Micturation reflex from guyton and halls
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Here is a complete summary of the Micturition Reflex directly from Guyton and Hall Textbook of Medical Physiology (Chapter 26):
Micturition
Micturition is the process of emptying the urinary bladder when it becomes filled. It involves two main steps:
- Progressive bladder filling until wall tension rises above a threshold
- A nervous reflex - the micturition reflex - that empties the bladder or creates a conscious urge to urinate
Although it is an autonomic spinal cord reflex, it can be inhibited or facilitated by centers in the cerebral cortex or brain stem.
Physiological Anatomy
- The bladder consists of a body (collects urine) and a neck (funnel-shaped, connects to the urethra)
- The smooth muscle is the detrusor muscle - its fibers extend in all directions and, when contracted, raise bladder pressure to 40-60 mm Hg. Muscle cells are electrically fused, so an action potential spreads through the entire bladder at once
- The trigone is a triangular area on the posterior wall above the bladder neck; its mucosa is smooth (no rugae)
- The internal sphincter (in the bladder neck) maintains tonic tone to keep the posterior urethra empty until pressure exceeds a critical threshold
- The external sphincter passes through the urogenital diaphragm - it is voluntary skeletal muscle, controlled by the pudendal nerve
Cystometrogram and Pressure Changes
As the bladder fills:
- 0-50 mL: pressure rises slightly
- 50-300 mL: pressure stays nearly constant (stress relaxation of the detrusor)
- >300-400 mL: pressure rises rapidly
Superimposed on these are micturition waves - acute pressure spikes (a few cm to >100 cm H2O, lasting seconds to >1 minute) caused by the micturition reflex.
The Micturition Reflex
Trigger: Stretch receptors in the bladder wall, especially in the posterior urethra, when it begins to fill at higher bladder pressures.
Arc:
- Sensory signals travel via pelvic nerves to the sacral segments of the spinal cord
- Reflexively back to the bladder via parasympathetic fibers through the same pelvic nerves
Self-regenerative nature:
- Initial bladder contraction activates more stretch receptors → more sensory impulses → stronger reflex contraction → repeat
- This cycle builds progressively until the bladder contracts strongly
- After seconds to >1 minute, the reflex fatigues and the cycle ceases, allowing the bladder to relax
A single complete micturition reflex cycle consists of:
- Progressive, rapid increase in pressure
- A period of sustained pressure
- Return of pressure to basal tone
If the reflex fails to empty the bladder, the nervous elements remain inhibited for minutes to >1 hour before another cycle occurs. As the bladder fills more, reflexes recur more frequently and powerfully.
External Sphincter Inhibition
When the micturition reflex becomes strong enough, it triggers a reflex through the pudendal nerves that inhibits the external sphincter. Voiding occurs if this inhibition is stronger than voluntary constrictor signals to the external sphincter.
Higher Center Control
The higher brain centers exert final control over the micturition reflex:
- They keep the reflex partially inhibited except when micturition is desired
- They can prevent voiding by tonic contraction of the external sphincter, even during an active reflex
- They can facilitate the sacral micturition centers when it is time to urinate, while simultaneously inhibiting the external sphincter
Voluntary initiation: The person contracts the abdominal muscles → raises bladder pressure → forces urine into the bladder neck and posterior urethra → stretches walls → stimulates stretch receptors → excites the micturition reflex → inhibits the external sphincter. Rarely >5-10 mL is left after voiding.
Abnormalities of Micturition
| Condition | Cause | Mechanism | Result |
|---|---|---|---|
| Atonic Bladder / Overflow Incontinence | Destruction of sensory nerve fibers (e.g., sacral crush injury, tabes dorsalis) | No stretch signals reach the cord; efferent fibers intact | Bladder fills to capacity, overflows drop by drop |
| Automatic Bladder | Spinal cord damage above the sacral region (sacral cord intact) | Micturition reflexes still occur but are uncontrolled by the brain; initially suppressed (spinal shock), then return | Periodic, unannounced bladder emptying |
| Uninhibited Neurogenic Bladder | Partial damage to spinal cord or brain stem interrupting inhibitory signals | Facilitative impulses keep sacral centers overexcitable; even small urine volumes trigger reflex | Frequent, relatively uncontrollable micturition |
Source: Guyton and Hall Textbook of Medical Physiology, Chapter 26 - Urine Formation by the Kidneys
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