Micturition Reflex

Two muscular sphincters surround the urethra. The upper sphincter, composed of smooth muscle, is called the internal urethral sphincter; the lower sphincter, composed of voluntary skeletal muscle, is called the external urethral sphincter. The actions of these sphincters are regulated in the process of urination, which is also known as micturition.

Renal pelvis

Renal pelvis

Micturition Reflex

Ureter

Urinary bladder

Ureter

Urinary bladder

■ Figure 17.3 A pseudocolor radiograph of the urinary system. In this photograph, shades of gray are assigned colors. The calyces of the kidneys, the renal pelvises, the ureters, and the urinary bladder are visible.

Micturition is controlled by a reflex center located in the second, third, and fourth sacral levels of the spinal cord. Filling of the urinary bladder activates stretch receptors that send impulses to this micturition center. As a result, parasympathetic neurons are activated, causing rhythmic contractions of the detrusor muscle of the urinary bladder and relaxation of the internal urethral sphincter. At this point, a sense of urgency is perceived by the brain, but there is still voluntary control over the external urethral sphincter. When urination is consciously allowed to occur, descending motor tracts to the micturition center inhibit somatic motor fibers to the external urethral sphincter. This muscle then relaxes, and urine is expelled. The ability to voluntarily inhibit micturition generally develops between the ages of 2 and 3.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Responses

  • NADINE
    What is physiology of micturation in human?
    7 years ago
  • roma
    Where is the micturition reflex center located?
    7 years ago
  • j
    Where in the brain is the micturation reflex center?
    6 years ago
  • Maire
    Where are the atrecth receptors of the micturition reflex located?
    6 years ago
  • Fiyori
    How will Herbert’s micturition be controlled from this point on?
    5 years ago

Post a comment