Introduction
Micturition also known as urination is the process of removing urine from the urinary bladder and maintaining proper fluid balance. The function of urination is to eliminate products and toxic wastes from the body that have been filtered by the kidneys. This appears to be a simple process, but on the other hand, it is a complex neurophysiological process that involves coordination by the sympathetic, parasympathetic, and somatic nervous systems. Therefore proper coordination between the bladder, the spinal cord, and the brain can facilitate the voluntary and involuntary control of urination.
What Is Micturition?
Micturition or urination is the process of excreting the urine from the storage organ which is the urinary bladder. This process relies on both the lower urinary tract micturition areas of the central nervous system (CNS). This whole process is done in two phases which are - the storage phase, where urine from the kidneys is stored in the bladder until it is appropriate, and the voiding phase, where the urine is expelled from the bladder via the urethra.
What Is the Relevant Anatomy and Physiology in Micturition?
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The urinary tract consists of two mutually dependent parts, which include - the upper tract comprising of the kidneys and ureters and the lower tract consisting of the bladder and urethra. The lower tract is the one that is involved in the micturition reflex.
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The bladder is a hollow organ that acts as a reservoir for the storage and regular elimination of urine.
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The urinary bladder walls are formed of three layers of smooth muscle known as detrusor.
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At the bottom of the bladder, an outlet going into the urethra is seen known as the bladder neck, which is surrounded by smooth muscles known as the internal urethral sphincter, which is involuntarily controlled.
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The pelvic floor muscles surround the urethra forming the external urethral sphincter, from where the urine is excreted out of the body. This sphincter has voluntary control or conscious control. In the storage phase, both of these sphincters are closed to prevent leakage.
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The detrusor muscle is usually relaxed, which helps the bladder to expand the store the urine. A normal healthy adult bladder can hold anywhere between 300 to 500 milliliters of urine for two to five hours. When the bladder inflates, it sends signals to the brain that the bladder is full.
What Is the Micturition Reflex?
The micturition reflex is the pathway of removal of urine from the body. It is a highly coordinated process that includes both involuntary and voluntary control. The reflex is primarily controlled by the autonomic nervous system and is regulated by the central nervous system (CNS) which includes the brain and spinal cord. The micturition reflex is divided into two phases which include -
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Storage Phase - In this phase, the bladder gets filled with urine and gets inflated to accommodate the urine. The detrusor muscles get relaxed and allow the bladder to expand while maintaining low pressure. This relaxation process of the detrusor muscle is controlled by sympathetic nerves that release norepinephrine, which acts on beta-adrenergic receptors in the bladder wall. At the same time, the internal urethral sphincter present at the junction of the bladder and urethra remains contracted to prevent any leakage.
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Voiding Phase - As in the storage phase, the bladder gets filled to its capacity, and after that micturition reflex gets initiated. In this phase, the collective interplay of the bladder, spinal cord, and brain takes place. The receptors present on the walls of the bladder detect the increased volume of urine and send the signals to the spinal cord via pelvic nerves. The synapse or reflex pathway formed between afferent fibers or pelvic nerves and the spinal cord is called the spinal micturition center.
This spinal micturition center is responsible for involuntary urination in infants, and as the individual grows, the micturition reflex becomes subject to voluntary control due to the development of connections between the spinal micturition center and higher brain centers.
What Are the Pathologies Associated With Micturition Reflex?
There are few pathologies or disorders related to the micturition reflex. One of the most commonly seen conditions is urinary incontinence.
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Urinary incontinence means involuntary loss of urine. This usually occurs due to weakened pelvic floor muscles, damage of nerves involved in bladder control, or some other factors that may impair the coordination of the micturition reflex.
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Urinary retention is another disorder related to the micturition reflex. In this condition, the person is not able to empty the bladder completely. This can occur as a result of an obstruction in the urinary tract due to weakened bladder muscles or neurological disorders that affect the micturition reflex.
Both of these disorders can severely affect the quality of life of the patient and may require medical intervention and treatment.
Neurological Disorders:
Various neurological disorders such as spinal cord injury, multiple sclerosis, or stroke can disrupt the normal functioning of the micturition reflex.
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Injury of the spinal cord can lead to interruption in the communication between the bladder and brain, leading to impaired voluntary control over micturition. This can further lead to loss of bladder control, involuntary urination, or urinary retention.
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Likewise, multiple sclerosis can cause demyelination of nerve fibers, affecting the transmission of signals involved in the micturition reflex.
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Stroke can impact certain areas of the brain and may lead to urinary dysfunction.
Clinical Evaluation and Management:
- For patients experiencing urinary symptoms or dysfunctions, a complete clinical evaluation is crucial for diagnosis and appropriate management. Therefore various diagnostic tests are advised by health care professionals, which may include bladder ultrasound, cystoscopy (a technique used to visualize the inside of the bladder), or neurological assessments to assess the underlying cause of the urinary dysfunction.
- The treatment options for micturition reflex disorders usually depend upon the type of underlying condition. In the case of urinary incontinence, certain treatment options, like lifestyle modifications, pelvic floor exercises, and behavioral techniques, may be recommended.
- Medicines may be advised to relax the bladder muscles or strengthen the sphincter.
- In the case of neurological disorders or injuries that may affect the micturition reflex, the focus is on managing the underlying condition and maximizing bladder function.
Conclusion
The micturition reflex is a complex process that works by the coordinated interaction between the bladder, spinal cord, and brain. The whole process is regulated by the autonomic nervous system and influenced by voluntary control from higher brain centers. Any disruption in the micturition reflex can lead to various urinary dysfunctions and disorders impacting the individual's quality of life. Therefore early diagnosis and appropriate management are crucial factors in addressing or improving bladder function.
