Neuromuscular Physiology of the Pelvic Floor

24.2.1

Pelvic Floor

The pelvic floor is a dome-shaped muscle structure that consists mainly of striated muscle with midline defects enclosing the bladder anteriorly, the uterus in women, and the rectum posteriorly. These defects are enclosed by connective tissue, which is located anterior to the urethra and both anterior and posterior to the rectum (perineal body and postanal plate, respectively). Together with the bladder and anorectum, the pelvic floor is responsible for storage and evacuation of urine and stool.

The muscle components of the pelvic diaphragm are the levator ani, coccygeus, and puborectalis muscles. The levator ani complex consists of the pubococcygeus and ileococcygeus muscles, which originate at the pubis bone, the arcus tendineus fascia pelvis, and the ischial spine and insert at the levels of the rectum, the levator plate, and the coccyx ( Fig. 24.1 ).

There has been some controversy as to whether the puborectalis muscle is a component of the levator ani complex or of the external anal sphincter (EAS). Whereas developmental, innervation, and histological studies suggest that the puborectalis muscle is distinct from the other muscles of the levator ani, it is also true that the EAS and the puborectalis muscle have separate and distinct innervations, as do the levator ani and the EAS. In most animal species and probably humans, the levator ani and puborectalis are innervated by specific nerves emanating from the sacral cord, whereas the EAS is innervated by the pudendal nerve derived from S2,3 with possible contributions from S1 and S4. Recent studies also suggest that the activities of the puborectalis muscle and the EAS do not uniformly overlap, indicating important differences between them.

24.2.2

Rectum and Anal Canal

The rectum is a tubular structure, approximately 15–20 cm in length, extending from the rectosigmoid junction of the colon to the anal orifice. The rectum has a dual embryologic origin; the upper rectum is derived from the embryologic hindgut and is able to distend toward the peritoneal cavity whereas the lower rectum is derived from the cloaca and is surrounded by dense extraperitoneal connective tissue.

The anal canal is an anteroposterior slit. The literature describes a longer “surgical” anal canal (4.0–4.5 cm) and a shorter “anatomical” or “embryological” canal. The proximal 1 cm of the anal canal is lined by columnar epithelium; the middle 1.5 cm is lined by stratified (or modified columnar) epithelium; and the distal 1.5–2 cm is lined by stratified or squamous epithelium.

The anal canal is surrounded by the internal anal sphincter (IAS) and the EAS. The former is a thickened extension of the circular smooth muscle of the colon. In contrast to the rest of the gastrointestinal tract in which the interstitial cells of Cajal (ICCs) are located in networks along the submucosal and myenteric borders, the ICCs in the IAS are located along the periphery of the circular smooth muscle bundles.

The EAS consists of superficial, subcutaneous, and deep layers of striated muscle, the latter blending with the puborectalis. The EAS and IAS are separated by the longitudinal anal muscle, which runs vertically and consists of mixed smooth and striated elements. EAS fibers are oriented circumferentially, are small in size, and are separated by profuse connective tissue. There are few enteric ganglia in the rectum compared with the colon and very few in the anal canal. Recently, it has been recommended that the term “anal rhabdosphincter” be used by physiologists in place of the term “EAS,” although the latter is the commonly accepted term among clinicians. These muscles do not have attachments to skeletal structures and as with all true sphincters, contraction produces constriction of the lumen with little if any other movement.

24.2.3

Bladder

The urinary bladder is a hollow, tetrahedron-shaped, muscular organ that, when filled with urine, is spherical. The superior surface of the bladder is covered by the peritoneum whereas the posterior surface, or base of the bladder, lies on the ventral aspect of the rectum in the male and on the vagina in the female. The remaining bladder is surrounded by an intermediate stratum of retroperitoneal connective tissue. The bladder in the male is supported by the prostate and a condensation of intermediate stratum termed the puboprostatic ligaments. A similar condensation of endopelvic fascia, termed the pubovesical or pubourethral ligament, occurs in the female.

In most general descriptions of the gross anatomy of the bladder wall, three muscular layers are noted: outer longitudinal, middle circular, and inner longitudinal. It is probable that the arrangement most closely approaches a meshwork of musculature. A prominent detrusor band thickens toward the prostate as it progresses caudally where it divides and spreads around the neck of the bladder and base of the prostate. A further bundle of musculature that progresses from the anterior vesical neck posteriorly has been termed the bundle of Heiss. On the inner surface of the bladder is a mucosal layer composed primarily of transitional epithelium. Anatomically, the trigone is the triangle-shaped internal base of the bladder formed by ureteric muscles. In surgical practice, because the ureteral muscles are not particularly visible, the trigone is defined as a triangle formed by the ureteral orifices and the dorsal urethra. The muscular band that forms the base of the trigone is termed the inter-ureteric ridge. The ureteral orifices themselves appear slit-like as they enter into the bladder.

The bladder is innervated by the vesical plexus, which is part of the pelvic plexus located on the lateral aspects of the rectum. Sympathetic innervation is derived from T10 to L2 cord segments, whereas parasympathetic innervation is derived from S2 to S4 spinal cord segments, which reach the pelvic plexus via the pelvic splanchnic nerves. The detrusor is primarily supplied by parasympathetic nerves; the bladder neck receives sympathetic innervation in the male, in contrast to parasympathetic innervation in the female. The urethral sphincter is supplied by the pelvic splanchnic nerves. The sensory afferent fibers from the bladder accompany both the sympathetic and parasympathetic nerves to their respective spinal cord segments, as will be discussed later.

24.2.4

Urethra

24.3.1