Micturition Reflex Notes

Micturition is the physiological process of urine expulsion from the bladder, which is regulated by the intricate micturition reflex involving multiple components of the urinary system. This reflex is crucial for maintaining urinary control and contributes significantly to homeostasis and fluid balance within the body.

Anatomy of the Bladder

  • Ureters: Two long, muscular tubes (left and right) that transport urine from the kidneys to the bladder. They utilize peristaltic motion to move urine, assisted by gravity, and are lined with a mucous membrane to facilitate this process.

  • Bladder: A hollow muscular organ that temporarily stores urine. It comprises a robust muscular layer known as the detrusor muscle (red layer) that contracts to expel urine. The bladder is a highly distensible organ capable of expanding significantly as its volume increases, typically holding 600-800 mL of urine when full.

  • Internal Urethral Sphincter

    • Composed of smooth muscle surrounding the entry point of the bladder to the urethra

    • This sphincter involuntarily regulates the flow of urine from the bladder to the urethra and is influenced by autonomic nervous system signals.

    • Males have a larger internal urethral sphincter due to the increased length of the male urethra, which provides additional support and control during micturition.

  • External Urethral Sphincter: Made of striated muscle and part of the urogenital diaphragm, this sphincter provides voluntary control over urination, allowing for conscious relaxation to release urine when appropriate.

Initial Urine Accumulation

  • An empty bladder generally contains 10-20 mL of urine post-urination, which allows for a margin of low volume.

  • As the bladder fills, stretch receptors embedded in the detrusor muscle respond to stretching due to the increasing urine volume by sending afferent sensory impulses to the spinal cord and higher brain centers. This signaling is crucial for the perception of bladder fullness and the regulation of urination.

Nervous System Involvement

  • Spinal Cord Regions:

    • Sacral Region (S2-S4): This area of the spinal cord primarily receives sensory input from the bladder regarding fullness and initiates reflexive actions.

    • Thoracolumbar Region (T11-L2): Coordinates sympathetic responses to maintain bladder contraction and urine retention until a suitable time for micturition is reached.

  • Brainstem - Pons: This area contains vital centers that control micturition:

    • Pontine Storage Center: Activated when bladder volume is low, facilitating urine retention and suppressing the urge to urinate.

    • Pontine Micturition Center: Activated when the bladder is full, signaling the need to void.

Micturition Reflex Process - Empty Bladder

  • With minimal urine within the bladder, few impulses are sent from stretch receptors to the spinal cord.

  • In response to low stretch signals, the Sympathetic Nervous System is activated through the hypogastric nerve, which releases norepinephrine into:

    • Detrusor Muscle: Norepinephrine activates beta-3 adrenergic receptors causing relaxation of the detrusor muscle as K+ leaves the cell, which inhibits involuntary contractions.

    • Internal Urethral Sphincter: Alpha-1 adrenergic receptors are activated, maintaining sphincter contraction to prevent urine flow.

  • Concurrently, the cerebral cortex becomes aware of the bladder's state, stimulating the pontine storage center, a center in the pons, which inhibits the pontine micturition center.

  • Pontine storage center sends signals through the descending fibres to the thoraco-lumbar plexus and the sacral plexus to modulate the activity of the sacral spinal cord, enhancing the sensory feedback related to bladder fullness and coordinating the storage of urine.

  • Acetylcholine is released in the detrusor muscle, promoting its contraction when it is time to initiate micturition, while also inhibiting the internal urethral sphincter to allow for urine passage.

  • Pudendal nerve (from the somatic system) plays a critical role in controlling the external urethral sphincter, allowing voluntary control over urination and ensuring that micturition occurs at an appropriate time.

  • The impulses generated by the pontine micturition center are slow in nature.

Micturition Reflex Process - Full Bladder

As the bladder fills to approximately 200-500 mL, stretch receptors are highly activated, significantly increasing sensory signals sent to the spinal cord.

This process initiates sensory fibers stimulate the spinal cord to send high-frequency impulses to the pons and cerebral cortex, signaling bladder fullness.

The cerebral cortex reacts in the following ways:

  • Activating the pontine micturition center, initiating the urge to urinate.

  • Inhibiting the pontine storage center, ceasing signals that prevent urination.

  • The axons from the pontine micturition center goes down to the thoracolumbar region of the spinal cord, where they facilitate the coordination of motor signals to the detrusor muscle of the bladder and inhibit sympathetic activity, allowing for bladder contraction and triggering micturition.

  • Stimulation of parasympathetic nerves, which contract the detrusor muscle while also inhibiting somatic motor neurons in the ventral gray horn of the spinal cord to relax the external urethral sphincter, allowing urine to flow out.

  • Less norepinephrine is released on the alpha-1 adrenergic receptors of the internal urethral sphincter, which further promotes bladder contraction and assists in the effective voiding of urine.

Kegal muscles increase urine flow by providing additional support to the pelvic floor, which helps maintain proper bladder function and assists in the control of urination.