Ultrastructure of the somatic motor system in Bothrops jararaca (Reptilia, Squamata, Viperidae).

Although the ultrastructural characteristics of motor neurons, their myelin-forming cells, and skeletal muscle fibers are very well described in mammals, much less is known in reptiles, especially snakes. In this context, two specimens of Bothrops jararaca were euthanized for morphological analysis of the somatic motor system (SMS). Samples of the thoracic spinal cord, thoracic spinal nerves, and epaxial muscles were collected and processed for investigation by transmission electron microscopy. In the central nervous system (CNS), motor neurons and their myelin sheaths produced by oligodendrocytes presented ultrastructural characteristics similar to those previously described in mammals. Likewise, thoracic somatic nerves, their Schwann cell-derived sheaths, and the innerved skeletal fibers showed comparable features. Morphological similarities between reptilian and mammalian neural cells are probably linked to their evolutionary conservation throughout vertebrate phylogeny.