Micturition, How does the micturition reflex occur – The course is designed for the basic understanding of anatomical structures and physiological functions of human body, musculoskeletal system, digestive system, respiratory system; cardiovascular system; urinary system, endocrine system, reproductive system, nervous system, hematologic system, sensory organs, integumentary system, and immune system.The aim of the course is to acquire knowledge and skills regarding anatomy and physiology.
Micturition, How does the micturition reflex occur
The urinary bladder stores urine prior to its elimination and then expels urine into the urethra by an act called micturition, commonly known as urination. Micturition requires a combination of involuntary and voluntary muscle contractions. When the volume of urine in the urinary bladder exceeds 200 to 400 mL, pressure within the bladder increases considerably, and stretch receptors in its wall transmit nerve impulses into the spinal cord. These impulses propagate to the lower part of the spinal cord and trigger a reflex called the micturition reflex.
In this reflex, parasympathetic impulses from the spinal cord cause contraction of the detrusor muscle and relaxation of the internal urethral sphincter muscle. Simultaneously, the spinal cord inhibits somatic motor neurons, causing relaxation of skeletal muscle in the external urethral sphincter. Upon contraction of the urinary bladder wall and relaxation of the sphincters, urination takes place.
Define GFR and Renal threshold
Glomerular filtration rate (GFR):
The amount of glumerular filtrate formed in each minute by all the nephrons of both kidneys is called GFR.
Or,
The volume of plasma which is filtrated by all the nephrons of both kidney per minute is called GFR.
Normal value: About: 125 ml/min.
(Ref-Guyton & Hall, 12th edition, P-314.)
Renal threshold :-
The kidneys maximum capacity for Reabsorption of a substance is known as the renal threshold.
For example, renal threshold for glucose is 180 mg/dl. If plasma concentration of glucose rises above this, glucose appear in urine.
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