Learning Modules - Medical Gross Anatomy
Autonomics of the Pelvis - Page 1 of 12
All pelvic viscera receive dual
innervation, meaning both
sympathetic and parasympathetic
contributions (visceral efferent fibers).
When thinking about the autonomic
innervation of the pelvic viscera, what
should instantly spring to mind is the
inferior hypogastric plexus. It is here,
where all sympathetic and
parasympathetic signals converge
and are redistributed either directly to
target organs within the pelvis or via
smaller subsidiary plexuses.
To use an analogy, the inferior
hypogastric plexus can be compared
in many ways to a main branch post
office. A post office is responsible for
taking in many types of
correspondence from distant locations
and redistributing them either directly
to their final destinations, or to small
branch post offices who then deliver
them to their final destinations.
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Learning Modules - Medical Gross Anatomy
Autonomics of the Pelvis - Page 2 of 12
In addition to parasympathetic and sympathetics, visceral afferent (sensory) fibers are often included during a
discussion of autonomics because of their similar pathways of distribution. Visceral afferent (sensory) fibers also
need to reach all organs within the pelvis. These fibers, originating within the spinal cord, accompany visceral efferent
fibers throughout their pathways. Afferent fibers, important in reflex and visceral function, are also important in
conducting visceral pain impulses. We are usually unaware of the sensory input of these fibers, which provides the
central nervous system information about the condition of the body's internal environment. Conscious visceral
sensation is usually categorized as pain and is poorly localized. (For more information, please see the introduction to
autonomics module.)
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Learning Modules - Medical Gross Anatomy
Autonomics of the Pelvis - Page 3 of 12
In order to understand the autonomic innervation of the pelvis, one
must recall that the sympathetic outflow of the autonomic nervous
system (ANS) originates in the lateral or intermediolateral column of
gray matter within the thoracolumbar region of the spinal cord (T1
through L2). Some sympathetic fibers travel in the sympathetic chain
into the pelvis, while other sympathetic fibers enter the pelvis from
the superior hypogastric plexus, which passes over the common iliac
arteries. Recall that the different regions of the sympathetic chain are
named according to the portion of the vertebral column along which
the chain lies. So, there is a cervical, thoracic, lumbar, and a sacral
portion of the sympathetic chain. Therefore sympathetics ultimately
reaching the pelvis have descended into the inferior hypogastric
plexus from the lumbar region.
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Learning Modules - Medical Gross Anatomy
Autonomics of the Pelvis - Page 4 of 12
The inferior hypogastric plexus, the hub
of all autonomic control within the pelvis,
is a paired meshwork of nerves located
on either side of the rectum lying medial
to the internal iliac vessels. The
sympathetic contributions into the inferior
hypogastric plexuses stem from two
sources. The largest sympathetic
contribution is from the superior
hypogastric plexus. The superior
hypogastric plexus contains no
parasympathetic fibers, so it is a purely
sympathetic plexus. The superior
hypogastric plexus itself receives upper
lumbar contributions from the
intermesenteric plexus, as well as
contributions from L3 and L4 splanchnic
nerves. Located within the abdomen at
the bifurcation of the aorta, the superior
hypogastric plexus descends into the
pelvis and bifurcates as the right and left
hypogastric nerves. Please note these
"nerves" aren’t really what is traditionally
thought of as a nerve, they are more like
meshworks than solid nerve trunks.
These nerve meshworks diverge lateral to
the rectum on either side and curve
outward and backward as they make their
way down about 7.5 – 10 centimeters into
the pelvis. They interconnect the superior
and inferior hypogastric plexuses and
contain no ganglia. The hypogastric
nerves convey the majority of the
sympathetic contribution from the
superior hypogastric plexus into the
inferior hypogastric plexuses.
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Learning Modules - Medical Gross Anatomy
Autonomics of the Pelvis - Page 5 of 12
The second, much smaller, contribution of
sympathetics to the pelvic viscera comes
from the sacral splanchnic nerves. (Recall
that a splanchnic nerve is a nerve,
parasympathetic OR sympathetic, that
brings innervation to viscera.) The sacral
splanchnic nerves are very small, and are
anteriorly directed branches from the
sacral portion of the sympathetic chain.
Just like the other splanchnic nerves in the
thorax and abdomen, they leave the chains
(on each side of the vertebral column) and
course anteromedially into the inferior
hypogastric plexus. These small, mostly
postganglionic**, fibers are collected
together, with the contribution from the
respective hypogastric nerve, into the
inferior hypogastric plexuses, from which
postganglionic fibers are distributed to
either smaller plexuses or directly to pelvic
viscera.
**Sympathetic outflow originates within
the thoracic and upper lumbar region. The
further a fiber travels away from this
region, the less likely that it would remain
preganglionic. For example, thoracic
splanchnic nerves, coming directly off the
sympathetic trunk, are primarily
preganglionic and will synapse in a
prevertebral ganglion. As one proceeds to
lower level splanchnics, such as lumbar or
sacral, the proportion of pre- to
postganglionic fibers change. For this
reason lower lumbar splanchnics will have
a much higher proportion of postganglionic
fibers than preganglionic fibers. By the time
sacral splanchnics arise, they are mostly
postganglionic fibers and have already
synapsed prior to reaching the inferior
hypogastric plexus. In all diagrams within
this module sacral splanchnics will be
represented as postganglionic fibers that
are coming into the inferior hypogastric
plexus.
***Don't forget gray rami
communicantes. They travel laterally from
the sacral sympathetic trunk to reach the
sacral ventral primary rami.
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Autonomics of the Pelvis - Page 6 of 12
Now that the sympathetic
contribution is accounted
for within the inferior
hypogastric plexuses, the
parasympathetics can be
considered. The pelvic
splanchnic nerves
represent the sacral
contribution of the
parasympathetic
(craniosacral) outflow of
the ANS. The pelvic
splanchnic nerves
contribute
parasympathetic supply
for the pelvic viscera,
along with the
parasympathetic supply
to the hindgut, which
begins at the distal third
of the transverse colon
(see abdomen module).
Pelvic splanchnic nerves
are preganglionic
parasympathetic fibers
whose cell bodies are
located in a small lateral
horn of gray matter in
spinal cord levels S2, 3,
and 4. These
preganglionic fibers leave
the lateral horn, and
subsequently pass
through the ventral horn,
ventral root, spinal nerve,
and ventral ramus
associated with S2, 3,
and 4. These fibers then
directly branch off the
ventral rami as pelvic
splanchnic nerves and
join the inferior
hypogastric plexus.
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Learning Modules - Medical Gross Anatomy
Autonomics of the Pelvis - Page 7 of 12
After all the parasympathetics and sympathetics have successfully reached the inferior hypogastric
plexus they must still find their way to their final destinations and synapse in the correct locations.
Recall that both sympathetics and parasympathetics are each two neuron systems, but that the
location of synapses in each of these systems is very different. Most of the sympathetic fibers
reaching the inferior hypogastric plexus are postsynaptic, having synapsed within the lower lumbar or
sacral portions of the sympathetic trunk prior to leaving it. Those preganglionic sympathetic fibers that
reach the inferior hypogastric plexus synapse within small, diffuse ganglia within this plexus and then
send out postganglionic fibers to their target organs. Parasympathetics, on the other hand, travel
through the plexuses, but wait to synapse until they reach the wall of their target organ and then send
out very short postganglionic fibers.
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Learning Modules - Medical Gross Anatomy
Autonomics of the Pelvis - Page 8 of 12
Sympathetics reaching the inferior hypogastric plexuses
have several options for reaching their final destination.
Most of these sympathetics are already postsynaptic,
having synapsed within the sympathetic trunk before
leaving it. They can proceed directly to their target
organ. Sympathetic fibers that haven’t synapsed already
will synapse in diffuse ganglion cells (not identifiable
ganglia) within the inferior hypogastric plexus or its
subsidiary plexuses and then travel on to their target
viscera. Parasympathetics do the same, except as you
know, they don’t synapse until reaching their target
organ, so they simply travel through the inferior
hypogastric plexus and its subsidiary plexuses.
Subsidiary plexuses (which are really just extensions of
the inferior hypogastric plexus) help the autonomics to
more easily become distributed to pelvic viscera and as
a whole are referred to as the pelvic plexuses. These
subsidiary plexuses include the rectal, vesical, and the
uterovaginal plexus in the female, and the prostatic
plexus in the male.
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Learning Modules - Medical Gross Anatomy
Autonomics of the Pelvis - Page 9 of 12
Autonomic Innervation
of the Rectum: The
rectal plexus is an
offshoot of the posterior
portion of the inferior
hypogastric plexus.
Nerves run from the
inferior hypogastric
plexus and penetrate the
walls of the rectum
providing enteric
plexuses like those found
in other parts of the
intestines. Pelvic
splanchnics
(parasympathetics)
innervate the internal
anal sphincter that
surrounds the upper twothirds of the anal canal.
During most times the
internal anal sphincter is
tonically contracted to
prevent leakage of feces.
Following distention of
the rectal ampulla, the
sphincter relaxes and
then it is the job of the
external anal sphincter,
which is under voluntary
control and supplied by
the inferior rectal
branches of the pudendal
nerve, to prevent
defecation.
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Autonomics of the Pelvis - Page 10 of 12
Autonomic Innervation of
the Bladder: The vesical
plexus, the anterior portion of
the inferior hypogastric plexus,
forms loops around the end
portion of the ureter and
terminates within the bladder
wall. Parasympathetic fibers
(derived from pelvic
splanchnics) travel through this
plexus to provide motor
innervation to the detrusor
muscle and inhibitory signals to
the internal sphincter.
Therefore when the sensory
fibers (visceral afferent) are
stretched, the bladder
contracts reflexively and the
internal sphincter relaxes so
urine can flow to the urethra.
Sympathetics, on the other
hand, cause vasoconstriction
of renal vessels to slow urine
formation and maintain
contraction of the internal
sphincter of the bladder to
allow for urinary continence.
The fibers of the vesical
plexus, which also supply the
fundus of the bladder, have
offshoots to the seminal
vesicle, the ductus deferens
and the ejaculatory duct in the
male.
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Autonomics of the Pelvis - Page 11 of 12
Autonomic Innervation of the Uterus: The uterovaginal plexus of the female stems from the intermediate
part of the inferior hypogastric plexus. Its branches innervate the uterus, uterine tube, cervix and upper
vagina. Sympathetic innervation originates in segments of the lower thoracic spinal cord and passes through
lumbar splanchnics and the inferior mesenteric/hypogastric series of plexuses and finally to the uterovaginal
plexus. Parasympathetic innervation originates in the S2 through S4 spinal cord segments and passes
through pelvic splanchnics to the inferior hypogastric plexus and then into the uterovaginal plexus.
(Remember that the pudendal nerve (S2-S4) does not carry parasympathetic fibers.) Visceral afferent fibers
originating in the fundus and body of the uterus travel retrograde with sympathetics along the hypogastric
nerves and superior hypogastric plexus to reach the lower thoracic segments of the spinal cord. Afferent
fibers of the upper vagina and cervix travel back along the pelvic splanchnics (S2-S4) and on the pudendal
nerve.
Clinical
Sidenote: There are
several ways in which
pain is regionally
managed during
childbirth.
1. A spinal block, in
which
anaesthetic
agent placed
into the L3/L4
subarachnoid
space numbs
everything
inferior to the
waist. In this
case a mother
would not be
conscious of
anything below
her waist and
would need to
rely on
electronic
monitoring of
uterine
contractions to
help deliver the
baby.
2. Pudendal nerve
block involves
direct administration of anaesthetic agent into the nerve as it exits the greater sciatic foramen. This
type of anaesthetic provides a nerve block over the S2 through S4 dermatomes only, which includes
the perineum and the lower one-fourth of the vagina. It does not block pain felt from the body or fundus
of the uterus because those afferent fibers travel retrograde along sympathetics heading to the
thoracic region. By not numbing these regions, a mother can still feel contractions and play an active
role in labor.
3. A caudal epidural block can be administered via an in-dwelling catheter into the sacral canal. Within the
sacral canal, the agent bathes the sacral nerve roots, which reach the cervix, upper vagina and form
the pudendal nerve.
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Learning Modules - Medical Gross Anatomy
Autonomics of the Pelvis - Page 12 of 12
Autonomic
Innervation of the
Prostate: The prostatic
plexus of the male is
derived from the larger
nerves of the anterior
portion of the inferior
hypogastric plexus and
lies alongside the
prostate gland. It
supplies the prostate
gland, the prostatic
urethra and the
ejaculatory duct. The
prostatic plexus also
gives rise to the
cavernous nerves of the
penis, which are mainly
parasympathetic and
responsible for
relaxation of smooth
muscle allowing blood to
flow into cavernous
spaces in the corpora of
the penis resulting in
erection. Sympathetic
stimulation causes
ejaculation and
vasoconstriction
resulting in remission of
an erection.
An easy way to
keep this straight is to
remember that P
arasympathteics = Point
and Sympathetics
= Shoot
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