Sensitivity

SENSITIVITY (sensibilitas), subjective experience associated with the effect on the organism of various kinds of stimuli. The physiological basis of sensitivity consists of the receptor function, i.e., the process of propagation of an impulse arising under the influence of stimulation from peripheral sensory apparatus to the central nervous system. The process of excitation of these impulses of central apparatus constitutes the physiological basis of sensitivity. In the form in which it is represented in humans, sensitivity consists of qualitatively different experiences corresponding to the specificity of various stimuli acting on the organism (light, sound, pressure, touch, heat, cold, etc.). This diversity and complexity of the receptor function of higher animals and humans represents the result of gradual complication and refinement of forms of reaction of the organism to external stimuli developed in the course of evolution, in the process of better adaptation to the environment. In its most primitive form, at the lowest stages of phylogenetic development, the receptor apparatus is represented in the form of a single nerve cell, which with one of its processes comes into contact with the external environment (body surface), and with the other—with the apparatus of motor reaction (muscle). In animals with such a primitive nervous system, reactions to stimuli are non-specific and are determined mainly by quantitative factors; they are carried out according to the physiological principle of "all or nothing," i.e., at a certain intensity of stimulation, a certain motor reaction occurs as a result of the direct propagation of excitation from the body surface to the muscle. The entire process of evolution of the receptor function can be defined as a gradual transition from automatic, anatomically predetermined reactions of the "all or nothing" type to reactions characterized by an element of choice, greater specificity, qualitative adequacy between the form of stimulation and the method of reaction. Anatomically, this complication of the receptor function corresponds to an increase in the number of neurons both between the periphery and the central apparatus and between these latter, as well as a gradual increase of these centers (development of the cortex), and physiologically—a gradual decrease in the threshold of stimulation, development of specific apparatus for various kinds of stimulation, and establishment of abundant associative connections of the central apparatus of sensitivity with other parts of the cerebral cortex. The main factor determining the evolutionary changes of the receptor function is the adaptation of the organism to the forms of external stimuli existing in the environment for a given way of life: the presence of certain forms of stimulation and their frequent repetition leads to a decrease in the threshold with respect to these stimuli. Thus, the evolution of the receptor function is inextricably linked with the evolution of functions in general and with changes in the relations of the animal to the external environment. Thus, with the transition from an aquatic way of life to a terrestrial one, one of the receptor apparatus necessary for maintaining equilibrium in water disappears—the lateral line organ; under the influence of the abundance of sound stimuli associated with the terrestrial way of life, this organ, adapted to perceiving the effect of vibrational oscillations of water, is transformed into a receptor of vibrational oscillations of air, i.e., into the cochlear apparatus. This explains the at first glance strange topographical closeness in the inner ear of such apparatus as the vestibular and cochlear, which appear to be qualitatively completely heterogeneous. This closeness is due to the origin of the cochlear apparatus from the lateral line organ, which as an organ of equilibrium has an anatomical-physiological closeness to the vestibular apparatus. With the transition of some mammals from a terrestrial way of life to an arboreal one, the importance of the olfactory function decreases; in connection with this, a regression of the olfactory function occurs and greater perfection of vision, hearing, and touch begins. With the transition of man to an upright position of the body, which led to the freeing of the hands from supporting the body with their adaptation to the function of grasping, an extraordinary refinement of tactile sensitivity develops and a new complex form of sensitivity is developed in the form of the so-called stereognosis. Transformations of sensitivity occur not only in the process of phylogenesis but also in the process of ontogenesis, i.e., during individual life. The degree of excitability of various receptors, the distribution of sensitivity over various areas of the body surface, the methods of reaction to various stimuli—all this is subject to certain individual variations and depends on a number of conditions. And even modern data on the study of sensitivity in normal and pathological conditions increasingly lead to the conviction that the receptor function cannot be regarded as something static; for explaining many phenomena in the area of normal and pathological conditions, it appears essentially necessary to approach the analysis of these phenomena from a dynamic point of view. Both in qualitative and quantitative respects, the receptor function is not something predetermined anatomically-physiologically. The relationships between different kinds of sensitivity, the excitability of peripheral and central receptor apparatus, the ratios between the quality and intensity of external stimuli and reactions to them—all this is in a certain dependence on dynamic moments. This fact has particularly important significance for the interpretation of certain clinical phenomena. The latest data on the study of sensitivity show that the previously existing anatomical-physiological schemes, although they appear correct, do not however exhaust all the complexity of the receptor function. The basis for the classification of various receptor functions, which has not lost its significance to this day for clinical practice, is the concept of the quality of external stimuli, of the subjective experiences associated with these stimuli, as well as of the area of action of these stimuli. Accordingly, all receptor organs can be subdivided into exteroceptors, i.e., receptors intended for stimuli affecting the organism from the outside, and into interoceptors, or receptors for stimuli arising in the organism itself, more precisely—in internal organs.--Exteroceptors can in turn be subdivided into: 1) distance receptors, i.e., receptors perceiving stimulation at a distance without direct contact of an external object with the surface of the organism; these include the olfactory, visual, and auditory receptors; 2) contact receptors—receptor apparatus for stimuli arising from direct contact of an external object with the surface of the organism, hence the synonym "superficial" sensitivity; to this form of receptor functions belong: tactile sensitivity, localization of touch, painful sensitivity, and temperature sensitivity with its varieties—sensation of cold and heat; 3) proprioceptors—receptors for stimuli arising in the deep parts of tissues, hence the synonym "deep" sensitivity; however, the concept of deep sensitivity is not entirely identical with the concept of proprioceptive function, which includes mainly receptors perceiving impulses arising in connection with changes in the tension of muscles, tension or relaxation of tendons, joint capsules, changes in the contact of bone articular surfaces, etc.; the totality of receptor functions corresponding to the indicated impulses is also denoted by the term "sense of position" or muscle-joint sense (see Proprioceptive elements). To the area of deep sensitivity, besides proprioceptors, belong the sense of pressure, painfulness of deep tissues on pressure, and vibratory sensitivity. As for the receptors of the vestibular apparatus, they represent a special receptor mechanism, which however in view of its relation to the function of equilibrium and regulation of the position of the body in space can be attributed to the system of proprioceptive sensitivity. Along with this purely descriptive classification of sensory functions, another classification based on biological data is beginning to acquire importance in recent times. The basis for this classification was provided by observations by Head on the phenomena of restoration of sensitivity during nerve regeneration. Observing the process of restoration of sensitivity after cutting a nerve with its subsequent suturing to itself, for which purpose for the sake of the experiment Head cut one of the sensory nerves (superficial radial nerve) in himself, he established the following facts. First, it turned out that after cutting a sensory nerve only superficial sensitivity is lost, while the sensation of pressure on deep parts (muscles, tendons) is not lost at this time, but becomes even more painful than in normal conditions. From this Head concluded that receptor impulses from deep parts of the body go, at least in the distal parts, as part of motor fibers. And from the fact of intensification of deep pain after the elimination of superficial sensitivity, Head drew the conclusion about the existence in normal conditions of an inhibitory influence of superficial sensitivity on deep pain.--In regard to the restoration of superficial sensitivity, Head established the following facts.

After a certain period of complete anesthesia, the first stage of recovery of S. sets in; it is characterized by the following features: the threshold of irritation appears sharply elevated, but upon reaching this threshold, external stimuli are still perceived by the subject, but the character of these perceptions differs sharply from what is the case in the normal state. Sensations caused by external irritation have a diffuse character, i.e., they cannot be localized with such precision as is the case in the normal state; the quality of the external object is not recognized with sufficient clarity, but at a certain intensity of these irritations, they cause a very sharp feeling of unpleasantness. Painful irritations are not recognized as a prick, scratch, or pinch, but as something unpleasantly painful in general. Likewise, temperature effects within the range of warm and cool (27-37°) are not recognized at all, and at greater intensity they cause a sharply unpleasant feeling without a clear sensation of the quality of the temperature effect. The form of sensitivity observed in this first stage Head designated by the term protopathic S. In the process of nerve regeneration, the stage of protopathic S. lasts for several months, after which the restoration of normal S. occurs. It manifests itself in the appearance of elements of a finer receptor function, which Head designated by the term epicritic S. Epicritic S. is characterized by the following features: a lower threshold of irritation, the ability to perceive light touch, precise localization of external irritation, and more perfect recognition of the quality of the external stimulus. Head explained the phenomena of restoration of S. that he established by the fact that for protopathic and epicritic S. there exist separate fibers and that in the process of nerve regeneration, protopathic fibers, which are phylogenetically older, primitive in structure, are restored earlier than epicritic. The facts established by Head are confirmed by all subsequent observations. As for their explanation, there are some insignificant disagreements in this respect. Comparing different viewpoints (Head, Forster, Stopford), one can summarize the relevant data in the following form. Protopathic S. is a phylogenetically old form of receptor function; it is a primitive form of S.; its central organ is the thalamus opticus. Epicritic S. is a phylogenetically later, cortical function. Both in the process of phylogenesis and in the process of nerve regeneration, the appearance of epicritic S. has an inhibitory effect on protopathic S.—a fact which is merely a manifestation of the general law of the inhibitory influence of cortical functions on subcortical ones. The concept of the existence of two basic forms of S., a coarser protopathic (thalamic), and a finer epicritic (cortical), was later extended (Stopford) to deep S. Accordingly, the classification of various types of S. can be presented as follows: I. Superficial sensitivity a) Thalamic, or protopathic S., feeling of unpleasantness with sharp painful and temperature irritations b) Cortical, or epicritic S. Perception of the lightest forms of touch; precise localization of touch, ability to distinguish non-intense temperature irritations (warm-cool); normal recognition of Weber's compass points II. Deep sensitivity a) Thalamic, or protopathic S. Sensation of pressure; pain with pressure; perception of tuning fork vibrations b) Cortical, or epicritic S. Precise localization of pressure. Normal recognition of passive movements in joints (muscle-joint sense) Along with the terms 'protopathic' S. and 'epicritic', other terms later came into use as their synonyms. Protopathic S. represents an effective experience of an unpleasant feeling associated with intense thermal and painful irritations; therefore it is also designated by the terms 'pathic' or 'affective' S. (Forster). The term 'nociceptive' S. (Sherrington) is also very appropriate, i.e., perceiving 'harmful', threatening destruction irritation. In contrast, as synonyms for epicritic S., the terms are used: 'perceptual', 'discriminative' (distinguishing), 'gnostic' (cognitive); all these terms contain an indication of the ability for fine recognition of the quality of external irritation associated with epicritic S. The classification of S. presented above, based on biological data, is of extreme importance both theoretically and practically. Many unclear questions of the symptomatology of disorders of S. received explanation through the work of Head; new guiding data were obtained for the diagnosis and prognosis of certain diseases, especially of peripheral nerve injuries. The theoretical importance of these studies is determined by establishing the role of the thalamus in the function of perception of elementary painful sensations. Through Head's research on disorders of S. in brain lesions, it was established that the thalamus opticus and specifically its medial nucleus (nucleus essentialis) represents the anatomical substrate of perception of elementary painful sensations (center of protopathic or nociceptive S.). This undermined the old viewpoint that all 'sensations' are a function of the cerebral cortex and that the role of the thalamus in the function of S. is limited to the role of an intermediate stage between the periphery and the cortex. At the present time, it can be considered established that for certain types of S., the thalamus opticus (nucl. essentialis) represents not an intermediate, but a final 'station'. The relationship of different types of S. to the cortex and thalamus is shown schematically in Fig. 1 (according to Levy-Valensi). From this scheme it is evident that pain and vibratory stimuli can be perceived independently of the cortex; tactile sensations can in their elementary form be perceived by the thalamus, but for fine qualitative recognition of tactile sensations, the participation of the cortex is necessary; coarse perception of temperature irritations (hot, cold, i.e., protopathic sensations) is carried out by the thalamus; fine distinctions of warm from cool (epicritic temperature S.) are carried out by the cortex. Muscle-joint sense (sense of position), precise localization of touch, stereognosis, and normal recognition of Weber's compass points are functions of the cortex. In addition to the basic forms of S. mentioned above, there exist types of sensation that are merely varieties of the first and have not yet acquired significant clinical importance. These include: electrocutaneous S., the sense of itching, and the sense of moisture (hygresthesia). Different forms of current cause different sensations. Dantek identifies these sensations with different forms of superficial S.: faradic current causes a sensation corresponding to touch ('faradic cutaneous' S.); galvanic current causes a sensation corresponding to

temperature S.; different centers: 1 and 2- rows of static S. (final stage-thalamus); 3-sense of position (cortex); 4-

Fig. 1. Diagram of the relationships of different types of S.

electricity (FRANKLIN'S SPARKS) SO-

with thalamic and cor-

Weber's stereognosis. Patients usually experience sensations of electric current passing through the body, especially along the back when bending the head forward; this symptom, which is generally quite rare, is sufficiently pathognomonic for disseminated sclerosis. Itching is a variety of tactile sensitivity. Hygresthesia (feeling of moisture) should apparently be considered as a combination of tactile sensation with temperature sensation (touch + cold). Hygresthesia has clinical significance: in certain organic diseases, patients experience sensations of skin moisture, movement of drops of liquid, etc. ('hygroparesthesias'). The conducting pathways of sensitivity, i.e., the aggregate of receptor conductors from the periphery to the brain centers, appear according to modern data as follows: fibers beginning from peripheral sensory endings go as part of peripheral nerves together with motor fibers, but as they approach the spinal cord, sensory fibers separate from motor fibers and enter the intervertebral ganglion; sensory fibers of peripheral nerves represent processes of intervertebral ganglion cells directed toward the periphery; each of these cells, in addition to processes directed toward the periphery, gives processes directed toward the central nervous system, i.e., toward the spinal cord; these processes, which are anat.-physiol. continuations of peripheral sensory fibers, form a common trunk in the form of the so-called posterior root, in the composition of which they enter the spinal cord; upon entering the spinal cord, fibers corresponding to various types of Sensitivity go separately from each other (fig. 2); conductors of conscious proprioceptive impulses (muscle-joint sense) enter the posterior columns, which consist mainly of conductors of this category. Since at each given level conductors of conscious proprioceptive impulses enter the outer part of the posterior columns, the corresponding lower-lying sections become increasingly displaced toward the midline, i.e., at each given level of the spinal cord, the lateral parts (Burdach's bundle) correspond to higher sections of the spinal cord, while the medial parts (Goll's bundle) correspond to lower sections. Fibers of the posterior roots corresponding to painful and temperature Sensitivity enter directly into the gray matter of the posterior horns of the spinal cord, where their endings come into contact with cells of the posterior horns; these cells represent the beginning of the second sensory neuron; their processes cross through the anterior commissure to the opposite side and enter here into the white matter of the lateral columns, in the composition of which they are directed toward the brain (tractus spino-thalamicus (fig. 2 d, e). Part of the conductors of tactile Sensitivity also goes in the composition of tr. spino-thalamicus; the remaining (larger) part of tactile conductors passes in the spinal cord in the posterior columns together with conductors of muscle-joint sense. With regard to the details of the entry of posterior root fibers into the spinal cord, it is necessary to note the following: fibers corresponding to proprioceptive Sensitivity enter the posterior columns on their side immediately upon their entry into the spinal cord, whereas the crossing to the opposite side of fibers forming tr. spino-thalamicus (painful and temperature conductors) does not occur at the level of entry of the given posterior root, but gradually over 2-3 segments (fig. 3). This fact has important clinical significance; with unilateral destruction of tr. spino-thalamicus, the loss of painful and temperature Sensitivity on the opposite side of the body is observed not from the level of the lesion, but below it. In fig. 4, the arrangement of conductors of various types of Sensitivity is shown on a cross-section of the spinal cord with indication of the topographic relationships of conductors corresponding to different segments of the spinal cord. As can be seen from this scheme, in the area of the posterior columns, medial parts correspond to lower segments, while in the area of the lateral columns (superficial Sensitivity), the conductors are located more laterally, the lower segments they correspond to. The further course of sensory conductors in their direction toward the brain undergoes significant changes in the area of the stem part of the brain. In the medulla oblongata, the entry of fibers of the posterior roots.

Figure 2. Diagram of the entry of posterior roots into the spinal cord and the course of sensory conductors in it: 1-fibers of conscious proprioceptive sensitivity, directly entering the posterior columns (a-funiculus gracilis, or Goll's bundle; b-funiculus cuneatus, or Burdach's bundle); 2 and 5-conductors of reflex proprioceptive impulses, ending in the so-called columns of Clarke (c) and the adjacent part of gray matter; from here begin fibers of new neurons directed along the outer surface of the lateral columns to the cerebellum [tractus spino-cerebellaris, or the so-called dorsal (Flexig's) and ventral (Govers') cerebellar bundles]; 3-conductors of pain and temperature sensitivity, ending in the posterior horn; fibers of new neurons beginning here cross to the opposite side and go further in the lateral columns (d-tractus spino-thalamicus lat.); 4-part of tactile conductors crossing to the opposite side but going in the anterior columns (e-tractus spino-thalamicus ant.); another part of tactile conductors goes in the posterior columns together with proprioceptive ones.

department of the posterior columns, then the corresponding lower-lying sections are increasingly displaced toward the midline, i.e., at each given level of the spinal cord, the lateral parts (Burdach's bundle) correspond to higher sections of the spinal cord, while the medial parts (Goll's bundle) correspond to lower sections. Fibers of the posterior roots corresponding to painful and temperature Sensitivity enter directly into the gray matter of the posterior horns of the spinal cord, where their endings come into contact with cells of the posterior horns; these cells represent the beginning of the second sensory neuron; their processes cross through the anterior commissure to the opposite side and enter here into the white matter of the lateral columns, in the composition of which they are directed toward the brain (tractus spino-thalamicus (fig. 2 d, e). In the composition of tr. spino-thalamicus goes part of the conductors of tactile Sensitivity; the remaining (larger) part of tactile conductors passes in the spinal cord in the posterior columns together with conductors of muscle-joint sense. With regard to the details of the entry of posterior root fibers into the spinal cord, it is necessary to note the following: fibers corresponding to proprioceptive Sensitivity enter the posterior columns on their side immediately upon their entry into the spinal cord, whereas the crossing to the opposite side of fibers forming tr. spino-thalamicus (painful and temperature conductors) does not occur at the level of entry of the given posterior root, but gradually over 2-3 segments (fig. 3). This fact has important clinical significance; with unilateral destruction of tr. spino-thalamicus, the loss of painful and temperature Sensitivity on the opposite side of the body is observed not from the level of the lesion, but below it. In fig. 4, the arrangement of conductors of various types of Sensitivity is shown on a cross-section of the spinal cord with indication of the topographic relationships of conductors corresponding to different segments of the spinal cord. As can be seen from this scheme, in the area of the posterior columns, medial parts correspond to lower segments, while in the area of the lateral columns (superficial Sensitivity), the conductors are located more laterally, the lower segments they correspond to. The further course of sensory conductors in their direction toward the brain undergoes significant changes in the area of the stem part of the brain. In the medulla oblongata, the entry of fibers of the posterior roots.

t.

pressure and movement

Figure 4. Diagram of the relationship between various conductors of Sensitivity on a cross-section of the spinal cord. The diagram corresponds to the upper part of the cervical section. (After Forster.) conductors of deep Sensitivity (fibers of posterior columns) end in the area of accumulations of gray matter located in the dorsal parts of the medulla oblongata-the so-called nuclei of posterior columns (fig. 5); fibers beginning from cells of these nuclei, representing the second neuron of proprioceptive conductors, undergo crossing to the opposite side and are located in the base of the medulla oblongata between the olives (interolivary layer, fig. 5). Conductors of superficial Sensitivity are located in the medulla oblongata in the outer part of the middle level (fig. 5). In the area of the posterior parts of the pons, proprioceptive and superficial conductors merge into a common trunk (lemniscus medialis-medial lemniscus), located in the middle part of the base of the pons (fig. 5). The medial lemniscus in the area of the cerebral peduncles is located upward and outward from the red nuclei and further enters the lateral nucleus of thalami optici; here ends the second neuron of sensory conductors; endings of corresponding fibers come into contact with cells, from which begins the aggregate of fibers of the third sensory neuron; part of these fibers ends in the medial nucleus (ncl. medialis, s. essentialis) and has here its terminal point (protopathic, nociceptive form of Sensitivity), while another part in the form of the superior brachia of the thalamus is directed through the posterior part of the internal

Figure 5. Diagram of the course of sensory conductors from the intervertebral ganglion to thalamus opticus: I-area of thalami optici; II-cerebral peduncles; III-pontes Varolii; IV-medulla oblongata; V-spinal cord; 1-superior brachium of thalami optici (thalamo-cortical sensory bundle); 2-thalamus opticus; 3-capsula int.; 4-chiasma; 5 and 6-lemniscus med.; 7-tractus pyramidalis; 8-tuberculum acusticum and entrance of n. cochlearis; 9 and 16-tractus spino-thalam.; 10-nucleus of posterior columns; 11-librae arcuatae; 12-interolivary layer; 13-Burdach's bundle; 14-substantia gelatinosa of posterior horn; 17-muscle and joint sense

ganglion; 18-sensory fibers; 19-fibers of pain and temperature sensitivity, and then as part of the radiant crown into the sensitive area of the cerebral cortex (postcentral gyrus; epicritic, discriminative Sensitivity). Regarding the details of cortical localization of Sensitivity, it is necessary to mention the following data. In the past, since the time of the research of Fritsch and Hitzig, it was believed that both the anterior and posterior central gyri represent areas related to both motor and sensory functions ('sensorimotor' area). Now it can be considered established that the cortical projection area is only the postcentral gyrus with its part extending into the depth of the Rolandic fissure. If the Rolandic fissure can thus be considered the anterior boundary of the sensitive area of the cortex, then the centers extend posteriorly beyond the postcentral gyrus—into the parietal area, into the area of g. supramarginalis, and perhaps partly into the area of the posterior parts of the two upper temporal gyri. In the postcentral gyrus, the sensory centers are located identically to the motor centers in the anterior central gyrus, i.e., in its upper parts are located the centers corresponding to the lower limb, and these centers extend into the gyrus paracentralis of the inner surface of the cerebral hemisphere; the middle part of the postcentral gyrus corresponds to the upper limb, and in the very lowest part of the postcentral gyrus are localized the sensory centers of the face. When various parts of the postcentral gyrus are affected, losses of cortical forms of sensitivity are observed in the corresponding areas of the body. Along with this focal cortical type of Sensitivity disorder, it is necessary to keep in mind the segmental type, i.e., the predominance of disorder in a certain part of the limb; thus, for cortical Sensitivity disorder, the predominance in the distal parts of the limbs is characteristic. When the postcentral gyrus is affected, the so-called 'axial' type can also be observed, i.e., loss of sensitivity in areas of the limb corresponding to its axis; thus, in some cases, cortical Sensitivity disorder is limited to the 'preaxial' area of the hand, i.e., its outer surface, (Cv, Cvi, Cvn); in contrast to this, sometimes cortical losses of Sensitivity of the 'postaxial' type are observed, limited to the inner side of the hand (Cush, Di). The possibility of such false-root (pseudoradicular) disorders of cortical origin should be kept in mind to avoid mistakenly taking them for the result of root damage. In contrast to these limited Sensitivity disorders caused by lesions of the postcentral gyrus, lesions of the parietal parts of the sensory area always lead to disorders affecting the entire half of the body ('half-type'). Regarding the forms of cortical Sensitivity localized in the parietal gyri, there are some disagreements. According to the most widespread view, the superior parietal gyrus (more precisely, its anterior part) is the center for all forms of cortical Sensitivity, and the inferior one—mainly for deep (proprioceptive) sensitivity and stereognosis; some consider g. supramarginalis to be the center of stereognosis. According to the data of Benisti (A. Benisti), when the parietal sensory area is affected, a peculiar disorder of spatial orientation is observed without impairment of the sense of position, i.e., without impairment of muscle-joint sense. These data allow us to recognize the existence of an independent 'sense of space'. In addition to the forms of Sensitivity listed above, the so-called general sense should be mentioned. The term 'general sense' is used in two different meanings. In the past, this term was used to denote sensations that do not have a definite topographic connection with any part of the body and depend on disturbances in the processes of nutrition and functions of visceral organs; thus, hunger, thirst, the feeling of nausea during dizziness, vague sensations from individual organs, etc., were referred to as general sense. Recently, the term 'general sense' is also used in another sense. Under general sense (synonym—kinaesthesia) is meant 'the elementary sense concerning the entire organism, the aggregate of constantly changing sensations in which changes in the state of one's own body are manifested. In the normal state, these sensations, due to their usualness, remain subconscious and pass into the sphere of consciousness in pathological cases (kinaesthopathy). Kinaesthopathy, or disorder of general sense, consists of unusual, unpleasant sensations and is not a sense of pain, but a peculiar experience, the essential psychological element of which is the feeling of anxiety or fear, which is a consequence of the tormenting inexplicability of the experienced unusual sensations. Often clinically it is necessary to note how these disorders sum up and form a delusional system of hypochondriacal character'. The anatomical-physiological data presented should be supplemented by some data on psycho-physiological relationships between sensation and irritation. The basis for ideas about these relationships in the past was Fechner's law, based on the dependence of the increase in sensation on the intensification of irritation, established by Weber (1834): the stronger the sensation, the greater the intensification of irritation is required for a noticeable increase in sensation. If for a sensation caused by irritation whose strength is equal to 1, an increase of 1/w is required for a noticeable increase in sensation, then for a sensation caused by irritation twice as strong, an increase in irritation of 2/w is required for a noticeable increase in sensation. Thus, with twice the irritation, twice the increase in irritation is required to obtain a noticeable increase. These relationships were expressed mathematically by Fechner in the so-called 'basic psycho-physical formula': an increase in sensation in arithmetic progression corresponds to an intensification of irritation in geometric progression. However, this relationship by no means represents a law. Fechner himself already noted that the formula he established is applicable only to irritations of medium strength and is not justified for weak and strong irritations. However, there are a number of other facts contradicting the assumption of exact mathematical relationships between irritation and sensation. Here, first of all, the phenomenon of summation should be mentioned, the essence of which is the enhancement of excitation under the influence of repeated irritations. Summation can manifest in two forms: an irritation insufficient to cause a certain sensation can cause a sensation if this irritation is repeated at short intervals ('hidden addition', addition latente), secondly, with repetition of irritations, each of which causes a weak sensation, a stronger sensation can be obtained. Thus, an increase in sensation can occur without an increase in irritation. An even greater contradiction to Fechner's formula is encountered in the phenomenon of protopathic sensitivity. In the latter, as indicated above, the reactions approach in character the type of 'all or nothing' reactions: in this case, the sensation does not show a gradual increase with the intensification of irritation, but upon reaching a certain threshold, it manifests itself in its maximum degree. The ineffectiveness of Fechner's formula is an additional confirmation of the fact that the complexity of biological processes excludes the possibility of their mathematical formulation. The study of Sensitivity occupies a very important place in the general process of examining the patient. Since data on the state of Sensitivity are based mainly on the subjective statements of the subject, certain precautions must be observed both in the methods of examination and in the evaluation of the results obtained. For better concentration of the patient's attention and exclusion of visual perception of the irritations applied, the examination of Sensitivity must be performed with the patient's eyes closed. Due to the fact that the accuracy of the patient's statements can be affected by fatigue, it is desirable first to conduct a general examination and to stop at the details after some rest of the patient. It is necessary to carefully avoid forms of questions that contain an element of suggestion. For example, one should never, when applying a painful irritation, ask the patient: 'does it hurt?', or, when applying a tactile irritation, ask: 'do you feel?'. One should never apply irritations rhythmically. The subject should be informed of the need to immediately report any sensation experienced, since a delay in the patient's responses may give rise to the erroneous assumption of 'delayed conductivity of sensitivity', sometimes observed in certain pathological cases. Tactile Sensitivity is examined by light touch to the patient's skin with a brush, a piece of cotton wool, etc. With eyes closed, the patient should say 'yes' with each sensation of touch. For the examination of painful Sensitivity, pinpricks or the point of a goose quill are used.

To the patient, of course without a definite sequence and not rhythmically, tactile and painful irritations are applied, and he is asked to immediately upon receiving the corresponding sensation say: 'dull' or 'sharp'. For the investigation of temperature sensitivity, test tubes containing cold (or cool) and hot (or warm) water are applied to the skin, and the patient must report the sensation received with the words 'hot' or 'cold'. In the investigation of localization, light tactile irritations are applied, and with the patient's eyes closed, he must indicate with his finger the place of contact. In the investigation with Weber's compass, the instrument is applied with either two or one point; the subject must report whether he feels paired or single contact. The results obtained can be recorded as follows: a horizontal line is drawn, before which the distance between the points of the compass in millimeters is marked; above this line the readings for single-point contact are marked, and below the line the readings corresponding to two points; correct answers are marked with vertical lines, and incorrect ones with diagonally crossing lines. Thus, if the patient on a certain area of skin gave correct answers for all single and double points at a distance of 12 mm from each other, this is depicted by the following formula:

This means that the patient was first given three single irritations, then two paired, then two single, etc., and that in all cases the sensation received was correct. If the patient at a distance of points of 10 mm correctly recognized all single irritations, but took all paired ones for single ones, this is depicted as: 10 mm -I--------!----------II----------II.. 2 XX XXX XXX If the patient made mistakes both with single and paired contacts at a distance of points of 15 mm, this is depicted as: -15

This means that out of three single stimuli applied, the patient correctly perceived one as single, and two incorrectly, i.e., as paired. Then four paired stimuli were applied, of which he incorrectly identified the first and fourth as single, and correctly identified the second and third, and so on. It should be kept in mind that in regard to the fineness of tactile sensitivity, which constitutes the essence of recognizing Weber's points (tactile localization of touch), different areas of the skin surface show sharp differences. For approximate orientation, the following minimum distances between points that can be recognized in different skin areas can be indicated: lips - about 5 mm, pulp of the third finger of the hand - 3 mm, palm - 12 mm, back of the hand - 30 mm, forearm, shin - 40 mm, back - 60 mm, thigh - 70 mm. - For investigating muscle-joint sense, passive movements in the joints are performed; with eyes closed, the patient should report on the corresponding changes and their direction. The investigation should begin with the distal parts of the extremities (fingers), where muscle-joint sense is developed with particular fineness. - For investigating vibratory sensitivity (pallesthesia), a small tuning fork (A 256) with sleeves on the vibrating prongs can be used. The vibrating tuning fork is applied by the handle to the periosteum; the patient should determine the sensation experienced; with normal vibratory sensitivity, patients perceive this sensation as vibration, 'trembling,' 'like electricity'; with loss of vibratory sensitivity, only the tactile sensation of contact of the tuning fork handle with the skin surface is perceived. - For investigating stereognostic sense, which actually does not belong to the area of elementary sensitivity, but to the area of associative activity of the cortex, various objects (coin, key, pencil, etc.) are placed in the patient's hand with eyes closed, and they are asked to identify them. Pathological disorders of sensitivity may involve quantitative and qualitative changes in sensations. Quantitative changes include decrease in intensity of sensation, i.e., lowered sensitivity (hypoesthesia), complete loss of sensitivity (anaesthesia), and increased sensitivity (hyperaesthesia). Hypoesthesia and anaesthesia are based on partial or complete disruption of conductivity; hyperaesthesia is caused by the presence of pathological irritation along the nerve or in the area of sensory centers; the irritation applied during investigation, summing with the existing pathological irritation, leads to the phenomenon of hyperaesthesia. Accordingly to the form of quantitatively disordered sensitivity, the following terms are used: analgesia (loss of pain sensitivity), thermoanaesthesia (loss of temperature sensitivity), stereoanaesthesia, synonym astereognosia (loss of stereognostic sense), loss of localization ability (topanaesthesia). - The area of qualitative disorders of sensitivity includes incorrect perceptions of external irritations, for example, painful sensation of cold (psychralgesia) or heat (thermalgia); tactile sensation of an object being larger than it actually is - macroesthesia (e.g., the patient perceives a matchstick placed in his hand as a stick); erroneous perception of several objects instead of one - polyesthesia; synalgia - concomitant pain, consists in sensation of pain, besides the site of application of painful irritation, in some other area; a variety of synalgia is paralgia: when painful irritation is applied, pain is felt in the symmetrical area of the other half of the body. - A special form of qualitative change in sensitivity is hyperpathy, the essence of which lies in painful perception of various kinds of irritations; hyperpathy differs from hyperaesthesia in that with hyperaesthesia there is lowering of the irritation threshold; conversely, with hyperpathy the irritation threshold is elevated. In the area of hyperpathy, light irritations are perceived less clearly than in the symmetrical healthy area, but when significant intensity is reached, all irritations in the hyperpathic area are perceived as sharply painful, unpleasant, diffuse, leaving prolonged after-effects. Hyperpathy is a manifestation of protopathic sensitivity (see thalamic sensitivity above). - From the above objective forms of sensitivity disorders, i.e., those that are discovered when investigating sensitivity by applying artificial external irritations, subjective sensitivity disorders should be distinguished, which manifest independently of visible external irritations. A typical example of subjective sensitivity disorder is pain. Furthermore, a very extensive area of subjective sensitivity disorders is represented by so-called paresthesias, or dysesthesias, the essence of which consists in patients experiencing abnormal sensations independently of external irritations: numbness, sensation of crawling ants, sensation of heat, cold, tension of the skin, pain in the roots of hair (trichalgia), etc. As for the quality and topographical distribution of sensory disorders depending on the localization of the pathological process, the relevant data can be summarized in the following points. Lesions of peripheral nerves are characterized by sensitivity disorders limited to the area of distribution of the given nerve; these disorders are expressed in the acute period by pains, hyperesthesias, and paresthesias; in case of destruction of fibers, persistent anaesthesia develops; when during the period of recovery of sensitivity only protopathic sensitivity is present, phenomena of hyperpathy are observed. '29 With lesions of the posterior roots, sensitivity disorders have a root distribution. With diffuse transverse lesions of the spinal cord, loss of sensitivity is observed below the level of the lesion; hemisections of the spinal cord lead to loss of deep sensitivity on the side of the lesion and to loss of pain and temperature sensitivity on the contralateral side (see Brown-Séquard syndrome). For the correspondence between brain segments and skin surface, as well as for details of spinal cord sensitivity disorders - see Spinal Cord. Lesions of the brain stem are characterized by loss of all types of sensitivity on the opposite side of the body; their diagnosis is based on involvement of cranial nerves in the process. With lesions of the thalamus, loss of all types of sensitivity occurs on the entire opposite side of the body; the internal capsule is involved to a greater or lesser degree in the process (hemiplegia, hemianopia); if there is irritation of the medial nucleus, severe pains and hyperpathia are observed on the opposite side of the body. Among sensitivity disorders caused by lesions of the cerebral cortex, cortical and transcortical disorders should be distinguished. By cortical disorders are meant changes in sensitivity associated with lesions of projection cortical centers of sensitivity (postcentral gyrus); these disorders are characterized by loss of fine forms of sensitivity (light touch with precise localization, distinguishing warm from cool, sense of position) while preserving coarse protopathic sensitivity (pain without precise recognition of the quality of painful irritation, sense of unpleasantness from hot and cold, strong pressure, vibration). - By transcortical sensitivity disorders are meant changes in the perception of external irritations depending on lesions of association areas or on disconnection of these areas from projection sensitivity centers. Transcortical sensitivity disorder is one of the forms of agnosia; a typical manifestation of it can be tactile agnosia ('tactile paralysis,' Tastlahmung). The essence of this disorder is loss of the ability to recognize objects by touch, despite preservation of all types of sensitivity, including tactile. The cause of tactile agnosia is disruption of the connection between cortical projection sensitivity centers and association areas, as a result of which the patient, despite correct recognition of various properties of the object, cannot 'recognize' it (e.g., when a watch is placed in his hand, he identifies the object as round, hard, smooth, etc., but does not recognize that it is a watch). Tactile gnosis is a complex sensory function; in recognizing an object by palpation, tactile sensitivity, muscle-joint sense, pressure, localization, etc., participate. Loss of each of these types of sensitivity can lead to disruption of the ability to recognize an object by palpation; such a disorder of tactile gnosis, caused by disorder of one of the elementary types of sensitivity, is designated by the term 'perceptory astereognosia.' In contrast to this, with 'tactile paralysis' (Tastlahmung), the ability to recognize objects is lost, despite preservation of all elementary types of sensitivity. The anatomical basis of this disorder consists of lesions of the cortex behind the postcentral gyrus, i.e., in the parietal area.