100 Neuroanatomy: Draw It to Know It
The Urinary System (Advanced)
Here, we will draw the innervation of the bladder and
urethra, which are responsible for micturition (urina-
tion). We will address three classes of fi ber circuitry:
parasympathetic, sympathetic, and somatomotor. We
will address the urinary system in detail, but quite simply,
the sympathetic and somatomotor eff erents inhibit uri-
nation and the parasympathetic fi bers activate it. First,
draw a bladder and urethra; then, label the bladder wall
as the detrusor muscle; next, draw the internal and exter-
nal urethral sphincters; and then, draw viscerosensory
aff erents from the bladder and urethra — aff erents pass
through the hypogastric, pelvic splanchnic, and puden-
dal nerves to innervate select spinal cord and supraspinal
nervous system regions. Now, let’s draw the eff erent
pathways of the lower urinary system; we will draw the
pathways, fi rst, and then label their function.^11
First, indicate that S2 to S4 parasympathetic eff erents
project to the parasympathetic vesical ganglion. Th en,
show that postganglionic parasympathetic fi bers inner-
vate the bladder wall and also the internal urethral
sphincter. Next, indicate that preganglionic sympathetic
eff erents from T12 to L2 synapse in the paravertebral
chain and the inferior mesenteric prevertebral ganglion,
and then show that postganglionic fi bers from these
ganglia project to three separate targets: the bladder wall,
the parasympathetic ganglion, and the internal urethral
sphincter. Finally, show that somatomotor neurons in
Onuf ’s nucleus, a circumscribed region of the sacral ven-
tral horn from S2 to S4, project to the external urethral
sphincter.
Next, let’s include the function of each of these fi ber
types (parasympathetic, sympathetic, and somatomo-
tor), but again, we can reason out these functions if we
remember that the sympathetic and somatomotor fi bers
inhibit urination and the parasympathetic fi bers activate
it. First, show that the parasympathetic fi bers excite blad-
der wall contraction and inhibit internal urethral sphinc-
ter constriction. Th en, show that the sympathetic fi bers
inhibit bladder wall contraction, excite internal urethral
sphincter constriction, and tonically inhibit the para-
sympathetic ganglion. Lastly, show that the somatomo-
tor eff erents provide tonic activation of the external
urethral sphincter.
Now, we need to address the supraspinal control of
urination. Th e command center for bladder emptying
(micturition) and the command center for bladder fi ll-
ing (continence) lie within neighboring regions of the
pons. Th e pontine micturition center lies in the medial
(M) region of the dorsolateral pontine tegmentum and
the pontine continence center lies ventro-lateral to it in
the lateral (L) region. Th ese regions receive aff erents
from many brain regions, including the periaqueductal
gray area, the hypothalamus, cerebellum, and certain
limbic system regions. Barrington fi rst described the
pontine micturition center, so it is oft en referred to as
the Barrington nucleus.
Lastly, let’s add to our diagram the peripheral nerves
that carry the aforementioned autonomic and somatic
fi bers. Show that postganglionic thoracolumbar sympa-
thetic fi bers initially travel within the superior hypogas-
tric plexus and then emerge as hypogastric nerves. Next,
show that the sacral parasympathetic fi bers travel via the
pelvic splanchnic nerve. Th en, indicate that both the
hypogastric and pelvic splanchnic nerves join within
the inferior hypogastric plexus (aka the pelvic plexus),
which provides both sympathetic and parasympathetic
innervation to the bladder and urethra. Finally, show
that somatomotor eff erent fi bers travel as the pudendal
nerve to provide somatomotor innervation to the exter-
nal urethral sphincter.
Note that the inferior hypogastric plexus innervates
the reproductive organs, as well. In regards to sexual
function, the parasympathetic nervous system is respon-
sible for penile and clitoral engorgement and the sympa-
thetic nervous system is responsible for penile ejaculation.
We remember this functional relationship with the
mnemonic Parasympathetic — Point and Sympathetic —
Shoot.^4 – 8 , 10 , 12 – 14