Micturition (Urinary) Reflex

Original Editor - Laura Ritchie, posting on behalf of Julia Wiebe, MPT Class of 2022 at Western University, project for PT9581.

Top Contributors - Laura Ritchie

Introduction[edit | edit source]

Micturition, also known as urination, is the process of expelling urine from the bladder. The purpose of urination is to eliminate metabolic products and toxic wastes from the body that have been filtered from the blood by the kidneys. it is a complex mechanism that involves coordination by the sympathetic, parasympathetic and somatic nervous systems,[1] [2]and it is the higher centers within the brain that allow for urination to occur at the right time. Normal muscle tone, absence of physical obstructions and psychological inhibition are other important factors that play a role in this process.[3]

Relevant Anatomy & Physiology[1][edit | edit source]

The urinary tract comprises of two mutually dependent components: the upper tract, which contains the kidneys and ureters, and the lower tract consisting of the bladder and urethra. The lower tract is involved in the micturition reflex.

The bladder is a hollow organ that functions as reservoir for the storage and periodic elimination of urine. The bladder’s walls are made up of three layers of smooth muscle, known as the detrusor.

At the bottom of the bladder is an outlet into the urethra known as the bladder neck, which is surrounded by a ring of smooth muscle known as the internal urethral sphincter and is involuntarily controlled. The urethra the passageway in which urine is excreted out of the body. The pelvic floor musculature encircles the urethra creating the external urethral sphincter, which operates under conscious control. During storage, both the internal and external sphincters are contracted to prevent leakage.

Normally, the detrusor muscle is relaxed, which allows the bladder to expand as it fills with urine. When the bladder is full, the detrusor contracts to expel urine out of the body through the urethra. A normal healthy adult bladder can hold anywhere from 300-500mL of urine for 2-5 hours.

Sex Differences[edit | edit source]

Differences do exist between the male and female anatomy of the urinary tract; however, these differences mostly concern the urethra. More specifically, the male urethra passes through the prostate gland at the base of the bladder. The urethra then passes through the penis, making it much longer than the female urethra. The female urethra exits just anterior to the vaginal opening.

Mechanism[edit | edit source]

Micturition is a complex and highly distributed process, involving pathways at multiple levels of the brain, spinal cord and PNS, in addition to being mediated by multiple neurotransmitters. At the most basic level, the micturition reflex is triggered when the bladder fills with urine. The detrusor muscle remains relaxed until the bladder fills to about 250 mL with urine.[3] As the bladder fills, the detrusor muscle contracts, which increases the pressure within the bladder, stimulating stretch receptors within the detrusor and exciting parasympathetic fibres to relay this information to the sacral nerves of the spine. These fibres help convey the bladder’s degree of fullness. Within the spine, this information is integrated and the relayed to the brain via two different sets of neurons. The brain determines the urgency and responds by sending command signals back to the bladder to either hold on or void. If the response is to void, parasympathetic motor neurons cause contraction of the detrusor muscle, which causes the intra-bladder pressure to increase. As a result of the increased pressure, the internal sphincter opens by relaxing, thus allowing urine to enter the bladder neck and flow into the urethra. Simultaneously, somatic motor neurons inhibit the pudendal nerve which causes the external sphincter to also relax, allowing urine to be expelled from the body. Typically, around 50mL remains in the bladder after a voluntary void in a normal healthy adult.[4]

References[edit | edit source]

  1. 1.0 1.1 Andersson KE, Michel MC. Urinary Tract. 1st Ed. Springer Berlin Heidelberg; 2011.
  2. Lorenz MD, Coates JR, Kent M. Chapter 3: Disorders of Micturition. In: Lorenz MD, Coates JR, Kent M, ed. Handbook of Veterinary Neurology. 5th ed. Elsevier; 2010:58-74.
  3. 3.0 3.1 Richardson, M. The physiology of micturition. Nurs Times. 2003;99(29):46-48
  4. Ballstaedt L, Woodbury B. Bladder Post Void Residual Volume. [Updated 2021 May 10]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan. Available from: https://www.ncbi.nlm.nih.gov/books/NBK539839/