Chapter 11. Posterior Abdominal Wall

Big Picture

The diaphragm forms the superior and much of the posterior border of the posterior abdominal wall. In addition, the psoas major, iliacus, and quadratus lumborum muscles form the posterior abdominal wall. These muscles function in respiration (diaphragm) as well as trunk and lower limb motion.

Muscles and Fascia

The muscles that form much of the structure of the posterosuperior abdominal wall are as follows (Figure 11-1):

• Diaphragm. A dome-shaped muscle that separates the abdominal cavity from the thoracic cavity. The origin of the muscles of the diaphragm is along the internal circumference of the ribcage, sternum, and lumbar vertebrae. This portion of muscle consists of the esophageal hiatus, at the T10 vertebral level, and the aortic hiatus, at the T12 vertebral level. The muscle fibers are directed to the center of the diaphragm, to the central tendon. The central tendon consists of collagen tissue and the venal caval hiatus, which is at the T8 vertebral level. When the muscle is stimulated to contract (phrenic nerve, C1–C4), the muscle fibers shorten, causing the central tendon to move inferiorly and flatten. This action results in inspiration. The structural components of the diaphragm are as follows:
  • Right crus. Forms part of the aortic hiatus. The right crus also loops around the esophagus to form the esophageal hiatus and contributes to the suspensory ligament of the duodenum (ligament of Treitz).
  • Left crus. Forms part of the aortic hiatus.
• Quadratus lumborum muscle. Attaches to the iliac crest, lumbar transverse processes, and the 12th rib. The quadratus lumborum muscle is the bed on which the kidneys lie. The muscle laterally flexes the vertebral column and stabilizes the 12th rib during breathing. The subcostal and lumbar intercostal nerves provide innervation.
• Psoas major muscle. Attaches to the lumbar vertebrae superiorly and to the lesser trochanter of the femur inferiorly. Between these attachments, the psoas major muscle courses deep to the inguinal ligament and lateral to the femoral nerve. The psoas major muscle flexes the hip joint (when the vertebrae are stabilized) and the lumbar vertebrae. The psoas major muscle is innervated by the L1–L3 spinal nerves.

• Psoas minor muscle. Attaches to the L1 vertebra and pubic crest. The psoas minor muscle helps to tilt the pelvis and is innervated by the L1–L2 spinal nerves. This muscle is not present in everyone.
• Iliacus muscle. Attaches within the iliac fossa and lesser trochanter of the femur. Between its attachments, the iliacus muscle courses deep to the inguinal ligament and joins with the psoas major muscle to attach to the lesser trochanter of the femur. The combination of these two muscles in the thigh is often referred to as the iliopsoas muscle. The iliacus muscle flexes the hip. This muscle is innervated by the femoral nerve (L2–L3 spinal nerves).

Big Picture

The abdominal aorta and inferior vena cava course vertically in the retroperitoneal space, providing the vascular supply for the abdomen, pelvis, and perineum.

Abdominal Aorta

The aorta enters the abdomen from the thorax by traversing the aortic hiatus of the diaphragm at the T12 vertebral level. The aorta courses along the midline, on the anterior surface of vertebral bodies to the left of the inferior vena cava. The abdominal aorta has the following branches, from superior to inferior (Figure 11-2):

• Inferior phrenic arteries. The first paired branches of the aorta in the abdominal cavity. The inferior phrenic arteries supply the inferior surface of the diaphragm.
• Middle suprarenal arteries. One of three pairs of arteries supplying the adrenal glands.
• Gonadal arteries. Paired arteries that supply the gonads.
• Lumbar arteries. Usually, four pairs of arteries that supply the abdominal wall, similar to the intercostal arteries of the thorax.
• Celiac trunk. Unpaired artery that is located approximately at the L1 vertebral level. Supplies the foregut and organs associated with the foregut.
• Superior mesenteric artery. Unpaired artery that is located immediately below the celiac trunk. The superior mesenteric artery supplies the midgut.
• Inferior mesenteric artery. Unpaired artery that is located 4 to 5 cm superior to the bifurcation of the abdominal aorta into the common iliac arteries. The inferior mesenteric artery supplies the hindgut.
• Common iliac arteries. At the L4 vertebral level, the abdominal aorta bifurcates into the left and right common iliac arteries.

Inferior Vena Cava

The inferior vena cava is located to the right of the abdominal aorta. The union of the left and right common iliac veins forms the inferior vena cava. The inferior vena cava ascends along the right side of the vertebral bodies. Before entering the thoracic cavity, the inferior vena cava courses within a groove on the posterior surface of the liver. This portion of the inferior vena cava receives the hepatic veins. Along its course in the abdomen, the inferior vena cava receives the following tributaries:

• Right gonadal vein. Drains the right testis or ovary by entering the inferior vena cava, inferior to the right renal vein.
• Renal veins. Drain the kidneys. The gonadal veins are not bilaterally symmetrical. The left gonadal vein drains into the left renal vein, in contrast to the right gonadal vein.
  • Adrenal veins. Drain the adrenal glands, typically by entering the left and right renal veins.
• Inferior phrenic veins. Drain the inferior surface of the diaphragm.

The lumbar veins drain into a pair of ascending lumbar veins, which ascend posterior to the diaphragm to empty into the azygos system of veins in the thoracic cavity. Connections exist between the ascending lumbar veins and the inferior vena cava. Therefore, blood in the posterior abdominal wall may drain through the azygos vein or through the inferior vena cava. Remember, blood from the abdominal viscera drains through the hepatic portal system.

• Blood from the lower limbs and pelvis drains into the superficial epigastric veins in Camper's fascia. These veins anastomose with other epigastric veins.
• Blood flows from the epigastric veins to the internal thoracic veins, the brachiocephalic veins, the superior vena cava, and then the right atrium.
• The epigastric veins may swell because they accommodate so much blood that they appear in the skin as irregular veins coursing vertically on the abdominal wall.
  

Big Picture

The ventral rami of the lower thoracic and lumbar spinal nerves provide somatic innervation to the abdominal wall muscles and skin.

Somatic Nerves

The somatic nerves of the posterior abdominal wall are the ventral rami of the subcostal and lumbar spinal nerves (Figure 11-3).

• Subcostal (T12), iliohypogastric (L1), and ilioinguinal (L1) nerves. These three nerves emerge along the lateral surface of the psoas major muscle and course between the internal oblique and transverse abdominis muscles, innervating these abdominal wall muscles.
  • Iliohypogastric nerve (L1) In addition the: provides cutaneous innervation to the skin in the hypogastric region.
  • Ilioinguinal nerve (L1) traverses the superficial inguinal ring, enters the spermatic cord, and provides cutaneous innervation to the anterior surface of the scrotal sac and labia majora.
• Genitofemoral nerve (L1–L2). Pierces through the anterior surface of the psoas major muscle and courses along its surface before bifurcating into the femoral branch (cutaneous innervation of anterior thigh) and the genital branch (courses through the superficial inguinal ring). In males, the genital branch courses within the spermatic cord and innervates the cremaster muscle and the skin of the scrotum. In females, the genital branch innervates the skin of the mons pubis and labia majora.
• Lateral cutaneous nerve of the thigh (L2–L3). Emerges along the lateral border of the psoas major muscle and crosses anteriorly to the iliacus muscle, deep to the inguinal ring. This nerve provides cutaneous innervation to the skin over the lateral aspect of the thigh.
• Femoral nerve (L2–L4). Emerges from the lateral surface of the psoas major muscle and provides innervation to the anterior compartment muscles of the thigh responsible for knee extension (also known as the quadriceps).
• Obturator nerve (L2–L4). Emerges from the medial surface of the psoas major muscle and provides innervation to the medial compartment muscles of the thigh responsible for hip adduction.
• Lumbosacral trunk (L4–L5). Branches of the L4 and L5 ventral rami that unite and course inferiorly over the pelvic brim into the pelvic cavity and contribute to the sciatic nerve (L4–S3).

Big Picture

The prevertebral plexus is a network of sympathetic and parasympathetic fibers that innervate the digestive, urinary, and reproductive systems. Sympathetic nerves contribute to the prevertebral plexus via splanchnic nerves from the sympathetic trunk. Parasympathetic nerves contribute to the prevertebral plexus via the vagus nerve [cranial nerve (CN X)] and pelvic splanchnics from the S2–S4 spinal nerves.

Sympathetic Contributions to the Prevertebral Plexus

The sympathetic trunk in the abdomen is continuous with the sympathetic trunk in the thorax (Figure 11-4A). The sympathetic trunk is located along the anterolateral surface of the vertebrae. The trunks are bilateral and descend over the sacral promontory to enter the pelvic cavity. There are approximately four sympathetic ganglia in the posterior abdominal wall on each side of the body, and each ganglion houses the cell bodies of postganglionic sympathetic neurons. The following splanchnic nerves carry preganglionic sympathetic fibers and visceral afferent fibers to and from the prevertebral plexus (Figure 11-4B):

• Greater splanchnic nerve. Courses from the sympathetic trunks in the thorax at the T5–T9 spinal nerve levels and contributes primarily to the celiac ganglion and to a lesser extent to the superior mesenteric ganglion.
• Lesser splanchnic nerve. Courses from the sympathetic trunks in the thorax at the T10–T11 spinal nerve levels and contributes primarily to the superior mesenteric and aorticorenal ganglia and to a lesser extent to the celiac ganglion.
• Least splanchnic nerve. Courses from the T12 sympathetic trunk and contributes primarily to the renal plexus.
• Lumbar splanchnic nerves. Course from the sympathetic trunks at the lumbar spinal nerve level and contribute primarily to the inferior mesenteric plexus.
• Sacral splanchnic nerves. Course from sympathetic trunks in the sacral spinal nerve level and contribute primarily to the inferior hypogastric plexus.

Parasympathetic Contributions to the Prevertebral Plexus

Parasympathetic fibers from the vagus nerve (CN X) innervate all of the abdominal viscera, distally to the transverse colon; the remainder of the gastrointestinal tract is innervated by pelvic splanchnics (Figure 11-4B).

• Vagus nerve (CN X). The right and left vagus nerves form the esophageal plexus in the thoracic cavity and enter the abdomen through the esophageal hiatus as the anterior and posterior vagal trunks. The anterior vagal trunk is derived from the left vagus nerve and primarily innervates the stomach, liver, and gallbladder. The posterior vagal trunk is derived from the right vagus nerve and primarily contributes to the celiac plexus of the prevertebral plexus.
• Pelvic splanchnics (S2–S4). Preganglionic parasympathetic neurons originate in the S2–S4 levels of the spinal cord and course in the ventral root and rami and into the pelvic splanchnic nerves, which contribute to the inferior hypogastric plexus en route to innervate the distal part of the transverse colon, descending colon, sigmoid colon, and rectum, as well as the urinary and reproductive systems.

All preganglionic parasympathetic fibers synapse within the wall of the end organ. In the gastrointestinal system, the synapse with the postganglionic parasympathetic fibers occurs in the myenteric (Auerbach's) plexus and the submucosal (Meissner's) plexuses.

Prevertebral Ganglia and Plexus

The prevertebral (preaortic) plexus is a network of autonomic nerve fibers covering the abdominal aorta and extending into the pelvic cavity between the common iliac arteries. This plexus serves as a common pathway for the following autonomics (Figure 11-4A and B):

• Preganglionic sympathetic nerves. From greater, lesser, least, and lumbar splanchnic nerves.
• Preganglionic parasympathetic nerves. From the vagus nerve (CN X) and pelvic splanchnics (S2–S4).
• Visceral afferents. From both sympathetic and parasympathetic pathways.

Sympathetic (prevertebral) ganglia that are named for the associated branch of the abdominal aorta are located within the prevertebral plexus. These ganglia are collections of postganglionic sympathetic neurons. Neurons within the prevertebral plexus connect autonomics to and from the digestive, urinary, and reproductive organs.

The prevertebral plexus is subdivided into smaller plexuses and ganglia. Many of these plexuses and ganglia are located very close together and are variable and interrelated. Their principal features can be described as follows:

• Celiac ganglia and plexus. Located around the origin of the celiac trunk and distributed along its branches. The celiac plexus receives preganglionic sympathetic nerves from the greater splanchnic nerve. Parasympathetics from the vagus nerve course through the celiac ganglion en route to the viscera without synapsing. Nerves exiting the celiac ganglion supply postganglionic sympathetic nerves to the liver, gallbladder, stomach, spleen, pancreas, and proximal part of the duodenum.
• Superior mesenteric ganglia and plexus. Located around the origin of the superior mesenteric artery and distributed along its branches. The ganglia receive preganglionic contributions from the lesser splanchnic nerve. The superior mesenteric plexus supplies postganglionic sympathetic fibers to the head of the pancreas; the distal part of the duodenum; the jejunum, ileum, and cecum; the ascending colon; and the transverse colon.
• Aorticorenal ganglia and plexus. Located at the origin of the renal arteries and distributed along their branches. The ganglia receive contributions from the lesser splanchnic nerves and supply the adrenal glands, the kidneys, and the proximal part of the ureters.
• Inferior mesenteric ganglia and plexus. Located at the origin of the inferior mesenteric artery and distributed along its branches. The inferior mesenteric ganglion receives contributions from the superior mesenteric plexus, the first and second lumbar splanchnic nerves (sympathetics), and fibers from the superior hypogastric plexus (sympathetics and parasympathetics). The inferior mesenteric plexus supplies the descending colon, the sigmoid colon, and the upper portion of the rectum.
• Superior hypogastric plexus. Located inferior to the bifurcation of the aorta, between the common iliac arteries. The superior hypogastric plexus is formed by contributions of the inferior mesenteric plexus, the third and fourth lumbar splanchnic nerves (sympathetics), and the parasympathetic nerves that ascend from the inferior hypogastric plexus. The superior hypogastric plexus continues caudally into the pelvis via the hypogastric nerves.
• Inferior hypogastric plexus and pelvic ganglia. In males, the inferior hypogastric plexus is posterolateral to the bladder, seminal vesicles, and prostate. In females, it is posterolateral to the bladder and cervix. The inferior hypogastric plexus and pelvic ganglia are formed primarily from the following fibers:
  • Sympathetics. Preganglionic sympathetic fibers enter the inferior hypogastric plexus through the sacral splanchnic nerves. Additionally, some sympathetic fibers arising from the lumbar splanchnic nerves descend from the superior hypogastric plexus into the inferior hypogastric plexus. Preganglionic sympathetic neurons usually synapse with postganglionic sympathetic neurons somewhere in the prevertebral plexus (i.e. inferior hypogastric plexus).
  • Parasympathetics. Preganglionic parasympathetic fibers enter the inferior hypogastric plexus through the pelvic splanchnic nerves. Once most of the parasympathetic fibers enter the inferior hypogastric plexus, they ascend out of the pelvis and into the superior hypogastric plexus to innervate the hindgut. However, some nerves exit the pelvis to innervate the urinary and reproductive systems. Preganglionic parasympathetic neurons usually synapse with postganglionic parasympathetic neurons somewhere in the wall of the target organ (i.e., Aurbachis plexus).

Big Picture

The adrenal (suprarenal) glands are responsible for regulating stress through the production and secretion of hormones such as adrenalin (epinephrine), glucocorticoids, mineralocorticoids, and androgens. The kidneys filter systemic blood to produce urine, which is transported to the bladder by the ureters.

Adrenal Glands

The adrenal (suprarenal) glands consist of a cortex (secretes mineralocorticoids, glucocorticoids, and androgens) and a medulla (secretes adrenalin). The adrenal glands lie at the T12 vertebral level, on the superior pole of each kidney, and are separated from the kidneys by the renal capsule. The left adrenal gland is positioned more medially than the right adrenal gland (Figure 11-5).

• Vascular supply. Each gland receives an arterial supply from three arteries: superior adrenal artery (branch of the inferior phrenic artery), middle adrenal artery (branch of the aorta), and inferior adrenal artery (branch of the renal artery). Venous drainage of the right adrenal gland empties directly into the inferior vena cava, whereas that of the left adrenal gland empties into the left renal vein.
• Innervation. Sympathetic nerves innervate the adrenal medulla. The least splanchnic nerve, through the aorticorenal ganglion, provides innervation. However, unlike other organs innervated by the sympathetic nerves, only preganglionic neurons enter the adrenal medulla. The adrenalin (epinephrine) hormones released into the blood stream act like a systemic postganglionic neuron to stimulate tissues around the body that have receptors for adrenalin.

The adrenal cortex is primarily regulated within the endocrine system by hormones from the pituitary gland (e.g., adrenocorticotropic hormone).

Kidneys

The kidneys are the functional unit of the urinary system. Within the kidneys, millions of nephrons maintain body water, salt, and pH balance and eliminate solutes from the blood. The kidneys accomplish these functions by filtering blood plasma into urine, which is eliminated from the body via the ureters, bladder, and urethra. The kidneys are a pair of bean-shaped organs located adjacent to the vertebrae at the T12–L2 vertebral level. The right kidney is slightly lower than the left rib because of the presence of the liver. The ureter, bladder, and kidney are all retroperitoneally positioned. Perirenal fat surrounds and thus cushions the kidneys (Figure 11-5).

• Vascular supply. Renal arteries supply blood to the kidney; renal veins drain blood from the kidneys.

Ureters

After the nephrons filter the blood plasma and produce urine, a series of tubes collect the urine and transport it to the bladder. Urine is received by collecting ducts, which in turn drain into minor calyces and then into major calyces. The major calyces in the kidney form the renal pelvis, which exits the kidney as the ureter. The ureter descends anterior to the transverse processes of the lumbar vertebrae and course over the sacral promontory and into the pelvis. In the female, the ureters course inferior to the uterine arteries before entering the base of the bladder.

• Vascular supply. Is provided by branches from the renal arteries and veins.

Innervation of the Kidneys and the Proximal Part of the Ureters

The autonomic innervation of the kidneys and proximal ureters is as follows:

• Sympathetic innervation. Preganglionic neurons course through the lesser, least, and lumbar splanchnic nerves. They synapse in the aorticorenal and superior mesenteric ganglia, where postganglionic neurons course in a renal plexus to the kidneys.
• Parasympathetic innervation. Preganglionic parasympathetic neurons originate in the brainstem and course in the vagus nerve to enter the abdomen. The vagus nerve branches to become the posterior vagal trunk, which enters the celiac plexus. The neurons join the renal plexus and synapse in the wall of the kidney.

• The kidney continues to produce urine, which backs up behind the stone and may cause the kidney to swell.
• The pressure buildup causes much of the pain associated with kidney stones.
• However, the pressure also helps to push the stone along until it enters the bladder.
• Depending on the level of the location of the stone, which will vary as pressure moves it along, pain will most likely be referred from the lumbar region or to the hypogastric region to the groin.
• The pain is referred to the cutaneous areas supplied by the same spinal cord segments that supply the ureter (i.e., lesser splanchnic, T10–T11; least splanchnic, T12; lumbar splanchnic nerves, L1–L2). The urethra has a larger lumen than the ureter, and therefore, the symptoms are usually relieved at this point.