The Integrative Action of the Nervous System

Figure 6i. — Flexion-reflex. The reflex was being elicited (by unipolar faradization applied to a point in the plantar skin of the outermost digit) with intervals of about 60 seconds between the end of one reaction and the commencement of the next. A shows the com- mencement of the third reaction in the series, and this reaction was continuously elicited for 50 seconds ; B shows it in its latest stage. The stimulation was then stopped for 70 seconds and then recommenced : C shows the opening of the next reaction, the fourth of the series. In the 70 seconds interval the reflex has fully recovered from the " fatigue " exhibited in B. Signal above shows interruptions in primary circuit, 40 per second. Time below in seconds. An abscissa indicates the latent periods in A and C.

longer for that reflex, contract in response to the scratch-reflex which also employs them. Similar results are furnished by the scratch-reflex with certain differences in accord with the peculiar character of its dis- charge.^^*^ One of these latter is the feature that the individual beats of the scratch-reflex usually become slower and follow each other at slower frequency (Fig. 62). Also the beats, FiGURK 6a. — The scratch-reflex evoked in spinal dog by mechanical stimulation at a spot in the skin of the shoulder. The reflex shows the rapid waning of the movement under continued application of stimuli to one sp>ot of skin — this occurs with the same general features both under mechanical or electrical stimulation. The beats become of slower rhythm and more irregular and smaller amplitude and finally occur in groups. Time is marked below in seconds.

instead of remaining fairly regular in amplitude and frequency tend to succeed in somewhat regular groups. The beats may disappear altogether for a short time, and then for a short time reappear, the stimulus continuing all the while (Fig. 63). Here, again, the phenomena are not referable to the muscle, for when excited through other reflex channels, or through its motor- nerve directly, the muscle shows its contraction well. Part of the decline of these reflexes under electrical stimulation in the spinal dog may be due to reduction of the intensity of the stimulus itself by physical polarization. That does not account in the main for the above described effects. The graphic record of fatigue of the flexion of the scratch-reflex obtained by continued mechanical stimulation does not appreciably differ from that yielded under electrical stimulation. The diff*erent speed of the decline due to fatigue proceeds characteristically in

Figure 63. — The scratch-reflex toward the end of a Ions-maintained mechanical stimulation applied to a point of the skin of the shoulder. The lowest signal line marks the time of application of this stimulation, which, when the reflex was nearly tired out, was remitted for about 10 seconds and then repeated. The reflex then returns, showing considerable reeorery. The second signal line from the bottom shows the time of application of a nmilar stimulus applied to a point of skin two centimeters distant from the long-maintained one. The reflex elicited from that neighbouring point shows little evidence of fatigue. Time is marked above in seconds.

different kinds of reflex, and in the same kind of reflex under different physiological conditions e. g. " spinal shock " : this in- dicates its determination by other factors than electrical polar- ^ ization. Polarization has in a number of cases been deferred as Si far as possible by using equalized alternate shocks applied in opposite directions through the same gilt needle ; this precau- i tion has not yielded results differing appreciably from those given by ordinary double shocks or by series of make or break shocks of the same direction. The slowing of the beat in " fatigue " is also against the explanation by polarization, since merely weakening the stimulus does not lead to a slower beat.

When the scratch-reflex elicited from a spot of skin is fatigued, the fatigue holds for that spot but does not implicate the reflex as obtained from the surrounding skin.^s"*^^ The reflex is when tired out to stimuli at that spot easily obtainable ^ by stimulation two or more centimeters away (Fig. 63). This is seen with either mechanical or electrical stimuli. When the spot stimulated second is close to the one tired out, the reflex shows some degree of fatigue, but not that degree obtaining for the original spot. This fatigue may be a local fatigue of the nerve-endings in the spot of skin stimulated, to which in experi- ments making use of electric stimuli some polarization may be \ added. Yet its local character does not at all necessarily imply its reference to the skin. It may be the expression of a spatial arrangement in the central organ by which reflex-arcs arising in adjacent receptors are partially confluent in their approach i toward the final common path, and are the more confluent the closer together lie their points of origin in the receptive field. The resemblance between the distribution of the incidence ! of this fatigue and that of the spatial summation previously described argues that the seat of the fatigue is intraspinal and ^ central more than peripheral and cutaneous ; and that it affects * the afferent part of the arc inside the spinal cord, probably at the first synapse. Thus, its incidence at the synapse Ra — Pa , and at RyS — PyS (Fig. 13, B, or 39, B) would explain its restric- | tions, as far as we know them, in the scratch-reflex.

The local fatigue of a spinal reflex seems to be recovered from with remarkable speed, to judge by observations on the reflexes of the limbs of the spinal dog. A few seconds' re- mission of the stimulus suffices for marked though incom- plete restoration of the reaction (Fig. 63). In a few instances I have seen return of a reflex even during the stimulation under which the waning and disappearance of the reflex occurred. The exciting stimulus has usually in such cases been of rather weak intensity. In my experience, these spinal reflexes fade out sooner under a weak stimulus than under a strong one. This seeming paradox indicates that under even feeble intensities of stimulation the threshold of the reaction gradually rises, and that it rises above the threshold value of the weaker stimulus before it reaches that of a stronger stimulus. This is exemplified by Fig. 64, where the scratch-reflex which

Figure 64. — The scratch-reflex evoked by a relatively feeble stimulation and disappearing under that stimulation. On increasing the intensity of the stimulus the reflex reappears, and does not reappear on reverting again to the original intensity of stimulation. The signal line at top marks the stimulation by an electromagnet in the primary : the arma- ture is arranged so that the increase in intensity of the stimulation is shown by greater amplitude of excursion. Time in seconds below.

has ceased to be elicited by the stimulus A is immediately ). evoked — often without any sign of fatigue in its motor response { — by increasing the intensity of the stimulus (applied at the | same electrode) to A + a, 5 ohms having been short-circuited ^ from the current in the primary circuit. But the occurrence of : "fatigue" earlier under the weaker stimulus than under the | weaker stimulus may often be relatively to the production of \ the natural discharge greater than when a stronger stimulus is j

employed. This which has been of frequent occurrence in my \ observations on the leg of the spinal dog if obtaining widely in | It is easy to avoid in some degree the local fatigue associated I with excitation of the scratch-reflex from one single spot in the ^ Figure 65. — A scratch-reflex reappearing after having lapsed completely under a stimulation (unipolar faradization by double-induction shocks) which has been maintained unaltered although the reflex it originally evoked had lapsed. The reappearances occur at irregular intervals of long duration, e.g. 50 seconds, and the reflex on reappearance may last for 20 seconds at a time. The stimulation was applied at ten separate points in the skin surface and at each point by unipolar faradization from a separate secondary circuit.

skin by taking advantage of the spatial summation of stimuli applied at different points in the receptive field. Brief-lasting stimuli can be shifted from point to point in the field. When this is done, a curious result has met me. The provocation of the reflex has been made through ten separate points in the receptive field, the distance between each member of the series of points and the point next to it being about four centimeters. Each point is stimulated by a double-induction shock delivered twice a second. When this is done a series of scratch move- ments is elicited, and continues longer than when the stimuli i-e applied at the same interval, not to succeeding series of :ln points but to one point. Thus three or four hundred ;ats can be elicited in unbroken series. But the series tends ►mewhat abruptly to cease. If then, in spite of the ces- ition of the response, the stimulation be continued without teration during three or four minutes or more, the scratch- ing movement breaks out again from time to time and gives another series of beats (Fig. 65), perhaps longer than the first. These experiments indicate that physical polarization at the stigmatic electrode is not answerable for the fading out of the scratch-reflex. It shows also the complexity of the central mechanisms involved in the reflex. The phenomenon recalls Lombard's ^^^ phases of briskness and fatigue in series of records obtained with the ergograph.

It is interesting to note certain differences between the cessation of a reflex under " fatigue " and under inhibition. Figs. 60 and 43 can be compared. The reflex ceasing under inhibition is seen to fade off without obvious change in the fre- quency of repetition of the ** beats," or in the duration of the individual beats. The reflex ceasing under fatigue is seen to show a slower rhythm and a sluggish course for the latter beats, especially for the terminal ones.

Among the signs of fatigue of a reflex action are several suggesting that in it the command over the final common path exercised for the time being by the receptors and afferent path in action becomes less strong, less steady, and less accurately adjusted. Under prolonged excitation their hold upon the final common path becomes loosened. This view is supported by the fact that its connection with the final common path is then more easily cut short and ruptured by other rival arcs compet- ing with it for the final common path in question. The scratch- reflex interrupts the flexion-reflex more readily when the latter is tired out than when it is fresh.

In the hind limb of the spinal dog the extensor-thrust is inelicitable during the flexion-reflex. That is to say, when the flexion-reflex is evoked with fair or high intensity I have never succeeded in evoking the extensor-thrust, though the flexed posture of the limb is itself a favouring circumstance for the production of the thrust if the flexion be a passive one. But when the flexion-reflex is kept up by appropriate stimulation of a single point over a prolonged time, so that it shows fatigue, the "extensor-thrust" becomes again elicitable. Its elicitabih'ty is, then, not regular nor facile, but it does become obtainable, usually in quite feeble degree at first, later more powerfully. In other words, it can dispossess the rival reflex from a common path when that rival is fatigued, though it cannot do so when the rival action is fresh and powerful.

Again, the crossed ** extension-reflex " cannot inhibit the flexion of the flexor-reflex under ordinary circumstances if the intensity of the stimulation of the competing arcs be approxi- mately equal ; but it can do so when the flexion-reflex is tired. The waning of a reflex under long maintained excitation is one of the many phenomena that pass in physiology under the name of " fatigue." It may be that in this case the so-called fatigue is really nothing but a negative induction. Its place of incidence may lie at the synapse. It seems a process elaborated and preserved in the selective evolution of the neural machin- ery. One obvious use attaching to it is the prevention of the too prolonged continuous use of a " common path " by any one receptor.^ It precludes one receptor from occupying for long periods an efiector organ to the exclusion of all other receptors. It prevents long continuous possession of a common path by any one reflex of considerable intensity. It favours the receptors taking turn about. It helps to insure serial variety of reaction. The organism, to be successful in a million-sided environment, must in its reactions be many-sided. Were it not for such so- called " fatigue," an organism might, in regard to its receptivity, develop an eye, or an ear, or a mouth, or a hand or leg, but it would hardly develop the marvellous congeries of all those vari- ous sense-organs which it is actually found to possess.

The loosening of the hold upon the common path by so- called "fatigue" occurs also in paths other than those leading to muscle and effector organs. If instead of motor effects sen- sual are examined, analogous phenomena are observed. A visual image is more readily inhibited by a competing image in the same visual field when it has acted for some time than when it is first perceived (W. Macdougall).^^^ One point, on a priori grounds a natural corollary from the " principle of the common path," is indicated by the experi- mental findings relative to the incidence of fatigue. The reflex- arcs, each a chain of neurones, converge in their course so as to impinge upon and conjoin in links (neurones) common to whole varied groups — in other words, they conjoin to common paths. This arrangement culminates in the convergence of many sepa- rately arising arcs in the final efferent-root neurone. This neu- rone thus forms the instrument for many different reflex arcs and acts. It is responsive to them in various rhythm and in various grades of intensity. In accordance with this, it seems from experimental evidence to be relatively indefatigable?^ It thus satisfies a demand that the principle of the common path must make regarding it.

III. Intensity. In the transition from one reflex to another a final common path changes hands and passes from one master to another. A fresh set of afferent arcs becomes dominant on the supercession of one reflex by the next. Of all the condi- tions determining which one of competing reflexes shall for the time being reign over a final common path, the intensity of re- action of the afferent arc itself relatively to that of its rivals is probably the most powerful. An afferent arc strongly stimu- lated is caeteris paribus more likely to capture the common path than is one excited feebly. A stimulus can only establish its reflex and inhibit an opposed one if it have intensity. This explains why, in order to produce examples of spinal inhibi- tion, recourse has so frequently been made in past times to strong stimuli. A strong stimulus will inhibit a reflex in progress, although a weak one will fail. Thus in Goltz's inhibition of mic- turition in the " spinal" dog 2l forcible squeeze of the tail will do

it, but not, in my experience, a weak squeeze. So likewise any '| condition which raises the excitability and responsiveness of a ' nervous arc will give it power to inhibit other reflexes just as it 'i would if it were excited by a strong stimulus. This is much as j in the heart of the Tunicate. There the prepotent spot whence t starts the systole lies from time to time at one end and from l time to time at the other. The prepotent region at one end | which usually dominates the common path is from time to time i displaced by local increase of excitability at the other under j local distension of the blood-sinuses there. f

In judging of intensity of stimulus the situation of the i stimulus in the receptive field of the reflex has to be remem- [; bered. One and the same physical stimulus will be weak if | applied near the edge of the field, though strong if applied to t] the focus of the field. ^ Crossed reflexes are usually less easy to provoke, less reliable | of obtainment, and less intense than are direct reflexes. Conse- ^ quently we find crossed reflexes usually more easily inhibited .• and replaced by direct reflexes than are these latter by those i former. Thus the crossed stepping-reflex is easily replaced by ■ the scratch-reflex (Fig. 52), though its stimulus be continued all 1 the time, and though the scratch-reflex itself is not a very • potent reflex. But the reverse c^ occur with suitably adjusted j intensity of stimuli.

Again, the flexion-reflex of the dog's leg is, when fully devel- ^ oped, accompanied by extension in the opposite leg. This : crossed extensor movement, though often very vigorous, may be considered as an accessory and weaker part of the whole reflex ^ of which the prominent part is flexion of the homonymous limb. ] When the flexion-reflex is elicitable poorly, as, for instance, in ; spinal shock or under fatigue or weak excitation, the crossed ex- ; tension does not accompany the homonymous flexion and does i not appear. But, where the flexion-reflex is well developed, if : not merely one but dotk feet be stimulated simultaneously with ; stimuli of fairly equal intensity, steady flexion at knee, hip, and i ankle results in dotA limbs (Fig. 66} and extension occurs in ! neither limb.^^ The contralateral part of each reflex is in- j

Figure 66. — Diagram (cat) of the predominant uncrossed flexor-reflex of the hind limb in- hibiting the crossed extensor-reflex otherwise obtainable by stimulation of the opposite limb. I. The initial pose of the spinal animal ; 2. The pose assumed after stimulation of the left hind foot, the flexors of the left hip, knee, and ankle, and the extensors of the right hip, knee, and ankle are in active contraction ; 3. The pose assumed after simulta- neous stimulation of both hind feet. The extensor action of the hip, knee, and ankle that would appear from either side as a crossed reflex is bilaterally inhibited and the an- tagonistic flexor-reflexes bilaterally prevail.

hibited by the homolateral flexion of each reflex. In other words, the more intense part of ^ach reflex obtains possession of the final common paths at the expense of the less intense portion of the reflex. But if the intensity of the stimuli applied to the right and left feet be not closely enough balanced, the crossed extension of the reflex excited by the stronger stimulus is found to exclude even the homonymous flexion that the weaker stimu- lus should and would otherwise evoke from the leg to which it is applied.

It was pointed out above that in a number of cases the transference of control of the final common path FC from one afferent arc to another is reversible. The direction of the trans- ference can caeteris paribus be easily governed by making the stimulation of this receptor or that receptor the more intense. A factor largely determining whether a reflex succeed another or not is therefore intensity of stimulus.^^ IV. Species of reflex. A fourth main determinant for thei issue of the conflict between rival reflexes seems the functional | species of the reflexes.^^

Reflexes initiated from a species of receptor apparatus that] may be termed ^^ noci-ceptive'' '^'^ appear to particularly domi-j nate the majority of the final common paths issuing from the spinal cord. In the simpler sensations we experience from ! various kinds of stimuli applied to our skin there can be : distinguished those of touch, of cold, of warmth, and of pain, i The adequate stimuli for the first mentioned three of these are' certainly different ; mechanical stimuH, applied above a certain; speed, which deform beyond a certain degree the resting con-l tour of the skin surface, seem to constitute adequate stimuli for| touch. Similarly the cooling or raising of the local tempera-^ ture, whether by thermal conduction, radiation, etc., are ade-^\ quate for the cold and warmth sensations. The organs for- these three sensations have by stigmatic stimuli been traced to.'i separate and discrete tiny spots in the skin. In regard to skin- i pain it is held by competent observers, notably by v. Frey^^! and Kiesow,^'^ that skin-pain likewise is referable to certain specific nerve-endings. In evidence of this it is urged thatj mechanical stimuli applied at certain places excite sensations! which from their very threshold upward possess unpleasantness, < and as the intensity of the stimulus is increased, culminate inj "physical pain." The sensation excited by a mechanical^ stimulus applied to a touch-spot does not evoke pain, however* intensely applied, so long as the stimulation is confined to the^ touch-spot.

The threshold value of mechanical stimuH for* touch-spots is in general lower than it is for pain-spots; and? conversely the threshold value of electrical stimuli for touch- ^ spots is in general higher than it is for the spots yielding pain..: Similarly it is said that stimulation of a cold spot or of a warm.^ spot does not, however intense, evoke, so long as confined to^ them, sensations of painful quality. But pain can be excited!^ not only by strong mechanical stimuli and by electrical stimuli,^ but by cold and by warmth, though the threshold value o^ these latter stimuli is higher for pain than for cold and warmt^

spots. If these observations prove correct there exist, there- fore, numerous specific cutaneous nerve-fibres evoking pain. A difficulty here is that sensory nerve-endings are usually provided with sense-organs which lower their threshold for stimuli of one particular kind while raising it for stimuli of all other kinds; but these pain-endings in the skin seem almost equally excited by stimuli of such different modes as mechan- ical, thermal conductive, thermal radiant, chemical, and elec- trical. That is, they appear anelective receptors. But it is to be remarked that these agents, regarded as excitants of skin-pain, have all a certain character in common, namely this, that they become adequate as excitants of pain when they are of such in- tensity as threatens damage to the skin. And we may note about these excitants that they are all able to excite nerve when applied to naked nerve directly. Now there are certain skin surfaces from which, according to most observers, pain is the only species of sensation that can be evoked. This is alleged, for instance, of the surface of the cornea — a modified piece of skin. The histology of the cornea jeveals in its epithelium nerve-endings of but one morphological kind ; that is, the ending by naked nerve-fibrils that pass up among the epithelial cells. Similar nerve-endings exist also in the epidermis generally. It may therefore be that the nerve-endings subserving skin-pain are free naked nerve-endings, and the absence of any highly evolved specialized end-organ in connection with them may explain their fairly equal amenability to an unusually wide range of different kinds of stimuli. Instead of but one kind of stimulus being their adequate excitant, they may be regarded as adapted to a whole group of excitants, a group of excitants which has in relation to the organism one feature common to all its com- ponents, namely, a nocuous character.

With its liability to various kinds of mechanical and other damage in a world beset with dangers amid which the individual and species have to win their way in the struggle for existence we may regard nocuous stimuli as part of a normal state of affairs. It does not seem improbable, therefore, that there should under selective adaptation attach to the skin a so-to-say specific sense of its own injuries. As psychical adjunct to the reactions of that apparatus we find a strong displeasurable affec- tive quality in the sensations they evoke. This may perhaps be a means for branding upon memory, of however rudimen- tary kind, a feeling from past events that have been perilously critical for the existence of the individuals of the species. In other words, if we admit that damage to such an exposed sentient organ as the skin must in the evolutionary history of animal life have been sufficiently frequent in relation to its importance, then the existence of a specific set of nerves for skin-pain seems to offer no genetic difficulty, any more than does the clotting of blood or innate immunity to certain diseases. That these nerve-endings constitute a distinct species is argued by their all evoking not only the same species of sensation but the same species of reflex movement as regards " purpose," intensity, re- sistence to " shock," etc. And their evolution may well have been unaccompanied by evolution of any specialized end-organ, since the naked free nerve-endings would better suit the wide and peculiar range of stimuli, reaction to which is in this case required. A low threshold was not required because the stimuli were all intense, intensity constituting their harmfulness ; but re- sponse to a wide range of stimuH of different kinds was required, because harm might come in various forms. That responsive range is supplied by naked nerve itself and would be cramped by the specialization of an end-organ. Hence these nerve- endings remained free.

It is those areas stimulation of which, as judged by analogy, can excite pain most intensely, and it is those stimuli which, as judged by analogy, are most fitted to excite pain which, as a general rule, excite in the ** spinal " animal — where pain is of course non-existent — the prepotettt reflexes. If these are reactions to specific pain-nerves, this may be expressed by saying that the nervous arcs of pain-nerves, broadly speaking, dominate the spinal centres in peculiar degree. Physical pain is thus the psychical adjunct of an imperative protective reflex. It is preferable, however, since into the merely spinal and re- flex aspect of the reaction of these nerves no sensation of any

kind can be shown to enter, to avoid the term " pain-nerves." Remembering that the feature common to all this group of stimuli is that they threaten or actually commit damage to the ssue to which they are applied, a convenient term for appli- ation to them is nocuous. In that case what from the point of view of sense are cutaneous pain-nerves are from the point of Iiew of reflex reaction conveniently termed noci-ceptive nerves. In the competition between reflexes the noci-ceptive as rule dominate with peculiar certainty and facility. This ex- lains why such stimuli have been so much used to evoke :flexes in the spinal frog, and why, judging from them, such fatality " belongs to spinal reflexes. One and the same skin surface will in the hind limb of the spinal dog evoke one or other of two diametrically differ- ent reflexes according as the mechanical stimulus applied be of noxious quality or not, a harmful insult or a harmless touch.^^ A needle-prick to the planta causes invariably the drawing up of the hmb — the flexion-reflex. A harmless smooth contact, on the other hand, causes extension — the extensor- thrust above described. This flexion is therefore a noci-ceptive reflex. But the scratch-reflex — which is so readily evoked by simple light irritation of the skin of the shoulder — is relatively mildly noci-ceptive. When the scratch-reflex and the flexion-reflex are in competition for the final neurone com- mon to them, the flexion-reflex more easily dispossesses the scratch-reflex from the final neurone than does the scratch- reflex the flexion-reflex. If both reflexes are fresh, and the stimuli used are such as, when employed separately, evoke their reflexes respectively with some intensity, in my experi- ence it is the flexion-reflex that is usually prepotent (Fig, 43). Yet if, while the flexion-reflex is being moderately evoked by an appropriate stimulus of weak intensity, a strong stimu- lus suitable for producing the scratch-reflex is applied, the steady flexion due to the flexion-reflex is replaced by the rhythmic scratching movement of the scratch-reflex (Fig. 51), and this occurs though the stimulus for the flexion-reflex is maintained unaltered. When the stimulus producing the

scratch is discontinued the flexion-reflex reappears as before. The flexion-reflex seems more easily to dispossess the scratch- reflex from the final common paths than can the scratch-reflex dispossess the flexion-reflex. Yet the relation is reversible — by heightening the intensity of the stimulus for the scratch- reflex or lowering that of the stimulus for the flexion-reflex. In decerebrate rigidity, where a tonic reflex is maintaining contraction in the extensor muscles of the knee, stimulation of the noci-ceptive arcs of the limb easily breaks down that reflex. The noci-ceptive reflex dominates the motor neurone previ- ously held in activity by the postural reflex. And noci-ceptive reflexes are relatively Httle depressed by " spinal shock."