Intramuscular pathway and fascicular characteristics of the segmental intercostal innervation to rectus abdominis

Rectus abdominis is a muscle that is commonly used clinically as a muscle flap, especially in reconstructive surgery. Its intramuscular innervation, however, has rarely been examined in detail or accurately mapped. The purpose of this study was to use biopsied, histological specimens complemented with a staining technique to investigate intramuscular nerve connections and distribution of the rectus abdominis.

Four fresh human cadavers were included in the study and rectus abdominis was dissected bilaterally. Nerve sections innervating the rectus abdominis were biopsied and histologically processed. Sections were viewed under the microscope, and axons within each fascicle were counted using imaging software. All specimens were stained with a modified Sihler's staining technique. Intramuscular innervation was observed and the number as well as distribution was recorded.

Macroscopically stained specimens showed that the eighth, ninth, 10th and 12th intercostal nerves innervated the eight muscle bellies of rectus abdominis. The greatest number of minor nerve branching as well as intramuscular nerve communications originated from nerve roots T9 and T10. Minor nerve branches crossed tendinous intersections to communicate with adjacent nerves and innervate adjacent muscle bellies. Nerves originating from T9 had the greatest number of nerve fascicles and the highest axon count in each cadaver.

The rectus abdominis is divided into four compartments with each receiving its own independent nerve supply. Minor nerve branches crossed tendinous intersections to communicate with adjacent muscle bellies and nerves suggesting that rectus abdominis can be used as a whole in innervated free flap transfer procedures.