The micturition reflex is a complex process that controls the act of urination, allowing for the voluntary and involuntary control over the expulsion of urine from the bladder.
This reflex involves an intricate interplay between the central nervous system (brain and spinal cord) and the urinary tract (bladder and urethra).
Components of the Micturition Reflex
1. Bladder:
Function: The detrusor muscle in the bladder walls stores urine by relaxing and expels urine by contracting during urination.
2. Internal Urethral Sphincter:
Function: Involuntarily controls urine flow from the bladder into the urethra. It relaxes during urination.
3. External Urethral Sphincter:
Function: Under voluntary control, it allows individuals to start or delay urination.
Phases of the Micturition Reflex
1. Filling Phase:
Process: As the bladder fills, the detrusor muscle remains relaxed due to sympathetic nervous system activity, which also keeps the internal urethral sphincter contracted to store urine.
2. Storage Phase:
Process: Afferent nerves send signals to the brain indicating bladder fullness, but the central nervous system inhibits the reflex, allowing voluntary control.
3. Voiding Phase:
Process:
I. Afferent signals indicate bladder fullness to the brain.
II. The brain sends efferent signals via the parasympathetic nervous system.
III. The detrusor muscle contracts, and the internal sphincter relaxes.
IV. Voluntary relaxation of the external urethral sphincter allows urination.
Control of Micturition
1. Voluntary Control:
Brain centers like the pontine micturition center (PMC) and the cerebral cortex control urination based on social and situational factors.
2. Involuntary Control:
In infants and individuals with certain neurological conditions, urination can occur involuntarily when the bladder reaches a specific volume, indicating that the control over micturition can be affected by the development and integrity of neural pathways.