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Cortical potentials evoked by stimulation of tooth pulp afferents in the cat
SHORT COMMUNICATIONS
211
Cortical potentials evoked by stimulation of tooth pulp afferents in the cat Analysis of the pathways activated from tooth pulp primary afferents is of increasing interest as all available evidence shows that tooth pulp nerve fibers represent the only pure afferent input which is exclusively engaged in transmitting the perception of pain 1,6,a. In cats, it has been shown that the tooth pulp primary afferents project into the rostral subdivision of the trigeminal spinal tract 2 and none of them terminate in the pars caudalis of the spinal trigeminal tract which has been suggested as relay station for nociceptive impulse activity in man 7. In several earlier studies attempts were made to analyze the central pathways activated from the tooth pulp afferents 4,5. However, these results were criticized later s and doubts were expressed about the selective stimulation of tooth pulp afferents. The very short latencies of some of the responses made it likely that an admixture of the fast trigeminal fibers was being stimulated in addition to the tooth pulp nerve. Taking advantage of the averaging technique, Van Hassel 8 was able to demonstrate convincingly bilateral evoked potentials in the cortical facial somatosensory area from tooth pulp afferents in primates with a latency of 12-22 msec. The present report gives an account of the gross potentials evoked in the somatosensory cortex in the cat when all precautions had been made to stimulate tooth pulp afferents selectively and to prevent current escape to soft tissue. Cats were anesthetized with chloralose (75 mg/kg). Small holes were drilled into the dentine up to the near vicinity of the pulp on opposite sides of each upper canine tooth. Thin copper wires, insulated up to the tip, were pressed into the holes with a small amount of amalgam and the whole tooth was covered with cement to prevent short-circuiting with saliva. The finding that selective stimulation of tooth pulp afferents produces the jaw opening reflex but no twitches in the mimic musculature 3 was used to check that no fast conducting fibers of other trigeminal branches were stimulated. In some of the cats the potentials from the cortex were recorded simultaneously with the surface potential from the oblongata caudally to the obex. A compound trigeminal potential can be recorded in this region only in response to stimulation o f the Aa fibers in the trigeminal branches but not to the stimulation of the A~ fibers of the tooth pulpL After these criteria for selective stimulation of tooth pulp afferents were followed, the animal was immobilized with gallamine triethiodide (Remyolan Spofa) and ventilated artificially with a mixture o f 9 8 ~ 02 and 2 ~ CO~. The skull and dura were opened extensively on one side and maprecordings were made with a silver ball electrode from the surface of the cortex covered with warm paraffin oil. Evoked potentials due to stimulation of both the contralateral and the ipsilateral tooth pulp afferents were recorded from the lateral part of the coronary gyrus. Single pulses of 3-4 mA and 10-50 #sec were usually sufficient for eliciting distinct evoked potentials. A short train of such pulses (three at the frequency 400/sec) usually increased the amplitude of the evoked potential. In most of the preparations, however, the spontaneous EEG interfered with the evoked responses so that only potentials averaged on the LINC 8 computer were used for evaluation. Fig. 1 shows specimen records of the summated gross potentials recorded at 4 selected Brain Research, 41 (1972) 211-213
212
SHORT COMMUNICATIONS
A
B
Ti
C
~
D
~-~
~'--..~..,... ~
Fig. 1. Averaged cortical potentials (n = 64) evoked by single pulses in the contralateral tooth pulp nerve (Tc), ipsilateral tooth pulp nerve (Ti), contralateral infraorbital nerve (Ic) and ipsilateral infraorbital nerve (Ii) at 4 selected points at the surface of the cortex of the cat. Dotted area in the diagram shows the part of the cortex from which evoked potentials from both the ipsilateral and the contralateral tooth pulp nerve could be recorded. Positivity upwards in all recordings. Calibration 100/~V in the right lower corner relates to all recordings except Ic-C for which the calibration is given separately.
regions of the cortex. The shaded part of the coronary gyrus is the area where distinct evoked potentials were recorded from contralateral and ipsilateral tooth pulp nerves, as exemplified by recordings in the sets C and D. No distinct potentials were evoked from the tooth pulp afferents when the recording electrode was placed on the surface of the suprasylvian or ectosylvian gyrus (A, B). For comparison, recordings of evoked potentials from the low threshold contralateral ([c) and ipsilateral (Ii) infraorbital nerve fibers were added to the sets A - D . The evoked potentials from the tooth pulp afferents were mostly negative and the maximum amplitude recorded exceeded 100 #V. The mean latency of the evoked potentials was 11.5 4 - 2 . 0 m s e c and 11.9 i 2.8 msec when the contralateral and ipsilateral tooth pulp afferents were stimulated respectively. These latencies are much longer than those evoked from low threshold fibers of other trigeminal branches which were 4.6 4- 2.3 msec and 9.7 + 3.2 msec for the contra- and ipsilateral infraorbital nerve respectively. It can be concluded that there is bilateral cortical representation of the impulse activity from the tooth pulp primary afferents in the cat. This finding, however, does not imply that this representation is necessarily involved in the mechanism of pain. It is possible that cortical representation is essential only for the localization of nociceptive stimuli. Institute of Physiology, Czechoslovak Academy of Sciences, Prague 4 (Czechoslovakia) Institute of Physiology, Academy of Sciences of the GSSR, Tbilisi ( G.S.S.R.) Brain Research, 41 (1972) 211-213
L. VYKL1CKY O. KELLER G. BRO~EK
S. M. BUTKHUZI
SHORT COMMUNICATIONS
213
1 BROOKHART,J. M., LIVINGSTON,W. K., AND HAUGEN,F. P., Functional characteristics of afferent fibers from tooth pulp of cat, J. Neurophysiol., 16 (1953) 634-642. 2 DAVIES,W. I. R., SCOTT, JR. D., VESTERSTROM,K., AND VYKLICK~',L., Depolarization of the tooth pulp afferent terminals in the brain stem of the cat, J. Physiol. (Lond.), 218 (1971) 515-532. 3 KELLER, O., VYKLICK3(, L., AND SYKOV.~, E., Reflexes from the A8 and Aa trigeminal afferents, Brain Research, 37 (1972) 330-332. 4 KERR, D. I. B., HAUGEN, F. P., AND MELZACK, R., Responses evoked in the brain stem by tooth stimulation, Amer. J. Physiol., 183 (1955) 253-258. 5 MELZACK,R., AND HAUGEN,F. P., Responses evoked at the cortex by tooth stimulation, Amer. J. Physiol., 190 (1957) 570-574. 60RaAN, B. J., Oral Histology and Embryology, 4th ed., Mosby, St. Louis, Mo., 1966. 7 SJ6QVIST,O., Studies of pain conduction in the trigeminal nerve, Acta psychiat, scand., Suppl. 17 (1938) 1-139. 8 VAN HASSEL,H. J., CorticalPotentials evoked by Tooth Pulp Stimulation in Primates, Ph.D. Thesis, Univ. of Washington (Hlth Sci., Dentistry), Seattle, Wash., 1969. (Accepted February llth, 1972)
Brain Research, 41 (1972) 211-213
211
Cortical potentials evoked by stimulation of tooth pulp afferents in the cat Analysis of the pathways activated from tooth pulp primary afferents is of increasing interest as all available evidence shows that tooth pulp nerve fibers represent the only pure afferent input which is exclusively engaged in transmitting the perception of pain 1,6,a. In cats, it has been shown that the tooth pulp primary afferents project into the rostral subdivision of the trigeminal spinal tract 2 and none of them terminate in the pars caudalis of the spinal trigeminal tract which has been suggested as relay station for nociceptive impulse activity in man 7. In several earlier studies attempts were made to analyze the central pathways activated from the tooth pulp afferents 4,5. However, these results were criticized later s and doubts were expressed about the selective stimulation of tooth pulp afferents. The very short latencies of some of the responses made it likely that an admixture of the fast trigeminal fibers was being stimulated in addition to the tooth pulp nerve. Taking advantage of the averaging technique, Van Hassel 8 was able to demonstrate convincingly bilateral evoked potentials in the cortical facial somatosensory area from tooth pulp afferents in primates with a latency of 12-22 msec. The present report gives an account of the gross potentials evoked in the somatosensory cortex in the cat when all precautions had been made to stimulate tooth pulp afferents selectively and to prevent current escape to soft tissue. Cats were anesthetized with chloralose (75 mg/kg). Small holes were drilled into the dentine up to the near vicinity of the pulp on opposite sides of each upper canine tooth. Thin copper wires, insulated up to the tip, were pressed into the holes with a small amount of amalgam and the whole tooth was covered with cement to prevent short-circuiting with saliva. The finding that selective stimulation of tooth pulp afferents produces the jaw opening reflex but no twitches in the mimic musculature 3 was used to check that no fast conducting fibers of other trigeminal branches were stimulated. In some of the cats the potentials from the cortex were recorded simultaneously with the surface potential from the oblongata caudally to the obex. A compound trigeminal potential can be recorded in this region only in response to stimulation o f the Aa fibers in the trigeminal branches but not to the stimulation of the A~ fibers of the tooth pulpL After these criteria for selective stimulation of tooth pulp afferents were followed, the animal was immobilized with gallamine triethiodide (Remyolan Spofa) and ventilated artificially with a mixture o f 9 8 ~ 02 and 2 ~ CO~. The skull and dura were opened extensively on one side and maprecordings were made with a silver ball electrode from the surface of the cortex covered with warm paraffin oil. Evoked potentials due to stimulation of both the contralateral and the ipsilateral tooth pulp afferents were recorded from the lateral part of the coronary gyrus. Single pulses of 3-4 mA and 10-50 #sec were usually sufficient for eliciting distinct evoked potentials. A short train of such pulses (three at the frequency 400/sec) usually increased the amplitude of the evoked potential. In most of the preparations, however, the spontaneous EEG interfered with the evoked responses so that only potentials averaged on the LINC 8 computer were used for evaluation. Fig. 1 shows specimen records of the summated gross potentials recorded at 4 selected Brain Research, 41 (1972) 211-213
212
SHORT COMMUNICATIONS
A
B
Ti
C
~
D
~-~
~'--..~..,... ~
Fig. 1. Averaged cortical potentials (n = 64) evoked by single pulses in the contralateral tooth pulp nerve (Tc), ipsilateral tooth pulp nerve (Ti), contralateral infraorbital nerve (Ic) and ipsilateral infraorbital nerve (Ii) at 4 selected points at the surface of the cortex of the cat. Dotted area in the diagram shows the part of the cortex from which evoked potentials from both the ipsilateral and the contralateral tooth pulp nerve could be recorded. Positivity upwards in all recordings. Calibration 100/~V in the right lower corner relates to all recordings except Ic-C for which the calibration is given separately.
regions of the cortex. The shaded part of the coronary gyrus is the area where distinct evoked potentials were recorded from contralateral and ipsilateral tooth pulp nerves, as exemplified by recordings in the sets C and D. No distinct potentials were evoked from the tooth pulp afferents when the recording electrode was placed on the surface of the suprasylvian or ectosylvian gyrus (A, B). For comparison, recordings of evoked potentials from the low threshold contralateral ([c) and ipsilateral (Ii) infraorbital nerve fibers were added to the sets A - D . The evoked potentials from the tooth pulp afferents were mostly negative and the maximum amplitude recorded exceeded 100 #V. The mean latency of the evoked potentials was 11.5 4 - 2 . 0 m s e c and 11.9 i 2.8 msec when the contralateral and ipsilateral tooth pulp afferents were stimulated respectively. These latencies are much longer than those evoked from low threshold fibers of other trigeminal branches which were 4.6 4- 2.3 msec and 9.7 + 3.2 msec for the contra- and ipsilateral infraorbital nerve respectively. It can be concluded that there is bilateral cortical representation of the impulse activity from the tooth pulp primary afferents in the cat. This finding, however, does not imply that this representation is necessarily involved in the mechanism of pain. It is possible that cortical representation is essential only for the localization of nociceptive stimuli. Institute of Physiology, Czechoslovak Academy of Sciences, Prague 4 (Czechoslovakia) Institute of Physiology, Academy of Sciences of the GSSR, Tbilisi ( G.S.S.R.) Brain Research, 41 (1972) 211-213
L. VYKL1CKY O. KELLER G. BRO~EK
S. M. BUTKHUZI
SHORT COMMUNICATIONS
213
1 BROOKHART,J. M., LIVINGSTON,W. K., AND HAUGEN,F. P., Functional characteristics of afferent fibers from tooth pulp of cat, J. Neurophysiol., 16 (1953) 634-642. 2 DAVIES,W. I. R., SCOTT, JR. D., VESTERSTROM,K., AND VYKLICK~',L., Depolarization of the tooth pulp afferent terminals in the brain stem of the cat, J. Physiol. (Lond.), 218 (1971) 515-532. 3 KELLER, O., VYKLICK3(, L., AND SYKOV.~, E., Reflexes from the A8 and Aa trigeminal afferents, Brain Research, 37 (1972) 330-332. 4 KERR, D. I. B., HAUGEN, F. P., AND MELZACK, R., Responses evoked in the brain stem by tooth stimulation, Amer. J. Physiol., 183 (1955) 253-258. 5 MELZACK,R., AND HAUGEN,F. P., Responses evoked at the cortex by tooth stimulation, Amer. J. Physiol., 190 (1957) 570-574. 60RaAN, B. J., Oral Histology and Embryology, 4th ed., Mosby, St. Louis, Mo., 1966. 7 SJ6QVIST,O., Studies of pain conduction in the trigeminal nerve, Acta psychiat, scand., Suppl. 17 (1938) 1-139. 8 VAN HASSEL,H. J., CorticalPotentials evoked by Tooth Pulp Stimulation in Primates, Ph.D. Thesis, Univ. of Washington (Hlth Sci., Dentistry), Seattle, Wash., 1969. (Accepted February llth, 1972)
Brain Research, 41 (1972) 211-213