basic process of normal urination (or "micturition") can be broken
Urine is made in the
Urine is stored in the
The sphincter muscles
The bladder muscle (detrusor)
The bladder is emptied
through the urethra and urine is removed from the body.
Of course, beyond those
5 steps there are elaborate layers of control. The picture on the right is a
"snapshot" of the bladder during the urine storage phase. The text
below explains the micturition process, and there is a more indepth animated
graphic at the bottom of the page. We invite you to "interact" with
it to explore some of the more complex issues that arise when a person
has a spinal cord injury.
The bladder is composed
of bands of interlaced smooth muscle (detrusor). The innervation of the body
of the bladder is different from that of the bladder neck. The body is rich
in beta adrenergic receptors. These receptors are stimulated by the sympathetic
component of the autonomic nervous system (ANS). Beta
stimulation, via fibers of the hypogastric nerve, suppress contraction of
the detrusor. Conversely, parasympathetic
stimulation, by fibers in the pelvic nerve, cause the detrusor to contract.
Sympathetic stimulation is predominant
during bladder filling, and the parasympathetic
Two sphincters control
the bladder outlet. The internal sphincter is composed of smooth muscle like
the detrusor and extends into the bladder neck. Like the detrusor, the internal sphincter
is controlled by the ANS and is normally closed. The primary receptors in
the bladder neck are alpha-adrenergic.
Sympathetic stimulation of these alpha
receptors, via fibers in the hypogastric nerve, contributes to urinary continence.
The external sphincter
is histologically different from the detrusor and internal sphincter. It is
striated muscle. Like skeletal muscle, it's under voluntary control. It receives
its innervation from the pudendal nerve,
arising from the ventral horns of the sacral cord. During micturition, supraspinal
centers block stimulation by the hypogastric and pudendal nerves. This relaxes
the internal and external sphincters and removes the sympathetic
inhibition of the parasympathetic receptors.
The result is unobstructed passage of urine when the detrusor contracts.
The ureters pass between
the layers of the detrusor and enter the bladder through the trigone. The
ureters propel urine into the bladder. The bladder passively expands to accept
urine. As the bladder expands and intravesicular pressure increases, the ureters
are compressed between the layers of muscle, creating a valve mechanism. This
valve mechanism limits the backflow of urine.
The normal adult bladder
can hold about 500 cc of urine. After emptying, the bladder may still retain
about 50 cc residual volume. At about 150 cc of volume, stretch receptors
in the detrusor begin signaling the CNS via afferent nerves; at 400 cc we
are "seeking" an appropriate toilet
Summary: Normally, we are able to control where and when we void. This is
largely because the cerebrum is able to suppress the sacral micturition reflex.
If the sacral reflex is unrestrained, parasympathetic stimulation via the pelvic
nerve causes detrusor contraction. Detrusor contraction is suppressed by alpha
stimulation via the hypogastric nerve. In response to afferent
stimulation, the cerebrum becomes aware of the need to void. If it is appropriate,
the cerebrum relaxes the external sphincter, blocks sympathetic inhibition,
the bladder contracts and urine is expelled.