CONDUCTIVITY
_Contents_: Only processes of excitation are conducted, not processes
of depression. Conduction of excitation in its two extreme instances.
Conduction in undifferentiated pseudopod protoplasm of rhizopoda.
Conduction of excitation with decrement of intensity and rapidity.
Conduction of excitation in the nerve. Rapidity of conduction of
excitation without decrement. Relation between irritability and
conductivity. Conduction of excitation with decrement of the nerve
after artificial depression of irritability by narcosis. Theory of the
decrementless conduction of the normal nerve. Proof of the validity of
the “all or none law” in the medullated nerve. Theory of the process
of the conductivity of excitation. Theory of core model (Kernleiter).
Electrochemical theory of conduction based on the properties of
semi-permeable surfaces.
When the response to a stimulus is studied in a living system, whether
it be a single cell, a tissue, or a complex organism, the indicator
used, either that of movement, current of action, production of certain
substances, the development of light, of heat or the alteration of
form, is the result of two distinct processes. The first of these is
primary excitation, brought about by the stimulus at a local point, and
the second is an extension of the excitation to the surrounding tissue.
We are not in a position to experimentally bring about a response
to stimulation, in which the primary excitation occurs and not the
secondary process, that of conductivity. All living substance contains
this property, although to a very different degree, as all living
substance possesses irritability, and this presents the condition not
only for the taking place of the process of excitation but also that of
its conduction.
If I here speak only specifically of the conduction of excitation
instead of the conductivity of response to stimulation this is not
only primarily for the reason that we intend especially to analyze the
conductivity of excitation on this occasion, but also because no other
effects of stimulation except those of excitation can be conducted from
the part affected by the stimulus to the surroundings.
Although considered on theoretical grounds it appears more or less
improbable that depression is extended from the place of its origin,
it is very easy to convince one’s self experimentally of the fact
that depression following a stimulus is invariably localized to that
portion directly affected by the stimulus. The nerve furnishes a very
favorable object for this purpose. If a nerve muscle preparation of the
frog is made and introduced in the glass chamber previously described
containing platinum electrodes, and another pair is applied to the
nerve between the chamber and the muscle, it is possible to subject
the stretch of nerve in the chamber to various agents, producing a
paralyzing effect. In this way it may be exposed to an atmosphere of
pure nitrogen for example, or to narcosis as by ether, chloroform,
carbon dioxide and other gases, to an increase in temperature or to
other agents, without these in any way affecting the irritability of
the nerve stretch situated over the electrode between the chamber and
the muscle. The contractions of the muscle, which are produced by
stimulation of the periphery region of the nerve with stimuli of a
definite strength, remain unaltered, even when the asphyxiated stretch
of nerve in the chamber is already completely degenerated. The central
depression of a ganglion cell of a motory neuron is likewise wholly
without influence on the degree of excitability of its nerve fiber, as
I was able to demonstrate[79] in the reflex inhibition of the motor
neurons of the spinal cord of the dog. (Figure 14.) That which is
conducted by the nerves is solely the process of excitation.
[79] _Max Verworn_: “Zur Physiologie der nervösen
Hemmungserscheinungen.” Arch. f. Anat. u. Physiol. physiol. Abt.
Suppl. 1900.
It is our task to analyze in detail the conditions involved in the
conduction of excitation in order to obtain a deeper insight into the
physics of this process. A comparative survey of a series of various
types of living substance shows us that they differ in respect to the
conduction of excitation in the following points: In regard to the
rapidity with which the excitation is conducted, the extent of the area
over which it spreads, and the intensity with which it extends. These
conditions may be best illustrated by citing two extreme examples. The
one is formed by the rhizopods, the other by the nerve fibers. Between
these two extremes we have manifold gradations in the conditions of
conductivity. Not all cell forms are suitable objects for the study of
conductivity. There are forms of rhizopods which are as favorable to
investigation as the nerve; this is due to the fact that, although
they are often of microscopic dimensions, they possess elongated
fingerlike or threadlike pseudopods.
[Illustration: Fig. 14.
Contractions of the musculus extensor digitorum communis longus of the
dog, brought about by rhythmic stimulation of the nervus peroneus. The
muscle is in the condition of tonic excitation which proceeds from
the center. The arrows indicate the point where reflex inhibition of
the central tonus is produced. The height of the single contraction
undergoes no diminution. ]
Indeed, a rhizopod cell, with its straight, elongated pseudopods,
is preëminently fitted as an object of comparison with a neuron.
Although the difference in respect to the individual points is so
far-reaching, still, based on their outward morphological similarity
various physiological parallels in both are forced on our observation.
A comparison of the rhizopod cell with the neuron can consequently
guard us from many erroneous generalizations which we might be
inclined to deduce from a one-sided investigation of the nerve. This
is especially the case in regard to the conductivity of excitation,
which was formerly studied almost exclusively on the nerve and only
occasionally on the muscle, which offers similar conditions. The nerve,
in which the function of the conductivity of excitation is particularly
highly developed, was considered at the same time as the type in which
this process could be most readily analyzed, and from which it was
believed general information of the process of the conductivity of
excitation could first be gained. This view has led to serious errors,
as the nerve, resulting from the high development of its conductive
capability, shows quite one-sided specialized conditions, which can by
no means be transferred to other forms of living substance.
A very suitable object among rhizopods for the study of conductivity,
and which is everywhere easily procured, is _Difflugia_. This species
living in small pools has a delicate urn-shaped, pear-shaped or
flask-shaped capsule built up of sand grains, diatomes or material
produced by the organism itself. From the opening the protoplasm
extends often to a considerable length its finger-shaped hyaline
pseudopods. When _Difflugia_ is placed in a flat dish in water and
observed under the microscope, it is frequently seen to extend from the
opening long pseudopods in exactly opposite directions, which reach
for a considerable distance on the bottom. These offer particularly
favorable conditions for the study of the conduction of excitation.
When this animal is placed under a microscope, the pseudopods are
very readily stimulated at any position to a desired extent by means
of a sharp needle, to which fat has been previously applied and
subsequently the excess removed. The extension of the response from
one point toward the other can then be followed with great ease.
The pseudopod of the rhizopod has the great advantage over the
nerve that its excitation can be directly observed. The excitation
following weaker stimulation is manifested by a wrinkling of the
previously completely smooth surface; stronger stimulation produces
differentiation of the hyaline protoplasm to a strongly refractive
strand in the axis and a turbid myelinlike mass at the periphery, the
pseudopod at the same time retracting toward the central cell body. In
spite of all these occurrences being of microscopic dimensions, still
with some practice it is quite possible to experiment on them under the
microscope. In this way I found it comparatively simple to study the
fundamental principles of conductivity.[80]
[80] _Max Verworn_: “Psycho-physiologische Protistenstudien.
Experimentelle Untersuchungen.” Jena 1889.
[Illustration: Fig. 15.
_Difflugia urceolata._ A--Weak local stimulation at the end of a long
extended pseudopod. B--Stronger local stimulation applied to the end of
a long pseudopod.]
[Illustration: Fig. 16.
_Difflugia urceolata._ A--In non-stimulated condition. B--The same
individual locally stimulated in the middle of a long extended
pseudopod. The excitation spreads in both directions, centripetal as
well as centrifugal. ]
All these factors, the intensity with which the excitation extends from
the point of stimulation, the rapidity of the extension, and finally
the area over which conduction takes place, are manifestations of the
intensity of stimulus, and as such alter with these in corresponding
manner. If the end of a pseudopod is barely touched and thereby weakly
stimulated, the response is restricted to a slight wrinkling of the
surface, which slowly extends to the immediate neighborhood, whilst
the more distant parts of the pseudopod are not affected at all by
the excitation. (Figure 15, A.) On a stronger stimulation of the
pseudopod by slight pressure, the response is likewise stronger, and
the characteristic differentiation of the protoplasm, consisting in
the strongly refractive strand in the axis and the turbid myelinlike
outer mass, appears at the point of stimulation. From here a peculiar
alteration spreads gradually further over the pseudopod, in that
first upon its smooth surface a few myelinlike droplets are seen,
which become larger and with the development of the strand in the
axis, dissolve into a wrinkled mass on the surface. The further this
process extends from the point of stimulation, the weaker it becomes
and the more slowly it proceeds, until at last there is complete
disappearance. (Figure 15, B.) The pseudopod has at the same time
retracted to a considerable degree. If a still stronger stimulus is
applied by firm pressure at the end of the pseudopod the process takes
place with much greater violence. The differentiation of the protoplasm
spreads centripetally from the point of stimulation over the whole
pseudopod with great rapidity, and produces a quick retraction in the
same, then involves the oppositely directed pseudopod, in which it
then extends more and more slowly, until, proceeding in a centrifugal
direction, it is at last gradually completely obliterated. When strong
stimulation is applied, the process occurs with such rapidity that the
contraction of the pseudopod is almost twitchlike. As the rapidity of
the conduction alters within a wide limit according to the strength
of the stimulus and the distance from the point of stimulation, it
is self-evident that no constant figure can be stated. To give a
general idea of the rapidity, they might be estimated according to
observations I have made with second watch and ocular-micrometer as
from within a slight fraction to that of a millimeter in the second.
When a very long extended pseudopod is locally stimulated in the
middle, the response spreads from the point affected in both directions
diminishing in intensity and rapidity. The excitation extends equally
in all directions. (Figure 16.) These facts show very clearly that
in _Difflugia_ the excitation following a localized stimulus is
dependent on the intensity of the stimulus, and that according to
the degree of this, the wave progresses in either stronger, more
rapid and extended, or weaker, slower and more limited manner. With
the greater distance from the point of stimulation the excitation
undergoes an increasing decrement of its intensity and rapidity of
conduction. Different species of _Difflugia_ which I have investigated,
_Difflugia lobostoma_, _urceolata_, _pyriformis_, have shown a
complete conformity in this respect. A great number of other fresh
water and marine rhizopods, the pseudopods of which I have used for
analogous experiments, although differing in the manner of reaction
in regard to the extent and rapidity of the course of excitation,
manifest exactly the same fundamental principles. A very favorable
form is, for instance, the much smaller _Cyphoderia margaritacea_,
which is distinguished by a somewhat higher degree of excitability and
rapidity of reaction.[81] The long straightly extended pseudopods are
thinner and more threadlike than those of _Difflugia_ and show upon
stimulation as a result of their local excitation a simple contraction
into clumps of the stimulated protoplasm without the characteristic
differentiation of that of _Difflugia_. (Figure 17.) In the case of
the marine rhizopods, _Orbitolites_ (Figure 19), _Amphistegina_, etc.,
which I investigated at the Red Sea, the conduction of excitation takes
place also as in _Difflugia_ with a decrement of intensity and rapidity
becoming larger with the distance from the point of stimulation until
the wave of excitation is obliterated.
[81] _Max Verworn_: “Die Bewegung der lebendigen Substanz.
Eine vergleichend physiologische Untersuchung der
Contractionserscheinungen.” Jena 1892.
[Illustration: Fig. 17.
_Cyphoderia margaritacea._ Result of localized mechanical stimulation
at the end of a long extended pseudopod. A, B, C--three successive
stages.]
[Illustration: Fig. 18.
_Cyphoderia margaritacea._ Result of localized mechanical stimulation
in the middle of a long extended pseudopod.]
[Illustration: Fig. 19.
A pseudopod of Orbitolites complanatus (cf. Fig. 7). _a_--In normal
condition. _b_--Severed by a cross section near the end. _b-f_--Five
successive stages of the effect. _b-d_--The pseudopod retracts by
centripetal flowing of the protoplasm contracted in the shape of
microscopic balls and spindles. _e_ and _f_--The pseudopod begins to
extend again. The centripetal flowing balls and spindles begin to
disappear. ]
A sharp contrast to this type is formed by the other extreme as
represented by that of the medullated nerve. As an indicator of the
course of excitation we will take the action current in an isolated
nerve of the frog. If this is stimulated at one end, we can test the
intensity of the conducted excitation by leading off the action current
from two points at varying distances from the one influenced by the
stimulus. Since the classical discovery of _Du Bois-Reymond_ of the
action current of the nerve, we know that in the fresh medullated
nerve, if observed under favorable experimental conditions, no
decrement of intensity of excitation during its course from the point
of stimulation along the length of the nerve can be demonstrated.[82]
If unpolarizable electrodes are applied to a nerve in such a position
that they are equidistant from the cross section and are connected with
apparatus for testing the current, it will be found that there exists
an “unwirksame Ableitung” in the sense of _Du Bois-Reymond_, that is,
in which there is no demarcation current. When a tetanizing current is
applied to one end of the nerve, no difference of potential between the
two nonpolarizable electrodes is observed, which indeed would be the
case if excitation with its current of action would have a decrement
on its way from one to the other point of leading _off_ the current.
_This fact, which has been repeatedly confirmed, shows us that the
medullated nerve, under normal conditions, conducts excitation without
a perceptible decrement of the intensity._
[82] _Du Bois-Reymond_: “Untersuchungen über tierische Electricität.”
II Band. 1849.
This specific property of a medullated nerve is in conformity with
the conditions in connection with the rapidity of conductivity. Since
_Helmholtz_[83] has devised the method for measuring the rapidity of
conduction in the nerve, this investigator himself and numerous others
have studied the rate in different nerves.[84] _Helmholtz_ found the
rate for motor nerves of the frog to be 27 meters per second, for
the sensory nerves of man 60 meters, and the motor nerves of man 34
meters. Other investigators have obtained quite different results;
_Hirsch_, for the sensory nerves of man, 34 meters; _Schelske_, for the
same, 25–33 meters; _De Jaager_, 26 meters; _v. Wittich_, 34–44 meters,
and _Kohlrausch_, 56–225 meters; _v. Wittich_ for the motor nerves of
man, 30 meters; _Piper_[85] finally in the most recent investigations
about 120 meters in the second.
[83] _H. Helmholtz_: “Messungen über den zeitlichen Verlauf der
Zuckung animalischer Muskeln und die Fortpflanzungsgeschwindigkeit
der Reizung des Nerven.” Müller’s Archiv. 1850.
The same: “Messungen über die Fortpflanzungsgeschwindigkeit der
Reizung in den Nerven.” Zweite Reihe, Müller’s Arch. 1852.
[84] Compare: _Hermann_: “Handbuch der Physiologie.” II, 1 Leipzig
1879.
[85] _Piper_: “Ueber die Leitungsgeschwindigkeit in dem markhaltigen
menschlichen Nerven.”
The same: “Weitere Mitteilungen über die Geschwindigkeit der
Erregungsleitung im markhaltigen menschlichen Nerven.” Pflügers Arch.
Bd. 127, 1909.
These differences may be explained in a _large_ measure by the variety
of the methods used, in part also by the difference in the structures.
The methods employed for the study of the velocity have also been used
to solve the question, whether the velocity of the excitation wave in
its course over the nerve meets with a decrement as it moves further
and further away from the point of stimulation. Here the endeavor was
made to study the difference in time of the latent period, which is
observed by the indicator, when the nerve is stimulated at two points
at different distances from the muscle, used as an indicator, or
from the wires leading the current to the indicator. The more recent
investigators, as _René Du Bois-Reymond_,[86] _Engelmann_,[87] _G.
Weiss_,[88] have arrived at the same conclusion, that the rate of
conductivity in the medullated nerve under normal conditions is the
same at all distances from the point of stimulation. (Figure 20.)
[86] _R. Du Bois-Reymond_: “Ueber die Geschwindigkeit des
Nervenprincips.” Arch. f. Anat. u. Physiol. physiol. Abt. Suppl. 1900.
[87] _Engelmann_: “Graphische Untersuchungen über die
Fortpflanzungsgeschwindigkeit der Nervenerregung.” Arch. f. Anat. u.
Physiol. physiol. Abt. 1901.
[88] _G. Weiss_: “La conductibilité et l’excitabilité des nerfs.” In
Journ. de Physiol. et de Pathol. générale 1903.
The medullated nerve shows, therefore, under normal conditions
neither a decrement of its conductivity, nor of its _irritability_,
as the distance of the wave of excitation increases from that of the
position of stimulation; this means, in other words, that excitation is
conducted with the same intensity with which it is started, and with a
constant rate throughout the entire course of the nerve.
[Illustration: Fig. 20.
Curves of muscle contraction obtained by stimulation of 3 and 4 points
situated at equal distances from each other on the sciatic nerve of the
frog. The increase of length of the nerve stretch corresponds with an
equal increase of the latent period of contraction. From this follows,
that the rapidity of the wave of excitation is the same at all points
of the entire length of the nerve. (After _Engelmann_.) ]
There is, nevertheless, a third point of considerable difference
between the types of conduction of excitation in the rhizopods and
in the nerve. Whereas in the rhizopods the rapidity of conduction
is dependent upon the _intensity_ of the stimulus, it has been long
known as the result of investigation by _Rosenthal_, _Brücke_ and
_Lautenbach_ and at a more recent date by _Gotch_[89] and _Piper_,[90]
that in the nerve of the frog, as well as in man, the velocity is _not_
dependent upon the intensity of stimulation. (Figure 21.) Contrary
results have been obtained by a few early observers wherein the latent
period was shorter when the stimulation was strong. _Nicolai_[91]
explains this shortening of the latent period, resulting from the
application of strong electrical stimuli, to a spreading out of the
“Stromschleifen” from the point of application and consequently
there is a shortening of the stretch of nerve between the point of
stimulation and the indicator.
[89] _Gotch_: “The submaximal electric response of nerve to a single
stimulus.” Journal of Physiology, Vol. XXVIII, 1902.
[90] _Piper_: Ueber die Leitungsgeschwindigkeit in den markhaltigen
menschlichen Nerven. Pflügers Arch. Bd. 124, 1908, und Bd. 127, 1909.
[91] _Nicolai_: “Ueber Ungleichförmigkeiten in der
Fortpflanzungsgeschwindigkeit des Nervenprincips, nach Untersuchungen
am marklosen Riechnerven des Hechts.” Arch. f. Physiologie 1905.
[Illustration: Fig. 21.
Course of the action current of the nerve. The thin line indicates the
action current produced by a weak, the thick line the action current
produced by a strong stimulus. The duration of the action current is
the same in both cases. (After _Gotch_.) ]
This conspicuous difference in the conduction of the two extreme types
of living substance, which we have already observed, arouses the
question as to what properties of living substance bring about these
differences. In order to answer this question, it is necessary, first
of all, to make some general statements concerning the processes of
conductivity.
As already emphasized, all living substance possesses the capability of
conducting excitations to a definite degree. We may, therefore, assume
that the same fundamental _property_ of conductivity exists in all
substances. A fact to be considered in the conduction of excitation, is
that the primary breaking down of the complex molecules at the position
of stimulation act in turn as exciting stimuli upon the neighboring
portion of the living substance, which in turn undergoes a similar
decomposition. And so this process continues. This fact is evident from
the observations on the process of excitation. But the nature of the
stimulus which produces the breaking down of the complex molecules
upon the surrounding molecules is a problem which can only be studied
later. Here only one point will be mentioned in advance concerning the
intensity of the stimulus. It is apparent from the experiments on the
rhizopods, that the greater the intensity of the stimulus the more
extensive must be the breaking down of the living substance. A stronger
primary stimulation must also secondarily produce a stronger stimulus
in the neighborhood. In other words: the _conduction of excitation_
is a function of irritability. The greater the irritability, that is,
the greater the number of molecules broken down in a unit of time
and space by a stimulus of a certain intensity, the greater also is
the conductivity of the living system, that is, the stronger, the
more rapidly and the further excitation is extended. Conductivity
of excitation is, therefore, unthinkable without irritability. Both
are inseparably connected. The conclusion forced upon us by this
chain of reasoning admits of no argument. Nevertheless the endeavor
has been made, because of certain evidence at hand, to show that
the property of conductivity could exist without irritability. A
number of authors, such as _Schiff_,[92] _Erb_,[93] _Grünhagen_,[94]
_Effron_,[95] _Hirschberg_[96] and _G. Weiss_,[97] have observed the
fact that in spite of a more or less strong decrease of _excitability_
of a stretch of nerve, stimuli applied above this stretch can still
produce a conduction of excitation through the affected part. They have
concluded from this that it is possible to separate the conductivity
from irritability. _Erb_ and _G. Weiss_ have even gone so far as
to directly express the opinion that capability of conduction and
irritability involve two different histological elements. In contrast
to this, other investigators, such as _Hermann_,[98] _Szpilmann_ and
_Luchsinger_,[99] _Gad_,[100] _Piotrowski_[101] and _Wedenski_,[102]
have more or less decidedly taken the stand that an actual separation
of irritability and of conductivity does not exist. The apparently
contradictory evidence as well as the conflicting theoretical views
have been cleared up by _Werigo_,[103] _Dendrinos_,[104] _Noll_[105]
and _Fröhlich_.[106] These investigators have shown that the length
of the narcotized stretch of the nerve plays an important rôle in the
obliteration of conductivity. It has been found by the application
of a stimulus above the narcotized stretch of nerve, that the longer
this stretch is, the less is the reduction of irritability which
obliterates the excitation wave reaching this area. Further: The
shorter the stretch, the greater must be the reduction in irritability
before this result is brought about. (Figure 22.) In other words, the
conductivity in the narcotized nerve is dependent upon the length and
the irritability of the narcotized stretch. From this observation the
important fact is evolved, that the wave of excitation meets with a
decrement of its intensity in the narcotized area. This decrement
becomes larger as the wave progresses through the involved stretch.
Further it is progressively increased as the amount of the irritability
is reduced. Finally, when the stretch is long enough, the wave of
excitation is obliterated. This important fact has been further
established by the experiments of _Boruttau_ and _Fröhlich_,[107] in
which they studied the intensity of the current of action, produced by
a wave of excitation, from two points in the narcotized stretch. The
wave of negative variation, brought about by the excitation, gradually
decreases in the narcotized stretch as the electrode is further removed
from the point of entrance. Beside a decrement of _intensity_, as
the investigations of _Fröhlich_[108] prove, the wave of excitation
shows a decrement of the velocity in the narcotized stretch. And it
is probable that the wave of excitation extends with _progressive_
reduction in the velocity, corresponding to the decrement of intensity.
The work of _Koike_[109] under the direction of _Garten_, in which the
conclusion arrived at is that the reduction in the velocity is the same
throughout the narcotized area, should not be accepted as conclusive
in spite of the delicate method employed. These investigations are
extremely difficult, being in the field of the most delicate of
present-day methods. The decrement, which the wave of excitation meets
with in its progress in the narcotized stretch, makes the conflicting
testimony concerning the apparent separation of irritability and
conductivity intelligible. It depends entirely upon the length of
the narcotized area, and the amount of reduction in irritability on
the one hand, and the strength of the stimulus used for testing the
irritability on the other, whether the conductivity will disappear
_before_ the irritability or _vice versa_. If I test the irritability
in the narcotized stretch with a weak stimulus, just slightly _above_
the threshold, then by slight reduction in the irritability complete
absence of response occurs, when the same stimulus is applied. This
occurs at a time when excitation reaches the narcotized area from
above and meets with a decrement so slight that it can pass through
the whole narcotized stretch, that is, when the narcotized stretch is
short enough. If I test the irritability of the narcotized area with a
strong stimulus, far above that of the threshold, irritability will be
found to be present at a time when the conductivity for the excitation,
coming from above, is already obliterated. This is due to the fact that
the decrement in the narcotized area is already great enough to bring
about the complete disappearance of the wave of excitation coming
from above. This, of course, only occurs provided the length of the
narcotized stretch is great enough. The separation of conductivity
and irritability is, therefore, only an apparent one. In reality, the
facts obtained from experimentation indicate that with the reduction of
irritability the decrement of the wave of excitation increases, whilst
the shorter the stretch, the smaller is the decrement. This shows that
_conductivity is a manifestation of irritability_.
[92] _Schiff_: “Über die Verschiedenheit der Aufnahmsfähigkeit und
Leitungsfähigkeit in dem peripherischen Nervensystem.” Henle u.
Pflügers Zeitschr. 1866.
[93] _Erb_: “Zur Pathologie und pathologischen Anatomie
peripherischer Paralysen.” Deutsches Arch. f. Klin. Med. 1869.
[94] _Grünhagen_: “Versuche über intermittierende Nervenreizung.”
Pflügers Archiv. Bd. 6, 1872.--_Funke-Grünhagen._ Lehrbuch der
Physiologie Bd. I, 1876.
[95] _Effron_: “Beiträge zur allgemeinen Nervenphysiologie.” Pflügers
Arch. Bd. 36, 1885.
[96] _Hirschberg_: “In welcher Beziehung stehen Leitung und Erregung
der Nervenfaser zu einander?” Pflügers Arch. Bd. 39, 1886.
[97] _G. Weiss_: “La conductibilité et l’excitabilité des nerfs.”
Journ. de physiol. et de pathol. générale. T. V. 1903.--“Influence
des variations de temperature et des actions méchaniques sur
l’excitabilité et la conductibilité des nerfs.” _Ibidem._
[98] _Hermann_: “Handbuch der Physiologie.” Bd. II, I Leipzig 1879.
[99] _Szpilmann und Luchsinger_: “Zur Beziehung von Leitungs- und
Erregungsvermögen der Nervenfaser.” Pflügers Arch. Bd. 24, 1881.
[100] _Gad_: “Ueber Trennung von Reizbarkeit und Leitungsfähigkeit
des Nerven.” (Nach Versuchen des Herrn Sawyers) Arch. f. Anat. u.
Physiol. physiol. Abt. 1888.
Derselbe: “Ueber Leitungsfähigkeit und Reizbarkeit des Nerven in
ihren Beziehungen zur Längs- und Querschnitts erregbarkeit.” Nach
Versuchen des Herrn Piotrowski Arch. f. Anat. und Physiol. physiol.
Abt. 1889.
[101] _Piotrowski_: “Ueber Trennung von Reizbarkeit und
Leitungsfähigkeit des Nerven.” Arch. f. Anat. u. Physiol. physiol.
Abt. 1893.
[102] _Wedenski_: “Die fundamentalen Eigenschaften des Nerven unter
Einwirkung einiger Gifte.” Pflügers Arch. Bd. 82, 1900.
The same: “Excitation, inhibition et narcose.” Compt. rendus du v.
Congres internat. de Physiologie à Turin 1901.
[103] _Werigo_: “Zur Frage über die Beziehungen zwischen Erregbarkeit
und Leitungsfähigkeit des Nerven.” (Nach Versuchen von stud.
Rajmist.) Pflügers Arch. Bd. 76, 1899.
[104] _Dendrinos_: “Ueber das Leitungsvermögen des motorischen
Froschnerven.”
[105] _Noll_: “Ueber Erregbarkeit und Leitungsvermögen des
motorischen Nerven unter dem Einfluss von Giften und Kälte.” Zeitsch.
f. Allgem. Physiol. Bd. III, 1907.
[106] _Fr. W. Fröhlich_: “Erregbarkeit und Leitfähigkeit des Nerven.”
Zeitschr. f. allgem. Physiol. Bd. III, 1904.
[107] _Boruttau und Fröhlich_: “Erregbarkeit und Leitfähigkeit des
Nerven.” Zeitschrift f. allgem. Physiologie Bd. IV, 1904. The same:
“Electropathologische Untersuchungen ueber die Veränderungen der
Erregungswelle durch Schädigung des Nerven.” Pflügers Arch. Bd. 105,
1904.
[108] _Fröhlich_: “Die Verringerung der Fortpflanzungsgeschwindigkeit
der Nervenerregung durch Narkose and Erstickung des Nerven.”
Zeitschrift allgem. Physiologie Bd. III, 1904.
[109] _Izuo Koike_: “Ueber die Fortleitung des Erregungsvorgangs in
einer narkotisierten Nervenstrecke.” Zeitsch. f. Biologie Bd. 5, 1910.
[Illustration: Fig. 22.
Scheme of the decrement of the excitation wave in the narcotized
stretch of a nerve. A--The narcotized stretch (indicated by the cross
section of the chamber) is 30 mm. long. The ordinates of the dotted
lines indicate the amount of the decrement. If the decrement is slight
(upper dotted line), the excitation wave passes the narcotized stretch
and increases again on entering the normal stretch. If the decrement is
great (lower dotted line), the excitation wave is obliterated towards
the end of the narcotized stretch and the muscle remains at rest.
B--The narcotized stretch is 15 mm. long. The decrement is slight.
The excitation wave can therefore pass into the normal stretch and
here increase again. C--The narcotized stretch is 15 mm. long. The
decrement is great. The excitation wave is obliterated, therefore, in
the narcotized stretch, and the muscle remains at rest. ]
The facts just mentioned have, however, a much deeper meaning. They
show us that it is possible by means of narcosis to convert an extreme
type of a living system, with decrementless conductivity, into another
extreme type of living substance, in which excitation in its progress
meets with a strong decrement, like that seen in the rhizopods. The
same results may also be obtained by asphyxiation and other forms of
temporary and permanent injury of the nerve. We are, therefore, in the
fortunate position in the case of the medullated nerve of having a
substance to study, which, depending upon conditions which are under
our control, may become a type in which conductivity occurs with
or without the presence of a decrement. We can likewise reduce the
irritability to various degrees, producing all intermediate gradations
between the two extremes. This latter is particularly valuable in that
it permits us to study the conditions in one and the same substance
necessary to bring about the various peculiarities of conductivity. The
great differences in the conductivity of excitation are conditioned by
variations in the degree of irritability. If the irritability of the
nerve is at the normal level the wave of excitation progresses to the
end of the nerve without manifesting a decrement of its intensity or
rapidity.
If the level of irritability of the intact nerve is artificially
reduced, the wave of excitation meets with a greater decrement and
reduces in velocity, and in fact disappears the more quickly in the
stretch of nerve, as the reduction in irritability is increased.
These three factors, irritability, intensity and velocity of the
progress of the wave of excitation, are inseparable. All living
substances may be grouped according to their capability of conducting
excitation into a long series, starting with those possessing the
least irritability, as we found in the rhizopods, then those having
greater irritability, as the smooth muscle and ganglion cells, then
those with still greater irritability, as the striped muscle, and
finally those having the greatest degree of irritability, as the
medullated nerves of the warm-blooded animal. Should the processes of
excitation, as we saw, result from the energy production following the
disintegration of the labile molecules of the living substance, then
the degree of irritability is determined by the chemical constitution
of the disintegrating molecules, by the number of molecules which are
broken down in a definite space and a given time, and by the nature
of the disintegration itself. All of these individual components, if
we observe the problem from the physical standpoint, are manifested
by the quantity of energy production. The higher the irritability of
a living system, the greater is the amount of energy production in a
given time and space which the stimulus produces. This has particular
interest from the standpoint of the extreme cases of medullated nerves
of the vertebrates with their most highly developed conductivity,
and which will be analyzed in somewhat greater detail. How are we to
explain their decrementless conductivity? When we study the decrement
of the excitation wave in the series of living substances, before
alluded to, we see that this reduces with a progressive increase of
irritability. Consequently the extreme irritability of the nerve is
a manifestation of its decrementless conductivity. If we study the
course of a process of excitation and its conduction in its molecular
details, the fact of the decrementless conduction indicates that
in excitation, produced by a stimulus, the same number of specific
molecules capable of disintegration are broken down in the same manner
at every following cross section, as at the point of stimulation; or
in other words: an equal amount of energy is set free at every cross
section, which, in its turn, acts as stimulus to the next, etc. Such a
condition presupposes, however, in an elementary fiber of the nerve,
that by the conduction of the wave of excitation from cross section
to cross section, all those molecules capable of disintegration are
broken down. If it is assumed that the entire number of molecules
capable of disintegration do not break down, but only a certain per
cent. of the same, then it would not be possible to conceive of a
molecular structure of the nerve in which this would take place
without decrement of the wave of excitation. With the assumption of
a generally homogeneous molecular structure (Figure 23, a) of the
elementary fibers it would be entirely incomprehensible how, with the
decrementless extension of the excitation, individual molecules capable
of breaking down could escape disintegration. If, on the contrary, the
molecular structure is not homogeneous it only is possible to explain
a conduction, on each cross section of which an equal per cent. of
irritable molecules break down, by the hypothesis that the irritable
molecules are in their turn ordered in fiber-shaped series (Figure 23,
b) within the elementary fiber and are thus protected to a certain
degree from one another and from transverse conduction of excitation.
This hypothesis would, therefore, only mean that the elementary fiber
is not such in reality and would thus transfer the difficulty to
the ultimate fiber unit, for which a homogeneous molecular structure
would have to be presumed. In short, whatever may be the assumption on
which molecular structure of elementary fibers is based, the fact of
the decrementless conduction peremptorily demands, from the physical
standpoint, that from cross section to cross section the entire number
of irritable molecules are broken down. This conclusion is highly
important, for it indicates very clearly that the “all or none law” is
applicable to the nerve.
[Illustration: Fig. 23.]
This gives us occasion to return to the discussion of the question, if
living systems really exist which respond in accordance with the “all
or none law.” The medullated nerve forms an object particularly suited
to serve as a starting point for the treatment of this especially
important problem. The question arises in this connection, if the
validity of this law for the nerve can be tested by other means.
At first it would seem as if the application of the “all or none law”
to the nerve were in contradiction to the well-known fact that a
weak stimulation of the nerve produces a weak, a strong stimulation,
a strong response. In this connection _Gotch_[110] has pointed out,
as the result of experimental studies of the wave of activity of the
nerve, that the difference in response, following the application of
stimuli of varying strengths, is understandable from the fact that
threshold stimuli stimulate only a few of the fibers of the nerve
trunk, whereas progressively increasing the intensity of the current
involves more and more fibers. There can be no doubt that this factor
explains the difference in the strength of the response. Therefore, in
reality we do not find here a contradiction of the “all or none law.”
On the other hand, the fact that the nerve, in contradistinction to
many other forms of living substance, the ganglion cell, for example,
upon a weak stimulation does not show the phenomena of summation, even
when the stimuli follow each other in a rapid succession, indicates
very strongly that the weakest operable stimulus produces maximal
excitation, so that the response cannot be further increased. But
above all, there is a series of facts, which have been gained in the
Göttingen laboratory, which demonstrate apparently without doubt the
validity of the “all or none law” for the medullated nerve. These
observations I wish now to consider in greater detail.
[110] _Gotch_: “The submaximal electrical response of nerve to a
single stimulus.” Journal of Physiology, Vol. XXVIII, 1902.
If a nerve of a nerve muscle preparation is drawn through a specially
devised glass chamber so that the middle portion can be narcotized or
asphyxiated and the nerve so arranged that it rests upon a pair of
electrodes in the chamber and upon a second pair without the chamber
and centrally located, then the nerve can be narcotized or asphyxiated
and thereby the alterations in the irritability as well as the
conductivity can be followed. In order to obtain as distinct a picture
of this alteration as possible, I tested continuously the threshold of
stimulation, which just produced minimal contraction in the muscle, and
_Fröhlich_[111] continued these observations. As a result the following
very remarkable conditions were observed. During the increase of the
depth of narcosis or asphyxia the irritability sinks more and more with
regularity. The conductivity remains unaltered for a long time, as the
strength of the threshold stimulus is not changed until irritability
has fallen to a definite point. When this is reached, conductivity
disappears. (Figure 24.) The most important point in this connection,
however, is, that the conductivity disappears simultaneously and
practically momentarily for the excitation produced by both weak and
strong stimuli. When the stimulation at the electrode placed centrally
to the chamber does not bring about response for threshold stimuli,
maximal stimuli at the same time also become inoperative. This is a
very interesting point, the importance of which has not until now been
recognized. This fact is not in harmony with the view held until now,
that in the nerve fiber different strengths of stimuli bring about
excitation of different intensity, and are then conducted. Let us now
clearly comprehend this problem.
[111] _Fröhlich_: “Erregbarkeit und Leitfähigkeit des Nerven.”
Zeitschr. f. allgem. Physiologie, Bd. III, 1904.
[Illustration: Fig. 24.
Curves of the changes in irritability (p) and conductivity (c) of
a nerve under the influence of narcosis or asphyxiation. (After
_Fröhlich_.)]
We have already seen that the wave of excitation meets with a decrement
of its intensity in the narcotized stretch, which increases in
strength as the irritability diminishes. If the value of the threshold
is learned by stimulating the nerve at the electrodes centrally placed
to the chamber with minimal stimuli, it would necessarily follow that
this weak stimulus would bring about a corresponding weak excitation
of the individual fibers and the wave of excitation already in the
beginning of narcosis would be obliterated, for it would meet with a
decrement, the result of the reduction in the irritability. A wave of
excitation of minimal strength could under these conditions no longer
reach the muscle, even in the beginning of narcosis. In spite of this
the excitation, even when produced with threshold stimuli, passes
through for a long time, even when the irritability in the chamber is
greatly reduced, as shown by testing with the electrodes within the
chamber. This is not consistent with the assumption that a threshold
stimulus brings about the minimal excitation, even in the individual
nerve fiber. But further: with a definite decrease of irritability of
the narcotized stretch the capability of conductivity disappears, and
indeed simultaneously for the weakest as well as the strongest stimuli.
If it is assumed that weak stimuli bring about weak excitations in the
nerve fiber, it must most certainly be expected that on the cessation
of the response, weak stimuli applied at the central nerve end would
still, by slight increase of the intensity of stimulation, be followed
anew by reaction in the muscle. This is all the more to be expected,
because the irritability of the narcotized stretch, as shown by
stimulation with the electrodes inside the chamber, very gradually
decreases, so that within the chamber stimuli of moderate strength are
still effective. Instead the capability of conduction is completely
obliterated, and even the strongest stimuli, applied to the end of
the nerve, produce no response in the muscle. This in turn does not
agree with the assumption that the intensity of excitation varies with
the strength of the stimulus in the individual nerve fiber. The facts
here alluded to are, therefore, either not correct, or the intensity
of excitation in the individual nerve fibers is independent of the
strength of the stimulus, and the view which we have entertained up to
the present in this respect is incorrect.
[Illustration: Fig. 24.]
In order to examine these facts once more on an extensive scale, and
at the same time obtain an understanding of the development of the
decrement in the narcotized stretch, I have requested _Dr. Lodholz_ to
register as many accurate curves as possible in which the positions of
the secondary coil of an inductorium are the ordinates indicating the
threshold of stimulation at four points of a nerve stretch. Of these
points three are situated at prescribed distances from each other in
the narcotized or asphyxiated stretch; the fourth is centrally placed.
(Figure 24.) As might be expected the result was the same as in former
investigations. They show however even more strikingly the abruptness
of the disappearance of conductivity simultaneously for the weakest
and the strongest stimuli. The curve produced by the centrally placed
electrode remains at the same height for a considerable period, then
suddenly abruptly declines. Those of the electrodes within the chamber
likewise sink, at first slowly, then with increasing rapidity in
successive order corresponding to the distance which they are situated
from the point of exit of the nerve, so that the curve of the most
distant electrode reaches the abscissa first, that of the electrode
nearest the muscle in the chamber, last. The experiments demonstrate
with complete clearness that in contrast to all those points within the
affected stretch, where the conductivity, though already obliterated
for weaker stimuli, still exists for stronger, that with stimulation
of a point towards the center _above_ the affected stretch, conduction
ceases simultaneously for all different strengths of stimuli. This
last state at the points within the affected stretch might be ascribed
to the diminution of the excitability of this stretch, and the idea
entertained that the weak stimuli no longer produce excitation capable
of further conduction.
This assumption is contradicted, however, by the fact that subsequently
to the disappearance of the response at a point situated at the
_greatest distance_ from the place of exit, an effect of stimulation
can be obtained at the _nearest_ point to the exit with the same or
even less strength of the current. As the irritability in the affected
stretch is reduced at all points in equal measure, the fact of a weaker
stimulus becoming inoperative whilst a stronger remains effective can
only be attributed to the circumstance that the wave of excitation
set free at some point of the influenced stretch by a weaker stimulus
is sooner obliterated on its way to the muscle than that produced at
the same point by a stronger stimulus. These experiments, in which
the manifestations of the nerves in response to stimuli applied
centrally above the chamber in the normal stretch are compared to those
in response to a stimulus acting on the affected stretch, clearly
demonstrate the totally different effect in both cases. In stimulation
of the centrally situated normal stretch, the wave of excitation, which
enters from here into the influenced stretch, is obliterated at the
same point simultaneously for the weakest as well as for the strongest
stimulus; stimulation of the affected stretch, the wave of excitation
which is set free at one point by a weak stimulus, is obliterated
sooner and after passing through a shorter stretch than that which is
produced by a stronger stimulus. It is self-evident that in the first
instance, in which the stimulus acts on the centrally situated normal
stretch, the wave of excitation, thereby set free, must in passing
through the affected stretch undergo a decrement of its intensity. If,
therefore, the wave of excitation, coming from above, is obliterated
exactly at the same point, whether brought about by weak or strong
stimuli, the inevitable conclusion must be drawn that, whether either
a weak or a strong stimulus is operative, the wave of excitation must
have entered into the influenced stretch from the normal stretch with
exactly the same intensity. In other words: the weakest as well as the
strongest stimuli produce excitations of equal intensity in the normal
nerve, that is, the “_all or none law_” is _valid for the nerve_.
This information can no longer be doubted in the presence of such
evidence as was presented above. This indeed is a fact of far-reaching
importance in the understanding of the functional activity of our
nervous system, for it is evident that the difference of intensity in
the conduction of excitation is not, as has been assumed until now, the
result of the conduction of varying strengths of a single excitation
in the same elementary fibers, but rather the involvement of a various
number of fibers, and that a series of processes which we have to the
present attributed to the varying intensities are now to be explained
by difference in the duration and form of excitation. This gives us
an entirely different but nevertheless a more definite picture of the
physiological character of the processes in the nervous system. Still,
this question belongs to another chapter of physiology. Here we are
interested in the fact that we have in the nerve a form of living
substance, in which irritability has reached a high degree, and every
stimulus which is at all operative brings about disintegration of all
the material involved in excitation, and consequently the property of
conductivity in the nerve reaches the state of highest development
of all living structures, in that the medullated nerve conducts with
the greatest rapidity on the one hand, and on the other, there is
no decrement of the velocity and intensity of excitation. All these
characteristics: the existence of the “all or none law,” the rapidity
of the conduction of excitation, the absence of a decrement in the
velocity, the absence of a decrement of the intensity of the excitation
wave, the want of the capability of summation of excitation, are all
dependent upon one another, for they are the combined expression of
one and the same factor, that of the high state of irritability. When
the irritability is artificially reduced, then the nerve approaches
more and more, depending upon the amount of reduction, to the series
of living substances in which we found the protoplasm of the rhizopoda
to occupy the other extreme. Between the normal medullary nerve with
its maximal, and the pseudopods of the rhizopods with their minimal
capability of reaction, we find innumerable gradations in groups of
living substances. Whether or not other forms of living substances
follow the type of the nerve, whether for example the “all or none law”
can be applied to the skeletal muscle as the investigations of _Keith
Lucas_[112] seem to show, requires further investigation.
[112] _Keith Lucas_: “On the gradation of activity in a skeletal
muscle fiber.” Journal of Physiology, Vol. IX, 1888. The same: The
“all or none” contractions of the amphibian skeletal muscle-fiber.
Journ. of Physiology, Vol. XXXVIII, 1909.
Finally, there arises the important question as to the finer mechanism
of conductivity. The progression of excitation from cross section to
cross section in a living system is brought about by the decomposition
of the molecules in one region acting as a stimulus and producing
a disintegration of the molecules in another region, etc. We have
already seen that the intensity is dependent upon the amount of energy
produced by the disintegration of the molecules following the stimulus,
that is, upon the amount liberated in a definite space in a definite
time. The question which now arises is this: What form of energy is
produced by the stimulus at the point of stimulation, which acts upon
the neighboring molecules? The conduction of excitation is a property
of all living substance, and we may presume that this occurs in all
living systems in the same manner. If one examines the forms of energy
which are produced in a living substance by the breaking down of the
molecules, we find that chiefly three forms of energy may be taken in
consideration in the problem of conductivity: heat, electricity and
osmotic energy. Light cannot be looked upon as a form of energy which
is produced by all living substance, and the other forms of energy,
as the chemical energy and surface tension, remain local. At a first
glance one is inclined to assume that heat is the form of energy
which is liberated by the breaking down of the stimulated molecule
and which spreads to the neighboring molecules and brings about their
decomposition. For we know that heat facilitates dissociation, and the
analogy between living substance and explosive material is very close.
In both instances the decomposition, which extends over a great mass
of molecules, is accomplished by the heat produced in the breaking
down of a few molecules. In fact, the conduction of excitation of a
nerve can in many respects be compared with the burning of a fuse.[113]
Nevertheless, it must not be forgotten that this analogy, which on
first glance seems so apt, upon closer observation presents serious
difficulties. It can be experimentally shown that an increase in the
temperature in the living substance follows stimulation, but it is
also known that in momentary excitation following a single stimulus,
as in the muscle after the application of an induction shock, the heat
production is extremely small. This difficulty becomes particularly
apparent if we endeavor to gain an approximate idea of the numerical
proportions of the irritable, that is the disintegrating molecules to
the remaining mass of a living system. The water content above all
represents an enormous proportion. If we calculate this to be for
the nerve, for instance, roughly about 75 per cent., which is a low
estimate, only 25 per cent. of dry substances remain. Even of this
25 per cent. by far the largest part is apportioned to connective
tissue, for which 15 per cent. is certainly not too high a figure.
Neither can the remaining 10 per cent. of dry substances be regarded
as consisting entirely of molecules capable of decomposition. For in
this is also included the organic reserve material of the axis cylinder
protoplasm, which is doubtless of very considerable amount. Further,
the salts and products of disintegration, for which the estimate for
the sum total would probably not be too low if we assume the amount to
be equal to that of the group specially concerned in the process of
excitation. As, however, a constant metabolism of rest takes place,
these last molecules or atom groups are certainly not at the moment
of entrance of the stimulus in their entirety in a condition capable
of decomposition. It is quite certain, therefore, that we are still
overestimating the amount of the molecules capable of disintegration,
if we put them down as 5 per cent. of the entire nerve substance.
If we now suppose that this 5 per cent. of irritable molecules are
broken down as a result of stimulation, 95 per cent. of nonirritable
substance, separating these irritable molecules, must become heated to
such a degree by the disintegration of the latter that the amount of
heat suffices to bring about decomposition of the nearest surrounding
molecules or atom groups, for otherwise conduction of disintegration
could not take place in this manner. This condition presents a
serious difficulty for the assumption that heat is the form of energy
responsible for the conduction of disintegration. It is true that we
cannot reject this view at once as being completely incorrect, as the
possibility of conduction does not depend upon the absolute amount of
heat which reaches the next molecule capable of decomposition, but
upon the relative amount of heat in regard to the degree of lability
of the irritable molecules, of which we cannot even approximately make
an estimate. However, by a comparison with other highly explosive
substances, such as iodide of nitrogen, we find that a slight trace
of water applied to the iodide of nitrogen suffices to prevent the
extension of the disintegration process, and with this the explosion of
the whole mass. Nor does the view of _Pflüger_ remove this difficulty,
which assumes that the atom groups capable of breaking down are joined
together by a chemical linking of atoms to long fiber-shaped giant
molecules through the whole nerve fiber, for this assumption of a firm
structure can hardly be reconciled with the principles concerned of
metabolism.
[113] Compare _Pflüger_: “Ueber die physiologische Verbrennung
in den lebendigen Organismen.” In Pflügers Archiv. Bd. 10, 1875.
Further: _L. Hermann_: “Handbuch der Physiologie, Bd. II, Allgemeine
Nervenphysiologie,” 1879.
In consideration of this difficulty it seems easier to assign the
rôle of mediator of disintegration not to heat but to electricity.
Production of electricity is likewise a property of all living
substance. Differences of electrical potential between two points may
be equalized in the stretch by conduction through the intervening
space. Electricity would then fulfil the important conditions, which
must be demanded for the form of energy, acting as mediator for the
conduction of disintegration from cross section to cross section.
[Illustration: Fig. 25.
Model of a “Kernleiter.” A, B--Glass tube, with a number of side tubes
filled with saline solution, through which a wire is passed. _c_ and
_d_--Side tubes with electrodes for stimulation. _e_ and _f_--Tubes for
connection with a galvanometer. (After _Hermann_.) ]
Physiologists even at an early date, misled by the apparent likeness
in the conduction of excitation, especially in the nerve, to that of
electricity in a metal wire, regarded both processes as identical.
When, however, _Helmholtz_ first demonstrated experimentally the
rapidity of the conduction in the nerve, the thought that electrical
conduction was concerned, such as takes place in a metal wire, had to
be abandoned, as the velocity shows too great a difference in the two
cases.
[Illustration: Fig. 26.
Scheme of the conduction by local electric currents in a “Kernleiter.”
(After _Hermann_.)]
The observations, on the other hand, on the conductivity in the
so-called “core model,” seemed to offer another possibility of
attributing the conduction of excitation in the nerve to electric
processes. _Matteucci_, later _Hermann_ and finally _Boruttau_[114]
have endeavored to apply the results obtained when electricity is
introduced in a wire covered with a moist envelope (saline solution),
to the explanation of conductivity in the nerve. (Figure 25.) The fact
has been shown, that in such a model the application of electricity
to a point, as a result of polarization between the moist envelope
and the metal, produces a weak local current, which in turn disturbs
the electrical potential in the next cross section and consequently
a new local current is produced and so on through the whole length
of the wire. (Figure 26.) This fact, in connection with the apparent
similarity in the differentiation of the axial fibers and peripheral
envelope in the nerve, has led _Boruttau_ to apply the principles of
conductivity in the “core model” to that of the nerve. Then, however,
_Nernst_ and _Zeyneck_ brought forward their theory, according to which
the galvanic current is operative as a stimulus in that it brings
about an alteration in the concentration of the ions at the junction
of two different electrolites which, in turn, produce local currents.
_Boruttau_ then dropped the assumption of the existence of a simple
physical polarization between the wire and the envelope and replaced it
by the assumption of an alteration in the concentration of the ions at
this position. Thereby the “core model explanation” was already altered
in principle, in that only the differentiation of a central fibrilla
and a peripheral enveloping substance was appropriated. It seems to me
that this factor can likewise be considered as completely dispensable
and may, therefore, be omitted; thus nothing remains of the “core model
explanation” of the conduction of excitation in the nerve.
[114] The enormously extensive literature on this subject up to the
most recent date is quoted in _Cremer_: “Die allgemeine Physiologie
der Nerven.” In _Nagels_ Handbuch der Physiologie des Menschen, Bd.
IV, 1909. Braunschweig.
The results of continually increasing numbers of investigation in
recent times make it appear almost as a certainty that the elementary
fibrillæ in the axis cylinder are nothing else but skeletal substances.
_Wolff_,[115] _Verworn_[116] and others have first expressed the
view that the neurofibrillæ must be looked upon as skeletal fibers
for the soft neuroplasm, and more recently _Lenhossek_[117] and
especially _Goldschmidt_[118] have confirmed this assumption in detail.
_Goldschmidt_ has shown by extensive comparative studies of cell
mechanism the rôle played by the neurofibrillæ in a physical connection
as internal skeletal formations, and has proved at the same time, in
complete unanimity with other investigators, their continuity with
other undoubted skeletal fibrillæ. By this the numerous combinations
and speculations of _Apathy_ and _Bethe_ concerning the part taken by
the neurofibrillæ have been rendered untenable. In no case is there the
slightest justification to regard the apparent “Kernleiterstructur” of
the nerve as the principal condition for the process of conductivity,
for should we dispense completely with this point for the theory of the
conduction of the nerve, we can obtain, solely by the aid of the facts
known today in physical chemistry, the foundations for a theory of the
conductions of excitation which not merely renders the specific case of
the conduction of the nerve intelligible, but contains at the same time
the principles of the process of the conduction of excitation for all
living substance.
[115] _M. Wolff_: “Ueber die fibrillaren Structuren in der Leber des
Frosches.” Anatom. Anzeiger Bd. 26, 1905.
[116] _Max Verworn_: “Bemerkungen zum heutigen Stand der
Neuronlehre.” Medicin. Klinik, Jahrg. IV, 1908.
[117] _M. v. Lenhossek_: “Ueber die physiologische Bedeutung der
Neurofibrillen.” Anatom. Anzeiger Bd. 36, 1910.
[118] _Richard Goldschmidt_: “Das Nervensystem von Ascaris
lumbricoides und megalocephala. Ein Versuch in den Aufbau eines
einfachen Nervensystems einzudringen.” III Teil. Festschrift zum 60
Geburtstage Richard Hertwigs Bd. II, 1910, Jena.
[Illustration: Fig. 27.
Scheme of the foam structure of living substance. A--In
undifferentiated protoplasm. B--In fibrillae protoplasm.]
On the basis of investigation in the physical chemistry on the
properties of semi-permeable membranes, we know that such membranes
produce an elective effect on the diffusion of dissolved substances.
This is in the way that the two different solutions, separated by a
semi-permeable surface, do not follow the known laws of diffusion, but
are altered in that certain substances in contrast to their rapidity
of diffusion pass through the membrane or are prevented from entering
by the latter. This applies likewise to the two kinds of ions, which
are dissociated in diluted substances. If the surface exercises a
selection in the way, for instance, that the positive kations are
allowed to pass through, whilst the negative anions are held back, a
difference of potential must exist between the two. In this manner,
wherever two different solutions are separated from each other by a
semi-permeable surface, an opportunity occurs for the taking place
of galvanic currents. As we know, living protoplasm by reason of
its colloidal components possesses, in common with all colloidal
substances, on its surface the properties of semi-permeable membranes.
Between the cell and the medium, therefore, there is always the
opportunity for the occurrence of differences of electric potential.
But more. We likewise know that protoplasm itself represents a
mixture of colloid substances and actual solutions. Frequently, if
not always, living structure presents a morphological differentiation
of two types, when seen under the microscope, in the form of a foam
structure described by _Bütschli_. (Figures 27 and 28.) If we suppose
that with the disintegration of complex molecules, which we must assume
as taking place in the material of the walls of the protoplasm network,
substances are formed which are subjected to electrolytic dissociation,
the anions and kations hereby liberated must be diffused from the place
of their separation into the surroundings. Their diffusion, however,
is restricted by the protoplasmic network. The positive ions may pass
through, but the negative ions may not. As a result: the reticulated
substance is the seat of electric discharge, which in turn gives the
impact to the breaking down of new molecules and with this to the
occurrence of new potential differences, and so on, consequently the
disintegration is extended further and further through the connected
masses of the protoplasmic framework.
[Illustration: Fig. 28.
Protoplasm of different cells, showing foam structures. A--Pseudopod
of a marine rhizopod. The protoplasm only shows foam structure at the
point of stimulation. B--Epidermic cell of lumbricus. C--Nerve fiber.
D--Part of the cell body of a ganglia cell. (A-C after _Bütschli_, D
after _Held_.) ]
This theory, founded on facts gained entirely from investigation, would
involve those forms of energy which play the rôle of activator in the
extension of the breaking down of the molecule from cross section to
cross section, namely, the osmotic and the electrical energy. Based on
the general properties of physical chemistry and those of morphology
of the living substances, they would be applicable to all vital
systems. It would be premature to attempt to extend this assumption and
further develop its specific details, above all to make it responsible
for the specific differences in the process of the conduction of
excitation in various forms of living substance. For this our knowledge
of the properties of living substance is still far too incomplete.
Nevertheless, it furnishes us even now with various points of view
for the further analysis of a series of vital manifestations, as,
for instance, the facts concerning the production of electricity, of
galvanotaxis, chemotaxis and so on. This, however, exceeds the limits
of the task we have here mapped out. We are concerned here solely with
the general principle on which the conductivity of excitation in the
living substance is founded.