The sympathetic nervous system of anamniotes

The sympathetic nervous system develops as an evolutionary trait with gnathostomes (jawed vertebrates), but not with agnathan fishes (i.e., hagfishes and lampreys). Organization of the sympathetic preganglionic neuronal columns is different in teleosts and anurans. In the teleosts so far examined, the majority of sympathetic preganglionic neurons (SPNs) are located in the dorsal part of the spinal central gray matter. In Tetraodontiformes, the cell column occupies only two rostral spinal segments, which are distinct in their cytoarchitecture and projections. On the other hand, the SPNs of anurans form two cell columns segregated mediolaterally. The lateral and medial columns are also distinct in their cytoarchitecture and projections. The neuroactive substances expressed in the SPNs both in teleosts and anurans are coded to the projections. In anurans, the SPNs containing gonadotrophin-releasing hormone and those containing calcitonin gene-related peptide are involved in the regulation of blood vessels and cutaneous glands, respectively. In the filefish, the SPNs containing galanin project specifically to non-adrenergic non-cholinergic postganglionic neurons in the cranial sympathetic ganglia. Therefore, both anuran and teleost systems have different morphological and chemical-coded patterns for functional variation, although the anuran sympathetic nervous system has more organizational similarity with that of amniotes.