia of thoracic, lumbar and sacral nerves are located in the
intervertebral foramen that corresponds in number to the spinal cord segment of
the nerve.
d) Afferent
fibers reach the spinal cord via the dorsal roots of spinal nerves.
e) Afferent
fibers are responsible for visceral pain and visceral reflexes and they form a
basis for referred pain.
3. A typical
spinal nerve (e.g. Tl through L2; S2-4) has four functional components (SA, SE,
VA, VE) with the efferent fibers coursing within the ventral root of the nerve
and afferent fibers in the dorsal root.
4. The ventral
roots of L3 to S5 do not contain preganglionic sympathetic fibers but each L3
to S5 spinal nerve acquires postganglionic sympathetic fibers via gray rami
communicantes. The preganglionic cell
bodies of these visceral efferent sympathetic fibers are in the lateral horn
(intermediolateral cell column) of the lower thoracic and upper lumbar segments
of the spinal cord; the fibers reached the sympathetic chain via white rami
communicantes and they descended down the chain to eventually synapse in a
sympathetic chain ganglion; the postganglionic fibers are then distributed to a
spinal nerve via gray rami communicantes.
In this manner the sympathetic fibers are distributed to all of the
peripheral nerves that pass through the pelvic region as well as those that
innervate the external genitalia (e.g. pudendal nerve). The
sympathetic chain extends along the entire length of the vertebral column and
in so doing supplies each spinal nerve with postganglionic sympathetic fibers.
SYMPATHETIC NERVES PASSING TO THE PELVIC VISCERA
1. The
sympathetic nerves which innervate the pelvic viscera have their cell bodies in
the intermediolateral cell column of lower thoracic and upper lumbar
regions. The fibers are accompanied by
afferent (pain) fibers. Instead of
synapsing in the sympathetic chain ganglia the preganglionic fibers pass
through the ganglia and travel medially to form a plexus of nerves that is
intimately associated with the abdominal aorta. This aortic plexus ends at the bifurcation of the aorta and the
nerves continue downward as the superior hypogastric plexus (sometimes called
the presacral nerve) which divides into right and left inferior hypogastric
nerves (plexuses). These fibers then
mingle with pelvic splanchnic nerves (parasympathetics and accompanying sensory
nerves) to form the pelvic plexus.
2. The
sympathetic fibers that descend into the pelvis to join the pelvic plexus
synapse on isolated ganglia located within the aortic, hypogastric and pelvic
plexuses.
3. The
sympathetic fibers that reach the pelvic plexus terminate on blood vessels, the
prostate gland and seminal vesicle and they are important in ejaculation (i.e.
emission).
PARASYMPATHETIC NERVES ASSOCIATED WITH THE PELVIS
1. In a
typical spinal nerve the VE component in the ventral root is sympathetic; in
the sacral nerves (S2 through S4) the VE component is parasympathetic.
2. Throughout
the body the nerves distributed to the periphery contain all four functional
components while those distributed to the blood vessels and organs of the body
cavities contain only VA and VE fibers; the same applies to the nerves
associated with the internal organs of the pelvis (only VE and VA). The external genitalia, in contrast to the
internal genitalia, receive all four components.
3. Throughout
the thorax and abdomen, VE fibers reach the internal organs by traveling in
“splanchnic nerves” or in plexuses that are closely associated with blood
vessels. The same is true of the
pelvis.
4. There is
one main route (bilaterally represented) by which VE parasympathetic fibers and
accompanying VA fibers reach the pelvic viscera and external genitalia: the pelvic splanchnic nerves (nervi
erigentes). These arise from sacral
spinal levels S2, 3, and 4 (same as pudendal nerve, “S2, 3 and 4, lift your
pudendum off the floor”).
5. The cell
bodies of these fibers are located in the lateral part of the gray matter in
the sacral segments of the spinal cord (at Ll and L2 vertebral level). The fibers travel in the long ventral roots
of the spinal nerves and after they pass through the pelvic (anterior) sacral
foramina they, and their accompanying afferents, course directly toward the
pelvic viscera as “pelvic splanchnics”.
6. The pelvic
splanchnics become mixed with fibers from the hypogastric plexus to form the
pelvic plexus; this plexus splits to envelop the pelvic organs and in so doing
a number of subsidiary plexuses are formed (i.e. vesical, rectal, prostatic,
uterine, vaginal).
7. Parasympathetic
fibers are responsible for erection and the fibers travel with the deep dorsal
vein to reach the penis or clitoris.
THE PUDENDAL NERVE
1. The main
(but not exclusive) nerve associated with the UG diaphragm and external
genitalia is the pudendal nerve.
2. The
pudendal nerve, like the pelvic splanchnics, arises from S2 through S4, but it
is a peripheral nerve and carries the typical four functional components.
3. The
pudendal nerve should be remembered for:
a) its sensory innervation to the external genitalia and anal region and
b) its motor innervation to the muscles of the UG diaphragm and muscles of the
penis or clitoris
4. It is important to remember that other sensory nerves supply the external genitalia (ilioinguinal, posterior femoral cutaneous, genital branch of genitofemoral); be sure that you know their areas of innervation.