Physiology of the special senses

Front matter

M. GREENWOOD, JUNR.

OP THE LONDON HOSPITAL STATISTICAL DEPARTMENT J LATE SENIOR

DEMONSTRATOR OF PHYSIOLOGY IN THE LONDON HOSPITAL

Contents

CHAP. PAGE

I. INTRODUCTION — THE LAWS OF MULLER, WEBER, AND

FECHNER 1

Chapter II General Physiology Of Cutaneous Sensation

IN studying any branch of knowledge it is an obvious advantage to begin with its simplest and most readily intelligible manifestations, and to approach only by gradual stages its more complex parts. In most cases this plan can be readily followed, but in sense physiology peculiar difficulties are encountered. To judge from the number of pages allotted in a general text- book to the sense organs of the skin, on the one hand, and to the eye, on the other, it might be supposed that the physiology of the former is much easier than that of the latter. Such is by no means the case; hi fact, the converse proposition is quite arguable. At the same time, the study of cutaneous sensation is in many respects peculiarly instructive, and it will therefore be convenient to undertake it at once.

At the outset we shall see that the tendency of modern workers has been to conceive of the cutaneous surface as a universe of points, each reacting in a specific manner to the application of stimuli. We shall study the results which have led up to this, and then examine important modifications of the conception which have been rendered necessary by more recent discoveries.

Pre- Aristotelian writings on cutaneous sensations are too vague to be of service to the modern reader. The scientific history of the subject really commences with Aristotle.1

Aristotle recognised distinctly that the sense of " touch " is separable into many sense modalities, although his limited means of investigation did not permit him to render the

1 For information as to Greek Sense Physiology the reader is strongly advised to consult Professor J. I. Beare's admirable work, Greek Theories of Elementary Cognition from, Alcmccon to Aristotle. Clarendon Press, 1906.

distinctions precise. He also anticipated the view that the organs are affected by a change of state. He regarded the sense of touch as fundamental, and the only one necessary to existence.

In both these conclusions Aristotle has been followed by the majority of subsequent writers. Without attempting to detail the course of investigations which have led up to the modern epoch, I shall describe in as few words as possible the state of knowledge gained up to a few years ago.

At least four distinct qualities of sensation can be distinguished when stimuli are applied to the skin — those of touch or pressure, warmth, cold, and pain. Some writers have advocated an even finer analysis, regarding, for instance, the sensation of itching as specific, but this is hardly warranted by experiment. It is quite clear that a series of points can be mapped out over the skin, stimulation of which produces the sensations of heat, cold, pain, and touch ; the question of pain, however, will occupy us subsequently. The principles of experimental methods for this sort of work are simple enough, although the actual instruments vary enormously in detail. For experimenting on tactile perception, v. Frey, who has done pioneer work in cutaneous sense physiology, devised a series of valuable instruments.1 They consist of a number of wooden rods, to the ends of which hairs of differing thickness are fixed at right angles. If the hair be applied perpendicularly to a point on the skin and pressure exerted, then with the application of a certain pressure the hair will be visibly bent. The amount of pressure required to bend visibly any given hair can be experimentally determined, and with a large series of hairs of different thickness we can use pressures measured and graded with some accuracy.

Toulouse and Vaschide have justly remarked2 that different readings will be given with the same hair unless the pressure is arrested at the instant of bending, a point not

1 Bloch described a similar but less perfect instrument in 1891 (Arch. dePhys., Avrill891).

- Toulouse, Vaschide, and Pieron, Technique de Psychologic Experimentale, Paris, 1904, p. 64.

easy to determine with absolute precision. I also think that the degree of moisture in the air makes some difference. Notwithstanding these and certain other objections, the method seems the most accurate at present in use. To study temperature sensations, a hollow pencil-shaped rod through which water at a definite temperature can be circulated is the best, although French workers claim to have obtained good results by using minute drops of water at various temperatures as stimuli.

The topography of sensation points was first completely worked out by Magnus Blix, and his conclusions have been endorsed by the best modern observers (Sommer).

These conclusions are : —

(1) Pressure points are closely related to the distribution of hairs, each hair having a pressure point near its site of eruption (v. Frey), the point corresponding on the surface to the situation of a hair follicle. There may, however, be some pressure points which do not correspond to hairs.

(2) Cold reacting points are more numerous than warm points, Sommer finding 12-13 cold points sq. cm. against 1-2 warm points, but the distribution is by no means uniform.

Respecting the anatomical structures which correspond to the sense points, hardly anything is certainly known. For instance, it is not easy to tell whether any given nerve fibre conveys centripetal or centrifugal impulses, the ordinary methods of determining the matter being generally impossible of application. The relation of pressure points to hairs renders it probable that the nerve fibre entering the root sheath and forming ring-like arborisations round the upper part of the follicle are especially associated with the pressure sense.

It is also possible that the Meissner corpuscles are pressure end organs, and may even, in the hairless parts of the skin, replace the hairs.1

It has been stated that Krause's end bulbs and Ruffini's organs are associated respectively with sensations of cold and warmth, but experimental evidence is of the slenderest.

Sensations of pressure can normally be evoked from the whole cutaneous surface, the mucous membrane of the mouth, the tongue, teeth, and nares. Respecting the cornea and glans penis much difference of opinion has existed, but the reason probably may be sought in a failure to distinguish the exact forms of stimuli employed, as will be clear subsequently.

Striimpell has recently1 asserted that sensations of pressure can be set up in internal parts, tendons, fasciae, muscles, and periosteum. Thunberg2 has suggested that in this case tactile impressions are confused with those of pain.

The adequate (vide supra) stimulus for the sense of pressure is of course mechanical, but since any stimulus applied to the surface does not operate directly upon the end organs — if such there be — the question arises as to what physical conditions in the immediate vicinity of the possibly recipient structures give rise to sensations of pressure.

This problem was solved by v. Frey and Kiesow3 in 1899. When a point, e.g. the end of a hair, is pushed against the skin, the pressure is greatest at the part immediately in contact with the point, and diminished in the deeper and lateral regions ; that is to say, there is a fall of pressure from without inwards. Conversely, if a small plate be affixed to the skin and traction made upon it, the pressure is least externally, and increases from without inwards. In the first case we have a negative and in the second a positive slope of pressure from without inwards. V. Frey and Kiesow found that the sensations experienced in the two cases appeared to be identical, provided the magnitude of the pull in one case were equal to that of the push in the other; that is to say, the end organ is excited by a change in the existing pressure relations of end organ and superficies, whether the change be positive or negative, increase or decrease.

Although the variation in pressure in the neighbourhood

1 Deutsche med. Wochen., 1904, pp. 1411 and 1460. - Nagel's Ifandb. d. Phys. d. Menschcn, vol. iii. p. 657. 3 Z.P.P.S.O., vol. xx. pp. 126, etc., especially p. 153. Those desirous of expeiimenting on the skin should note the cautions given in this paper.

of an end organ is an essential condition for the latter's excitation, other factors modify the result.

For instance, if the stimulus be a falling weight, its velocity is a factor. The greater the velocity, within limits, the less the weight required to produce a just noticeable sensation.

The sensation does not, however, vary as the kinetic energy of the exciting mass. Two weights allowed to drop from heights inversely proportional to their masses would possess equal amounts of kinetic energy on reaching the skin, but the smaller weight produces the more intense sensation.

The extent of surface to which the stimulus is applied is also an important matter. It might be thought that the sensations excited in two areas would be identical if the pressure per unit of surface were the same in both cases. This is not so if, for instance, an area of *5 sq. mm. be compared with one of '25 sq. mm. ; a greater pressure per unit is required in the second than in the first case to produce a just noticeable sensation. The optimum surface is approximately *5 sq. mm. ; as we increase or decrease the extent of surface stimulated from these dimensions, the liminal value increases (v. Frey and Kiesow, op. cit.). A partial explanation is found in the above-mentioned fact, that pressure sensations are excited by a slope of pressure from the point of application to the end organs. The greater the surface pressed upon, the less steep the slope from within outwards owing to lateral thrusts. This does not account for the fact that if the surface be diminished below a certain value the pressure must be increased, and the proposed solutions of the problem seem hardly adequate.

The above-mentioned experiments were made on hairless portions of the skin ; the conclusions, however, are, with some modification, valid for hair-clothed surfaces.

The tactile apparatus of the hairy skin is more excitable and more readily fatigued than that of the smooth areas. Stimuli not distinguishable on a smooth can be detected if applied to a hairy surface. This lowering of the magnitude of a threshold stimulus in the hairy regions perhaps depends on two factors,1 the diminution of the skin area stimulated, since the pressure is only effective through the base of the hair follicle, and the lever-like action of the hair itself.

The comparison and discrimination of two pressure stimuli have been the objects of many investigations. The quantitative results are discordant, but the following conclusion appears to be well founded.

It is easier to compare weights applied successively than when they act simultaneously ; i.e. a more accurate comparison can be made between the memory of a sensation and an actual sensation than between simultaneous sensations (Weber).

Again, the rapidity with which stimuli operate influences one's powers of discrimination. Generally, the slower the rate of change, the higher the liminal stimulus value (Dohrn).

It is also easier to perceive that a given weight differs from another than to say whether it is heavier or lighter. When such discrimination is possible, it is easier to detect an increase in weight than a decrease. If successive weights be applied with a rapidity such that, in equal times, the same fractions of the original weights are added, the ratios of increase of stimulus to original stimulus give results in fair accordance with Weber's law. It has, however, been shown (Seashore, Hall, and Motora) that the results are variable with the experimental conditions, and no definite proof of the law as applied to tactile impressions is forthcoming.

Experiments performed with the object of determining what frequency of pressure stimuli gives a fused sensation have yielded most discordant results,2 and need not be discussed. The length of time the sensation persists after removal of the stimulating body depends on the skin area ; in some places the sensation vanishes at once, in others, e.g. the forehead, the after-effect may with moderately strong stimuli persist some little time.

1 Aubert and Kammler, Pfliiyer's Arch., vol. lir. G01.

2 Sergi, Z.P P.S.O., iii. 179 ; v. Kries and Auerbach, Du Bois Reym. Arch., 1889 ; v. Vintschgau and Diirig, Pjl. Arch., Ixix. 377.

These, then, are the ': classical " results of investigations on the sense of touch, and we can pass to those dealing with the temperature senses.

Sensations of temperature can be elicited from the whole cutaneous surface — the skin of the external ear, parts of the nasal cavity, inclusive of the anterior nares, and the mucous lining of the beginning and end of the digestive tract. There are differences of opinion as to whether such sensibility exists in the cornea, conjunctiva, and glans penis; * we shall see in a later chapter how it is that confusion has existed on this point. There has been much dispute as to what constitutes the adequate stimulus of the temperature end organs, two theories, those of Weber and Herin, having been favoured at different times.

Weber supposed that a temperature sensation was dependent upon an alteration in the existing end organ temperature ; thus a rise of temperature in the heat end organ produces a sensation of warmth, and a fall in the temperature of the cold end organ one of cold. In either case such a change could be brought about in four ways. Thus a sensation of warmth might be produced by: (1) A diminution in heat loss with a constant influx of heat; (2) a constant heat loss and increased influx of heat; (3) increased heat loss combined with a proportionately greater influx of heat; (4) diminished heat loss and heat influx, the former diminution being the more considerable.

It has been shown that whichever of these conditions be fulfilled, the sensation is the same.

It is further known that, within certain limits, given a constant external temperature sensations originally present disappear, a constant rate of heat influx and efflux being presumably established.

These facts agree with Weber's hypothesis, but certain experiments seemed difficult to reconcile with it. If a metal disc at 2° C. be applied to the forehead for 30 seconds, the sensation of cold persists for some 20 seconds after removal of the stimulus, that is, while the heat loss is diminished and

1 See Thunberg, op. cit., p. 670.

the temperature of the end organs actually rising. Hering accordingly proposed a different theory.

When a certain skin area gives us no temperature sensations, the end organs are to be regarded as being at the physiological zero of temperature. Any alteration in the thermal condition will give rise to a sensation, the intensity of which will depend on the difference from this physiological point of reference. The actual situation of the physiological zero point is presumed to vary within tolerably wide limits, not being the same for all skin surfaces, nor for the same skin surface at different times.

On entering a warm room a sensation of warmth is elicited, because the physiological thermometer is set for a lower point ; but if the stay be prolonged, the physiological zero is readjusted to a higher level, and the temperature sensations disappear.

The difference between the theories is that Weber supposes sensations to arise during variation, Hering when the end organ temperature is constant.

Holm seems to have demonstrated that, if precautions be taken to prevent the spread of stimulation beyond the point originally involved, the sensation only persists for a short time after withdrawal of the stimulus, the time being so short as to correspond to the probable time during which the end organ temperature varies. The same investigator studied the experiment alluded to before, in which, after application of a cold piece of metal to the forehead, the sensation of cold persisted for some time after removal of the stimulus. Holm noticed that a distinct interval separates the appearance of the cold after-sensation from that at first experienced, and suggested that we are here dealing with a case of paradoxical sensation, that the cold end organs are stimulated by the warm blood which arrives in increasing amount owing to vasrodilatation, and that they respond specifically with a sensation of cold. The existence of such paradoxical sensations has long been known (Strilmpell, Alrutz, Thunberg, and others).

Holm also found that after-sensations of cold do not follow the application of moderately intense stimuli, but only that of strong ones. It may therefore be said that the experiment is too complex to be regarded as decisively negativing the theory of Weber, and that it is still questionable which of the hypotheses is to be preferred.

With respect to the factors which influence the threshold value of temperature stimuli, the following may be enumerated as the most important : —

(1) Situation of the point stimulated; warm and cold points are by no means uniformly distributed over the body surface. For instance, Goldscheider 1 found that cold points were far more numerous over the extremities than warm points. This author's methods have been adversely criticised, and there is no doubt that the topography recorded by different investigators is highly variable. I believe the inconstancy is partly due to real variation in the distribution of end organs, and partly to inexact methods of study.

(2) The actual size of the region stimulated. Thus Weber asserted that water at 29*5° Reaumur feels warmer than water at 32° R. if the whole hand be dipped into the former and only one finger into the latter. At the same time it is to be remembered that the considerations^enumerated under (1) may account for Weber's result. It is difficult to plan an experiment in which these sources of error are avoided.

(3) The physical condition of the stimulating body, its thermal capacity, conductivity, and more or less perfect application to the skin, are important, and play their part in the divergent results noted under (1).

Finally, it may be said that the numerous researches, dating back to the middle of the last century, in which the discrimination of temperature differences has been studied, have not yielded constant enough results to demand special attention in an elementary work. Attempts to establish the validity of Weber's law in the case of temperature sensations have not been successful.

1 Areh.f. Psychiatric und Nervenkr., xviii. 659.

BOOKS RECOMMENDED FOR FURTHER STUDY

An excellent account of modern work on the subject will be found in Professor Thunberg's article (Nagel's Handb. d. Phys., Bd. iii. p. 647). Of the original papers, those by v. Frey should be read first.

A valuable discussion of the experimental methods of dealing with the subject is contained in chapter i. of " The Grouping of Afferent Impulses within the Spinal Cord," by H. Head and T. Thompson (Brain, for 1906, part cxvi.).

Chapter III Pain

ALTHOUGH what is commonly termed pain is not exclusively associated with cutaneous stimulation, it will be found that the matter is best considered at this point. The discussion must be brief, since most of the problems which arise belong rather to the psychology than the physiology of the sense organs.

Many definitions of pain have been proposed by various writers, of which the following examples will suffice : —

"Pain is a change of sensibility repugnant to him who experiences it " (Mantegazza).

" Pain is any disagreeable sensation (sensation penible) perceived by the nervous centres, but varied in its modalities, effects, and causes (Eloy).

" A sensation such that one does not desire to experience it afresh " (C. Richet).

While it must be allowed that it is not an easy task to construct a satisfactory definition of pain, it appears that the authors cited confuse what should, if possible, be kept distinct. But little reflection is necessary to convince the reader that a sensation may be unpleasant, may be repugnant in the highest degree, without falling under the category which includes, for instance, the sensation experienced when a needle is thrust into the skin.

The extraction of a tooth without gas is a disagreeable experience, and so is the death of a friend, but it is surely an abuse of language to employ the word pain indifferently for the sensory phenomena associated with such disparate causes.

We have here an example of the " fallacy of words," discussed by Bentham and other logicians. German writers have attempted to remedy this evil by using the terms

PAIN 19

" Unlust " and " Schmerz." Any excessive stimulation of an afferent system is associated with "Unlust," and the word can also be made to cover what popular English writers convey by the phrase mental or moral pain. It is, however, true that many authors use " Schmerz " almost as loosely as " pain " or " douleur." Indeed, however one may strive for accuracy, the difficulties seem almost insuperable. Although there is fairly good evidence that the sensation associated with stimulation of the skin by a sharp instrument, such as a needle, is sui generis and dependent on the existence of specific organs, yet no absolute distinction can be drawn between this and the " pain " of visceral disease.

I think therefore, although it is a frank confession of weakness, that it will be convenient to use the word pain without a qualifying adjective somewhat in the sense of <; Unlust," while I shall speak of the sensation which appears to have a specific basis as cutaneous pain. I am not at all clear how far visceral pain is or can be separated from " Unlust."

Funcke appears to have been among the first physiologists actually to advance the opinion that cutaneous pain is distinguishable from other afferent impressions arising from the skin. He suggested that the separation might occur first in the spinal cord, or that the impulses are insulated throughout their whole course from periphery to sensorium. The researches of v. Frey made it possible to render these ideas more precise. It was found that by the employment of suitable mechanical stimuli clearly defined points not generally coincident with the touch spots could be mapped out which yielded maximal sensations of pain. In the ordinary way it is not possible to induce a sensation of cutaneous pain entirely free from one of touch, but this can be done if only very sharp needles or bristles are used and the epidermis is moistened.

From such experiments, and from the further observation that the points would also respond to chemical stimuli by sensations of cutaneous pain, it was possible to conclude that specific organs for cutaneous pain exist. The "pain points" are characterised by a very long latent period when subjected to weak stimulation, and a special inactivity of response to rapidly alternating or oscillating stimuli. They do not coincide with pressure points, than which they are at least four times more numerous.

The belief that cutaneous pain was induced by direct stimulation of nerve fibres, is contradicted by the fact that it can be elicited by the employment of stimuli too weak to act as ordinary nerve excitants. Further, weak mechanical stimuli, just above the liminal value, provoke a delayed sensation of pain, the delay being greater than can be accounted by the time required for the conduction of an impulse to the spinal cord, and greater than the interval which separates stimulus and response when the former is applied directly to a nerve fibre.

Although the interpretation of these experiments is more dubious than some think, they are not inconsistent with the hypothesis of specific organs for cutaneous pain. V. Frey suggests that the free nerve endings of the skin are the organs in question, and the following confirmatory evidence is adduced : —

(1) Diminution of the surface area stimulated does not diminish the effectiveness of the stimulus.

(2) The liminal value of an electrical stimulus is lower than that of any other form.

(3) Corrosives produce, first of all, a sensation of pain. These facts suggest that the pain end organs are more

superficial than those of touch or temperature. The only nervous elements definitely superficial to the end bulbs supposed to be associated with touch are the free intra-epithelial nerve endings, hence they should be the required organs.

It has also been asserted that, in the cornea, the only receptive elements are the intra-epithelial nerve endings, and that from the cornea painful sensations alone can be elicited. V. Frey suggests that the actual excitant might be fluid squeezed from the cells into the inter-cellular spaces. Such fluid not being sufficiently concentrated owing to the impermeability of the cell wall for certain salts, its low concentration may render it a chemical stimulus for the nerve endings. Apart from other objections which can be raised,

PAIN 21

it should be noticed that Donaldson finds sensations other than those of cutaneous pain can be derived from the cornea.

I have already remarked on the confusion which has existed respecting the meaning of the word pain. Even with respect to cutaneous pain, as above defined, there is room for further specialisation. Thunberg, for instance, seems to hold the view that aching and stabbing or pricking (cutaneous) pains depend on totally different mechanisms. He points out that on pinching up a large fold on skin, pressure, however slight, elicits a sensation of aching pain alone, while the application of superficial stimuli under normal circumstances excites only pricking sensations. Thunberg has not, however, shown satisfactorily that the difference of sensation may not be related to the very different stimuli employed and their conditions of application.

A few notes on visceral pain my fitly be introduced at this stage. It was formerly believed that the viscera are insensitive under normal circumstances, but become capable of originating painful sensations when diseased or injured.

The work of Lennander, Ramstrom, and others has proved that this conception is erroneous.

Lennander demonstrated that all painful sensations set up in the pelvic viscera can be assumed to arise in areas innervated by the lumbosacral and intercostal nerves, especially in the parietal layer of the peritoneum. For instance, increased peristalsis set up by an inflammatory condition of the gut acts as a mechanical stimulus, exerting traction on the peritoneum. According to Ramstrom, traction alone is an effective stimulus for the parietal peritoneum. Lennander believes that all organs innervated by the sympathetic or by the vagus below the origin of the recurrent laryngeal are insensitive ; he has found that pain is not excited by stimulation of the anterior wall of the vagina, the uterus, ovaries, and Fallopian tubes.

Next, as to referred pain. The fact that the pain associated with visceral disease is often localised in a cutaneous area rather than in the affected region, was noted many years ago by Lange. Henry Head was, however, the first to invest igate the subject systematically and render the results of careful observation and experiment available in practice.

It appears from Head's researches that certain stimuli applied to the viscera are capable of eliciting pain, but pain not localised in the viscera but referred to some cutaneous surface.

The area of reference was constant and, apart from actual pain, was often hyperasthetic. The constancy of these painful and hy penes thetic zones render them of much value for diagnostic purposes. Physiologically, the results can be best described in terms of the following hypothesis : It may be laid down as a fact of experiment and general observation that if a painful stimulus be applied to, e.g., a point of a normal foot, the resulting sensation is localised near to the point of stimulation ; if, however, the sensibility of the skin is impaired in that region, the sensation will be localised in the nearest (physiologically speaking) unaffected point. That is to say, if two spots, A and B, are physiologically associated, for instance, by nervous inter-connections, and if A be an area of lower sensibility than B — it may be normally, or pathologically, owing to disease of the afferent paths — then a stimulus applied to A is localised in B. A viscus is the A of our illustration, and B some corresponding or connected skin area. Following the train of ideas a little further, if A send up afferent impulses to some common "centre" C, an alteration of the state of equilibrium in C may result, with the consequence that if stimuli be now applied to B, an abnormal conscious response may be obtained owing to the modification of the recipient " centre."

The nature of the connecting link between our A and B has also been studied. It was first suggested that posterior root ganglia receiving fibres from viscus and corresponding skin area formed the link. Subsequent work suggested that the correlated parts could be grouped more satisfactorily by a schema derived from our knowledge of the primitive segmentation of the cord.

Recent work has, on the whole, rather tended to support the original hypothesis, but the matter is not yet definitely settled

PAIN 23

I have now described some of the main facts known as to cutaneous pain. In the next chapter we shall consider some important modifications of our beliefs due to recent studies.

BOOKS, ETC., RECOMMENDED FOR FURTHER STUDY

PAIN—

Thunberg, op. cit.

Psycho-physiologie de la Douleur, par I. loteyko d M. Stefanowska,

Paris, Alcan, 1909, pp. 249. (Contains a fairly good bibliography.

Must be read critically.) Richet. Art. Douleur, Diet, de Physiologic. (The same remarks

apply as in the previous case.)

REFERRED PAIN,- ETC.—

Consult the numerous papers by Head and his collaborators which have appeared in Brain from 1888 onwards.

Chapter IV

THE reader who has examined the results epitomised in the last two chapters will have noticed that, mainly in consequence of the researches initiated or inspired by Max v. Prey, a fairly coherent description of the sensory mechanisms of the skin has been reached. In effect, it has become customary to regard the cutaneous surface as a vast congeries of points each responding to stimuli by a specific afferent impulse, the heat spots responding by a sensation of heat, the touch spots by one of touch, and so on. The whole conception is conveniently resumed by the phrase "Punctate Sensibility."