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• Definition
• Micturition refers to the process of emptying urine from the urinary bladder through the urethra.
• Physiological nature
• It is a reflex process controlled by the spinal cord, but also influenced by higher brain centers.
• Functions of urinary bladder
• Temporary storage of urine
• Periodic elimination of urine.
• Control mechanisms involved
• Nervous control
• Muscular contraction of bladder wall
• Relaxation of urethral sphincters.
The act of urination may feel simple, but physiologically it is a finely tuned collaboration between smooth muscle, spinal reflexes, and conscious brain control.
• Nature of organ
• The urinary bladder is a hollow muscular organ located in the pelvic cavity.
• Main parts
• Apex
• Body
• Fundus (base)
• Neck.
• Wall structure
• Mucous membrane
• Muscular layer (detrusor muscle)
• Outer connective tissue layer.
• Important region
• Trigone – triangular area between the openings of ureters and urethra.
• Function of detrusor muscle
• Responsible for contraction of bladder during micturition.
The bladder wall is highly elastic. It can stretch remarkably, storing 400–600 mL of urine without a major rise in pressure.
• Continuous urine flow
• Urine produced by kidneys enters the bladder through ureters.
• Detrusor muscle during filling
• Remains relaxed allowing bladder expansion.
• Internal sphincter
• Remains contracted preventing urine leakage.
• Nervous control
• Sympathetic nerves help maintain bladder relaxation during storage phase.
• Bladder capacity
• Normally stores about 400–500 mL of urine.
• Sensation of fullness
• Stretch receptors in bladder wall send signals to spinal cord and brain.
The bladder behaves like a biological reservoir with elastic walls—it fills quietly for long periods before the nervous system decides it is time to empty.
• Trigger
• When bladder fills to about 300–400 mL, stretch receptors are activated.
• Reflex pathway
• Signals travel through pelvic nerves to the spinal cord.
• Parasympathetic activation
• Causes contraction of detrusor muscle.
• Sphincter relaxation
• Internal urethral sphincter relaxes.
• Voluntary control
• External urethral sphincter controlled by somatic nerves (pudendal nerve).
• Urine expulsion
• Coordinated contraction of bladder and relaxation of sphincters results in urination.
In physiological terms, micturition is a spinal reflex that learned to cooperate with consciousness. Infants rely entirely on reflex control, but the developing brain gradually gains the ability to delay or initiate the process.
• Definition
• Micturition reflex is a spinal reflex responsible for emptying the urinary bladder.
• Stimulus
• Stretching of the bladder wall due to accumulation of urine.
• Receptors involved
• Stretch receptors located in the bladder wall.
• Afferent pathway
• Impulses travel through pelvic nerves to the sacral spinal cord (S2–S4).
• Efferent pathway
• Parasympathetic fibers return through pelvic nerves.
• Response
• Contraction of detrusor muscle
• Relaxation of internal urethral sphincter.
• Nature of reflex
• It is automatic but can be voluntarily controlled by higher brain centers.
The reflex behaves like a rhythmic cycle—stretch → reflex contraction → partial emptying—until voluntary control either permits or suppresses the act of urination.
• Micturition is regulated not only by spinal reflexes but also by higher nervous centers.
• These centers help in coordination, voluntary control and inhibition of urination.
• Main centres involved
• Cortical centre
• Hypothalamic centre
• Brain stem centre.
These higher centres transform the primitive spinal reflex into a behavior under conscious control, allowing humans to decide the socially appropriate moment to void.
• Location
• Located in the cerebral cortex, particularly in the frontal lobe.
• Function
• Provides voluntary control over micturition.
• Mechanism
• Can either facilitate or inhibit the spinal micturition reflex.
• Physiological importance
• Enables conscious control of urination.
• Clinical relevance
• Damage to cortical areas may lead to loss of voluntary bladder control.
The cortex acts like the executive decision-maker, deciding whether the reflex should proceed or be temporarily suppressed.
• Location
• Situated in the hypothalamus of the brain.
• Function
• Integrates autonomic regulation of bladder activity.
• Role
• Coordinates bladder function with emotional and autonomic responses.
• Physiological significance
• Helps maintain appropriate bladder control during various physiological states.
The hypothalamus acts like a physiological coordinator, linking bladder activity with the body’s internal state—stress, hydration, and autonomic balance.
• Location
• Located in the pons of the brain stem.
• Name
• Known as the pontine micturition centre (PMC).
• Function
• Coordinates contraction of bladder and relaxation of urethral sphincters.
• Role in urination
• Ensures smooth and coordinated voiding of urine.
• Physiological importance
• Prevents simultaneous contraction of bladder and sphincters.
This small cluster of neurons in the pons functions like a biological conductor, synchronizing bladder contraction and sphincter relaxation so that urine flows efficiently rather than chaotically.
• Location
• Spinal centres for micturition are located in the sacral segments of the spinal cord (S2–S4).
• Function
• These centres control the basic micturition reflex.
• Afferent impulses
• Stretch receptors in bladder wall send signals to the sacral spinal cord via pelvic nerves.
• Efferent impulses
• Parasympathetic fibers return from spinal cord to bladder.
• Response produced
• Contraction of detrusor muscle
• Relaxation of internal urethral sphincter.
• Physiological importance
• Responsible for automatic bladder emptying in infants and spinal animals.
Before the brain’s higher centers develop control, the spinal cord alone manages urination through this primitive reflex circuit.
• Autonomic nerves involved
• Parasympathetic nerves (pelvic nerves)
• Sympathetic nerves.
• Effect of damage
• Loss of normal autonomic regulation of bladder function.
• Physiological consequence
• Bladder may become distended due to inability to empty properly.
• Urinary problems produced
• Urinary retention
• Overflow incontinence.
• Clinical significance
• Seen in spinal cord injury or neuropathies affecting pelvic nerves.
Damage to these nerves disrupts the delicate balance between storage and emptying phases of the bladder.
• Efferent nerves involved
• Parasympathetic fibers supplying the detrusor muscle.
• Effect of lesion
• Detrusor muscle cannot contract effectively.
• Result
• Bladder becomes large and flaccid.
• Urinary consequence
• Urine accumulates leading to retention of urine.
• Secondary effect
• Gradual overflow incontinence may occur.
Physiologically this produces a flaccid or atonic bladder—a reservoir that keeps filling but cannot generate the muscular force needed to empty itself.
• Origin
• Parasympathetic fibres arise from sacral segments of spinal cord (S2–S4).
• Pathway
• Travel through pelvic nerves to the urinary bladder.
• Function
• Stimulate contraction of the detrusor muscle.
• Effect on sphincters
• Promote relaxation of the internal urethral sphincter.
• Role in micturition
• Responsible for emptying of the urinary bladder.
These nerves are essentially the “voiding pathway” of the bladder—when activated, they convert the bladder from a storage organ into an expelling pump.
• Initial stage (spinal shock)
• Immediately after spinal cord transection, micturition reflex is abolished.
• Bladder condition
• Bladder becomes distended and urine accumulates.
• Urinary outcome
• Urinary retention occurs during early phase.
• Later stage
• Spinal reflex gradually returns.
• Result
• Bladder empties automatically without voluntary control.
• Condition produced
• Known as automatic bladder.
This phenomenon illustrates a curious principle in neurophysiology: when higher control is lost, primitive spinal reflexes eventually re-emerge and operate independently.
• Definition
• Artificial kidney refers to a dialysis machine used to remove waste products from blood when kidneys fail.
• Principle
• Based on diffusion and ultrafiltration through a semipermeable membrane.
• Process
• Blood flows through a dialyzer containing dialysis fluid.
• Substances removed
• Urea
• Creatinine
• Excess electrolytes
• Excess water.
• Clinical use
• Used in acute renal failure and chronic kidney disease.
• Purpose
• Maintains chemical balance of blood until kidney function recovers or transplantation is performed.
Dialysis machines are a fascinating example of physiology translated into engineering. They replicate one of the kidney’s core tricks—separating useful molecules from wastes using selective membranes and concentration gradients.
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