The rectal tunica muscularis, such as in the colon, is composed by one inner circular layer and one external longitudinal layer. The latter is an expansion of colon taenia, which, at the junction of sigmoid-rectal, creates a continuous muscular layer. Concerning rectal reservoir, there are links between the two kinds of musculature, since the longitudinal one opens like a fan within the circular muscle; this particular structure is easy to find at the level of the rectal valves. At the lower part of the rectum, the longitudinal muscular layer merges with the striated muscle of the levator ani muscle and with the fibroelastic tissue from the pelvic fascia overlying the pelvic diaphragm, in order to form the joint longitudinal muscle of the anal canal. In turn, at the level of the anal sinuses, the circular muscle thickening forms the internal sphincter muscle of the anus. This muscle is made by smooth muscle cells, innervated by the autonomic nerves of the intrinsic nervus plexuses (myenteric and submucosal), and terminates with an inferior boundary rounded at the intersphincteric line.

The longitudinal muscle joint (LMJ) broadens inferiorly surrounding the internal sphincter muscle of the anus and, in its turn, is surrounded by the external sphincter muscle of the anus. During its downward course, the LMJ emits a series of fibroelastic and muscular fascicles, which penetrate the internal sphincter muscle of the anus; some of these are joined to the muscularis mucosae of the anal canal composing the muscle of the anus submucosa (or “sustentator mucosae of Kohlrausch”), whose fibers anchor the anoderm of the dentate line to the underlying tissues and to the lower third of the internal sphincter muscle of the anus. Fixed in this way, the dentate line prevents the eversion of the anal canal and supports the overlying internal hemorrhoidal venous plexus during defecation [7]. The LMJ in its lower part before joining with the muscularis mucosa of the anal canal issues a series of fan-shaped fibro-muscular septa, which pass through the submucosal portion of the external sphincter muscle of the anus and are combined with perianal skin forming the corrugator muscle of the skin of the anus. A series of bundles in the opposite side is directed outward, passing through the superficial subcutaneous portion of the external sphincter muscle of the anus, lasting as transverse septum of the ischiorectal fossa. The importance of the LMJ consists in the fact that, along with the levator ani muscle, it exerts an action of lift on the sphincter and anal canal, preventing the spread of any anorectal infections.

The external musculature of the anal canal is formed by the external sphincter muscle of the anus (EAS), arranged in the shape of sleeve so as to surround 2–3 cm the terminal portion of the rectum and the anal canal. The EAS is composed by three parts: a subcutaneous, a superficial, and a deep part. The subcutaneous part about 3–5 mm in diameter surrounds the anal orifice directly above the edge of the anus, below, and slightly to the side to the internal sphincter muscle of the anus; in women, this part of the external sphincter muscle of the anus is more developed, especially anteriorly, where it forms a prominent annular bandage, which is sometimes engraved in the course of an episiotomy.

The subcutaneous part is functionally integrated with the levator ani, through expansions of the longitudinal muscle; they go through it like a fan and they terminate as fibers of corrugator muscle of skin and anus. The superficial part of the external sphincter muscle of the anus has an elliptical shape and is placed deeply and laterally to the subcutaneous part of the EAS. It is the most robust of the three parts of the EAS and originates independently from the rear face and from the tip of the coccyx, so it is sometimes called “coccygeal portion”; in the male it is connected at the front on the tendinous center of the perineum and on the median fascial raphe of bulbocavernosus muscles. In the female, muscle fibers are connected in lower part to the tendinous center; for the most part, however, they connect with the bulbocavernosus muscles. Posteriorly, the fibers form the anococcygeal raphe.

The deep part of the external anal sphincter muscle is for the most part an annular muscle bundle, not joined to the coccyx; posteriorly it is intimately fused with the pubo-rectal muscle since the fibers of this muscle, with a course in sling, pass around the terminal part of the rectum. Prior to the rectum, muscular fibers intersect with those of the opposite side mingling in part with the fibers of the superficial transverse perineal muscles; also in back fibers are mixed to fit in the anococcygeal raphe.

The essential supports that hold in position the recto-anal canal are derived from the muscles that form the pelvic floor: the levator ani muscle, consisting of three parts—the pubococcygeus muscle, the pubo-rectal muscle, and iliococcygeus. These three muscles together are located at the diaphragmatic and subdiaphragmatic floor; their organization is such that, when viewed from above, they appear as a funnel whose point is more declivous and corresponds to the gap of the rectal canal. The levator ani gives stability to the pelvic floor and acts as a fulcrum against the increased abdominal pressure, which is provoked by cough and defecation.

The pubococcygeal and the pubo-rectal part, forming a sling, becomes part of the anorectal muscular ring that surrounds part of the rectal reservoir and in part the upper portion of the anal canal. This ring is composed of the following: on its posterior half it is composed by fibers of the pubo-rectal ring; on the anterior part, by the internal sphincter muscle of the anus and fibers coming from the pubococcygeus muscle (pre-rectal or fibers of Luschka); and by the longitudinal muscle, surrounded by the deep part of the external sphincter muscle of the anus. The posterior half of that ring, along with the pubo-rectal sling, brings the rectum near to the pubic bone and increases anorectal torsion, shortens and narrows the pelvic opening, raises the anus, and collaborates to the closure of the anal canal (Fig. 2.2).

The sympathetic and parasympathetic fibers innervate rectum, anal canal, and genitourinary system. The sympathetic innervation is derived from the first three lumbar segments that form the preaortic plexus from which fibers arise that extend below the aortic bifurcation forming the superior hypogastric plexus or presacral nerve. The fibers of the superior hypogastric plexus will lead to the sides of the pelvis where they join with the branches of the parasympathetic nerves to form the inferior hypogastric plexus, or pelvic plexus, in close contact with the rectum [8].

The parasympathetic innervation is derived from the erigentes nerves, or pelvic splachnic, which originate from the II, III, and VI sacral root. The pudendal nerve arises from the sacral plexus at levels S2–S4, which provides motor innervation of the external sphincter and sensory innervation to the perianal skin. The pudendal nerve at the end of its course, at the side wall of the ischiorectal fossa, is divided into three branches: inferior hemorrhoidal, perineal, and dorsal of the penis/clitoris.

The external anal sphincter has a threefold innervation: the perineal muscular branch of the pudendal nerve for the inner or ventral side of the muscle, the anal nerve on the lateral side of the sphincter, and the perineal branch of the fourth sacral nerve for the posterior or caudal area [9]. The arrangement of these fibers is radial, numerous connections are intersegmental, and this explains the functional recovery after nerve section.

The internal sphincter essentially works as a reflex and is constituted by autonomous smooth muscle fibers. Both sympathetic and parasympathetic systems contribute to its innervation [10].

The levator ani muscle, intimately linked to the external sphincter by its pubo-rectal bundle, receives on its upper branches fibers of the levator ani. These nerves can come directly from the third and fourth sacral root or from internal pudendal nerve.

In the wall of the anal canal, there are numerous receptors, distributed all along the channel and also in the thickness of the wall layers.

Muscle receptors and polymodal nociceptive receptors exist in addition to specific mucosal receptors [11]. These muscular receptors are mechanoreceptors of two types: one for slow adaptation, in the internal sphincter, and the others for fast adaptation, in the external sphincter.

The sensory innervation of the anus is richer than the one of the rectum and especially at the mucosa level [12]. The sensitivity of the skin surface area of the anal canal, from the anorectal line, depends on isolated intraepithelial nerve fibers (region discriminative pain). Above this area, receptors with greatest diversity and density allow to analyze lots of information: genital corpuscles (friction), Golgi’s corpuscles (pressure), Meissner’s corpuscles (touch), Krause’s corpuscles (cold), and Pacini’s corpuscles (stretching). The sensory nerve fibers run in the hemorrhoidal side branches of the internal pudendal nerve but also with a parasympathetic group toward the hypogastric plexus and the sacral sympathetic trunk through the second and third sacral ganglion. The sensory role is particularly to discriminate accurately the quality of the content of the anal canal, of fundamental importance for continence and evacuation [13].

The processing of information takes place at three levels: the enteric nervous system, the paravertebral ganglia of the autonomic nervous system, and the cerebrospinal axis.

The enteric nervous system is the support of local muscle tone. This nervous system is localized in the submucosal plexus of Meissner and between the sphincter muscular layers of myenteric plexus of Auerbach, more voluminous than the last. Regardless of the extrinsic nervous system, a local reflex system, with the presence of interneurons, allows an adjustment control within the same wall. This local nerve plexus is characterized by a wide variety of chemical mediators; cholinergic neurons are present in spontaneous activity permanently, noradrenergic neurons are very common in sphincteric zone, and non-adrenergic and non-cholinergic (purinergic and serotonergic) neurons, as well as numerous neuropeptides, are present in large quantities in the anal canal [14].

Some neuropeptides have a relaxing effect on the internal sphincter, unlike opioid neuropeptides (enkephalinergic), which represent 25 % of the neuronal population and increase sphincter tone.

The autonomic nervous system (autonomic sympathetic and parasympathetic) guarantees a faster connection for the transmission of sensory information through the paravertebral ganglia and the hypogastric plexus.

The cerebrospinal axis receives afferent information from neurons of the ganglionar root placed in the back or in the plexiform ganglion of the vegetative system. At this point, the information can pass through the synapses of preganglionic sympathetic or parasympathetic origin of segmenting reflexes or continue its afference in tractus nervosi of posterior spinal cord, without interruptions up to the brain.

Brain areas of central regulation are still poorly defined and are located in the brain stem, hypothalamus, limbic system, and neocortex (Fig. 2.3).

The recto-anal canal is perfused from different arteries: the superior rectal artery, branch of the inferior mesenteric artery, middle rectal artery, branch of the internal iliac artery, inferior rectal artery branch of the internal pudendal artery, and branches of the lower sacral artery [15].

The veins that drain the recto-anal channel are the upper, middle, and lower. They come from the submucosal venous plexus or hemorrhoidal plexus. This plexus becomes richer of blood in two regions because they have ampullary expansions: above the dentate line, they form the internal hemorrhoidal plexus, and below the lower edge of the internal sphincter, the exterior hemorrhoidal plexus [16].

At the level of the dentate line, there is a communication between the portal and systemic circulation: the upper part of the anal canal is a tributary of the portal system through the superior hemorrhoidal vein and the inferior mesenteric vein; on the contrary, the effluent blood from the lower portion reaches the vena cava through the middle and inferior hemorrhoidal veins, tributary of the hypogastric veins (internal iliac).

As well as the venous system, the lymphatic goes in two directions: by simplifying it can be said that above the dentate line the drainage is toward the hypogastric lymph nodes (Fig. 2.4).