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Bladder Control www eng mu edu abdominal cavity

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Bladder Control www eng mu edu abdominal cavity

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									Bladder Control after Spinal Cord Injury
Erik Novotny
8/24/01

        The urinary system consists of two kidneys, two ureters, one urinary bladder, and
one urethra. After the kidneys filter blood and return most of the water and many of the
solutes to the blood stream, the remaining water and solutes are excreted from the kidney
as urine. The kidneys tend to do most of the work in the urinary system while the rest is
manly used for storage and passageways. The two ureters transport urine from the
kidney to the urinary bladder, which is a hollow muscular organ that is located in the
pelvic cavity behind the pubic bone. Folds of the peritoneum (membrane that lines the
abdominal cavity) help keep the bladder in place. There isn’t one set shape for the
urinary bladder. It depends on how much urine is being stored at the time. When it is
empty it looks deflated, kind of like a balloon. AS it fills up, it become more and more
spherical and once it starts to stretch out it takes on a pear like shape and rises into the
abdominal cavity. The capacity of the bladder is 700 to 800 ml or about ¾ quart. The
muscle layer of the bladder wall consists of three layers of smooth muscle called detrusor
muscle. Around the opening of the urethra, which takes the urine out of the bladder is an
internal urethral sphincter made of smooth muscle. This sphincter opens and closes
involuntarily. Below the internal sphincter is the external urethral sphincter, which is
made of skeletal muscles and is controlled voluntarily.

Figure 1 Ureters, Urinary Bladder, and Urethra (Female) (Tortora 523)




         The bladders main function is to store urine before it is released through a process
that is called micturition, or more commonly known as urination or voiding. When the
bladder fills up to a volume of more that 200 to 400 ml, pressure builds and the receptors


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in the bladder wall send signals to the spinal cord that triggers a reflex called a
micturition reflex. Impulses from the spinal cord cause contraction of the detrusor
muscle and relaxation of the internal urethral sphinctor. At the same time the spinal cord
inhibits the somatic motor neurons, which connects to and sends signals to the skeletal
muscles. This relaxes the skeletal muscles in the external urethral sphincter. This whole
process causes urination.
         The neural control system of the bladder is essentially a positive feedback system.
(Rijkhoff, 2) Bladder filling causes a pressure increase, which in turn activates
mechanoreceptors in the bladder wall. The detrusor muscles contract once the threshold
for micturition is exceeded causing the intravescular pressure to rise even further, which
then leads to the emptying of the bladder. A draw back of this system is that it can
become unstable fairly easily. Any stimulation to the mechanoreceptors can cause
microtuition reflex. In order to control this reaction the neural control system has
inhibitory systems at the spinal and supraspinal level. This is what causes the problems
after spinal cord injury. The supraspinal inhibitory systems are disconnected which
causes the bladder to become unstable.
         After spinal cord injury the bladder is referred to as a neurogenic bladder.
(Urinary Tract Management) It is called this because the bladder has been cut off from
the nervous systems control at the site of the lesion or injury. Because it is cut off the
message that is usually sent to the brain, that the bladder is full, can no longer be received
since it is blocked at the injury site. This means that the feeling that you get when you
have the urge to urinate is no longer felt. The muscle activities of the bladder and
external sphincter muscle are also effected by injury. In one case urination occurs by a
reflex that is still intact after the injury and is called reflex voiding. In this case when the
bladder fills it send nerve impulses to the spinal cord at S2-3-4. This is followed by a
responding impulse that is sent back to the bladder, which causes the bladder to contract.
If at this point the sphincter muscles are relaxed these contraction could expel urine. If
this reflex is destroyed in-patients with lower level injuries (usually T-12 and lower) the
bladder will remain relaxed and stretch as it fills. In this case there are no contractions to
empty the urine. In both of these cases the person has lost the conscious and
subconscious control over the storage function of their bladder.
         One successful FES system for assisting bladder control that is used and
medically perscribed is the Finetech-Brindley sacral anterior nerve-root simulator. The
main purpose of this system is to improve bladder emptying and to achieve continence in
those with neurogenic bladder-voiding dysfunction. (Lazar 622) The Finetech-Brindley
system consists of an external control unit with a radio-frequency transmitter for three
independent channels and a set of three receivers that are implanted over the lower rib.
(Lazar 622) Platinum-iridium helical cables connect these receivers to a set of U-shaped
platinum foil electrodes. The electrodes are mounted in a symmetrical tripolar
arrangement, with two anodes and one cathode in the middle, in slots of silicone-rubber
structures that are referred to as books. The books are implanted at the level of the last
intervertebral disc, and the S2 to S5 anterior roots are trapped in different slots of the
books. The sacral anterior roots S2-S4 innervate the detrusor by preganglionic
parasympathetic nerve fibers as well as the external sphincter of the bladder and pelvic
floor muscles by somatic nerve fibers through the pudendal nerve. When these roots are
stimulated the detrusor muscles contract, which results in an increase in intravesical



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pressure. However due to the stimulation the contractions of the external sphincter occur
simultaneously, which prevents the bladder from emptying. Once the stimulation is
interrupted the external sphincter relaxes faster than the detrusor muscles, resulting in a
portion of the urine to be released. In order to empty the bladder repetitive bursts of
stimulation pulses must be used.

Figure 2 Pudendal Nerve (Tortora 9th edition, 438)




Figure 3 Parasympathetic Preganglionic Fibers (Tortora 5th edition, 264)




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References

1) Grabowski, Sandra Reynolds, Tortora, Gerard J. Introduction to the Human Body
      Fifth Edition The essentials of Anatomy and Physiology Biological Science
      Textbooks Inc. 2001

2) Grabowski, Sandra Reynolds, Tortora, Gerard J. Principles of Anatomy and
       Physiology Ninth Edition. John Wiley & Son, Inc. 2000

3) Lazar, Richard B. Principles of Neurologic Rehabilitation McGraw Hill 1998

4) Rijkhoff, Nico, Sinkjaer, Thomas. Restoration Of Bladder Control In Spinal Cord
       Injury Patients Using Electrical Stimulation. 6th Internet world Congress For
       Biomedical Sciences. 2000 http://www.uclm.es/inabis2000/symposia/files/064/session.htm

5) Urinary Tract Management In Spinal Cord Injury: Urination And the Urinary Tract In
   SCI. Louis Calder Memorial Library of the University of Miami/Jackson Memorial
   Medical Center 1998. http://calder.med.miami.edu/pointis/urination.html




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