A variation on the subsidy problem

THE BLADDER: The bladder is simply a reservoir for storing and eliminating urine at the appropriate time. A normal bladder can store between 150 – 400ml of urine before the urge to micturate becomes unbearable.^15,16

The detrusor muscle is supplied by the parasympathetic fibres derived from the sacral S2 – 4 segments.^16,17 Efferent fibres pass via the posterior route of the same nerves to the spinal cord and are carried to the brain either in the spinothalamic tract or posterior columns. Sympathetic fibre arises from T11, T12 and L1 & 2. Their fibres traverse the lumbar sympathetic chain before

entering the hypogastric plexus. The pudendal nerve supplies the somatic and sensory fibres via S2 – 4 segments, which also supply the external urethral sphincter. Cholinergic receptors dominate in the detrusor while αia dominate the bladder neck.

2.3.2 CENTRAL CONNECTION OF THE BLADDER: These central connections are responsible for the conscious awareness and possible suppression of bladder activities. Bladder input passes via the spinothalamic tract and posterior column to the reticular formation. There is also input from nuclei in the septal region, hypothalamus, and areas in the cerebral hemisphere. Areas involved in micturition in the cortex are found in the frontal lobe anterior to the Rolandic fissure. It has been postulated that establishment of connections between the hypothalamic and cortical areas result in urinary continence in children; and its loss may account for incontinence in the elderly.

2.3.3 MICTURITION: This is the process by which the Urinary bladder empties when it is filled. This involves two main steps:

(i) the bladder fills progressively until the tension in its wall rises above a threshold level, eliciting the second step;

(ii) a nervous reflex called the micturition reflex occurs that empties the bladder or causes a conscious desire to urinate. Although this reflex

is an autonomic spinal reflex, it can be inhibited or facilitated by centres in the cortex or brain stem.

Urine flows from the collecting ducts into the renal calices. As the calices become distended, their inherent pace maker’s activity is increased, initiating peristaltic activities that spread to the pelvis and downward along the ureters forcing urine into the bladder. These peristaltic activities are inhibited by sympathetic, but facilitated by parasympathetic stimulation.

As the bladder fills, the intraluminal pressure remains constant but the wall tension increases. When a capacity of about 150 - 400ml is reached, a desire to void is experienced, and the intraluminal pressure begins to rise. At a capacity of 400ml and above, the desire is almost unbearable. When micturition is about to commence, the following event takes place:

contraction of the detrusor beginning from the dome, the bladder neck descends, relaxation of the perineal muscles, funneling of the bladder, shortening of the proximal urethra and dropping of the intraurethral pressure.

There is an associated increase in intra-abdominal pressure secondary to the contraction of anterior abdominal muscles and relaxation of the external urethral sphincter. The intravesical pressure thus generated ranges from 40-60mmHg. Micturition will thus commence until the available urine is expelled leaving a postvoid volume of usually ≤ 50ml.

These events are mediated through the nervous supply to the bladder derived from sacral 2-4 for parasympathetic and lumbar 2-4 for the sympathetic supply. Stretch reflexes from the posterior urethra carried via the pudendal and parasympathetic pathways (sacral 2 - 4) are mainly responsible for bladder emptying. These nerves are transmitted through the pelvic nerves.

They also supply the external urethral sphincter.

The contraction of abdominal muscles increasing intravesical pressure and expelling urine into the bladder neck and posterior urethra is the most critical event in the initiation of the micturition reflex. The inhibitory centres are located in the cerebral cortex, while the facilitatory centres are in the brain stem especially the pons.

The relationship between intravesical pressure and volume can be studied by cystometry: the bladder is first emptied using a catheter, and pressure recordings are made as it is filled with 50ml of water incrementally. A plot of intravesical pressure and volume (cystometrogram) can be made (see fig. 1)

ia

INTRAVESICAL PRESSURE (CM WATER)

ib II

80 60 40 20

100 200 300 400 500 0,0

Fig. 1

TRANSVERSICAL VOLUME

INTRAVESICAL VOLUME (ML): The curve shows an initial slight rise in pressure when the first increments in volume were made (ia), followed by a long flat segment (ib) as further increments are made, but with little change in intravesical pressure. Then a sharp rise in pressure (II) as the micturition reflex is triggered (at about 400ml volume). The first urge to micturate occurs at 150ml, and a marked sense of fullness at 400ml. The flat segment is explained by the law of LAPLACE^15,16 which states: Pressure in a spherical viscus is equal to twice the wall tension divided by the radius (P = 2T/r):

where P = pressure, T = tension and r = radius. However, certain intrinsic properties of the bladder like elasticity, compressibility of its wall tend to negate the perfect application of this law.

2.3.4 VOLUNTARY CONTROL OF MICTURITION: This is a complex process and one of the earliest events is the relaxation of pelvic floor which may produce a downward tug on the bladder to initiate contraction. Perineal muscles and the external urethral sphincter can be contracted voluntarily to inhibit micturition or interrupt it even if it has started. After micturition, the male urethra empties by contraction of the bulbocavernosus muscle, the female by gravity.

The autonomic and cerebral control of these activities are complex and not completely understood. The micturition reflex is integrated in the sacral portion of the spinal cord. Fibres in the pelvic nerves are the afferent limb,

while the parasympathetic fibres to the bladder which form the efferent limb also travel along these nerves (S2 – 4). The reflex mechanism has an inhibitory and a facilitatory area in the brain stem. The facilitatory area is in the pontine, while the inhibitory area is in the midbrain (Blok et al 1999).

Fibre from S2 – 4 project to these areas and coordinated activities involving these areas, the bladder musculature and the external urethral sphincter are responsible for micturition and continence. After transection of the brain stem above the pons, the reflex threshold is lowered and less bladder filling is required to trigger it, but when transection is above the midbrain, the reflex is essentially the same. The role of the cortical areas in micturition is not well understood, but evidence from pisitron emission tomography suggests only a modulatory role. ^18,19

In conclusion, for normal micturition to occur, there must be an intact bladder and urethra together with their spinal supply and integration as well as the pontine and midbrain facilitatory and inhibitory areas. The role of the cortex is probably modulatory.