Micturition Reflex

What is micturition?

The process of urinating (voiding the bladder).

What type of muscle is the detrusor muscle, and what is its function?

It is smooth muscle that contracts to expel urine from the bladder.

Which nervous system controls the detrusor muscle?

The parasympathetic nervous system (S2–S4).

What is the role of the internal urethral sphincter?

Involuntary control; contracts to retain urine.

What type of muscle makes up the internal urethral sphincter?

smooth muscle

what is the role of the external urethral sphincter

voluntary control; relaxes during urination

What type of muscle makes up the external urethral sphincter?

skeletal muscle

Which spinal segments are involved in parasympathetic control of micturition?

Sacral spinal cord segments S2–S4.

Which spinal segments are involved in sympathetic control of bladder storage?

Thoracolumbar spinal cord segments T11–L2.

What does the sympathetic nervous system do during bladder filling?

Inhibits detrusor contraction and keeps the internal sphincter contracted.

What does the parasympathetic nervous system do during voiding?

Stimulates detrusor contraction and relaxes the internal sphincter.

What part of the brain coordinates the micturition reflex?

The pons, specifically the Pontine Micturition Center (PMC).

What role does the cerebral cortex play in micturition?

It provides conscious awareness and voluntary control over urination.

What happens when the bladder is empty (10–20 mL)?

Low afferent signaling, detrusor relaxed, sphincters remain closed.

What allows the bladder to fill without contracting?

Sympathetic activity keeps the detrusor relaxed and sphincters contracted.

What triggers the micturition reflex when the bladder is full?

Stretch receptors signal the spinal cord and brain → activates parasympathetic output.

What are the normal components found in urine?

Water, electrolytes (e.g., sodium, potassium), urea, uric acid, toxins, drugs, protons, bicarbonate.

What does the presence of glucose in urine indicate?

Glycosuria, which suggests diabetes mellitus (Type 1 or Type 2).

What does proteinuria suggest?

It indicates high levels of protein in the urine, which may result from renal disease, glomerulonephritis, high blood pressure, or heart failure.

What does ketonuria indicate?

Ketonuria suggests uncontrolled diabetes, starvation, or a low-carb diet (e.g., ketogenic diet)

What condition is suggested by bilirubinuria?

Bilirubinuria is a sign of liver disease such as cirrhosis or hepatitis, or bile duct obstructions like gallstones.

• What can hemoglobinuria indicate?

Hemoglobinuria may be caused by hemolytic anemia, burns, or mismatched blood transfusions.

What does hematuria signify?

Hematuria (red blood cells in urine) may be caused by trauma, kidney stones, infections, tumors, or polycystic kidney disease.

What is pyuria and what does it suggest?

Pyuria is the presence of white blood cells in urine, indicating a urinary tract infection (UTI).

What is the first step in the micturition reflex when the bladder is empty?

Afferent fibers send signals to the spinal cord (S2-S4), activating the sympathetic nervous system to relax the bladder muscles and constrict the internal urethral sphincter.

What happens when the bladder fills to about 200 ml?

Sensory afferent fibers from the bladder signal the spinal cord, activating the pontine micturition center in the brain to initiate urination.

What occurs during the micturition reflex when the bladder is full?

• Sympathetic inhibition, bladder relaxation, internal urethral sphincter opening, parasympathetic stimulation of bladder contraction, and somatic inhibition of the external urethral sphincter.

What role does the sympathetic nervous system play in the micturition reflex?

The sympathetic nervous system keeps the bladder relaxed and constricts the internal urethral sphincter during storage and until urination is initiated.