The pelvic floor: Part 1 (control of pelvic floor)

According to modern imaging techniques and surgical techniques the classical concepts concerning the subdivision of the pelvic connective tissue and muscles need to be revised (3). A compartmental clinical model basing on clinical requirements subdivides the pelvic cavity into anterior, posterior, and middle compartments which is feasible (3). Further the pelvic structures can be divided in three groups: the hollow organs, the endopelvic fascia and the muscles (1). Let us review how the pelvic floor is organised

A. The pelvic floor & it’s important parts: The pelvic floor is the support of the pelvic visceras. Important pats of pelvic floor are
1. The levator ani muscle
2. Perineum & it’s myofascial structures
3. Pelvic fascia & it’s reinforcements

The M. levator ani is the muscle of the pelvic diaphragm. Its parts were given different names depending on their function or localization (1). The levator ani muscle (LA) with its two bundles (pubo- and ilio-coccygeus) is the major component of this pelvic floor (2). LA is also called pubiocaudalis & ilio-coccygeus is functunally named as abductor caudae internus. In anatomic studies (based on cadaver studies) the pelvic floor is described as basin-shaped. But dynamic MRI reveals: at rest the levator ani probably has the shape of a dome and differ when contract (1).

Perineum is called urogenital diaphragm by many. Under the LA lies the perineum with it’s includes the musculofascial structures as follows (2):

Ventrally: urethral sphincter, ischio-cavernous and bulbospongious (in males)
Caudally: fatty tissue filling the ischioanal fossa.
As this area is mostly a fascial structure and contains only fair muscular components. Therefore, many authors do not accept the term "diaphragm" and the physiologic function is still a matter of discussion (1).

Pelvic fascia or the endopelvic fascia covers the muscles. The endopelvic fascia has to fix the organs in the pelvis and forms "streets" for vascular and nervous supply (1). It has following 4 major reinforcements:

1. the uterosacral and cardinal ligaments
2. the arcus tendineus fascia pelvis (ATFP) and
3. the arcus tendineus levator ani (ATLA)

B. Development and organization of the pelvic connective tissue in the human fetus (4,5)

In both fetal and adult preparations pelvic connective tissue can be subdivided into a presacral, a perirectal and a paravisceral compartment. Whereas the presacral and the perirectal compartment contain connective tissue, adipose tissue and supplying structures, the paravisceral compartment is mainly composed of adipose tissue. While only a very thin rectal fascia was found at the border of the perirectal compartment. Moreover it is shown that the ligaments of the pelvic cavity are only composed of the sacrouterine ligaments and the pubovesical ligaments in the female and the puboprostatic ligaments in the male (5). Lte us discuss the IU development of pelvic connective tissue in fetus.

Three periods are important for the differentiation of the pelvic connective tissue in fetuses (4).

a. During a first or mesenchymal period (9-12-week-old fetuses), all pelvic regions identical with the so-called pelvic spaces in the adult are filled with loose undifferentiated mesenchyme. Some pelvic organs are covered by a layer of condensed mesenchyme which later constitutes the connective tissue sheath of these organs.

b. During a second or fibrous period (13-20-week-old fetuses), dense connective tissue predominates. It is arranged in circular and semicircular systems covering the rectum, the bladder and the urethra as well as the peritoneal pouches. The arrangement of dense connective tissue is the same in the male and in the female fetus. No ligaments were found within the pelvic cavity apart from the pubovesical and the puboprostatic ligaments. The connective tissue sheaths of the pelvic organs differ from one another. At the level of the pelvic floor only some of them are directly connected with the parietal pelvic fascia. The pelvic spaces are filled by loose connective tissue.

c. During a third or adipose period (21-38-week-old fetuses) adipose tissue develops within the different compartments of the pelvic cavity so that the clear organization found during the second period is abolished.
C. Nerve supply of adult pelvic floor:

The innervation of the pelvic floor comes from the 2nd, 3rd, 4th anterior sacral roots; denervation affects pelvic dynamism (2). One study also demonstrates a clear segregation of the segmental motor neuron pools innervating the different pelvic floor muscles (6). Pudendal nerve stimulation elicited only polysynaptic reflex responses from S2 ventral roots.

D. Spinal control of pelvic floor muscles (6):
A prevalent notion in the literature is that the pelvic floor muscles behave as a unitary mass (6).
Various muscles of the pelvic floor region could be reflexly activated either individually or as a mass unit depending on the intensity of stimulation. Tactile or electrical stimulation of pudendal regions on either side of the body elicited responses of the sphincter ani externus. Neural apparatus of the striated musculature of the pelvic floor is capable of activating individually the different muscles that make up the system. Further the sphincter ani externus from one side, and muscles of the diaphragm pelvis from the other, are subserved by different neuronal circuits (6).

E. Configuration of static & dynamic pelvic floor (1,2):
Especially the anatomic form of the levator plate does not conform with physiologic concepts (1).
Configuration of static pelvic floor: The pelvis statics is supported by the combined action of all this anatomical structures anteriorly forming the perineal "hammock", medially the uterosacral and cardinal ligaments, posteriorly the rectovaginal fascia and the perineal body. The angles formed by the pelvic visceras with their evacuation ducts participate to the pelvic statics (2).
Configuration of dynamic pelvic floor: During the pelvic dynamics the modification of these angles expresses the action of the musculofascial structures (2).

F. Clinical anatomy i. Muscle injury in pelvic floor

LA is formed essentially by type I fibers (with high oxidative capability and presence of slow myosin) as in postural muscles. The aerobic metabolism makes LA susceptible to injury caused by excentric contraction and mitochondrial dysfunction (2).

ii. Nerve injuries in pelvic floor
Injury to one pudendal nerve did not alter the level of tonic activity of the sphincter ani externus. But bilateral injury of the pudendal nerve entirely abolishes both tonic activity and phasic responsiveness of the SAE without affecting the activity of the levator ani and abductor caudae internus muscles.

Reference:
1. Schmeiser G et al; Radiologe. 2000 May;40(5):429-36.
2. Yiou R et al; Prog Urol. 2009 Dec;19(13):916-25. Epub 2009 Nov 6.
3. Fritsch H et al; Adv Anat Embryol Cell Biol. 2004;175:III-IX, 1-64.
4. Fritsch H; Ann Anat. 1993 Dec;175(6):531-9.
5. Fritsch H; Surg Radiol Anat. 1994;16(3):259-65.
6. Dubrovsky B et al; Exp Neurol. 1985 May;88(2):277-87.