8:23
Micturition reflex | Innervation of bladder | Renal physiology
Dr Raghavendra Sherikar
Overview
This video explains the micturition reflex, the process of urination, by detailing the nervous system's control over the bladder. It covers the bladder's innervation by sympathetic, parasympathetic, and somatic nerves, and how these systems interact during bladder filling and voiding. The explanation highlights the roles of the detrusor muscle, internal and external urethral sphincters, and the influence of higher brain centers like the pons and cerebral cortex in voluntary control.
How was this?
Save this permanently with flashcards, quizzes, and AI chat
Chapters
- The bladder is controlled by three sets of nerves: thoracolumbar (sympathetic), sacral (parasympathetic), and sacral (somatic).
- Sympathetic nerves (via hypogastric nerves) relax the detrusor muscle and contract the internal urethral sphincter, aiding bladder filling.
- Parasympathetic nerves (via pelvic nerves) contract the detrusor muscle and relax the internal urethral sphincter, initiating urination.
- Somatic nerves (via pudendal nerve) control the external urethral sphincter, allowing voluntary control over urination.
Understanding bladder innervation is crucial because it explains the complex interplay of nerves that allow the bladder to store urine and then release it controllably.
The sympathetic system causes the detrusor muscle to relax and the internal sphincter to contract, which is like keeping a lid on a pot to allow it to fill without overflowing.
- As the bladder fills, stretch receptors are activated, sending signals via pelvic nerves to the spinal cord.
- Low stretch signals stimulate the sympathetic nervous system, promoting detrusor muscle relaxation and internal sphincter contraction.
- Higher centers in the pons activate the 'storage center,' which further reinforces sympathetic activity and inhibits parasympathetic outflow.
- The somatic nervous system is also stimulated, contracting the external urethral sphincter to prevent leakage.
This phase explains how the bladder safely stores increasing volumes of urine without discomfort or leakage, preparing for eventual voiding.
When your bladder is only slightly full, the 'storage center' in your brain tells your body to keep the detrusor muscle relaxed and the sphincters tight, preventing any accidental leaks.
- When the bladder is sufficiently full (around 300-400 ml), strong stretch signals are sent to the spinal cord.
- These signals activate the parasympathetic nervous system, causing detrusor muscle contraction and internal sphincter relaxation.
- Higher brain centers, specifically the cerebral cortex, decide if the time and place are appropriate for urination.
- If appropriate, the pons stimulates the 'micturition center,' which further enhances parasympathetic outflow and inhibits sympathetic and somatic outflows.
- Inhibition of the somatic outflow leads to the relaxation of the external urethral sphincter, allowing urine to be voided.
This phase details the coordinated events that lead to the controlled release of urine, involving both spinal reflexes and conscious decision-making.
When you consciously decide to urinate, your brain signals the pons to activate the 'micturition center,' which then tells your bladder to squeeze and your external sphincter to relax, letting the urine out.
Key takeaways
- The micturition reflex is a complex process involving both involuntary spinal reflexes and voluntary control from higher brain centers.
- Sympathetic and parasympathetic nervous systems have opposing effects on the detrusor muscle and internal urethral sphincter to manage bladder filling and emptying.
- The somatic nervous system provides voluntary control over the external urethral sphincter, allowing us to consciously decide when to urinate.
- The pons contains distinct 'storage' and 'micturition' centers that modulate bladder activity based on sensory input and conscious decisions.
- The cerebral cortex ultimately determines the appropriateness of urination based on social context and internal signals.
Key terms
Micturition reflexDetrusor muscleInternal urethral sphincterExternal urethral sphincterSympathetic nervous systemParasympathetic nervous systemSomatic nervous systemPelvic nervePudendal nerveCerebral cortexPons
Test your understanding
- How do the sympathetic and parasympathetic nervous systems work together to control bladder function?
- What role does the somatic nervous system play in the micturition reflex?
- Describe the sequence of events that occur during the bladder filling phase.
- Explain how higher brain centers, like the pons and cerebral cortex, influence the decision to urinate.
- What happens to the detrusor muscle and sphincters when the micturition center is activated?