In the male, pseudostratified columnar epithelium lines the urethra between these two cell types. Voiding is regulated by an involuntary autonomic nervous system-controlled internal urinary sphincter , consisting of smooth muscle and voluntary skeletal muscle that forms the external urinary sphincter below it.
The external urethral orifice is embedded in the anterior vaginal wall inferior to the clitoris, superior to the vaginal opening introitus , and medial to the labia minora. Its short length, about 4 cm, is less of a barrier to fecal bacteria than the longer male urethra and the best explanation for the greater incidence of UTI in women. Voluntary control of the external urethral sphincter is a function of the pudendal nerve.
It arises in the sacral region of the spinal cord, traveling via the S2—S4 nerves of the sacral plexus. The male urethra passes through the prostate gland immediately inferior to the bladder before passing below the pubic symphysis see Figure 1b. The length of the male urethra varies between men but averages 20 cm in length. It is divided into four regions: the preprostatic urethra, the prostatic urethra, the membranous urethra, and the spongy or penile urethra.
The preprostatic urethra is very short and incorporated into the bladder wall. The prostatic urethra passes through the prostate gland. During sexual intercourse, it receives sperm via the ejaculatory ducts and secretions from the seminal vesicles. The mucus neutralizes the usually acidic environment and lubricates the urethra, decreasing the resistance to ejaculation.
The membranous urethra passes through the deep muscles of the perineum, where it is invested by the overlying urethral sphincters. The spongy urethra exits at the tip external urethral orifice of the penis after passing through the corpus spongiosum. Mucous glands are found along much of the length of the urethra and protect the urethra from extremes of urine pH.
Innervation is the same in both males and females. The urinary bladder collects urine from both ureters. The bladder lies anterior to the uterus in females, posterior to the pubic bone and anterior to the rectum. During late pregnancy, its capacity is reduced due to compression by the enlarging uterus, resulting in increased frequency of urination.
In males, the anatomy is similar, minus the uterus, and with the addition of the prostate inferior to the bladder. Figure 2. The bladder is a highly distensible organ comprised of irregular crisscrossing bands of smooth muscle collectively called the detrusor muscle. The interior surface is made of transitional cellular epithelium that is structurally suited for the large volume fluctuations of the bladder. Volumes in adults can range from nearly zero to — mL. The detrusor muscle contracts with significant force in the young.
Such voluntary contraction is also used in forceful defecation and childbirth. Micturition is a less-often used, but proper term for urination or voiding. It results from an interplay of involuntary and voluntary actions by the internal and external urethral sphincters. When bladder volume reaches about mL, an urge to void is sensed but is easily overridden. Voluntary control of urination relies on consciously preventing relaxation of the external urethral sphincter to maintain urinary continence.
As the bladder fills, subsequent urges become harder to ignore. To obtain the best experience, we recommend you use a more up to date browser or turn off compatibility mode in Internet Explorer.
In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript. Voluntary urination ensures that waste is eliminated when safe and socially appropriate, even without a pressing urge. Uncontrolled urination, or incontinence, is a common problem with few treatment options. Here we identify a small subset of Bar neurons that control the urethral sphincter in mice. These excitatory neurons express estrogen receptor 1 Bar ESR1 , project to sphincter-relaxing interneurons in the spinal cord and are active during natural urination.
Optogenetic stimulation of Bar ESR1 neurons rapidly initiates sphincter bursting and efficient voiding in anesthetized and behaving animals. Conversely, optogenetic and chemogenetic inhibition reveals their necessity in motivated urination behavior. The identification of these cells provides an expanded model for the control of urination and its dysfunction. Neural control of the lower urinary tract. Terjung, R. Valentino, R. The bladder-brain connection: putative role of corticotropin-releasing factor.
Holstege, G. The emotional motor system and micturition control. Article Google Scholar. Minassian, V. Urinary incontinence as a worldwide problem. To pee or not to pee: urine signals mediate aggressive interactions in the cooperatively breeding cichlid Neolamprologus pulcher. Reynolds, E. Urination as a social response in mice. Nature , — Hurst, J. Scent wars: the chemobiology of competitive signalling in mice. Bioessays 26 , — Heymann, E. Scent marking strategies of New World primates.
Gosling, L. Life history costs of olfactory status signalling in mice. Desjardins, C. Social rank in house mice: differentiation revealed by ultraviolet visualization of urinary marking patterns. Science , — Roberts, S. Scent-marking by male mice under the risk of predation. Barrington, F. The effect of lesions of the hind- and mid-brain on micturition in the cat.
Verstegen, A. Sasaki, M. Hou, X. Central control circuit for context-dependent micturition. Cell , 73— Sutin, E. Immunocytochemical localization of peptides and other neurochemicals in the rat laterodorsal tegmental nucleus and adjacent area.
Vanderhorst, V. Vincent, S. Corticotropin-releasing factor CRF immunoreactivity in the dorsolateral pontine tegmentum: further studies on the micturition reflex system.
Brain Res. McFadden, K. Rouzade-Dominguez, M. Tanagho, E. Initiation of voiding. Yalla, S. Initiation of voiding in humans: the nature and temporal relationship of urethral sphincter responses. Blok, B. Electrical stimulation of the sacral dorsal gray commissure evokes relaxation of the external urethral sphincter in the cat. Grill, W. Bladder and urethral pressures evoked by microstimulation of the sacral spinal cord in cats. Shefchyk, S. Sacral spinal interneurones and the control of urinary bladder and urethral striated sphincter muscle function.
Kruse, M. Pontine control of the urinary bladder and external urethral sphincter in the rat. Mallory, B. Pharmacological modulation of the pontine micturition center. Neuroscience , — Chang, H. Serotonergic drugs and spinal cord transections indicate that different spinal circuits are involved in external urethral sphincter activity in rats. Renal Physiol. Kaur, A. Murine pheromone proteins constitute a context-dependent combinatorial code governing multiple social behaviors.
Cell , — Differential effects of urethane and isoflurane on external urethral sphincter electromyography and cystometry in rats. Smith, P. Evidence of central modulation of bladder compliance during filling phase. Pudendal nerve stimulation induces urethral contraction and relaxation. PubMed Google Scholar. Cruz, Y. Sexually dimorphic micturition in rats: relationship of perineal muscle activity to voiding pattern.
LaPallo, B. At about ml of urine, the detrusor muscle begins to contract and the internal urethral sphincter muscle begins to relax.
If this urge is ignored, continence may be threatened. At about ml, detrusor muscle contractions begin to force open the internal urethral sphincter. Unless the external urethral sphincter is powerful enough to prevent it, micturition urination will occur involuntarily. Smooth muscle stretch initiates the micturition reflex by activating stretch receptors in the bladder wall. This autonomic reflex causes the detrusor muscle to contract and the internal urethral sphincter muscle to relax, allowing urine to flow into the urethra.
The stretch receptors also send a message to the thalamus and the cerebral cortex, giving voluntary control over the external urethral sphincter. We usually gain this control of urination between the ages of 2 and 3, as our brains develop. Download Urinary System Lab Manual. See more from our free eBook library. A description of the urinary bladder from the edition of Gray's Anatomy of the Human Body.
Using bladder ultrasound to detect urinary retention in patients from Nursing Times. Urinary System Structures. Give It Up for the Kidneys. The Three Steps of Urine Formation. Common Diseases and Disorders.
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