Physiology of micturation Flashcards
Outline the anatomy of the urinary bladder
- The urinary bladder is a smooth muscle chamber that consists of:
- Body: The place of urine collection, made of smooth muscle “detrusor muscle” which fuses to contract the bladder, the other layer of the bladder include (Mucosa made of “Transitional epithelium, lamina propria, submucosa” and the adventitia)
- Neck
- Trigone: A triangular area at the lowermost apex of the bladder
- Internal sphincter: 2-3cm which is formed of the bladder neck, its tone keeps the bladder neck and the posterior urethra empty (mostly constricted under the involuntary control)
- External sphincter: Beyond the posterior urethra, formed as the urethra passes through the urogenital diaphragm containing a muscle layer, it is under voluntary control
What is the sympathetic nerve supply of the bladder?
L1, L2, L3, via the hypogastric nerve, but they do not have a role in micturation; rather, they function to prevent the reflux of semen into the bladder
What is the parasympathetic innervation of the urinary bladder?
- The Pelvic nerve (S2, S3, S4), their sensory fibers, carries impulses from the stretch receptors found on the wall of the urinary bladder to the spinal center of micturtion
- The parasympathetic nerve supply leads to the passage of urine due to the fact that stimulating the parasympathetic efferent fibers causes the contraction of the detrusor muscle, leading to the emptying of the urinary bladder mainly through the pelvic nerve
What is the somatic nerve supply of the urinary bladder?
- S3, S4 which is the pudendal nerve
What is the action of the somatic innervation of the urinary bladder?
Causes the storage of urine by maintaing the tonic contraction of the skeletal muscle fibers of the external sphincter, making it always contracted unless inhibited, which occurs during micturation
Through which nerve does the bladder convey pain?
1) Pelvic (parasympathetic S2, S3, S4)
2) Hypogastric (SYMPATHETIC L1, L2)
3) Pudendal (Somatic S3, S4)
What is the motor innervation of the urinary bladder?
- Sympathetic (Hypogastric nerve)
- It only supplies the blood vessels, its influence on the bladder is unknown, and it has no role in micturation
- Parasympathetic (Pelvic nerve)
- Stimulates the bladder
What is the innervation of the ureters?
They are supplied by both sympathetic (which inhibits/relaxes) and parasympathetic (which constricts/causes peristalsis)
What is the function of the sympathetic (hypogastric nerve) innervation?
- It detects the fullness of the bladder
- It transmits impulses from the pain receptors to the upper lumbar segment resulting from the pain sensation from the urethra and the bladder
- It inhibits the bladder wall by relaxation of the detrusor muscle through the BETA-3 receptors
- It constricts (motor) the internal urethral sphincter, seminal vesicle, ejaculatory duct, and the prostatic musculature through alpha-1 receptors
- It mainly stimulates the BV’s, and it has little to do with bladder constriction
- The sensory nerve fibers of the sympathetic nerve mediates the sensation of fullness and pain of the badder
What are the receptors by which the sympathetic (hypogastric) nerve inhibits the bladder wall?
Beta-3 receptors
What is the receptor by which the sympathetic innervation constricts the internal urethral sphincter?
alpha-1 receptors
What sensation is conveyed by the sensory nerve of the sympathetic (hypogastric) nerve?
It mediates the sensation of fullness and pain
What is the function of the parasympathetic (pelvic) nerve of the urinary bladder?
- It gets its input from the stretch receptors, and it transmits impulses due to tension and pain, resulting in the reflex micturation and a sensation of bladder fullness
- Responds through ACh, which inhibits the muscarinic receptors (M3) of the internal urethral sphincter and contracts the bladder (detrusor muscle)
What is the function of the somatic (pudendal nerve) of the urinary bladder?
- Input: Transmits impulses for the sensation of distention of the urethra and passage of urine
- Motor: To the external urethral sphincter (which controls the voluntary skeletal muscle of the external sphincter); It keeps the external sphincter contracted until the conditions favor micturation
- Output: is through the efferent fibers to the external urethral sphincter, via ACh which stimulates the Nicotinic receptors
What are the phases of micturition?
- Empty bladder
- Filling of the bladder
- Emptying of the bladder
What is a cystometogram?
It is a graph that plots intravesical pressure against the volume of fluid in the bladder
- Initially, the curve shows an initial rise in pressure as the bladder fills with urine (100 ml), then from 100-300 ml, the graph shows a long, nearly flat segment, but the pressure does not increase (remains around 10), then around 400 ml there is a sudden sharp rise in pressure as the micturation reflex is triggered
What are the various triggers in emptying the bladder?
- The first urge to empty the bladder is felt at 150 ml
- A marked sense of fullness is reached at about 400 ml
- Between 150 and 400 the cytometogram shows a flat segment which is due to the law of laplace (pressure in a spheical vicus is equal to twice the tension divided by the radius, and as the bladder increases in tension with increased urine, the radius also increases as it stretches and thus it takes time to feel the fullness)
- Once urine reaches 400 ml, the micturation reflex is triggered, relaxing the perineal muscles and the external urethral sphincters and contracting the detrusor muscle to pass out urine
What are the steps in the micturation reflex?
1) Bladder fills
- Detrusor muscle relaxes
- Urethra sphincter contracts
- Pelvic floor is contracted
2) First sensation to void (150 ml)
- Partially filled bladder, the bladder is half full, and urination is inhibited voluntarily until an appropriate time
- The contractions usually relax after a fraction of a minute, the detrusor muscle stops contracting, and the pressure falls back to baseline
3) Normal desire to void
- As the bladder fills, the micturation reflex becomes more frequent and causes greater contraction of the detrusor muscles
4) Micturation
- Detrusor muscle contracts
- Urethral sphincter relaxe
- Pelvic floor relaxes
What is the nerve responsible for micturation?
Sensory signals from the stretch receptors are conducted to the sacral segments of the spinal cord through the PELVIC NERVE (parasympathetic S2, S3, S4)
What are the characteristics of the micturation reflex?
- Self-regenerative: The initial contraction of the bladder activates the stretch receptors and increases the sensory impulses from the bladder and the posterior urethra, which causes a further increase in the reflex contraction of the bladder
- Repeated cycles: The cycle keeps on repeating until the bladder reaches a strong degree of contraction
- Fatigue: Results after a few seconds of self-regenerative reflex and the cycle stops, and thus, the bladder relaxes
- Remains inhibited: Once the micturation reflex occurs but fails to empty the bladder, the nervous elements of this reflex remain inhibited for a few minutes to one hour until another reflex occurs
- More powerful and frequent: due to the bladder being more filled
- Another reflex to inhibit the external sphincter, when the reflex is powerful enough, if this reflex inhibition is more prone in the brain than the voluntary constrictors signals to the external sphincter, urination will occur, however if not; then urination won’t occur until the bladder fills further and the micturation reflex becomes more powerful
What is the role of the higher center in micturation?
- They exert the final control of micturition
- They keep the micturation reflex partially inhibited except when desired
- They can prevent micturition even if the reflex occurs via the tonic contraction of the external sphincter
- When in an appropriate time, they can facilitate sacral micturation centers by initiating reflexes and also inhibiting the external sphincter
What controls the micturation reflex?
- It is a complete autonomic apinal reflex to get the urine outside the body, and it is facilitated or inhibited by the higher brain centers
1) Unconditioned (automatic) in infants:
- Occurs through the spinal reflexes (micturation reflexes)
- The nerve tracts are not myelinated yet
- The stimulus for the reflexes is the stretch receptors through a rise in the intravesical pressure
2) Conditioned (voluntary) in adults:
- Urination occurs through the reflexes, but it can be voluntarily controlled by certain higher brain centers
- Inhibited in the midbrain
- Facilitated at the pontine area, posterior hypothalamus, and other cortical areas
What stimulates micturition reflexes in infants?
The stretch receptors (due to the rise in the intravesical pressure)
Which brain areas facilitate voluntary micturation in adults?
- Pontine area
- Posterior hypothalamus
- Other cortical areas
