Factors Affecting Transmission and Recovery in the Passive Iron Nerve Model

Abstract

The view that the essential factor in the transmission of the state of excitation from region to region in an irritable protoplasmic system (such as nerve) is a form of electrolytic distance action, dependent on the formation of local electrical circuits between the active and the resting regions of the protoplasmic surface, 1 has various implications, some of which can be tested by experiment. It is to be expected, for example, that the speed of transmission will be a direct function of the electrical conductivity of the external medium, since this medium is traversed by the local current between the active and the resting regions. This current, in order to have stimulating effect at a given distance from the active region, must have a certain intensit), at that distance, corresponding to the threshold intensity of electrical stimulation under the conditions; evidently this intensity will be decreased if the electrical conductivity of the medium is lowered. According to the local action theory of transmission, the speed (V) of the activation wave is proportional to the product of this critical distance (s) into the rate (r) of the local activation process (V -st); hence any decrease of external conductivity (other conditions remaining equal) must, by decreasing s, decrease correspondingly V. A close proportionality between the conductivity of the medium and the speed of transmission has in fact been observed in the nervous network of the medusa" in various dilutions of sea water, and in vertebrate and invertebrate muscle? Since 1LiUie, R. S., Am.

The “Lillie Transition”: models of the onset of saltatory conduction in myelinating axons

“…Impulses still propagated in continuous mode beneath the myelin but instead of traveling orthodromically in the same direction (which for convenience we will refer to as "proximal" to "distal") they spread in both directions from each node, the two oppositely-propagating impulses of a given internode meeting and annihilating in mid-internode. The same effect was reported by Lillie (1925) in his iron wire model. With further tightening, the firing of nodes (abrupt corners in the waveform of Fig.…”

“…We have examined theoretically the experiments of Lillie (1925) demonstrating that by insulating short segments of an active core conductor one can achieve a more rapid saltatory mode of impulse conduction. This we have done using the more physiologically realistic formulation for nerve impulse conduction developed by Hodgkin and Huxley in 1952, helping bridge Lillie's original studies to the issues of the evolution and development of myelin under biological constraints.…”

“…δ was decreased to 1.6 μm, conduction velocity steadily dropped to a minimum of 1.60 m s -1 . This 8% decrease can be attributed to the increased resistance in the extracellular submyelin current-return path as described by Lillie (1925). Further decreasing δ below 1.6 μm resulted in a reversal in the downward trend followed by a steep but not discontinuous increase in conduction velocity, along with a significant change in the behavior of signal propagation: "saltatory" conduction.…”

“…4. Were there no "nodes" in the myelin, the conduction velocity would continue to drop as the gap narrows, decreasing with the square root of the submyelin cross sectional area, as shown by Lillie (1925) in the iron-wire model (shown as the dotted curve in Fig. 4).…”

“…In 1925, Ralph Lillie discovered the mode of action of myelin (Lillie 1925). He had been studying a model of nerve impulse conduction consisting of an iron wire immersed in nitric acid, which when electrically or mechanically activated, triggered an electrochemically-mediated local breakdown and reestablishment of the nitrate coating that propagated along the wire much as occurs in unmyelinated nerve.…”