Renal system

Micturition

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 Second step, which is a nervous reflex called the micturition reflex that empties the bladder or, if this fails, at least causes a conscious desire to urinate


Physiologic anatomy and nervous connectionsof the bladder The urinary bladder, is a smooth muscle chamber composed of two main parts: (1) the body, (2) the neck, (a funnel-shaped extension of the body), - above the bladder neck, is a small triangular area (trigone) -the lower part of the bladder neck is also called the posterior urethra. the two ureters enter the bladder at the uppermost angles of the trigone. Each ureter, as it enters the bladder, courses obliquely through the detrusor muscle and then passes another 1 to 2 cm beneath the bladder mucosa before emptying into the bladder.

Bladder - Anatomy

The bladder neck (posterior urethra) is 2 to 3 cm long, and its wall is composed of detrusor muscle interlaced with a large amount of elastic tissue. The muscle in this area is called the internal sphincter. Its natural tone normally keeps the bladder neck and posterior urethra empty of urine and, therefore, prevents emptying of the bladder until the pressure in the main part of the bladder rises above a critical threshold.

Beyond the posterior urethra, a layer of muscle called the external sphincter. This muscle is a voluntary skeletal muscle prevent urination even when involuntary controls are attempting to empty the bladder. The muscle of the bladder body and neck is entirely smooth muscle.


Innervation of the bladder The principal nerve supply of the bladder is the pelvic nerves, which connect with the spinal cord through the sacral plexus, (S-2 and S-3). Both sensory nerve fibers and motor nerve fibers motor nerves transmitted in the pelvic nerves are parasympathetic fibers. The external sphincter is innervated by pudendal nerve (innervate and control the voluntary skeletal muscle of the sphincter).

The bladder receives sympathetic innervation from the sympathetic chain through the hypogastric nerves, connecting mainly with the L-2 segment of the spinal cord. These sympathetic fibers stimulate mainly the blood vessels and have little to do with bladder contraction. Some sensory nerve fibers may be important in the sensation of fullness and pain.

Peripheral Nervous System

Somatic (S2-S4) Pudendal nerves Excitatory to external sphincter Parasympathetic (S2-S4) Pelvic nerves Excitatory to bladder, relaxes sphincter Sympathetic (T10-L2) Hypogastric nerves to pelvic ganglia Inhibitory to bladder body, excitatory to bladder base/urethra


Transport of Urine The walls of the ureters contain smooth muscle and are innervated by: -sympathetic ,parasympathetic nerves -an intramural plexus of neurons and nerve fibers that extends along the entire length of the ureters. peristaltic contractions in the ureter are enhanced by parasympathetic stimulation and inhibited by sympathetic stimulation.


Normally, the ureters run obliquely for several centimeters through the bladder wall& normal tone of the detrusor muscle tends to compress the ureter& preventing backflow of urine from the bladder when pressure builds up in the bladder during micturition or bladder compression. In some people, the distance is less than normal, so that contraction of the bladder during micturition does not always lead to complete occlusion of the ureter & some of the urine in the bladder is propelled backward into the ureter, a condition called vesicoureteral reflux.


Pain sensation in the ureters, and the ureterorenal reflex The ureters are well supplied with pain nerve fibers. When a ureter becomes blocked (e.g., by a ureteral stone), intense reflex constriction occurs, associated with severe pain. Also, the pain impulses cause a sympathetic reflex back to the kidney to constrict the renal arterioles, thereby decreasing urine output from the kidney. This effect is called the ureterorenal reflex and is important for preventing excessive flow of fluid into the pelvis of a kidney with a blocked ureter.

Micturition Reflex

The micturition reflex is a single complete cycle of (1) progressive and rapid increase of pressure, (2) a period of sustained pressure, and (3) return of the pressure to the basal tone of the bladder. 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 becomes more and more filled, micturition reflexes occur more and more often and more and more powerfully. 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



The micturition reflex is a completely autonomic spinal cord reflex, but it can be inhibited or facilitated by centers in the brain. The micturition reflex is the basic cause of micturition, but the higher centers normally exert final control of micturition as follows: 1. The higher centers keep the micturition reflex partially inhibited, except when micturition is desired. 2. The higher centers can prevent micturition, even if the micturition reflex occurs, by continual tonic contraction of the external bladder sphincter until a convenient time presents itself. 3. When it is time to urinate, the cortical centers can facilitate the sacral micturition centers to help initiate a micturition reflex and at the same time inhibit the external urinary sphincter so that urination can occur.

Abnormalities of Micturition

If the spinal cord is damaged above the sacral region but the sacral cord segments are still intact, typical micturition reflexes can still occur. However, they are no longer controlled by the brain.


Uninhibited neurogenic bladder Frequent and relatively uncontrolled micturition. This derives from partial damage in the spinal cord or the brain stem that interrupts most of the inhibitory signals. Therefore, facilitative impulses passing continually down the cord keep the sacral centers so excitable that even a small quantity of urine elicits an uncontrollable micturition reflex, thereby promoting frequent urination.


Atonic bladder Micturition reflex contraction cannot occur if the sensory nerve fibers from the bladder to the spinal cord are destroyed, preventing transmission of stretch signals from the bladder. When this happens, a person loses bladder control, despite intact efferent fibers from the cord to the bladder and despite intact neurogenic connections within the brain. Instead of emptying periodically, the bladder fills to capacity and overflows a few drops at a time through the urethra. This is called overflow incontinence. A common cause of atonic bladder is crush injury to the sacral region of the spinal cord.