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Stimulus convergence in human teeth
ArchsoralBiol.Vol. IS, pp. 953-959,1970.Pergamon Press.Printed in GreatBritain.
STIMULUS
CONVERGENCE J. M.
MUMFORD
IN HUMAN
and A. V.
TEETH
NEWTON
Dental School, Liverpool University, Liverpool 7, England Summary-When human teeth are stimulated by brief electrical impulses to produce threshold pain, increasing the area of electrode contact is associated with progressively smaller increases in the current amplitude, the relationship between current and area being described by an exponential curve. To try to explain this, two investigations were carried out. The first showed that anatomical convergence of the tissues from the outer surface of the enamel to the pulpal surface of the dentine is a factor, but only a small one. In the second investigation two arrangements of multiple electrodes were used, one with contiguous contact areas; the other with discrete contact areas. The result indicates physiological convergence which might be due to receptor field overlap in the pulp and/or recoding at the 1st to 2nd, and 2nd to 3rd order neurones.
WHEN human teeth are subjected to brief electrical impulses, the pain perception threshold depends partly on the area of the electrode contact at the tooth surface, a relationship which has been described by MUMFORD and NEWTON (1969). They argued that increasing this area would be associated with a proportional increase in the area through which the current flowed at the pulpo-dentinal junction. Since the effective stimulus for sensory receptors is the current density, the increase in area would necessitate an increase in current. Eventually, it was supposed, the entire population of receptors in the pulp would be involved and/or current might reach such a value that nerves would be excited, perhaps in the pulp canal, so that no further increase in current would be needed. Experimentally, they obtained the result summarized in Fig. 1 which is based on average values. If the simple hypothesis described were completely valid, the curve would be an obliquely rising straight line, with an abrupt levelling off to a horizontal straight line. That it was not so might be due to several factors. The structure of enamel and dentine could be partly responsible, the effective stimulus being dependent on the degree of convergence of these tissues as they extend from the outer surface of the tooth to the pulp; that is, anatomical convergence. Another factor could be related to the receptor field, by which is meant the area (more strictly volume) in which lie the terminals of individual nerve fibres. A search of the literature has not yielded definite information on this, no doubt because investigators have encountered difficulties owing to the ramifications of the subodontoblastic nerve plexus. However, it is reasonable to suppose that there will be some overlap between adjacent receptor fields, and this might be sufficient in itself to give the gradually changing curve of the experimental result (Fig. 1). A third factor could be related to the possible convergence of the first order sensory neurones on to fewer
953
954
J.M.
MIJMF~RDAND A.V.
Area, FIG. 1. Relationship
NEWTON
mm’
of the pain perception threshold and contact area of the stimulating electrode.
second order neurones, and these in turn converging on fewer third order neurones; that is physiological convergence. The purpose of this investigation was to find the relative values of these factors. MATERIALS
AND
METHODS
The subjects were staff and students, aged IS-24 yr, of the Liverpool Dental School and Hospital. They were selected because they had upper central incisor teeth which were apparently normal. These teeth are easily accessible, easily isolated under rubber dam, and the middle of the labial surface is the largest fairly flat surface of any human teeth, which facilitates making good electrode contacts. The stimulator used was a mains powered, square wave constant current generator with an internal impedance of 50 MQ. It consisted of four basic sections which were the power supply, a blocking oscillator and polarity reversing amplifier, a pulse width generator and a constant current output. It was set to give continuous impulses of 10 msec duration at 50 Hz. To find if the physical structure of the tooth had any effect on the results the foliowing subsidiary investigation was made. An upper central incisor was isolated under rubber dam and its surface dried by a stream of air. A strip of adhesive paper, previously graduated in mm, was applied horizontally to the tooth. The stimulating electrode had an electrode tip of orangewood soaked in physiological saline, with a square cross-sectional area of 1 mm2. This was applied to the labial surface at position A (Fig. 2). The indifferent electrode was held in the subject’s hand. The electric stimulus was gradually increased until the pain threshold was reached, as indicated by the subject saying “ah”, whereupon the current reading was noted. The electrode was
SIIMGLIJS CGNVERGENCE IN HUMAN TEETH
955
FIG.2. Outlineof an upper central incisor showing a paper strip graduated in millimete.n, and four electrode areas. removed, the tooth surface dried and the electrode applied in the square mm adjacent to the tist one (position B, Fig. 2). The pain-perception threshold was again determined. The procedure was repeated for positions C and D. The pain threshold determinations were repeated in the reverse order for positions D, C, B and A, thus giving two readings for each electrode site. The graduated strip of adhesive paper was removed and applied vertically. A further series of threshold determinations was made in a vertical series as described above, the areas being designated E, F, G and H. The main investigation into possible overlapping of receptor fields was carried out as follows. The stimulating electrode was specially constructed (Fig. 3) and consisted of 11 stainless steel (orthodontic) tubes each with an external diameter of l-0 mm and an internal diameter of O-65 mm. These were clamped together by two stainless bands so that they were held in an approximately cylindrical manner. Electrical contact between the tubes was thus assured whilst still allowing them to slide one along another. One of the bands had soldered to it the negative lead of the stimulator. The positive lead was connected to a larger stainless steel tube (the indifferent electrode) held in the patient’s hand, the same material being used for both electrodes to avoid galvanic activity.
FIG. 3. Experimental arrangement of the multiple electrode applied to an upper central incisor tooth already isolated by a sheet of rubber dam.
956
J. M. MUMFORDAND A. V. NEWTON
This multiple stimulating electrode was mounted on a plastic impression tray which carried softened impression compound in the usual way. An impression was taken of the subject’s upper occlusal plane taking care to have the electrode accurately placed over the middle of the labial surface of the tooth being tested. The impression tray was removed from the mouth, cooled and dried. Rubber dam was applied to the individual tooth being tested and kept in place as the impression tray was replaced. The subject closed his mouth to keep the assembly in position. The enamel surface was dried by means of a brief stream of air and one electrode tube was pushed along to make contact with the tooth, contact being enhanced by having first pressed Cambridge electrode jelly into all the electrode tubes. The subject was instructed to say “ah”, without moving his jaws, when he first perceived a sensation in the tooth, at which point the microammeter reading was noted, this value of current being the pain perception threshold. The next electrode tube was pushed forward to make contact with the tooth and the threshold was again determined. The procedure was repeated for the remaining electrode tubes. Without disturbing the system, the tooth surface was cleansed of electrode jelly and dried. The experiment was repeated but only alternate electrode tubes were used, so that a space was left on the tooth surface between each electrode. After each experiment the subject was questioned regarding the intensity, quality and site of sensation.
RESULTS
In the subsidiary investigation on tissue convergence there were 4 horizontally arranged areas and 4 vertically arranged areas. For each area there were 14 readings on 7 teeth, the mean threshold values being as follows: Horizontal areas (A) 6.49 PA, (B) 6.37 PA, (C) 5.66 PA, (D) 4.82 PA Vertical areas (E) 7.49 PA, (F) 7.12 /LA (G) 6.88 PA, (H) 6.72 PA The underlined values are for the two areas (A and G) which corresponded approximately in the two series. The mean of these two values, 6.68 PA, was taken to be 100 per cent so that all the above values may be represented by the following percentages : Horizontal areas (A) (D) Vertical areas (E) (H)
100 per cent, (B) 98.2 per cent, (C) 87.2 per cent 74 *2 per cent 108.6 per cent, (F) 103.2 per cent, (G) 100 per cent, 97 - 6 per cent
The results of the main investigation on 20 normal upper central incisors are presented in Fig. 4. Curve A was obtained for separated electrodes, curve B for contiguous electrodes. In the latter it would be reasonable to expect a slightly larger area of contact than that of the tube areas alone because surface tension would tend to draw electrode jelly across the intervals between adjacent tubes; for this reason
STWULUS
I.5
CONVERGENCE
IN HUhWN
957
TEETH
r
I
5O
Arao,
mm2
I 7m
I 93
I IO
FIG. 4. Relationshipof the pain perception tbnzshold and contact area of the multiple electrode. A, curve obtained using discrete incrementsin area. B, curve obtained using contiguousincrementsin area. C, estimatedcorrection of curve B. Curve A ranged from 8.29-12.3 PA; curve B from 7.77-12.4 PA.
the curve B obtained experimentally has been corrected as shown in curve C, drawn for a maximum area estimated after tracing around the entire group of tubes on squared paper. The quality of sensation was mainly described as tingling, but mild pain, warm, cold, sharp, throb and sting were also mentioned. In any one subject the quality remained the same as the number of electrode tubes increased. The subject noticed only one site of sensation, whatever the number of electrode tubes applied. DISCUSSION It is common knowledge that, as a result of anatomical convergence of the dental hard tissues, any given area at the tooth surface corresponds to a smaller area at the pulpo-dentinal junction. The relative areas at the site under consideration are approximately 2 to 1 which means that an electrical stimulus of 10 PA passing through an electrode contact of 1 mm2 at the enamel surface, would be 20 PA/mm2 at the pulpo-dentinal junction; in other words, twice the current density and therefore twice the effective stimulus. Since it is this current density rather than current alone which is important, the anatomical convergence must be considered. This is not constant and the results show that it increased as the electrode area was moved peripherally. This anatomical convergence means that doubling the electrode area at the enamel surface may more than, or less than, double the corresponding area at the pulpo-dentinal junction, depending on the original site chosen. However, the effect is too small to explain the diminishing slope of the curve in Fig. 1. Moreover, the multiple electrode was placed on the middle of the labial enamel where the effect of A.O.B. 15/10-D
958
J. M. MWORD
AND
A. V. NEWTON
physical convergence was only about 1 per cent; yet the same kind of curve was obtained in Fig. 4 as in Fig. 1. It is concluded that the shape of this curve is mainly determined by physiological convergence. Comparison of the curves in Fig. 4 shows curve B (or C) to be less steep than curve A. It is suggested that for curve B there would be more overlapping of the underlying axon terminals. Some of these would already be subjected to electrical stimulation from the adjacent first electrode area, so the succeeding electrode area would be associated with a smaller increment of current. On the other hand, curve A was obtained for separated electrodes where overlap was reduced, but not necessarily eliminated. No significance can be attached to the lower threshold values in Fig. 1 compared with those in Fig. 4 because the former were obtained by a somewhat different method which included a microammeter reading on the cessation of pain as the current was reduced. It should be noted that the intensity, quality and site of sensation perceived by the subject were the same for all the electrode areas although the larger areas are presumed to involve excitation of more nerve fibres with possibly a greater total number of nerve impulses passing centripetally. If so, some of the additional nerve impulses are redundant in the informational sense. Presumably this is associated with re-coding of the sensory inflow so as to reduce its magnitude as suggested by BARLOW (1968), but this is beyond the scope of the present paper. The various qualities of sensation are the same as those previously reported and their significance has already been discussed (MUMFORD 1965). The subjects’ failure to observe sensation in more than one site of the tooth even with multiple separated electrodes is compatible with the clinical observation that pain arising from pulp disease is poorly localised by the patient. R&sun&-Lorsque les dents humaines sont soumises a des excitations dlectriques breves pour produire en seuil de douleur, l’augmentation de la surface de l’&ctrode de contact est associ&s avec des augmentations progressivement plus faibles de l’amplitude du courant, le rapport entre courant et surface &ant exprime par une courbe exponentielle. Deux types d’experiences sent me& pour expliquer ce fait. La premiere montre que la convergence anatomique des tissus, de la surface exteme de l’email vers la surface pulpaire de la dentine joue certes un role, mais il est peu important. Dans la deuxieme experience, deux dispositifs d’electrodes multiples sont utilisb; l’tm avec des surfaces de contact contigues: l’autre pr6sente des zones de contact disc&es. Une convergence physiologique est mise en evidence : elle peut etre li6e a un chevauchement du champ recepteur dam la pulpe et/au ii un enrigistrement au niveau des neurones du ler et 2&me ordre et du 2bme et 3eme ordre. Zusammen&sung-Wenn menschliche Zlhne durch kurze elektrische Impulse gereizt werden, urn einen Schwellenschmerz zu erzeugen, dann ist der vergriil3erte Rezirk des Elektrodenkontaktes mit fortschreitend geringerem Anwachsen der Stromamplitude verbunden, wobei die Beziehung zwischen Strom und Kontaktflache durch eine Exponentialkurve beschrieben wird. Urn dies zu erkl&ren, wurden 2 Untersuchungen ausgeftihrt. Die erste ergab, da8 die anatomische Konvergenz der zwischen Schmelzoberflache und pulpaler ObertXche des Dentins gelegenen Gewebe ein, wenn such kleiner, Faktor ist. Bei der zweiten Untersuchung wurden 2 Anordnungen ftir Mehr-
STIMULUS CONVERGENCE
IN
HUMANTEETH
959
fachelektroden benutzt, eine mit benachbarten Kontaktbezirken, die andere mit getrennten. Das Ergebnis ist eine physiologische Konvergenz; sie diirfte auf ein uberlappen der Rezeptorfelder in der Pulpa und/oder auf eine Perception im Bereich der Neuronen 1. und 2. sowie 2. und 3. Ordnung zurtickzufiihren sein. REFERENCES BARLOW,H. B. 1968. Sensory mechanisms, the reduction of redundancy, and intelligence. In Cybernef~cs, pp. 183-207 (edited by EVANS,C. R. and ROBERTSON,A. D. J.). Butterworths, London. M-an, J. M. 1965. Pain perception threshold and adaptation of normal human teeth, Archs oral Biol. 10,957-968. MUMFORD,J. M. and NEWTON,A. V. 1969. Zone of excitation when electrically stimulating human teeth. Archs oral Biol. 4,1383-1388.
STIMULUS
CONVERGENCE J. M.
MUMFORD
IN HUMAN
and A. V.
TEETH
NEWTON
Dental School, Liverpool University, Liverpool 7, England Summary-When human teeth are stimulated by brief electrical impulses to produce threshold pain, increasing the area of electrode contact is associated with progressively smaller increases in the current amplitude, the relationship between current and area being described by an exponential curve. To try to explain this, two investigations were carried out. The first showed that anatomical convergence of the tissues from the outer surface of the enamel to the pulpal surface of the dentine is a factor, but only a small one. In the second investigation two arrangements of multiple electrodes were used, one with contiguous contact areas; the other with discrete contact areas. The result indicates physiological convergence which might be due to receptor field overlap in the pulp and/or recoding at the 1st to 2nd, and 2nd to 3rd order neurones.
WHEN human teeth are subjected to brief electrical impulses, the pain perception threshold depends partly on the area of the electrode contact at the tooth surface, a relationship which has been described by MUMFORD and NEWTON (1969). They argued that increasing this area would be associated with a proportional increase in the area through which the current flowed at the pulpo-dentinal junction. Since the effective stimulus for sensory receptors is the current density, the increase in area would necessitate an increase in current. Eventually, it was supposed, the entire population of receptors in the pulp would be involved and/or current might reach such a value that nerves would be excited, perhaps in the pulp canal, so that no further increase in current would be needed. Experimentally, they obtained the result summarized in Fig. 1 which is based on average values. If the simple hypothesis described were completely valid, the curve would be an obliquely rising straight line, with an abrupt levelling off to a horizontal straight line. That it was not so might be due to several factors. The structure of enamel and dentine could be partly responsible, the effective stimulus being dependent on the degree of convergence of these tissues as they extend from the outer surface of the tooth to the pulp; that is, anatomical convergence. Another factor could be related to the receptor field, by which is meant the area (more strictly volume) in which lie the terminals of individual nerve fibres. A search of the literature has not yielded definite information on this, no doubt because investigators have encountered difficulties owing to the ramifications of the subodontoblastic nerve plexus. However, it is reasonable to suppose that there will be some overlap between adjacent receptor fields, and this might be sufficient in itself to give the gradually changing curve of the experimental result (Fig. 1). A third factor could be related to the possible convergence of the first order sensory neurones on to fewer
953
954
J.M.
MIJMF~RDAND A.V.
Area, FIG. 1. Relationship
NEWTON
mm’
of the pain perception threshold and contact area of the stimulating electrode.
second order neurones, and these in turn converging on fewer third order neurones; that is physiological convergence. The purpose of this investigation was to find the relative values of these factors. MATERIALS
AND
METHODS
The subjects were staff and students, aged IS-24 yr, of the Liverpool Dental School and Hospital. They were selected because they had upper central incisor teeth which were apparently normal. These teeth are easily accessible, easily isolated under rubber dam, and the middle of the labial surface is the largest fairly flat surface of any human teeth, which facilitates making good electrode contacts. The stimulator used was a mains powered, square wave constant current generator with an internal impedance of 50 MQ. It consisted of four basic sections which were the power supply, a blocking oscillator and polarity reversing amplifier, a pulse width generator and a constant current output. It was set to give continuous impulses of 10 msec duration at 50 Hz. To find if the physical structure of the tooth had any effect on the results the foliowing subsidiary investigation was made. An upper central incisor was isolated under rubber dam and its surface dried by a stream of air. A strip of adhesive paper, previously graduated in mm, was applied horizontally to the tooth. The stimulating electrode had an electrode tip of orangewood soaked in physiological saline, with a square cross-sectional area of 1 mm2. This was applied to the labial surface at position A (Fig. 2). The indifferent electrode was held in the subject’s hand. The electric stimulus was gradually increased until the pain threshold was reached, as indicated by the subject saying “ah”, whereupon the current reading was noted. The electrode was
SIIMGLIJS CGNVERGENCE IN HUMAN TEETH
955
FIG.2. Outlineof an upper central incisor showing a paper strip graduated in millimete.n, and four electrode areas. removed, the tooth surface dried and the electrode applied in the square mm adjacent to the tist one (position B, Fig. 2). The pain-perception threshold was again determined. The procedure was repeated for positions C and D. The pain threshold determinations were repeated in the reverse order for positions D, C, B and A, thus giving two readings for each electrode site. The graduated strip of adhesive paper was removed and applied vertically. A further series of threshold determinations was made in a vertical series as described above, the areas being designated E, F, G and H. The main investigation into possible overlapping of receptor fields was carried out as follows. The stimulating electrode was specially constructed (Fig. 3) and consisted of 11 stainless steel (orthodontic) tubes each with an external diameter of l-0 mm and an internal diameter of O-65 mm. These were clamped together by two stainless bands so that they were held in an approximately cylindrical manner. Electrical contact between the tubes was thus assured whilst still allowing them to slide one along another. One of the bands had soldered to it the negative lead of the stimulator. The positive lead was connected to a larger stainless steel tube (the indifferent electrode) held in the patient’s hand, the same material being used for both electrodes to avoid galvanic activity.
FIG. 3. Experimental arrangement of the multiple electrode applied to an upper central incisor tooth already isolated by a sheet of rubber dam.
956
J. M. MUMFORDAND A. V. NEWTON
This multiple stimulating electrode was mounted on a plastic impression tray which carried softened impression compound in the usual way. An impression was taken of the subject’s upper occlusal plane taking care to have the electrode accurately placed over the middle of the labial surface of the tooth being tested. The impression tray was removed from the mouth, cooled and dried. Rubber dam was applied to the individual tooth being tested and kept in place as the impression tray was replaced. The subject closed his mouth to keep the assembly in position. The enamel surface was dried by means of a brief stream of air and one electrode tube was pushed along to make contact with the tooth, contact being enhanced by having first pressed Cambridge electrode jelly into all the electrode tubes. The subject was instructed to say “ah”, without moving his jaws, when he first perceived a sensation in the tooth, at which point the microammeter reading was noted, this value of current being the pain perception threshold. The next electrode tube was pushed forward to make contact with the tooth and the threshold was again determined. The procedure was repeated for the remaining electrode tubes. Without disturbing the system, the tooth surface was cleansed of electrode jelly and dried. The experiment was repeated but only alternate electrode tubes were used, so that a space was left on the tooth surface between each electrode. After each experiment the subject was questioned regarding the intensity, quality and site of sensation.
RESULTS
In the subsidiary investigation on tissue convergence there were 4 horizontally arranged areas and 4 vertically arranged areas. For each area there were 14 readings on 7 teeth, the mean threshold values being as follows: Horizontal areas (A) 6.49 PA, (B) 6.37 PA, (C) 5.66 PA, (D) 4.82 PA Vertical areas (E) 7.49 PA, (F) 7.12 /LA (G) 6.88 PA, (H) 6.72 PA The underlined values are for the two areas (A and G) which corresponded approximately in the two series. The mean of these two values, 6.68 PA, was taken to be 100 per cent so that all the above values may be represented by the following percentages : Horizontal areas (A) (D) Vertical areas (E) (H)
100 per cent, (B) 98.2 per cent, (C) 87.2 per cent 74 *2 per cent 108.6 per cent, (F) 103.2 per cent, (G) 100 per cent, 97 - 6 per cent
The results of the main investigation on 20 normal upper central incisors are presented in Fig. 4. Curve A was obtained for separated electrodes, curve B for contiguous electrodes. In the latter it would be reasonable to expect a slightly larger area of contact than that of the tube areas alone because surface tension would tend to draw electrode jelly across the intervals between adjacent tubes; for this reason
STWULUS
I.5
CONVERGENCE
IN HUhWN
957
TEETH
r
I
5O
Arao,
mm2
I 7m
I 93
I IO
FIG. 4. Relationshipof the pain perception tbnzshold and contact area of the multiple electrode. A, curve obtained using discrete incrementsin area. B, curve obtained using contiguousincrementsin area. C, estimatedcorrection of curve B. Curve A ranged from 8.29-12.3 PA; curve B from 7.77-12.4 PA.
the curve B obtained experimentally has been corrected as shown in curve C, drawn for a maximum area estimated after tracing around the entire group of tubes on squared paper. The quality of sensation was mainly described as tingling, but mild pain, warm, cold, sharp, throb and sting were also mentioned. In any one subject the quality remained the same as the number of electrode tubes increased. The subject noticed only one site of sensation, whatever the number of electrode tubes applied. DISCUSSION It is common knowledge that, as a result of anatomical convergence of the dental hard tissues, any given area at the tooth surface corresponds to a smaller area at the pulpo-dentinal junction. The relative areas at the site under consideration are approximately 2 to 1 which means that an electrical stimulus of 10 PA passing through an electrode contact of 1 mm2 at the enamel surface, would be 20 PA/mm2 at the pulpo-dentinal junction; in other words, twice the current density and therefore twice the effective stimulus. Since it is this current density rather than current alone which is important, the anatomical convergence must be considered. This is not constant and the results show that it increased as the electrode area was moved peripherally. This anatomical convergence means that doubling the electrode area at the enamel surface may more than, or less than, double the corresponding area at the pulpo-dentinal junction, depending on the original site chosen. However, the effect is too small to explain the diminishing slope of the curve in Fig. 1. Moreover, the multiple electrode was placed on the middle of the labial enamel where the effect of A.O.B. 15/10-D
958
J. M. MWORD
AND
A. V. NEWTON
physical convergence was only about 1 per cent; yet the same kind of curve was obtained in Fig. 4 as in Fig. 1. It is concluded that the shape of this curve is mainly determined by physiological convergence. Comparison of the curves in Fig. 4 shows curve B (or C) to be less steep than curve A. It is suggested that for curve B there would be more overlapping of the underlying axon terminals. Some of these would already be subjected to electrical stimulation from the adjacent first electrode area, so the succeeding electrode area would be associated with a smaller increment of current. On the other hand, curve A was obtained for separated electrodes where overlap was reduced, but not necessarily eliminated. No significance can be attached to the lower threshold values in Fig. 1 compared with those in Fig. 4 because the former were obtained by a somewhat different method which included a microammeter reading on the cessation of pain as the current was reduced. It should be noted that the intensity, quality and site of sensation perceived by the subject were the same for all the electrode areas although the larger areas are presumed to involve excitation of more nerve fibres with possibly a greater total number of nerve impulses passing centripetally. If so, some of the additional nerve impulses are redundant in the informational sense. Presumably this is associated with re-coding of the sensory inflow so as to reduce its magnitude as suggested by BARLOW (1968), but this is beyond the scope of the present paper. The various qualities of sensation are the same as those previously reported and their significance has already been discussed (MUMFORD 1965). The subjects’ failure to observe sensation in more than one site of the tooth even with multiple separated electrodes is compatible with the clinical observation that pain arising from pulp disease is poorly localised by the patient. R&sun&-Lorsque les dents humaines sont soumises a des excitations dlectriques breves pour produire en seuil de douleur, l’augmentation de la surface de l’&ctrode de contact est associ&s avec des augmentations progressivement plus faibles de l’amplitude du courant, le rapport entre courant et surface &ant exprime par une courbe exponentielle. Deux types d’experiences sent me& pour expliquer ce fait. La premiere montre que la convergence anatomique des tissus, de la surface exteme de l’email vers la surface pulpaire de la dentine joue certes un role, mais il est peu important. Dans la deuxieme experience, deux dispositifs d’electrodes multiples sont utilisb; l’tm avec des surfaces de contact contigues: l’autre pr6sente des zones de contact disc&es. Une convergence physiologique est mise en evidence : elle peut etre li6e a un chevauchement du champ recepteur dam la pulpe et/au ii un enrigistrement au niveau des neurones du ler et 2&me ordre et du 2bme et 3eme ordre. Zusammen&sung-Wenn menschliche Zlhne durch kurze elektrische Impulse gereizt werden, urn einen Schwellenschmerz zu erzeugen, dann ist der vergriil3erte Rezirk des Elektrodenkontaktes mit fortschreitend geringerem Anwachsen der Stromamplitude verbunden, wobei die Beziehung zwischen Strom und Kontaktflache durch eine Exponentialkurve beschrieben wird. Urn dies zu erkl&ren, wurden 2 Untersuchungen ausgeftihrt. Die erste ergab, da8 die anatomische Konvergenz der zwischen Schmelzoberflache und pulpaler ObertXche des Dentins gelegenen Gewebe ein, wenn such kleiner, Faktor ist. Bei der zweiten Untersuchung wurden 2 Anordnungen ftir Mehr-
STIMULUS CONVERGENCE
IN
HUMANTEETH
959
fachelektroden benutzt, eine mit benachbarten Kontaktbezirken, die andere mit getrennten. Das Ergebnis ist eine physiologische Konvergenz; sie diirfte auf ein uberlappen der Rezeptorfelder in der Pulpa und/oder auf eine Perception im Bereich der Neuronen 1. und 2. sowie 2. und 3. Ordnung zurtickzufiihren sein. REFERENCES BARLOW,H. B. 1968. Sensory mechanisms, the reduction of redundancy, and intelligence. In Cybernef~cs, pp. 183-207 (edited by EVANS,C. R. and ROBERTSON,A. D. J.). Butterworths, London. M-an, J. M. 1965. Pain perception threshold and adaptation of normal human teeth, Archs oral Biol. 10,957-968. MUMFORD,J. M. and NEWTON,A. V. 1969. Zone of excitation when electrically stimulating human teeth. Archs oral Biol. 4,1383-1388.