Lecture 10 Dr. Janan Alrefaee
Innervation of the Bladder (fig. 9.2 last lecture)
1- Pelvic nerves are the principal nerve supply the bladder, which connect with the spinal
cord through the sacral plexus (S-2 and S-3) segments. Pelvic nerves contain both sensory
and motor nerve fibers. The sensory fibers detect the degree of stretch in the bladder wall.
Stretch signals from the posterior urethra are strong and mainly responsible for initiating the
micturition reflex. The motor nerves of this reflex are parasympathetic fibers terminate on
ganglion cells located in the wall of the bladder. Short postganglionic nerves then innervate
the detrusor muscle.
2- The skeletal (somatic) motor fibers in the pudendal nerve that innervate and control the
voluntary skeletal muscle of the external bladder sphincter.
3- hypogastric nerves are sympathetic fibers pass from L-2 segment of the spinal cord to the
sympathetic chain to innervate bladder. They stimulate mainly the blood vessels and have
little to do with bladder contraction. Some sensory nerve fibers also pass by way of the
sympathetic nerves are important for sensation of fullness and pain.
Transport of urine from the kidney through the ureters and into the bladder
Urine has same composition of fluid flowing out of the collecting ducts. Urine flowing from
the collecting ducts into the renal calyces stretches the calyces and increases their inherent
pacemaker activity, which in turn initiates peristaltic contractions that spread to the renal
pelvis and then downward along the length of the ureter, thereby forcing urine from the renal
pelvis toward the bladder. The walls of the ureters contain smooth muscle and are innervated
by both sympathetic and parasympathetic nerves as well as by an intramural plexus of
neurons and nerve fibers that extends along the entire length of the ureters. The peristaltic
contractions in the ureter are enhanced by parasympathetic stimulation and inhibited by
sympathetic stimulation (Fig. 9.2). The normal tone of the detrusor muscle in the bladder
wall tends to compress the ureter, thereby preventing back flow of urine from the bladder
when pressure increase in the bladder during micturition. Each peristaltic wave along the
ureter increases the pressure within the ureter so that the region passing through the bladder
wall opens and allows urine to flow into the bladder.
The ureterorenal reflex
It is important reflex occur when a ureter becomes blocked (ureteral stone), intense reflex
constriction occurs; associated with severe pain (the ureters are well supplied with pain nerve
fibers). Also, the pain impulses cause a sympathetic reflex back to the kidney to constrict the
renal arterioles, thereby decreasing urine output from the kidney with blocked ureter.
Micturition is the process by which the urinary bladder empties when it becomes filled. This
involves two main steps: First, the bladder fills progressively until the tension in its walls
rises above a threshold level; this elicits the second step, which is a nervous reflex called the
micturition reflex.
The cystometrogram Fig. 10.1
The basal intravesicular pressure is about 0 when there is no urine in the bladder, but by
the time 30 to 50 ml of urine has collected, this pressure rises to 5 to 10 cm of water.
Collection of urine 200 to 300 ml, only a small rise in basal pressure; this is caused by
intrinsic tone of the bladder wall (plasticity), so when the bladder fills, the tension increase
but the radius increase as well. Beyond 300 to 400 ml, collection of more urine in the
bladder causes the basal intravesicular pressure to rise rapidly.
The micturition waves (contractions) are the periodic acute superimposed increases in
intravesicular pressure that last from a few seconds to more than a minute during filling of
the bladder. The micturition waves may rise only a few cm of water up to more than 100 cm
of water. The micturition waves caused by the micturition reflex and they are as dashed
spikes in the cystometrogram.
Figure 10.1 Normal cystometrogram, showing basal intravesical pressure & the
micturition waves (dashed spikes)
Micturition Reflex
It is detrusor muscles contractions (micturition waves) due to a stretch reflex initiated by
sensory stretch receptors in the bladder wall, especially by the receptors in the posterior
urethra when this area begins to fill with urine at the higher bladder pressures. The first urge
to void is felt at a bladder volume of about 150 ml and a marked sense of fullness at about
400 ml.
When the bladder is partially filled, these micturition contractions cease spontaneously & the
bladder relax after a fraction of a minute and pressure falls back to the baseline. Once a
micturition reflex begins, it is “self-regenerative”.
Once a micturition reflex has occurred but has not succeeded in emptying the bladder, the
nervous elements of this reflex usually remain in an inhibited state for a few minutes to 1
hour or more before another micturition reflex occurs. As the bladder continues to fill, the
micturition reflexes become more frequent and cause greater contractions.
Once the micturition reflex becomes powerful enough, it causes another reflex, which passes
through the pudendal nerves to the external sphincter to inhibit it. If this inhibition is more
potent in the brain than the voluntary constrictor signals to the external sphincter, urination
will occur. If not, urination will not occur until the bladder fills still further and the
micturition reflex becomes more powerful.
Facilitation or inhibition of micturition by the brain
The micturition reflex is a completely autonomic spinal cord reflex, but it can be inhibited
or facilitated by centers in the brain (pons & cerebral cortex).
Voluntary urination is usually initiated in the following way: First, a person voluntarily
contracts his or her abdominal muscles, which increases the pressure in the bladder and
allows extra urine to enter the bladder neck and posterior urethra under pressure, thus
stretching their walls. This stimulates the stretch receptors, which excites the micturition
reflex and simultaneously inhibits the external urethral sphincter. Ordinarily, all the urine
will be emptied, with rarely more than 5 to 10 milliliters left in the bladder.