Bladder/urethral smooth muscle contraction is adjunctive to external striated muscle forces
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LETTER TO THE EDITOR
Bladder/urethral smooth muscle contraction is adjunctive to external striated muscle forces Peter Petros 1 Received: 6 February 2020 / Accepted: 26 February 2020 # The International Urogynecological Association 2020
Dear Editors, Re 1. Misry MA, Klarskov N, DeLancey JO, Lose G. A structured review on the female urethral anatomy and innervation with an emphasis on the role of the urethral longitudinal smooth muscle https://doi.org/10.1007/s00192-019-04104-7 International Urogynecology Journal (2020) 31:63 71. Video x-ray, EMG [1]: Ultrasound observations indicate there is much more to urethral closure/opening than longitudinal smooth muscle contraction as hypothesized [2]. Smooth muscle’s contribution to closure and micturition is adjunctive to the external striated muscle forces which close and open the urethra (Fig. 1). Smooth muscle fibers contract against their attachments: lower urethra to vagina, pubourethral ligament, pubovesical ligament, and vesicovaginal ligament inferiorly. These insertion points vary in position according to whether the striated pelvic muscles stretch the organs or not (Fig. 1).
Opening The trigone extends from the bladder base to the external meatus [3]. During micturition it is stretched backwards by striated muscle forces (LP/LMA; Fig. 1). This renders the longitudinal smooth muscle of the bladder and urethra semi-rigid, so that it can be actively opened out like a trapdoor, called “funneling” (Fig. 1), which drops the
* Peter Petros [email protected]; https://www.integraltheory.org 1
University of Western Australia School of Mechanical and Chemical Engineering, Perth, WA, Australia
urethral resistance exponentially and inversely by the fourth power of the radius (Poiseuille’s law). The pubovesical ligament inserts into the arc of Gilvernet (Fig. 1). It prevents the anterior bladder wall from collapsing inwards during opening by the LP/LMA.
Closure The trigone contracts and becomes semi-rigid. This allows the cresta urethralis to be pulled upwards by forward contraction of the pubococcygeus muscle, closing off the distal urethra against the rhabdosphincter, which also contracts to seal the urethra. Rotation of the bladder base around the arc of Gilvernet closes the urethra at the bladder neck, like kinking in a garden hose. The arc is held rigid by contraction of the pubovesical ligament. Rhabdosphincter (RS) contraction during effort is what pressure transducers measure. Huisman’s view [3] was RS is far too weak to close the urethra, especially as it atrophies considerably with age. A musculoelastic mechanism is a far more powerful closure mechanism, exponentially increasing resistance to flow (Poiseuille’s law). Halving the radius increases resistance to flow 16 times (2 × 2 × 2 × 2). Conversely, opening out the urethra (funneling) to twice its radius decreases resistance to detrusor emptying by a factor of 16.
Int Urogynecol J
Fig. 1 “Stretchability” of organ smooth muscle* is critical for transmission of striated muscle forces (arrows). These stretch the
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