Comparative Electro-Physiology: A Physico-Physiological Study

This is not, however, the same as that absolute zero at which the curve of sensation was initiated, for here the neutrality is not due to absence of effect, but to the fluctuating equilibrium of two opposite effects, one positive and the other negative. It is thus to be understood that, while in the positive region the tone is simple, in the region of the resultant negative it is complex, the sensation here being.compounded of positive and nega- tive (pleasure-pain). In this region, then, it is theoretically possible to bring out the positive element alone, by suppress- ing the negative. It is the predominance of either one of the two components which at any given moment determines the pleasure-pain character of the complex sensation. At a certain critical point it is the element of pain which will begin to appear as relatively conspicuous, this proceeding towards a climax with increasing stimulation.

It must be understood that we are dealing in general with nervous response under normal conditions of excitability. It will be sufficient here to make a cursory reference to certain exceptional cases which may occur. We have found that the character of the response given by a tissue is deter- mined by the two factors of (1) the effective intensity of stimulus, and (2) the excitability of the responding tissue. If the nature of a given stimulus be such as to produce but a moderate effect, there will be a greater likelihood of its evoking only positive response. Or the responding tissue, in another case, may possess exceptional excitability ; hence the responsive indication here will tend from the beginning to be negative. The different effects depending on the vary- ing excitatory characteristics of the tissue, we have already seen illustrated in several cases. The slightly excitable epidermis was seen to give a predominantly positive response, whereas nerve which had been rendered highly excitable tended, on the other hand, to give negative response.

We shall now proceed further to consider the corre- spondence between the responsive sensation and the degree of nervous change induced by stimulus. And here the first question that arises is that of the relation which sensation bears to the intensity of the stimulus that provokes it. The difficulty of this investigation lies in the generally unsatis- factory character of the subjective standard, and in the variations induced by stimulus itself in the sensitiveness of the neurile elements. .

According to what is known as Weber-Fechner’s Law, the strength of stimulus must increase in geometrical ratio in order that the intensity of the sensation may increase arithmetically. The method of experiment on which this result was based is not, however, altogether unexceptionable. Against the generalisation itself, many objections have been urged, and even its supporters claim for it only a very limited range of application. It has been pointed out that Fechner starts with the assumption that the change of

quantitative ; he does not take into account that the quality or sign of sensation is also liable to change. Confining our attention to that range of sensation which does not involve any change of quality, it is urged that, starting from the minimally effective intensity of stimulus, excitation at first increases very rapidly, and subsequently more slowly, under successive equal increments of intensity. This particular complexity led Fechner to suppose that such complex phenomena as the quantitative relation between stimulus and sensation were not determined by mere physical or physiological factors. He therefore maintained that his generalisation expressed an ultimate law concerning the relation between physical stimuli and psychical reactions, or, in other words, between body and soul.

It is now possible, however, turning away from specula- tive hypotheses, to subject this question to direct and simple experiment. We have seen how delicate and free from complication is the record of the mechanical response of nerve. In these tracings we have the record of the direct effect of stimulatory action on the nerve itself. In order, then, to study the effect of stimuli of varying intensities, I first chose a specimen of the sciatic nerve of gecko. A sliding electric inductorium was used for the purpose of stimulation. From a previous experiment with a ballistic galvanometer the absolute intensity of the electric shocks at different distances of the secondary from the primary was calibrated. Marks were then made on the sliding base, which gave various intensities of stimulation—1, 2, 3, 4, 5—- and so on. Thus, keeping the duration of the exciting

primary current the same, and bringing the secondary nearer and nearer the primary, the intensity of the stimulus could be gradually increased quantitatively. The effective intensity of stimulation might also. be increased in a graduated manner by fixing the primary inside the secondary, and gradually increasing the duration of stimulation. The following record shows the effect of stimuli in- creasing from one to ten, by increments of one at a time (fig. 400). It will be seen that at the beginning, owing to growing sub-tonicity, the nerve was undergoing a gradual relaxation, the downward slope of the beginning of the record. On application of stimulus of intensity as shown by 1, there is a sudden responsive relaxation, followed by a partial recovery. As the intensity of stimu- lus is successively in- creased, the positive response undergoes an increase, the maxi- mum-positive response being evoked when the FIG. 400. Record of Response in Nerve of

- : oye Gecko showing the Effect of Arithmetically stimulus-intensity is 3. increasing Stimulus After this the ampli- tude of response undergoes a progressive decline, the response to stimulus 8 being practically zero. This neutral point, as was shown earlier, is not to be regarded as the true zero, being in fact the balancing point of positive and negative. This will be seen when we inspect the record of intensity 9, where the increasing negative actually induces a minute diphasic response—positive followed by negative. The next response to stimulus 10 gives us a sudden large negative response. Above this point of transition I find, as will be seen in the following records, that there is a rapid enhancement of normal negative response. Still later, this increase of rate would decline, and later again, by the setting-in of fatigue, the responses might undergo an actual diminution.. ‘Thus, in that

part of the record which lies beyond the point of transition, we can see that the excitatory change at first increases very rapidly, and afterwards more slowly, under successive equal increments of stimulus-intensity. In order to test the universality of these characteristics of nerve-responses, I obtained records with many different specimens. The record just shown was taken, as already said, with a sciatic nerve of gecko, which was in somewhat sluggish condition. The next (fig. 401) was obtained from the sciatic nerve of a vigorous bull-frog. On subjecting this nerve to increasing stimuli, I, 2, 3, 4,.. . it was found that the positive response reached a maximum, after which it declined, and the response be- came diphasic. With gradu- ally increasing stimulus, the positive element in the re- sponse now became smaller and smaller, while the negative grew larger. Above. this the negative responses underwent

a very rapid increase. Fic. 401.—Response of Nerve of ; ; Bull-frog to Stimuli. 1, 2, 3, Up to this point, I have . . . 12, increasing in Arithmetical peen describing the peculiari- Progression : : ties of response, as seen in motor nerves. In order to show, however, that the same characteristics hold good of the responses of sensory nerves, I next took a specimen of the optic nerve of Ophzocephalus. As I here wished to demonstrate the possibility of the response after continuous increase, reaching a limit, I employed the very moderate magnification of only thirty times, in the recording Kunchangraph. With larger magnifications, the record exceeds the recording-plate, and such a demonstration is impossible. The object being thus to record the peculiarities of response in the negative region only, that stimulus which was taken as the unit was sufficiently strong to induce a contractile response which, under the given magnification,

appeared moderate. Records were then made under increas- ing stimulus 1, 2, 3, 4, 5. ... It will be seen from these (fig. 402) that increasing stimulus induces at first a rapid augmentation of the negative response, after which a limit is reached. The same characteristics I find to hold good of the response of plant-nerve (fig. 403). Here the response-records were commenced at a point just above that of transition. Now, if these particular relations between stimulus and response be due, in the case of man, to some specific psychic reaction, then the same must also be true not

Fic. 402.—Response of Optic Nerve FIG. 403. Mechanical’ Response of Ophiocephalus to Arithmetically of Nerve of Fern to Arithmetic- inereasing Stimuli 1, 2, 3, 4, 5, 6, 7 ally increasing Stimulus only of the animal but also of the plant. And further, the records themselves, being mechanical; were direct records of undeniably physical changes. Even in the case of the inorganic, again, the same characteristic relations obtain, as we shall find in the next figure. Hence it is not true that the relation between stimulus and the responsive reaction is different in the inorganic from what it is in those living nervous tissues whose changes we_ perceive .as_ sensation. That is to say, it is determined, in all cases alike, by the same underlying physical factors,

In fig. 404 we have a series of magnetic responses to arith- metically increasing magnetic stimulation. The magnetising current by which this was accomplished increased from :2 ampére to 2 amperes by steps of ‘2 ampere ata time. The responses at first, as will be seen, increased at a very rapid rate, and afterwards the rate declined. The remarkable similarity between this record and that obtained with the optic nerve of Ophzocephalus is at once apparent.

In the chapter on the modification of Response under Cyclic Molecular Variation, I have already shown how the responsive change differs in. intensity at different parts of the characteristic molecular curve. We there saw that just beyond the point of transition, in the B state, the rate of change was very rapid, and that it became slower in the higher part of the curve. The induced trans- formations in the condition of the nerve, by which it passes from the phase B to @, and so on, are of them- selves sufficient to elucidate not only these, but also other obscure charac- teristics of the phenomenon of sensa- Fic, 404. Piciogta ht tion. They will, for example, explain

sh is proves at first not unpleasant, and at the end of the series actually painful. Here sensation has become intensified, though the exciting stimulus remains the same. I may quote here an account of an experiment by Professor Sherrington, which exhibits this fact in a very interesting manner : ‘I found that by focussing the heat rays from a lamp upon a skin area of about 25 sq. mm. (on the back of the hand), and allowing a perforated screen to intermittently intercept the radiation, the heat-pain begins to be perceptibly intermittent when, the times of play and of interception

being equal, the intermittence falls below the rate of thirteen per minute. An intensity of this intermittent radiation stimuli, at first not painful, and yielding sensations strikingly discrete, soon becomes dolorific, and then the sensations remain discrete no longer, but are fused more or less to- gether,’ ! i This anomaly of gradual heightening of sensation, and later, fusion of effects, appears at first sight inexplicable. I shall give a satisfactory explanation of the latter point in the next chapter ; but as regards the former of its two elements— namely, the heightening of sensation under uniform intensity of stimulus—we have seen that, owing to the after-effect of stimulus, the condition of responding substances in general, and nerve in particular, is gradually transformed, from a point below the transition B to one above it. But, owing to this transformation, the character of the response is changed (cf. fig. 382). If the original response be positive, it will be

converted later into negative. If it be moderately negative, It will also be seen that the intensity of response is not solely determined by the intensity of stimulus, but is modified by the existing condition of the nerve. And this latter under- goes a change by the action of stimulus itself. Thus there are certain times of the day when, owing to sluggishness of the tissues, our power of perception is dull. But acuity of perception becomes enhanced with each successive stimulus responded to. This is also true, more or less, of each indi- vidual undertaking. These facts are easily understood from a consideration of the different responses which are charac- teristic of the ascent of the molecular curve above the point of transition.

We have also to consider the fact that this progressive molecular modification, under certain circumstances, imposes the anticipation of the responsive maximum, with increasing stimulus, or even, in other cases, the induction.of an actual decline. The molecular variation, in consequence of the after-effect of previous stimulation, is seen by the shifting upwards of the base-line. We may here refer back to fig. 31 where the responses of two different tissues to increasing stimulus, show, in the one case complete recovery, and in the other, a persistent after-effect. The base-line in the former coincides with the line of absolute equilibrium ; and increasing stimulus consistently shows an increasing response, till, owing to molecular distortion reaching a maximum value, a limit in the response was reached. In the second of these records the persistent after-effect shifts the base- line, which now becomes the line of modified equilibrium. Owing to this fact, the relative maximum of variation from the condition of modified equilibrium is reached much earlier than would otherwise have happened. The extent of individual response, after this, appears in fact to undergo an actual diminution, though, measured from the line of absolute equilibrium, this is not the case. In our sensation we are unable to take cognisance of the absolute zero, the changed condition of the nerve itself at any given moment providing us with a relative zero, which is our only standard of comparison, and our whole perception of intensity depends upon induced variations from this chang- ing zero,

The record which I have given in figs. 400 and 401, show- ing the mechanical responses in the nerves of gecko and frog, under increasing stimulus, not only explains the quantitative change in the sensation, but also that change of quality or sign, under appropriate conditions, which had not hitherto found any explanation. We see in‘the curve of response, as has been said, that with feeble stimulus response is foszdeve, and that this positive response attains a climax, after which it diminishes in amplitude, and then changes sign and passes over into negative under increasing stimulation. In the corresponding response by sensation, we find similarly, not only a difference of quality or sign, but also a transformation from one to the other, under increasing stimulation, from a climax where the sensation attains a maximum of the pleasurable tone. After

this the positive declines, and passes over the line of trans- ition into the region of the negative or painful. The fact, again, that a moderate stimulus, such as is efficient to induce the positive sensation, will, if tetanically applied, become negative or painful, finds satisfactory explana- tion from the peculiar characteristics of the molecular curve. For we have seen, under tetanisation, that the curve is raised from the positive region, below the point of transition, into — the negative, above it (cf fig. 383).

We have thus seen that by molecular transformation the excitability of the nervous tissue may be enhanced or depressed. There are other conditions also under which the conducting power of the nerve, as well as its excitability, appear to be modified by the exercise of will. Thus, by attention, a stimulus which was previously scarcely per- ceptible may be raised to sensory prominence. The reaction- time, again, may be diminished by attention. The very opposite of these effects, again, are induced by inhibition. As an example of the latter may be mentioned the inhibi- tion by the will of the muscular movements natural under a given stimulus. It was at one time thought that this inhibi- tion of movement was brought about by the in-nervation of antagonistic muscles. This theory, however, is held to be disproved by the fact that such inhibitory effects are seen, even where antagonistic muscles are not present. It would thus appear that the nerve is susceptible of being thrown into certain conditions, in response to internal action, of an unknown character, by which the transmission of excitation through it is either accelerated or inhibited. This power has been said to be ‘ unique and mysterious.’

That such opposite dispositions of the nerve may actually result from opposite incipient distortions of the molecules is well shown, however, by the action of electrotonus. Here we have seen that by the influence of one pole an incipient molecular distortion is brought about, which facilitates the transmission of the true excitatory wave, while by the oppo- site this transmission is hindered or blocked, Thus an identical nerve will be rendered acceleratory or inhibitory by the opposite.effects of the inducing tonus. |

If an external force be thus capable, according as it is positive or negative, of inducing opposite molecular disposi- tions, by which the conducting power of nerve is so pro- foundly modified as to render it for the time being-either an accelerator or an inhibitor, then it is not difficult to under- stand that these molecular dispositions may also be yaried — in a similar manner by impulses from an internal source. The molecular dispositions themselves, by which these effects are brought about, are no doubt curious, but they are neither mysterious nor unique; for we have seen that by molecular distortions of one sign or another, artificially induced in magnetic substances, a given magnetic impulse may be either accelerated or retarded.

We have seen that the transmission of excitatory changes is facilitated,.if the nerve be subjected to incipient molecular distortion in a favourable direction. Now in considering the attitude of attention, we can see at once, if only from the muscular indications which it induces, that the nervous channel is probably thrown by it into a state of moderate contraction. Inattention, on the other hand, must generally be attended by an attitude of relaxation. We have again found that while incipient contraction or K-tonus enhances conduction, an intense contraction, or very strong K-tonus, will inhibit, because the molecular distortion thus induced is already maximum, and external stimulus can produce little further effect (p. 610). It is here interesting to note that the expression ‘steeling the nerves to pain’ is not altogether fanciful or metaphorical. The attitude of pre- paredness thus denoted is one of rigid contraction.

In the course of the present chapter, then, it has been shown that there are two distinct nervous impulses, positive and negative. The former, induced by feeble stimulus, gives rise to a positive tone of sensation. The negative, on the other hand, due to stronger stimulus, gives rise to a sensation of painful or negative tone. The particular relation which exists between stimulus and sensation has been shown to be an expression of the peculiar characteristics of the molecular curve. The ascent in the curve, being rapid immediately above the point of transition, and slow later, equal increments of stimulation cause in this region an increase of sensation which is at first rapid and then slow. Below the point of transition responses are positive. But tetanisation, inducing a molecular transformation, carries the curve above the point of transition. Hence moderate stimulus, inducing positive sensation, is converted, when tetanically applied, into negative or painful. The positive response is simple, but the negative is complex, containing a masked positive; and we shall see in the next chapter how this complex sensation can be analysed into its component parts.

We have also seen that, in addition to moderate stimula- tion, there are other agencies by which the tonus of nerves may be altered. In a magnetic substance, the incipient molecular distortions of one sign induced by K-tonus, enhance the excitability and conductivity, while those due to A-tonus depress them. Strong K-tonus, again, inhibits conduction. By such polar actions, then, an impulse is either accelerated or inhibited. Similar polar changes are seen in the nerve, moreover, under the action of an- and kat-electrotonus. In like manner, by the exercise of the internal stimulus of will, the tonus of the nerve may be so varied that, under different circumstances, the transmission of excitatory impulses is accelerated or inhibited.

Conversion of pleasurable into painful sensation, and vice versa, by electrotonus— The Sensimeter—Mechanical stimulation--Stimulation by thermal shocks— Chemical stimulation—Opposite effects of anode and kathode--Normal effects reversed under feeble E.M. F.—Negative tone of sensation blocked by alcohol and anzesthetics—Separation of positive and negative sensations, by lag of one wave behind the other—Dissociation of sensation by depression of con- ductivity—-Abolition of the negative or painful element by block of conduction.

I SHALL next proceed to demonstrate, by means of decisive experiments, the fact that sensation and its variations are associated with certain physiological changes and _ their appropriate modifications. In the mechanical and electrical response of the nerve, we found that while moderate stimulus gave positive response, a stronger stimulus gave negative ; that while the positive was unmixed or elementary, the negative contained a masked positive ; and further, that the velocity of the transmission of the positive was greater than that of the negative wave.

We shall now study the correspondence of the responsive sensation with these various nervous changes induced by stimulus, and their modifications under different agencies. I shall next, therefore, show, in accordance with the chief aim of this chapter, that the same conditions which determine the exhibition of positive, mechanical, or electrical response, will also, pari passu, determine positive tone in the responsive sensation. Those conditions, on the other hand, which bring about a negative mechanical or electrical change in the nerve, will also induce a negative tone in the sensation. The resultant transmitted effect recorded in the responding apparatus, was, as we saw, determined by the receptive ex-

citability of the stimulated point, and the conductivity of the transmitting tissue. We shall, therefore, investigate separ- ately the effects of various agents in the modification of receptive excitability and of conductivity respectively. — We saw, again, in the last chapter, that as the intensity of stimulation is continuously increased, the response gradually passes from the positive, through the point of transition, into the negative. - This happens, as we have seen, immediately above the point B. This region, therefore, may be referred to as critical and indifferent, and must be regarded as having the peculiarity that if, by any means, the molecular curve be raised above it, the corresponding sensation tends to become actually painful, or if carried below, becomes pleasurable.