20 - Thoracodorsal Nerves Derive from Both the Anterior and Posterior Divisions of the Brachial Plexus

Poster Board Number: 20
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Co-authors:

Henry Aaker - Arizona College of Osteopathic Medicine; Daniel Burrola - Arizona College of Osteopathic Medicine; Brian Mayer - Arizona College of Osteopathic Medicine; Dominik Valdez - College of Graduate Studies, Midwestern University; Erin Simons - Arizona College of Osteopathic Medicine, Midwestern University College of Graduate Studies,; Justin Georgi - Arizona College of Osteopathic Medicine, Midwestern University College of Graduate Studies; Margaret Hall - Arizona College of Osteopathic Medicine, College of Graduate Studies, Midwestern University

Abstract Body : Objective: To visualize the thoracodorsal nerve and its inputs from the brachial plexus in donor human bodies and reference its anatomical distribution to current anatomical literature.



Introduction: The thoracodorsal nerve provides motor innervation to the latissimus dorsi muscle. Current anatomical literature describes the human thoracodorsal nerve as a branch off the posterior division of the brachial plexus with contributions from nerve roots C6, C7 and C8. In non-human mammals, the thoracodorsal nerve also receives a communicating branch from the lateral thoracic nerve, a branch of the anterior division caudal pectoral nerve that innervates m. cutaneous trunci, a muscle sheet that acts on the skin of the trunk found in most non-human mammals. In humans, this muscle layer has been replaced with Scarpa’s fascia. The lateral thoracic nerve innervates the m. latissimus dorsi (posterior division), the m. cutaneous trunci (anterior division), and the m. pectoralis profundus (anterior division), which in humans is part of m. pectoralis major. This contradicts the prevailing anatomical context that the brachial plexus is strictly divided into anterior and posterior divisions. Previous dissections show the relationship between lateral thoracic n. (anterior division) and thoracodorsal n. (posterior division) in several groups of mammals, including non-human primates, felids, canids, and marsupials. Although the human brachial plexus does not include a lateral thoracic nerve, we hypothesized that there would be a connection from the anterior division of the brachial plexus with the thoracodorsal nerve.



Methods: Standard dissection techniques were used to expose the brachial plexus and thoracodorsal nerve on human donor bodies.



Results: In all three human donor bodies, we observed the thoracodorsal nerve derives from both the anterior division medial cord and from the posterior division posterior cord. In two human donor bodies, we also observed a connection between the anterior division medial pectoral nerve (non-human caudal pectoral nerve) and the thoracodorsal nerve.



Conclusion: The human brachial plexus is not strictly divided between developmentally anterior and developmentally posterior divisions. In non-human mammals, we interpret the nerve connections between m. latissimus dorsi, m. cutaneous trunci, and m. pectoralis profundus as forming a lateral sling to pull the trunk forward while the limb is stable during locomotion. While the human forelimb is no longer part of locomotion, we observe that this ancestral connection between the anterior and posterior divisions of the brachial plexus is retained.