**3. Innervation of the abdominal wall**

The thoracic ventral rami (intercostal nerves) are allocated mostly in the region of the anterolateral walls of the chest area and abdominal wall. There are 12 of them on each side, but only 11 are intercostal. Twelve pairs are situated under the last ribs and end in the subcoastal area. The upper six pairs of intercostal nerves are limited to supplying the thoracic parietal region, and in addition, a significant number of fibers from the upper two pairs participate in forming the brachial plexus, innervation of the upper extremities. The lower five pairs of intercostal nerves and subcostal nerves supply the parietal region of the chest and abdomen area and provide fibers to the diaphragm [8].

Typically, the lower 7–12 thoracic and intercostal nerves extend longitudinally toward the thoracic abdominal wall under the respective ribs and intercostal veins. Behind, the nerves extend between the pleura and rear intercostal membrane, and then between the internal and most internal (last, rear) intercostal nerves. Each nerve provides collateral branches and lateral cutaneous branches. The last ones separate from the primary ramus only a few centimeters from the spine, lowering downward into the same intercostal layer as the larger nerves, extending longitudinally along the lower boundary of the intercostal area and ending in the front as small cutaneous nerves or as connected to the main ramus. The lateral cutaneous branch follows the main intercostal nerve to the middle axial line before penetrating the intercostal nerve obliquely and is divided into front and rear branches that have a larger cutaneous distribution. Intercostal nerves supply the intercostal, subcostal, and transversal thoracic muscles. The lower five or six intercostal nerves also supply filaments of the peripheral parts of the diaphragm [8].

The five lower intercostal nerves and subcutaneous nerves stem from the lower rib cartilage and enter the abdominal wall to supply oblique, transversal, and horizontal abdominal muscles and end as the anterior abdominal cutaneous branches. The tenth nerve serves as a dermatome at the umbilicus level. The lateral cutaneous branch of the subcostal nerve (T12) penetrates the internal and external oblique

#### *Fan-Shaped Application of Local Abdominal Wall Analgesia in Abdominoplasty Patients… DOI: http://dx.doi.org/10.5772/intechopen.100235*

muscles and then extends lower over the iliac crest so as to assist in supplying the skin on the lower lateral area of the groin [8].

The ventral primary ramus of the lower spinal nerves (five lumbar, five sacral, and one coccygeal) are divided and connect in a plexiform manner to form the lumbar, sacral, and coccygeal plexuses. They are interconnected as described earlier (rami communicantes) with the sympathetic trunks.

The lumbar plexus is formed with the ventral rami from the first three lumbar nerves and mostly the fourth (and also partly from the subcostal twelfth nerve). It is situated in front of the lumbar transverse vertebral body and sits in the rear section of the psoas major muscle which should be dissected for the plexus to be visible. The most common flow and distribution of plexus components and its relationship with bone structures as well as muscle and aponeurotic layers are shown on the next two pages, but consideration should be given to the fact that variations within the system of the lumbar plexus are frequent [8].

After gaining a branch from the subcoastal nerve, the first lumbar nerve is divided into the upper and smaller lower branches. The last section in the iliohypogastric and ilioinguinal nerve is later connected to the branch of the second lumbar nerve to form the genitofemoral nerve. The remainder of the second lumbar nerve, third and fourth nerve that contribute to this plexus are divided into front and rear sections that in turn are connected to form the obturator and femoral nerves. The accessory obturator nerve, when present, forms from branches in the front section of the third and fourth nerves, whereas the lateral femoral cutaneous nerve appears with the fusion of small parts of the rear sections belonging to the second and third lumbar nerves. The muscle branches from the subcostal and upper four lumbar nerves supply the quadratus lumborum muscle and those from the first and second extend to the psoas major and minor muscles. The psoas major muscle is further innervated with branches from the third and sometimes fourth nerve, which also supplies the muscle iliacus [8].

The iliohypogastric and ilioinguinal nerves are similar to thoracic nerves based on their position and distribution and are analogous to the main trunks and collateral branches of the intercostal nerves. The last nerve provides the lateral branch, which passes over the iliac crest a short distance behind the respective branch of the subcostal nerve, where both nerves then extend to the skin in the upper lateral section of the groin. Extending toward the front branch of the iliohypogastric nerve, it sends filaments toward the transversal and oblique abdominal muscles, passing through the external oblique aponeurosis some 3 cm above the surface inguinal ring and ending with innervation of the skin above the pubis [8].

The ilioinguinal nerve supplies the filaments of the adjacent muscles and after passing through the same muscles, as is the case with the iliohypogastric nerve, it passes under the funiculus spermaticus and through the superficial inguinal ring to supply the upper internal side of the groin area, the root of the penis and front section of the scrotum in men as well as the mons pubis and labia majora in women [8].

After leaving the lumbar plexus, the genitofemoral nerve passes through the psoas major muscle and descends to its anterior surface behind the peritoneum, which is then segmented at the fifth lumbar level into the genital and femoral branches. The last branch enters the inguinal canal through the deep inguinal ring, supplying the cremaster muscle, and provides some branches for the scrotum skin and labia major (external spermatic nerve). The femoral (lumboinguinal) branch passes laterally from the external iliac and femoral artery, passing behind the inguinal ligament and after passing through the anterior layer of the femoral sheath and fascia lata branches into the surface skin and skin of the upper section of the femoral (Scarp's) triangle. The genitofemoral nerve and its branches carry many efferent and afferent fiber to and from the common iliac, external iliac, and femoral arteries [8].

#### **Figure 6.**

*Innervation of the abdominal wall in the male patients [8].*

#### **Figure 7.**

*Innervation of the abdominal wall in the female patients [8].*

*Fan-Shaped Application of Local Abdominal Wall Analgesia in Abdominoplasty Patients… DOI: http://dx.doi.org/10.5772/intechopen.100235*

The other branches of the lumbar plexus (larger branches, femoral nerve) with the exception of the muscular branches for the quadratus lumborum, psoas major, and muscle iliacus are allocated at the lower extremities and that is why they are not discussed here [8] (**Figures 6** and **7**).
