The pelvic viscera (bladder, rectum, pelvic genital organs and terminal part of the urethra) reside within the pelvic cavity (or the true pelvis). This cavity is located within the lesser part of the pelvis, beneath the pelvic brim.
A number of muscles help make up the walls of the cavity – the lateral walls include the obturator internus and the pirformis muscle, with the latter also forming the posterior wall
In this article, we shall look at the anatomy of the muscles that make up the inferior lining of the cavity; the pelvic floor muscles. The pelvic floor is also known as the pelvic diaphragm.
We shall look at the individual roles of these muscles, their innervation and blood supply, and any clinical correlations.
Note – some texts consider the pelvic floor to include the perineal membrane and deep perineal pouch. We have considered these as a distinct and separate structures.
Pelvic Floor Structure
The pelvic floor is a funnel-shaped structure. It attaches to the walls of the lesser pelvis, separating the pelvic cavity from the perineum inferiorly (region which includes the genitalia and anus).
In order to allow for urination and defecation, there are a few gaps in the pelvic floor. There are two ‘holes’ that have significance:
- Urogenital hiatus – an anteriorly situated gap, which allows passage of the urethra (and the vagina in females).
- Rectal hiatus – a centrally positioned gap, which allows passage of the anal canal.
Between the urogenital hiatus and the anal canal lies a fibrous node known as the perineal body, which joins the pelvic floor to the perineum (described further here).
As the floor of the pelvic cavity, these muscles have important roles to play in the correct functioning of the pelvic and abdominal viscera.
The roles of the pelvic floor muscles are:
- Support of abdominopelvic viscera (bladder, intestines, uterus etc.) through their tonic contraction.
- Resistance to increases in intra-pelvic/abdominal pressure during activities such as coughing or lifting heavy objects.
- Urinary and faecal continence.The muscle fibres have a sphincter action on the rectum and urethra. They relax to allow urination and defecation.
When learning about the muscles of the pelvic floor, it is important to keep in mind its funnel-shaped structure. There are three main components of the pelvic floor:
- Levator ani muscles (largest component).
- Coccygeus muscle.
- Fascia coverings of the muscles.
We shall now consider each of these components in more detail.
Levator Ani Muscles
Innervated by branches of the pudendal nerve (roots S2, S3 and S4).
The levator ani is a broad sheet of muscle. It is composed of three separate paired muscles; pubococcygeus, puborectalis and iliococcygeus.
These muscles have attachments to the pelvis as follows:
- Anterior – pubic bodies of the hip bone.
- Laterally – thickened fascia of the obturator internus muscle, known as the tendinous arch.
- Posteriorly – ischial spines of the hip bone.
The puborectalis muscle is a U-shaped sling, extending from the bodies of the pubic bones, past the urogenital hiatus, around the anal canal. Its tonic contraction bends the canal anteriorly, creating the anorectal angle (90 degrees) at the anorectal junction (where the rectum meets the anus).
The main function of this thick muscle is to maintain faecal continence – during defecation this muscle relaxes.
Some fibers of the puborectalis muscle (pre-rectal fibers) form another U-shaped sling that flank the urethra in the male and the urethra and vagina in the female (in some textbooks they appear as pubovaginalis or sphincter urethrae / vaginae). These fibers are very important in preserving urinary continence, especially during abrupt increase of the intra-abdominal pressure i.e. during sneezing.
The muscle fibres of the pubococcygeus are the main constituent of the levator ani. They arise from the body of the pubic bone and the anterior aspect of the tendinous arch. The fibres travel around the margin of the urogenital hiatus and run posteromedially, attaching at the coccyx and anococcygeal ligament.
The iliococcygeus has thin muscle fibres, which start anteriorly at the ischial spines and posterior aspect of the tendinous arch. They attach posteriorly to the coccyx and the anococcygeal ligament.
This part of the levator ani is the actual “levator” of the three: its action elevates the pelvic floor and the anorectal canal.
Innervated by the anterior rami of S4 and S5.
The coccygeus (or ischiococcygeus) is the smaller, and most posterior pelvic floor component – as the levator ani muscles are situated anteriorly.
It originates from the ischial spines and travels to the lateral aspect of the sacrum and coccyx, along the sacrospinous ligament.
Clinical Relevance: Pelvic Floor Dysfunction
The pelvic floor support acts to support the pelvic viscera, and assist in their functions. If the muscles of the floor become damaged, then dysfunction of these viscera can occur.
The levator ani muscles are involved in supporting the foetal head during cervix dilation in childbirth. During the second phase of childbirth, the levator ani muscles and/or the pudendal nerve are at high risk of damage. Pubococcygeus and puborectalis are the most prone to injury due to them being situated most medially.
Due to their role in supporting the vagina, urethra and anal canal, injury to these muscles can lead to a number of problems. The primary problems include urinary stress incontinence and rectal incontinence. Urinary incontinence is most noticeable during activities where intra-abdominal pressure is increased – such as coughing, sneezing and lifting heavy objects.
Prolapse of the pelvic viscera (such as the bladder and vagina) can occur if there is trauma to the pelvic floor or if the muscle fibres have poor tone. Prolapse of the vagina can also occur if there is damage to the perineal body during childbirth.
This may be avoided by episiotomy (surgical cut in the perineum), which itself can cause damage to the vaginal mucosa and submucosa but helps prevent uncontrolled tearing of the perineal muscles. If the medial fibres of the puborectalis are torn within the perineal body, then rectal herniation can also occur.
There are a number of risk factors which can increase the chances of prolapse: –
- Number of vaginal deliveries
- Family history of pelvic floor dysfunction
- Chronic coughing (e.g from a lung disorder)
The pelvic floor can be repaired surgically, however a way to generally strengthen the muscles is to carry out pelvic floor exercises on a regular basis (Kegel exercises).