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).
Here's an overview of how the micturition reflex works:
Components of the Micturition Reflex
1. Bladder:
The bladder's walls are lined with a smooth muscle called the detrusor muscle, which relaxes to allow the bladder to store urine and contracts when urinating to expel urine.
2. Internal Urethral Sphincter:
A smooth muscle that involuntarily controls the flow of urine from the bladder into the urethra. It relaxes during urination.
3. External Urethral Sphincter:
A skeletal muscle that is under voluntary control, allowing individuals to initiate or delay urination.
Phases of the Micturition Reflex
1. Filling Phase:
As the bladder fills with urine, the detrusor muscle remains relaxed thanks to sympathetic nervous system activity, which also keeps the internal urethral sphincter contracted.
This phase allows the bladder to store increasing volumes of urine without a significant rise in bladder pressure.
2. Storage Phase:
Afferent nerves from the bladder wall send signals to the sacral spinal cord and brain, indicating the bladder's fullness as it expands.
However, urination does not occur yet because the central nervous system inhibits the micturition reflex, allowing voluntary control over urination.
3. Voiding Phase:
When the bladder reaches a certain level of fullness, stretch receptors in the bladder wall trigger the micturition reflex.
This involves a series of coordinated events:
I. Afferent signals to the brain increase, indicating that the bladder is full.
II. The brain processes these signals and, when appropriate, sends efferent signals back through the parasympathetic nervous system to the bladder.
III. The detrusor muscle contracts, and the internal urethral sphincter relaxes.
IV. Conscious relaxation of the external urethral sphincter allows urine to be expelled from the bladder.
Control of Micturition
1. Voluntary Control:
Higher brain centers, including the pontine micturition center (PMC) and the cerebral cortex, play a critical role in voluntary control over urination.
They can inhibit or stimulate the micturition reflex based on social appropriateness and the individual's desire to urinate.
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.