Micturition Process and Reflex

Micturition Process Overview

  • Definition: Micturition refers to the process of voiding or urination, which entails the emptying of the bladder.

Key Steps in the Micturition Reflex

  • Bladder Anatomy:

    • Transitional Epithelium: Lines the bladder.

    • Smooth Muscle: Known as detrusor muscle, responsible for bladder contraction.

    • Adventitia: Outer layer of the bladder.

    • Internal Urethral Sphincter: Composed of smooth muscle; regulates urine flow into the urethra.

    • External Urethral Sphincter: Composed of skeletal muscle; allows voluntary control over urination.

  • Urine Accumulation:

    • As urine fills the bladder, bladder wall distension activates stretch receptors.

    • Afferent Fibers: Carry signals from stretch receptors to the central nervous system, specifically the sacral portion of the spinal cord.

  • Parasympathetic Response:

    • Efferent Fibers: From the parasympathetic nervous system stimulate the detrusor muscle to contract.

    • Simultaneously, these fibers cause the internal urethral sphincter to relax, leading to micturition.

  • Communication with the Brain:

    • Interneurons within the spinal cord relay information about the full bladder to the micturition center located in the pons.

  • Cerebral Involvement:

    • If the situation is appropriate for urination, the brain sends impulses via the cerebral cortex.

    • This facilitates the relaxation of the external urethral sphincter, allowing urine to void.

    • If it is not appropriate to urinate, the signal to relax the external sphincter will not be sent, halting the micturition process.

Summary of Micturition Reflex Steps

  1. Bladder Filling: As urine fills and stretches the bladder wall, stretch receptors are activated.

  2. Signal Transmission: Afferent signals travel to the spinal cord.

  3. Involuntary Reflex Action:

    • Reflex arcs promote contraction of the detrusor muscle.

    • Internal urethral sphincter relaxes.

  4. Signal to Pons: Afferent signal also reaches the pons micturition center.

  5. Cortex Decision:

    • If appropriate: External urethral sphincter relaxes, and urine is voided.

    • If not appropriate: External sphincter remains contracted, preventing urine flow.