Innervation of Abdomen and Perineum
The segmentally arranged nerves are attached to the sides of the spinal cord by a series of anterior (ventral) and posterior (dorsal) roots. An anterior and posterior root at each spinal segment unite to form the spinal nerve, which emerges through the corresponding intervertebral foramen. The anterior roots contain axons from the motor nerve cells in the anterior horn of the spinal cord and the posterior roots contain the axons projecting from the pseudounipolar sensory cells located in the posterior (dorsal) root ganglia (spinal sensory ganglia).
The spinal nerve only exists for a short span before dividing into anterior and posterior rami, each of which carries both motor and sensory axons to their target tissues. Before splitting into rami, each of the spinal nerves gives off a small recurrent meningeal branch that is sensory to the nearby spinal dura mater and intervertebral disc. After emerging from the intervertebral foramen, each spinal nerve receives a branch or branches (gray rami communicantes) from an adjacent ganglion of the sympathetic trunk, which contains postganglionic sympathetic axons originating from the cells of that ganglion. Of the first thoracic through the first two or, occasionally, three lumbar anterior rami, each contributes a branch or branches (white rami communicantes), which contain preganglionic sympathetic fibers to the corresponding sympathetic ganglia.
In general, the posterior rami are smaller than the anterior rami and do not unite to form plexuses. They divide into medial and lateral branches that supply the muscles and skin of the back. The anterior rami supply the anterolateral aspects of the trunk as well as the limbs. In the cervical, lumbar, sacral, and coccygeal regions, the anterior rami converge to form plexuses, but in the thoracic region, they maintain their segmental character and each runs separately and independently to the site or structure it innervates.
The thoracic anterior rami, the intercostal nerves, are distributed chiefly to the anterolateral walls of the thorax and abdomen. They are 12 in number on each side, but only 11 are truly intercostal. The 12th pair lie below the last ribs and are termed subcostal nerves. The upper six pairs of intercostal nerves are limited in their supply to the thoracic body wall, although the first and second intercostal nerves also contribute to the brachial plexus, the innervation of the upper limbs. The fourth nerve innervates the skin at the level of the nipple. The lower five pairs of intercostal nerves and the subcostal nerves supply the thoracic and abdominal body wall and also contribute fibers to the diaphragm.
Typically, the 7th to 11th intercostal nerves course anteriorly along the thoracic wall below the corresponding rib and intercostal vessels. Posteriorly, the nerve lies between the pleura and the posterior intercostal membrane and passes between the internal and innermost intercostal muscles. Each nerve gives off a collateral branch and a lateral cutaneous branch. The former, separating from the primary ramus only a few centimeters away from the vertebrae, inclines inferiorly from the parent nerve, runs along the lower border of the intercostal space, and ends anteriorly as a small cutaneous nerve. The lateral cutaneous branch accompanies the main intercostal nerve as far as the midaxillary line before piercing the intercostal muscles and dividing into anterior and posterior branches, which are mainly cutaneous in distribution. The intercostal nerves supply intercostal, subcostal, and transverse thoracic muscles. The lower five or six inter- costal nerves also supply sensory axons to the peripheral parts of the diaphragm.
The lower five intercostal nerves and the subcostal nerves pass posterior to the costal cartilages and enter the abdominal wall to supply the external and internal abdominal oblique, transversus abdominis, and rectus abdominis muscles and end as anterior abdominal cutaneous branches. The 10th nerve serves the dermatome at the level of the umbilicus. The lateral cutaneous branch of the subcostal nerve (T12) pierces the internal and external oblique abdominal muscles and descends over the iliac crest to assist in supplying the skin over the upper lateral part of the thigh.
The anterior rami of the lower spinal nerves (five lumbar, five sacral, and one coccygeal) divide and reunite in a plexiform fashion to form the lumbar, sacral, and coccygeal plexuses. They are interconnected as described above with the sympathetic trunks via rami communicantes.
The lumbar plexus is formed by the anterior rami of the first three lumbar nerves and the greater part of the fourth lumbar nerve, along with a contribution from the subcostal nerve. It is situated anterior to the lumbar vertebral transverse processes and is embedded in the posterior part of the psoas major muscle, which needs to be dissected to make the plexus accessible. The most common course and distribution of the components of the plexus and its relationship are described and illustrated here, but it should be kept in mind that variations of the lumbar plexus are frequent.
The first lumbar nerve, after receiving a twig from the subcostal nerve, splits into an upper branch and a smaller lower branch. The former divides into the iliohypogastric and ilioinguinal nerves, and the latter unites with a twig of the second lumbar nerve to form the genitofemoral nerve. The rest of the second lumbar nerve, the third, and that part of the fourth which con- tributes to this plexus, each divide also into anterior and posterior sections, which combine to constitute the obturator and femoral nerves, respectively. The accessory obturator nerve, when present, is formed by branches from the anterior divisions of the third and fourth nerves, whereas the lateral femoral cutaneous nerve evolves by the fusion of small offshoots from the posterior divisions of the second and third lumbar nerves.
Muscular branches from the subcostal and upper four lumbar nerves supply the quadratus lumborum muscle, and those of the first and second reach the psoas major and psoas minor muscles. The psoas major muscles are further innervated by branches from the third and, sometimes, fourth lumbar nerves, which also supply the iliacus muscles.
The iliohypogastric and ilioinguinal nerves resemble the thoracic nerves in their course and distribution, being analogous, respectively, to the main trunk and the collateral branch of an intercostal nerve. The former nerve gives off a lateral branch, which crosses the iliac crest a short distance posterior to the corresponding branch of the subcostal nerve, both nerves supplying skin of the superior lateral part of the thigh. Continuing anteriorly, the anterior branch of the iliohypogastric nerve sends filaments to the transverse and oblique abdominal muscles, pierces the external oblique aponeurosis about 3 cm superior to the superficial inguinal ring, and terminates innervating the skin just superior to the pubis.
The ilioinguinal nerve supplies filaments to the adjacent muscles and, after piercing the same muscles as the iliohypogastric nerve, enters the inguinal canal, runs deep to the spermatic cord, and emerges through the superficial inguinal ring to supply the superior medial side of the thigh, the root of the penis, and the anterior part of the scrotum in the male, and the mons pubis and labium majora in the female.
The genitofemoral nerve, after emerging from the lumbar plexus, passes through the psoas major muscle and descends on its anterior surface, deep to the peritoneum, to divide into the genital and femoral branches at about the level of the fifth lumbar vertebra. The former branch enters the inguinal canal through the deep inguinal ring, innervates the cremaster muscle, and contributes some twigs to the skin of the scrotum, or the labium majora of the female. The femoral branch runs lateral to the external iliac and femoral arteries, passes posterior to the inguinal ligament, and, after piercing the anterior layer of the femoral sheath and the fascia lata, ramifies in the superficial tissues and skin over the femoral triangle. The genitofemoral nerve and its branches carry many of the efferent and afferent fibers to and from the common iliac, external iliac, and femoral arteries.
Other branches of the lumbar plexus (e.g., the femoral nerve), except for muscular rami to the quadratus lumborum, psoas major, and iliacus muscles, are distributed to the lower limb and, consequently, are not discussed in this volume.
The anterior rami of the sacral and coccygeal nerves, which, in contrast to the lumbar nerves, diminish in size as they progress inferiorly, divide and reunite to con- tribute to the sacral and coccygeal plexuses. These lie on the posterior wall of the pelvis, posterior to the ureters, internal iliac vessels, and intestinal coils, and anterior to the piriformis and coccygeus muscles. The inferior and smaller part of the fourth lumbar nerve unites with the anterior ramus of the fifth lumbar nerve as the lumbosacral trunk, which, together with the anterior rami of the first three and the upper part of the fourth sacral nerves, constitutes the sacral plexus. The lower part of the fourth sacral joins the fifth sacral and coccygeal nerves to form the small coccygeal plexus.
Each nerve entering into the composition of these two plexuses receives postganglionic sympathetic fibers by way of one or more gray rami communicantes from an adjacent ganglion of the sympathetic trunk. Preganglionic parasympathetic fibers originate in the second to fourth sacral levels of the spinal cord; they emerge with the second, third, and fourth sacral nerves and leave thereafter as pelvic splanchnic nerves.
The sacral plexus, by convergence and fusion of its roots, develops into a flattened band, from which many branches arise, before the large sciatic nerve passes through the greater sciatic foramen inferior to the piriformis muscle. This large nerve consists of a tibial section and a common fibular section, which usually remain fused until about the lower third of the thigh, but which may occasionally be separated at their points of origin or may divide before the nerve leaves the pelvis. The nerve of the sacral plexus splits into anterior and posterior divisions, which, in some individuals, unite again to produce the nerves. Most branches of the sacral plexus supply the lower limb and will be discussed in the volume addressing the musculoskeletal system. Others are distributed in the pelvic and perineal regions. The nerves to the piriformis, levatorani, and coccygeus muscles pierce the anterior or pelvic surfaces of these muscles. The nerve to the obturator internus muscle (not to be confused with the obturator nerve) leaves the pelvis through the greater sciatic foramen inferior to the piriformis muscle, crosses the ischial spine lateral to the pudendal nerve and internal pudendal vessels, reenters the pelvis through the lesser sciatic foramen, and sinks into the pelvic surface of the obturator internus muscle.
The pudendal nerve passes between the piriformis and coccygeus muscles, leaves the pelvis through the greater sciatic foramen, alongside the sciatic nerve, crosses posteroinferior to the ischial spine (medial to the internal pudendal artery), and accompanies that vessel through the lesser sciatic foramen into the pudendal canal on the obturator internus fascia. As the nerve enters the canal, it gives off the inferior rectal nerve and shortly thereafter terminates by splitting into the perineal nerve and the dorsal nerve of the penis or clitoris, respectively.
The inferior rectal nerve perforates the medial wall of the pudendal canal, crosses the ischioanal fossa obliquely with the inferior rectal vessels, and divides into branches that are the main supply of the external anal sphincter, the lining of the lower part of the anal canal, and the skin around the anus. Its branches communicate with the perineal branches of the posterior femoral cutaneous, fourth sacral, and perforating cutaneous nerves and the perineal nerve, which is the larger terminal branch of the pudendal nerve. This latter nerve runs anteriorly in the pudendal canal inferior to the internal pudendal artery, projecting toward the posterior border of the urogenital diaphragm, near which it divides into superficial and deep branches. The superficial one divides into medial and lateral posterior scrotal (or labial) nerves, which spread over the skin of the scrotum or labia majora, communicating with the perineal branch of the posterior femoral cutaneous nerve. The deep branches supply the anterior parts of the external anal sphincter, the superficial and deep transverse perineal, bulbospongiosus, and ischiocavernosus muscles, as well as the sphincter urethrae (and, in a subsidiary fashion, the levator ani). A twig, termed the nerve of the bulb, arises from the branch to the bulbospongiosus muscle and is distributed to the erectile tissue of the corpus spongiosum and the mucous membrane of the urethra.
The dorsal nerve of the penis accompanies the internal pudendal artery in its course through the deep transversal perineal muscle and passes anterior to the pubic arch under cover of the ischiocavernosus muscle and corpus cavernosum penis. Passing through a gap between the inferior fascia and the apex of the urogenital diaphragm, the nerve comes to lie alongside the dorsal artery of the penis and continues as far as the glans and the prepuce. In the female the dorsal nerve of the clitoris is smaller, but its distribution is similar.
The posterior femoral cutaneous nerve, besides innervating the skin of the posterior thigh, gives off a gluteal branch, the inferior cluneal nerve, supplying the skin area over the lower part of the gluteus maximus and, in the same region, a perineal branch that curves anteriorly and medially inferior to the ischial tuberosity to the skin and fasciae of the perineum, scrotum, and root of the penis. The distribution is similar in the female, to the perineum, labia majora, and root of the clitoris. Its terminal twigs communicate with the inferior rectal and perineal branches of the pudendal and terminal filaments of the ilioinguinal nerves. The perforating cutaneous nerve pierces the sacrotuberous ligament and turns around the lower margin of the gluteus maximus to become cutaneous a short distance lateral to the coccyx. Its origin and distribution, however, are not constant. It may be joined or replaced by branches from the pudendal nerve, posterior femoral cutaneous nerve, or perineal branch of the fourth sacral nerve, arising from a loop between the third and fourth sacral nerves. This branch reaches the posterior angle of the ischioanal fossa by perforating the coccygeus muscle and then divides into some twigs that run anteriorly to assist the innervation of the external anal sphincter and others that ramify in the overlying skin and fascia.
The coccygeal plexus is formed by the union of the inferior part of the anterior ramus of the fourth sacral nerve with those of the fifth sacral and coccygeal nerves. The plexus is small and really consists of two loops on the pelvic surface of the coccygeus and the levator ani muscles. It gives off fine twigs to the parts adjacent to both these structures, as well as the delicate anococcygeal nerves that pierce the sacrotuberous ligament and supply the skin in the vicinity of the coccyx.
Having discussed the nerves supplying the wall of the abdominal cavity, the lumbar, sacral, and coccygeal plexuses, and the nerves they release to innervate part of the abdominal viscera and floor (pelvis as well as perineum) of the abdominal cavity, it remains to consider the innervation of the diaphragm, which forms the roof of the abdominal cavity. The diaphragm is supplied by the phrenic and lower intercostal nerves. Each phrenic nerve contains both motor and sensory fibers; the latter convey afferent impulses from the pleura, pericardium, peritoneum, and other structures. The motor fibers are the axons of the phrenic nucleus in the third, fourth, and fifth cervical cord segments. If one phrenic nerve is destroyed, complete muscular atrophy occurs in the corresponding half of the diaphragm, so it is presumed the intercostal nerve supply must be sensory.
The phrenic nerves are distributed mainly on the inferior surface of the diaphragm. The right pierces the central tendon just lateral to the caval hiatus and divides into anterior and posterior branches that supply all the muscle fibers on the same side, including the crural fibers on the right side of the esophagus and those arising from the arcuate ligaments. The left nerve pierces the diaphragm about 3 cm anterior to the central tendon and thereafter supplies the left half of the muscle, including the fibers of the right crus lying to the left of the esophageal hiatus. The phrenic branches communicate with autonomic fibers from the celiac plexus accompanying the inferior phrenic arteries. On the right side a small ganglion marks one of these interconnections.