Vagal Nerve Stimulation and a bit of History on the Subject - Part 2

Galvani’s experiments changed how we look at nerves, from thinking that nerves function like waterpipes or channels as Descartes had proposed, to that of conductors that carry electricity generated by organic tissue. Sherrington acknowledged this in 1905 in his Silliman lectures at Yale.

In 1932 the Nobel Prize in Physiology or Medicine was shared by Lord Edgar Douglas Adrian and Sir Charles Scott Sherrington "for their discoveries regarding the functions of neurons" and their acceptance speeches summarize the state of knowledge 70 years ago.

In his Nobel lecture, entitled The Activity of Nerve Fibers, Adrian noted:

" The nerves do their work economically without visible change and with the smallest expenditure of energy. The signals which they transmit can only be detected as changes of electrical potential, and these changes are very small and of very brief duration….The grading and coordination of muscular activity is a subject which has been so greatly illuminated by my friend Sir Charles Sherrington that I mention my own work as a very small supplement to his. It has dealt as before with the signals which are sent by the individual nerve fibers and it’s results emphasize the close correspondence between sensory and motor activities of the nervous system…. On the whole it appears that the frequency of the impulses varies over a more restricted range in the motor than in the sensory discharge, but the two are so closely alike that the mechanism of the sense organ and of the motor nerve cell must have much in common. They have, of course, the common factor of nerve fiber which can only respond in one way, but the likeness goes beyond this. Also the particular frequencies which commonly occur are lower than they would be if determined solely by the characteristics of the nerve fibre. In quiet breathing for instance, at each expansion of the lungs the sense organs of the vagus send up a train of impulses rising to a frequency of about 20 a second at the height of inspiration, and simultaneously the movement of expansion is being produced by trains of motor impulses rising to a frequency of about 20 per second at the height of inspiration, and simultaneously the movement of expansion is being produced by indistinguishable from the discharge in the sensory fibers. In fact the motor nerve cells seem to be acting just like a collection of sense organs responding to a rhythmic stretch….. Resemblance’s of this kind show that there is an underlying unity of response in the various parts of the neurone in spite of their differentiation into axon, dendrites or terminal arborizations."

Sherrington’s lecture, Inhibition as a Coordinative Factor, is even more relevant to Vagal Nerve stimulation (VNS) as indicated by the following excerpts.

"That a muscle on irritation of it’s nerve contracts had already long been familiar to physiology when the 19^th century found a nerve which when irritated prevented it’s muscle from contracting. This observation seemed for a time too strange to be believed. It’s truth did not gain acceptance for ten years; but at last in 1848 the Webers accepted the fact at it’s face value and proclaimed the Vagus nerve to be inhibitory of the heart muscle. Two hundred years earlier Descartes, in writing the De Homine, had assumed that muscle was supplied with nerves which caused muscular relaxation. An analogous suggestion was put forward by Charles Bell in 1819. The inhibition suggested was in each case peripheral. The role of inhibition in the working of the central nervous system has proved to be more and more extensive and more and more fundamental as experiment has advanced in examining it. Reflex inhibition can no longer be regarded merely as a factor specially developed for dealing with the antagonism of opponent muscles acting at various hinge joints. It’s role as a coordinative factor comprises that, and goes beyond that. In the working of the central nervous machinery, inhibition seems as ubiquitous and as frequent as is excitation itself. The whole quantitative grading of the operations of the spinal cord and brain appears to rest upon mutual interaction between the two central processes "excitation" and "inhibition", the one no less important than the other."

It was by this circuitous route that electrical stimulation of nerve and muscle finally emerged as an investigation tool for the pursuit of medical application. The principles of depolarization or hyperpolarization of nerves and muscle membrane potentials serve as the basis of the effects noted. The nerve propagates signals that are not passive, but active and continually generated as an action potential along the length of the axon, which when it reaches the end of the axon, the signal changes from a digital all –or – none to a nonpropagated potential. At the synapse (term coined by Sherrington) it gives rise to an active/interactive phase of excitation or inhibition of adjacent neurons. These synapses resemble computer chips as they are components of a synergetic system that coordinates all the organs, tissues, and functions. They seem to have "memory" as it seems each synapse "knows" what every other synapse is doing…. Which is represented by the electroencephalogram (EEG). End of Part 2