Electron microscopic observations on the pit organ of a crotaline snake Trimeresurus flavoviridis.

The pit membrane in the pit organ of a crotaline snake is about 15 micron thick. Myelinated nerve fibers from the trigeminal nerve enter the pit membrane and swell into palmate structures on demyelination. Demyelinated fibers repeat branchings and their terminals contain many mitochondria. The terminal portion is not surrounded with Schwann cell processes. There are many small vesicles (30-60 nm in diameter) in the extra-cellular space in close association with the nerve terminal membranes. The spreading of branches from each palm seemed to have a definite territory which would correspond to the unit area detected with electrophysiological methods. But no "unit structure" was revealed by electron microscopy. intraepithelial free nerve endings exist in the outer epithelial layer. The nerve bundles from the trigeminal branches contain some unmyelinated fibers which come in close contact with vascular elements in the pit membrane.