Dominic Stone
Micturition, or urination, is a complex process involving the brain, spinal cord, and peripheral nervous system, as well as multiple muscles. The act of micturition is controlled by the detrusor muscle, which surrounds the bladder, and the internal and external urethral sphincters. When the bladder is full, the detrusor muscle contracts, and the internal urethral sphincter relaxes, allowing urine to pass out of the bladder and into the urethra. The external urethral sphincter is under voluntary control and must also relax for urine to exit the body.
| Characteristics | Values |
|---|---|
| Micturition | The act of emptying the bladder |
| Involuntary Micturition | Occurs in infants and young children until the age of 2 to 5 years |
| Voluntary Micturition | Gained between the ages of 2 and 3 as the brain develops |
| Involuntary Muscle | Sends signals to the nervous system, putting the decision to urinate under conscious control |
| Internal Urethral Sphincter | Involuntary muscle that surrounds the opening of the bladder to the urethra and relaxes to allow urine to pass |
| External Urethral Sphincter | Voluntary muscle that surrounds the urethra outside the bladder and must be relaxed for urination to occur |
| Detrusor Muscle | Surrounds the bladder and contracts to expel urine through the urethra |
| Neurological Control | The bladder receives input from the autonomic (sympathetic and parasympathetic) and somatic arms of the nervous system |
| Sympathetic | Hypogastric nerve (T12–L2) that causes relaxation of the detrusor muscle, promoting urine retention |
| Parasympathetic | Pelvic nerve (S2-S4) that causes contraction of the detrusor muscle, stimulating micturition |
| Somatic | Pudendal nerve (S2-4) that provides voluntary control over micturition by innervating the external urethral sphincter |
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What You'll Learn
The detrusor muscle contracts to expel urine from the bladder
The detrusor muscle is a smooth muscle that surrounds the bladder. It is normally relaxed, allowing the bladder to expand as it fills with urine. The bladder can hold anywhere from 300 to 500 ml of urine for 2 to 5 hours. When the bladder is full, stretch receptors in the bladder wall trigger the micturition reflex, and the detrusor muscle contracts. This contraction of the detrusor muscle increases the intra-bladder pressure, which results in the expulsion of urine from the bladder.
The contraction of the detrusor muscle and the relaxation of the urethral sphincter work together to allow for urination. The internal urethral sphincter is involuntary, while the external urethral sphincter is voluntary. When the detrusor muscle contracts, the internal urethral sphincter muscle relaxes, allowing urine to pass out of the bladder into the urethra. The external urethral sphincter must also be relaxed for urine to flow through the urethra and outside the body.
The detrusor muscle is under autonomic control and is composed of smooth muscle. The parasympathetic nervous system stimulates the muscarinic stretch receptors in the bladder through the pelvic nerve fibres. When the bladder is full of urine, the M3 receptors in the bladder become stretched and stimulated, leading to the contraction of the detrusor muscle for urination. At the same time, the parasympathetic fibres inhibit the internal urethral sphincter, causing relaxation and allowing for bladder emptying.
The detrusor muscle is also influenced by the sympathetic nervous system. When the bladder is empty, the sympathetic nervous system is stimulated to activate the beta-3 receptors through the adenylyl cyclase-cAMP pathway in the bladder. This process is achieved via the hypogastric nerve, which causes the relaxation of the detrusor muscle and promotes urine retention.
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The internal urethral sphincter relaxes to allow urine to pass
The process of micturition, or urination, is a complex and highly distributed process, involving pathways at multiple levels of the brain, spinal cord, and PNS, in addition to being mediated by multiple neurotransmitters. The internal urethral sphincter relaxes to allow urine to pass out of the bladder into the urethra. This is an involuntary reaction. The internal urethral sphincter is a ring of smooth muscle surrounding the bladder neck, or the outlet into the urethra. This muscle is involuntarily controlled.
The process of micturition begins when the bladder is full of urine. Stretch receptors in the bladder wall trigger the micturition reflex. The detrusor muscle, which surrounds the bladder, contracts. This contraction of the detrusor muscle causes the intra-bladder pressure to increase. As a result of the increased pressure, the internal urethral sphincter opens by relaxing, thus allowing urine to enter the bladder neck and flow into the urethra.
The internal urethral sphincter is under the control of the autonomic nervous system. The parasympathetic nervous system stimulates the muscarinic stretch receptors in the bladder through the pelvic nerve fibers. When urine fills the bladder, the M3 receptors located within the bladder become stretched and stimulated, which leads to the contraction of the detrusor muscle for urination. At the same time, the parasympathetic fibers inhibit the internal urethral sphincter, which causes relaxation and allows for bladder emptying.
The external urethral sphincter is under voluntary control. It surrounds the urethra outside the bladder and must be relaxed for urination to occur. The somatic nervous system is responsible for this voluntary control. During storage, both the internal and external sphincters are contracted to prevent leakage.
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The external urethral sphincter is under voluntary control
The external urethral sphincter is a voluntary muscle that surrounds the urethra outside the bladder. It is part of the pelvic floor musculature and must be relaxed for urination to occur. This muscle is under conscious control, which is typically gained between the ages of 2 and 3 as the brain develops. The process of micturition, or urination, begins when the bladder is full of urine. Stretch receptors in the bladder wall trigger the micturition reflex, causing the detrusor muscle to contract and the internal urethral sphincter to relax, allowing urine to flow into the urethra. The internal urethral sphincter is involuntary and surrounds the opening of the bladder to the urethra.
The external urethral sphincter is controlled by somatic motor neurons, which inhibit the pudendal nerve. This causes the external sphincter to relax, allowing urine to be expelled from the body. The somatic nervous system provides voluntary control over micturition, allowing for the conscious decision to urinate. This voluntary control is gained through toilet training in infants, where the spinal reflex that stimulates micturition is overridden by higher centres of the brain. The brain determines the urgency and responds by sending signals to either hold or void urine.
The neural circuitry that controls the micturition process is complex and highly distributed, involving pathways at multiple levels of the brain, spinal cord, and peripheral nervous system. The bladder receives input from both the autonomic (sympathetic and parasympathetic) and somatic arms of the nervous system. The sympathetic nervous system causes relaxation of the detrusor muscle, promoting urine retention, while the parasympathetic nervous system stimulates the contraction of the detrusor muscle, stimulating micturition. The pudendal nerve, innervated by the somatic nervous system, provides voluntary control over the external urethral sphincter.
During storage, both the internal and external sphincters are contracted to prevent leakage. The internal sphincter is composed of smooth muscle and is under involuntary control. The external sphincter, on the other hand, is under voluntary control and must be consciously relaxed to allow urine to flow through the urethra and outside the body. This voluntary control of the external urethral sphincter is essential for maintaining continence and preventing involuntary bladder emptying.
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The pontine micturition centre in the brainstem
The act of emptying the bladder, or micturition, involves a complex neural circuitry that is highly distributed. The Pontine Micturition Centre (PMC), also known as Barrington's nucleus, is a critical component of this circuitry, located in the brainstem.
The PMC is a collection of neuronal cell bodies found in the rostral pons region of the brainstem. It plays a crucial role in the supraspinal regulation of micturition, or urination. When the PMC is activated, it relaxes the urethral sphincter, allowing for micturition to occur. This activation involves stimulating the contraction of the detrusor muscle and the relaxation of the urethral sphincter, which are both involuntary reactions. The detrusor muscle surrounds the bladder and contracts when the bladder is full, initiating the micturition reflex. The internal urethral sphincter then relaxes, allowing urine to pass out of the bladder into the urethra.
The PMC coordinates with several other brain centres, including the medial frontal cortex, insular cortex, hypothalamus, and periaqueductal gray (PAG). The PAG acts as a relay station for bladder information from the spinal cord and signals from higher brain areas. In addition, the PMC receives input from the lumbosacral spinal cord and sends excitatory projections to spinal parasympathetic neurons controlling the detrusor muscle and inhibitory interneurons regulating Onuf's nucleus. These connections ensure the normal reciprocal relationship between bladder contraction and sphincter relaxation.
The discovery of the PMC is attributed to F. J. F. Barrington, whose pioneering work in 1925 involved studying the effects of lesions in the rostral dorsolateral pons on bladder function. Subsequent studies using positron emission tomography (PET) have confirmed the location of the PMC in the brainstem and its activation during voiding. The PMC's role in regulating micturition is well established, and it continues to be a focus of research, particularly in understanding urological issues faced by paraplegic individuals.
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The pudendal nerve provides voluntary control over micturition
Micturition, or urination, is the act of emptying the bladder. It occurs involuntarily in infants and young children until the age of 2 to 5 years, after which it is regulated voluntarily. The neural circuitry that controls this process is complex and highly distributed, involving pathways at multiple levels of the brain, spinal cord, and peripheral nervous system, in addition to being mediated by multiple neurotransmitters.
The process of micturition begins when the bladder is full of urine, and stretch receptors in the bladder wall trigger the micturition reflex. The detrusor muscle, which surrounds the bladder, contracts. This causes the internal urethral sphincter to relax, allowing urine to pass out of the bladder into the urethra. The detrusor muscle is under the control of the autonomic nervous system, specifically the parasympathetic nervous system, which stimulates the bladder to contract and the internal urethral sphincter to relax through the pelvic nerve fibers.
During micturition, the pudendal nerve is inhibited by somatic motor neurons, which cause the external urethral sphincter to relax and allow urine to be expelled from the body. This inhibition of the pudendal nerve is mediated by the pontine micturition centre in the rostral brainstem, which activates descending pathways that cause urethral relaxation and, subsequently, activation of the sacral parasympathetic outflow. This results in contraction of the bladder and an increase in intravesical pressure and the flow of urine.
The complex neural circuitry involved in micturition ensures that the process of urination is carefully regulated and controlled, allowing for the voluntary control of micturition in adults. The pudendal nerve plays a crucial role in this process by providing voluntary control over the external urethral sphincter, which is essential for the conscious regulation of urination.
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Frequently asked questions
Micturition, or urination, is the act of emptying the bladder.
The detrusor muscle surrounds the bladder and contracts to expel urine through the urethra. The internal urethral sphincter is involuntary and relaxes to allow urine to pass out of the bladder. The external urethral sphincter is voluntary and must be relaxed for urine to flow through the urethra and outside the body.
The parasympathetic nervous system stimulates the muscarinic stretch receptors in the bladder through the pelvic nerve fibres. When urine fills the bladder, the M3 receptors located within the bladder become stretched and stimulated, which leads to the contraction of the detrusor muscle for urination.
