General Morphology and Neurite Projection Patterns of Supramedullary Neurons in the Cunner, Tautogolabrus adspersus.

Supramedullary neurons (SMNs) are located on the dorsal surface of the hindbrain and spinal cord in many adults of teleost fishes. Comparative morphological studies have revealed differences in SMN size, number and distribution among fishes. However, little information exists on the projection patterns of SMN neurites within the central nervous system. We focus on a single species of adult teleost fish, the cunner, Tautogolabrus adspersus, a fish that has 25-46 SMN somata aligned dorsally along the midline in the hindbrain and rostral third of the spinal cord. Light and transmission electron microscopic techniques provide new information on the general morphology of SMNs and Lucifer yellow filled SMNs reveal that these neurons are not morphologically homogeneous. Rather, four different morphotypes can be identified based on projection patterns of major neurites in the hindbrain and spinal cord wholemounts. The morphological variations suggest that some SMNs exit the brain exclusively through the first dorsal root while others exit the central nervous system through a dorsal root and the trigeminal nerve. Despite the remarkable size and accessibility of SMNs, their function(s) remains unknown. One proposed role of these cells is eliciting mucous secretion in response to skin abrasion from predation. Variation in projection patterns in the cunner provides a morphological framework from which to design electrophysiological studies to help reveal SMN function.