Intriguing facts were obtained in the first electrophysiological investigations (1964) that the action potentials (AP) produced by direct depolarization of the cell membrane in different species of mollusks showed specific relations to changes in external ionic composition. In Helix neurons, the generation of AP was well maintained in sodium-free solutions with high calcium or barium content. The amplitude of the spike overshoot in the case was linearly related to the logarithm of calcium concentration. It is interesting that increase in external calcium ions decreased the ionic conductance of the resting membrane (R0) also in linear relation to the logarithm of Ca2+ or Ba2+ concentration. It was found for the first time (1965) that addition of Ba2+ to the external solution produced in the neurons well-developed prolonged (protracted) APs in all cases. However, the excitability of Planorbis and Limnea neurons was rapidly (during 3 min) reversibly depressed in sodium-free solutions. We found that, after injections of horse radish peroxidase (HRP) in the spinal cord of cats, the enzyme was transported retrogradely to brain stem neurons in the bulbar medial reticular formation, the vestibular complex, and the red nucleus. We obtained (1975) intriguing facts in our investigations: we recorded the labeled neurons in the locus coeruleus and subcoeruleus, as well as in the paraventricular hypothalamic nucleus. The existence of straight pathways from hypothalamus to the spinal cord has not been demonstrated earlier. The next step of our study was to determine the corresponding spinal funiculi where descending fibers (from various brain stem cell groups) are located. Indeed, in our pioneer studies (1977), we found that the fibers from the hypothalamus, which descend throughout the spinal cord, are located mainly in the lateral funiculus, ipsilaterally.