Description of the neuroanatomy of the brachial plexus in South American lizards. Phylogenetic implications

Abstract

The nervous system of reptiles can be divided into the central nervous system (CNS - brain and spinal cord), and the peripheral nervous system (PNS - the spinal nerves and autonomic ganglia). The spinal cord exhibits two intumescences at the level of the extremities from where nerves emerge, forming the brachial and lumbosacral plexuses. The nerves which form the brachial plexus constitute a complex network that innervates the muscles of the shoulder, forelimb, and some respiratory muscles. Very few studies on the anatomy of the nerve plexuses and musculature associated with them were performed in reptiles. Based on differentiated Sudan Black B staining and conventional dissections, we describe the neuroanatomy of the brachial plexus, its main associated nerves, and muscles. For that, representatives of the genera Diplolaemus, Liolaemus, Phymaturus, and Tropidurus were selected. Based on this, potential useful characters for phylogenetic analysis were described. In this regard, our results show that the brachial plexus can be formed by four, five, or six nerve branches. The brachial flexor trunk, circumflex, interosseous, median, radial, subscapulocoracoid, supracoracoid, and ulnar nerves were identified. Regarding themuscles innervated by the main nerves, the following muscles were identified: biceps brachii, deltoideus scapularis, latissimus dorsi, levator scapulae, pectoralis, serratus thoracis, trapezius, triceps longus caudalis, and triceps longus lateralis. Phylogenetic analyzes revealed 31 potential synapomorphies. There exists concrete evidence that neuroanatomy studies in a phylogenetic context could provide useful information helping to elucidate the relationships between taxonomic groups.