Fine structure and organization of the infrared receptor relays: lateral descending nucleus of V in Boidae and nucleus reticularis caloris in the rattlesnake.

The morphology of the lateral descending nucleus of V (LTTD) in three species of Boidae and nucleus reticularis caloris (RC) of the rattlesnake have been studied with the light and electron microscope. First- and second-order relays in the infrared receptor pathway to the tectum are contained within LTTD in the Boidae, whereas in the rattlesnakes the secondary relay to tectipetal neurons is in nucleus RC. The lateral descending nucleus in the boids contains small and large neurons. The larger cells project to the optic tectum and morphologically are quite similar to those of nucleus RC. It has been determined at the ultrastructural level that LTTD of the three species of boids studied have very similar morphology and organization. A marginal neuropil, located near the lateral descending tract, consists of terminals of thin unmyelinated axons in synaptic contact with thin dendrites. Deeper within the nucleus primary afferent terminals containing clear spherical vesicles form synaptic clusters with dendrites and are post-synaptic to other axon terminals containing pleomorphic vesicles. The large, tectipetal neurons are postsynaptic to two morphological types of synapses, one with clear spherical vesicles, asymmetric membranes, and subsynaptic web and the other with flattened vesicles and symmetrical membranes. Similar synapses are also present on the cells of nucleus RC in the rattlesnake. There is a close similarity in function, structure, and synaptic organization between the LTTD and boids and the LTTD plus nucleus RC of pit vipers, suggesting similar or identical evolutionary origin.