Pelvic floor and perineal muscles: a dynamic coordination between skeletal and smooth muscles on pelvic floor stabilization

Abstract

The purpose of this review is to present our researches on the pelvic outlet muscles, including the pelvic floor and perineal muscles, which are responsible for urinary function, defecation, sexual function, and core stability, and to discuss the insights into the mechanism of pelvic floor stabilization based on the findings. Our studies are conducted using a combination of macroscopic examination, immunohistological analysis, 3D reconstruction, and imaging. Unlike most previous reports, this article describes not only on skeletal muscle but also on smooth muscle structures in the pelvic floor and perineum to encourage new understanding. The skeletal muscles of the pelvic outlet are continuous, which means that they share muscle bundles. They form three muscle slings that pass anterior and posterior to the anal canal, thus serving as the foundation of pelvic floor support. The smooth muscle of the pelvic outlet, in addition to forming the walls of the viscera, also extends in three dimensions. This continuous smooth muscle occupies the central region of the pelvic floor and perineum, thus revising the conventional understanding of the perineal body. At the interface between the levator ani and pelvic viscera, smooth muscle forms characteristic structures that transfer the lifting power of the levator ani to the pelvic viscera. The findings suggest new concepts of pelvic floor stabilization mechanisms, such as dynamic coordination between skeletal and smooth muscles. These two types of muscles possibly coordinate the direction and force of muscle contraction with each other.

Twisted orientation of the muscle bundles in the levator ani functional parts in women: Implications for pelvic floor support mechanism

“…A part of the LA is known to attach to the lateral aspect of the vagina, and this structure is believed to influence the proximal position of the urethra (DeLancey & Starr, 1990 ). This connection between the LA and the vagina is not a result of LA muscle fibres inserting into the vaginal wall, but rather smooth muscle fibres extending from the vaginal wall interlocking with the medial surface of the LA skeletal muscle bundles to form a solid interface: skeletal muscle sandwiched by smooth muscle and smooth muscle inserting into skeletal muscle (DeLancey & Starr, 1990 ; Kato et al., 2020 ; Muro & Akita, 2023b ). Therefore, the LA‐a may also contribute to the support and elevation of the vagina and stabilisation of the proximal urethral position.…”

“…In particular, pelvic organ prolapse is strongly associated with injury to the levator ani (LA) (DeLancey et al., 2003 ; Lien et al., 2004 ; Miller et al., 2010 , 2015 ). The LA is the largest muscle that provides support to the pelvic organs and, in tandem with the coccygeus, constitutes the pelvic diaphragm (pelvic floor muscle) (Muro & Akita, 2023b ). LA function is indispensable for pelvic floor support and is closely related to pathologies such as urinary incontinence and pelvic organ prolapse in women; therefore, its anatomy has received much attention.…”

Anatomy of muscle connections in the male urethra and anorectal canal

“…We have previously reported that pelvic floor muscles, including perineal muscles and the external anal sphincter, share muscle bundles and are continuous [7–10]. Thus, we speculated that the external urethral sphincter may also be continuous with surrounding skeletal muscles.…”