Pflfigers Archiv
PflfigersArch. 381,201-207 (1979)
EuropeanJournal of Physiology
9 by Springer-Verlag 1979
Spread of the Dorsal Root Potentials in Lower Lumbar, Sacral and Upper Caudal Spinal Cord K. Lupa, G. W6jcik, M. OZ6g, and A. Niechaj Department of Human Physiology,MedicalSchool, Dymitrowa11, PL-20-080 Lublin, Poland
Abstract. Spread of the dorsal root potentials (DRPs)
along lower lumbar, sacral, and upper caudal segments of the cord has been studied in spinal cats. Ipsilateral DRPs produced by stimulation of L5 dorsal root with single volleys and recorded in consecutively more and more caudal segments gradually decrease and after passing 6 segments attain 47 % of amplitude observed in L6 dorsal root. DRPs spreading cranially from Ca2 dorsal root along 6 segments decrease virtually to zero. Depolarizations spreading caudally show greater conduction velocity, are maintained during repetitive stimulation in larger number of segments and display larger occlusion than DRPs spreading cranially. These findings show the preponderance of depolarization spreading from lower lumbar cord to sacral and caudal segments over that produced in caudal parts of the cord and spreading cranially. The extent of cranial spread of DRPs, appearance of sudden increases in latency of DRPs and changes in effectiveness of stimulation in maintaining prolonged depolarization show correlations with neuronal arrangement of the substantia gelatinosa. This suggests that substantia gelatinosa subserves the spread of DRPs along the spinal cord. Key words: Spread of dorsal root potentials Presynaptic inhibition - Sacral spinal cord - Caudal spinal cord.
Introduction
The dorsal root potentials (DRPs) reflect depolarization of presynaptic terminals of primary afferent fibres and are considered to indicate generation of presynaptic inhibition. The exact mechanism of presynaptic inhibition has not yet been elucidated. According to Eccles et al. [8] the terminal depolarization depends on activity in polysynaptic pathways.
The stimulation of the cutaneous afferent fibres activates the chain of at least two interneurones/of type C and D/situated at the base of the dorsal horn whose activity depolarizes the primary afferent terminals. Wall maintains that the depolarization is produced by activity in the cells of the substantia gelatinosa [29, 30] and units whose response pattern corresponds to the time cours of presynaptic inhibition have been identified [14, 32]. Following suggestion of Barron et al. [1] recent studies demonstrated that increase in concentration of extracellular potassium ions evoked by stimulation of afferent nerves may produce depolarization of primary afferent terminals [3, 17, 18]. However, the lack of correlation between some traits of this depolarization and extracellular potassium level shows that the latter cannot be the only factor responsible for presynaptic inhibiton. The depolarization of the dorsal roots has been most commonly measured in or very close to the fibres which transmit afferent volleys into the spinal cord. The DRPs evoked by these volleys were also detected in contralateral dorsal roots [1, 6, 11, 12, 15, 16, 19]. The spread of the DRPs along the spinal cord was studied to much lesser extent and only concerned ipsilateral side [1, 7]. In our previous experiments stimulation of the L6 dorsal root or of the cutaneous nerve entering the cord at the lumbar level was found to produce bilateral DRPs in $3 segment. The depolarizations were smaller but in many respects were similar to the DRPs evoked by stimulation of the neighbouring dorsal root [21]. It has been suggested that transmission of depolarization to the neighbouring dorsal root occurs via Lissauer tract which is propriospinal tract of the substantia gelatinosa [29]. Neuronal arrangement of the substantia gelatinosa may provide basis for spread of the primary afferent depolarization over considerably longer distances along the spinal cord [25, 27]. In the present experiments we have investigated the spread of the DRPs in lower lumbar, sacral and upper
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caudal segments of the cat spinal cord and attempted to c o r r e l a t e its p r o p e r t i e s w i t h t h e k n o w n p a t t e r n o f connections of the substantia gelatinosa.
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Methods Fourty-eight cats weighing 1.9-3.9kg were anaesthetized with tiamylal sodium (Surital, Parke-Davis, 30 mg/kg body weight, i.p.), supplemented every 2 h with 15 mg/kg, i.v. The spinal cord was transected at Thl 3. Following laminectomy all dorsal roots from L5 to Ca2 were cut on both sides of the cord at the points where they enter the dura. Then the distances between the points of entry of the most caudal rootlets of each dorsal root into the cord were measured on the right side. Preparations were de-efferented by bilateral section of the ventral roots from L6 through $3. In each dorsal root the most caudal rootlets of an average diameter of 0 . 5 - 0 . 7 r a m were separated. When the whole dorsal root had diameter less than 0.8 mm (as it occurred in some smaller preparations with Cal and Ca2 dorsal roots) its teasing into smaller strands was not attempted. For measuring the size of depolarization, its latency and effectiveness during prolonged stimulation we recorded bilateral DRPS at various distances between stimulating and recording electrodes. The stimulating electrodes were placed at dorsal roots of the most cranial and most caudal segments of the studied part of the cord. The recording electrodes were mounted on dorsal rootlets of the segment close to the cranial stimulating electrodes and after studying the DRPs elicited by volleys from both pairs of electrodes they were moved into caudal direction allowing to record depolarization from all segments lying between the points of entry of afferent volleys. When testing temporal facilitation and interaction of the DRPs the distance between stimulating and recording electrodes was fixed. The DRPs were recorded from $2 dorsal root. They were produced by stimulation of L6 and Ca2 dorsal roots, i.e. three segments cranially or caudally from the recording electrodes. Hooked platinum wire electrodes were used for stimulation and recording. To record the DRPs one electrode was placed close but not touching the cord and the other on the cut end. The inter-electrode distance was 15 mm. The DRPs were produced by single square wave pulses of 0.1 ms width or by repetitive stimulation at 250 c/s lasting 500 ms. The strength of stimulation was expressed in multiples of threshold (T). The strength of a single puIse applied to L5 dorsal root which produced liminal DRP in the L6 dorsal root was considered to be the threshold. Except where otherwise stated the intensity of stimulation was 4T. Single DRPs were evoked every 7 - 1 0 s. The interaction of the DRPs was produced every 1 2 - 1 5 s. The amplifiers Used for recording the DRPs were a.c. coupled with I s time constant. Potentials evoked by prolonged stimulation were recorded with a d.c. amplifier. The animals were immobilized with gallamine triethiodide (Flaxedil, Specia, 10 mg/kg, i.v.) and kept on artificial respiration. The temperature of the oil covering the cord and of the animals was kept at 3 6 - 3 8 ~ by radiant heat.
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Fig. 1. Dorsal root potentials produced by stimulation of L5 dorsal root (A) and Ca2 dorsal root (B) with single volleys and recorded bilaterally from dorsal roots of four segments. Symbol above records show the level of recording. Upper traces of each record represent ipsilateral potentials and lower traces contralateral potentials
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Fig. 2. Amplitudes of dorsal root potentials evoked by stimulation of L5 dorsal root (A) and Ca2 dorsal root (B) with single volleys and' recorded in consecutive segments of the cord. The magnitudes of DRPs in percentages in relation to control values taken as 100 are plotted against the distances between the points of entry of dorsal roots into the cord. Mean amplitudes of DRPs recorded in roots adjacent to stimulated dorsal roots were considered as contol values. They were calculated separately for ipsi- and contralateral potentials. Open circles represent ipsilateral DRPs, filled circles contralateral ones. Each point is arithmetic mean of eleven measurements
Results
DRPs Recorded at Various Distances from the Point of Entry of Afferent Volley into the Spinal Cord. D o r s a l roots L5 and Ca2 were stimulated one after the other, a n d b i l a t e r a l D R P s w e r e r e c o r d e d c o n s e c u t i v e l y a t all segments in between these roots. The sample DRPs evoked by stimulation of L5 dorsal root with single v o l l e y s a n d r e c o r d e d a t f o u r d i f f e r e n t levels a r e s h o w n i n Fig. I A . T h e c h a n g e s i n t h e size o f p o t e n t i a l s r e c o r d e d f r o m all s e g m e n t s a r e r e p r e s e n t e d i n Fig. 2 A .
It may be seen that with an increase in distance between stimulating and recording electrodes ipsilateral DRPs d e c r e a s e a n d i n C a 1 s e g m e n t t h e y a t t a i n a b o u t 47 % o f the level seen in L6 dorsal root, Quite unexpectedly contralateral depolarizations increase to more than 150% in segments just caudal to the stimulating electrode and than slowly diminish attaining at the most distant segment the value close to that observed in L6 dorsal root.
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Fig. 4. Changesin index of effectivenessof stimulation in maintaining prolonged depolarization of ipsilateral dorsal roots in consecutive segments of the cord. Index of effectivenessin percentagesis plotted against distances between points of entry of dorsal roots into the cord. Open circlesrepresent values calculated for DRPs produced by stimulation of L5 dorsal root and filledcirclesthose for DRPs evoked from Ca2 dorsal root. Each point is arithmetic mean of nine measurements
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(0.5 s at 250 c/s) of L5 dorsal root (A) and Ca2 dorsal root (B) and recorded in six segments of the cord. The amplitudes of DRPs in percentages in relation to control values taken as 100 are plotted against the distances between the points of entry of dorsal roots into the cord. Controls werecalculated as in Fig. 2. Open circlesrepresent ipsi- DRPs and filled circles contralateral DRPs. Each point is arithmetic mean of eleven measurements
decrease more slowly than in the case shown in Fig. 2B and in L6 dorsal root they reach 39.5 % and 56.6 % of their control on ipsi- and contralateral side. In none experiment did the D R P s decreased to zero in the most distant segments.
Effectiveness of Stimulation in Maintaining Prolonged Depolarization of Dorsal Roots. Depolarization of the The D R P s evoked by stimulation of the Ca2 dorsal root are shown in Fig. 1B and 2B. We will describe them beginning from the segment neighbouring to the stimulating electrodes (i.e. from right to left). When recording electrodes are moved to more distant segments the decrease in size of ipsi- D R P s is much greater than in the case illustrated in Fig. 1A and 2A and in L6 dorsal root the D R P s attain 17.6 % of the initial level Similar reduction is observed on the other side of the cord and in L6 segment potentials are reduced to 4.6 %. Because the course of changes of the D R P s on both sides of the cord is similar it is approximated by one curve. It should be noted that in 27.3 % of preparations stimulation of the Ca2 dorsal root did not produce any depolarization in L6 dorsal roots and in 18.2%of animals there were no bilateral D R P s in L7 dorsal roots. In all preparations bilateral D R P s evoked from L5 dorsal root were invariably observed at all segmental levels. The changes in the mean size of the DRPs produced by long-lasting repetitive stimulation are shown in Fig. 3. When L5 dorsal root is stimulated (Fig. 3 A) ipsiD R P s decrease more slowly then when evoked by single volleys and in the Cal segment they reach 84.1% of the control. The rise of contralateral D R P s in proximal segments is also less pronounced and attains 127 % of control. In almost all distant segments the level of depolarization is above the control value. The DRPs evoked by stimulation of Ca2 dorsal root (Fig. 3B)
dorsal roots maintained throughout the period of tetanization shows the potency of presynaptic inhibition in conditions of prolonged influx of sensory impulses. It has been demonstrated that ipsilateral potentials are maintained during all tetanus but contralateral D R P s are not sustained and after reaching maximum they rapidly decrease to zero [9, 10, 15]. When D R P produced by stimulation of the L6 dorsal root is recorded in ipsi- $3 dorsal root it falls to zero shorthly after the onset of tetanus being very similar to contralateral potential observed in lumbar segments [21]. This finding suggests that the effectiveness of stimulation in maintaining prolonged D R P is related to the distance between stimulating and recording electrodes. In this series of experiments we have determined the index of effectiveness for D R P s recorded in all segments between L5 and Ca2 dorsal root. It was calculated as the ratio of the level of depolarization measured at the end of stimulation to the m a x i m u m level of potential attained at its beginning and was expressed in percentages. Since it was found that the index calculated for all contralateral DRPs is zero, Fig. 4 shows only the changes in effectiveness of ipsilateral depolarization. The index of effectiveness for the DRPs produced by stimulation of L5 dorsal root (open circles) in the neighbouring L6 segment amounts to 15.5 ~ . It decreases when recording electrodes are moved to consecutive segments and this reduction is proportional to the distance between both pairs of
204 electrodes. Four segments away from stimulating electrodes ($2) the index amounts to 4.7 ~ and in the next segments it drops to zero. The decrease of the index calculated for DRPs evoked from Ca2 dorsal root is much more rapid (filled circles). After spreading two segments index drops to 4.6 ~ and at the $2 segmental level it is close to 0 ~.
Latencies ofDRPs. A significant increase in the time to peak and reduction in amplitude of the DRPs recorded at more distant segments made it difficult to obtain a complete set of the latency measurements in a single preparation. Figure 5 shows the latencies of the DRPs elicited by stimulation of the L5 dorsal root and spreading in the caudal direction. In all segments latencies of ipsi- potentials (open circles) are shorter than latencies of contralateral DRPs (filled circles). It should be noted that latencies of ipsi- DRPs recorded from two segments close to the stimulating electrode (L6 and L7) are almost the same amounting to 2.23 and 2.31 ms. Nearly identical latencies are also displayed by contralateral DRPs recorded from L7 and $I dorsal roots. In this experiment latencies of the DRPs evoked by stimulation of the Ca2 dorsal root and spreading cranially could only be measured in 4 segments of the cord (Fig. 5 B). It may be seen that in the Ca1 dorsal roots latencies are the same on both sides of the cord. Then the increase in the latency of contralateral DRPs becomes irregular and much more rapid than of the ipsi- potentials. Of particular interest is a large increase in the latency of contralateral DRPs occurring between $2 and $1 dorsal root where at a distance of about 5"ram it rises by 7.7 ms. The conduction velocities of the DRPs spreading caudally between L5 and Ca1 segments were calculated in 9 preparations. The mean conduction velocity ofipsiDRPs amounting to 4.12 _+ 0.31 m/s (2 _+ S.D.) is significantly higher than that of the contralateral potentials which is 2.23 _+ 0.27 m/s (P < 0.05). The mean conduction velocities of DRPs spreading cranially between Ca2 and $1 segments calculated in 7 preparations amounted to 2.71 + 0.48 m/s for ipsi- and 1.64 +_ 0.29 m/s for contralateral potentials. The difference between these values was statistically nonsignificant (P > 0.05). The sudden increases in latencies of the DRPs spreading cranially similar to those shown in Fig. 5 B were seen in 6 out of 7 preparations. They were observed on both sides of the cord and usually occurred 3 - 4 segments from the point of entry of an afferent v~?lley. The increases in latencies of the DRPs traveling cdudally were observed in 4 out of 9 preparations. They were seen on both sides of the cord after depolarization spreading 3 or 4 segments.
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Fig. 5. Latenciesof dorsal root potentialsevokedby stimulationof L5 dorsal root (A) and Ca2 dorsal root (B) and measuredin consecutive segments of the cord. Abscissa - distancesbetweenpoints of entry of dorsal roots into the cord. Ordinate - latencyin milliseconds.Open circles show ipsi- and filled circlescontralateral DRP. Each point is arithmetic mean of six to eight measurements
Temporal Facilitation of DRPs. In these experiments we have stimulated L6 or Ca2 dorsal roots with increasing number of volleys and recorded the resulting DRPs from $2 dorsal roots. The temporal facilitation of potentials spreading along three segments of the cord was compared with that of the DRPs elicited by stimulation of adjacent dorsal root. All depolarizations were evoked by submaximal stimulation. Figure 6A (triangles) shows that when the number of volleys in the neighbouring dorsal root is increased the ipsi-DRP rises fairly fast reaching 141 ~ of the control value following two volleys and 186 ~ after 5 or 6 volleys. Longer trains of volleys do not significantly affect the amplitude of depolarization. The increase of depolarization produced by stimulation of L6 dorsal root (open circles) is slower its plateau amounting to about 165 ~ being attained after 7 volleys. Temporal facilitation of the DRPs produced by volleys ascending from Ca2 dorsal root (filled circles) is simila/" to that resulting from stimulation of the S1 dorsal root. Two volleys enhance depolarization to 134 ~o and its maximals increase at 5 volleys is 193~. On the contralateral side of the cord (Fig. 6B) the increase of the DRPs evoked by growing number of afferent volleys is in general more rapid and peak depolarization higher than on the ipsilateral side. Two volleys applied to SI dorsal root (triangles) increase the DRPs to 184 ~ and after 5 volleys potentials reach the peak amounting to 294 ~ . Facilitation of the DRPs elicited from L6 dorsal root has similar time course but the maximum increase is lower and reaches 266 ~ of control. The volleys
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Fig. 6. A and B. Temporal facilitation of dorsal root potentials recorded in $2 dorsal roots and evoked by stimulation of $1 dorsal ~ root (triangles), L6 dorsal root (open circles), and Ca2 dorsal root (filled circles). Ipsilateral potentials are shown in A and contralateral potentials in B. Abscissa - number of volleys. Ordinate - amplitude of potentials in percentages in relation to the size of DRPs produced by single volleys taken as 100. Each point is arithmetic mean of nine to eleven measurements
Fig. 7A--G. Interaction of dorsal root potentials recorded from $2 dorsal root. Interval between conditioning and testing volleys was 20ms. Upper row of records represents conditioning potentials evoked by stimulation of S1 dorsal root (A), L6 dorsal root (B), Ca2 dorsal root (C), and testing potentials produced by stimulation of $1 dorsal root (D). Lower row of records (E, F, G)~hows interaction of conditioning potentials with testing depolarization. Upper traces in each record show ipsilateral and lower traces contralateral DRPs
ascending to $2 segment from Ca2 dorsal root produce relatively small augmentation of the contralateral DRP (filled circles). Following 2 volleys depolarization reaches 133 ~ which is value similar to that observed on the contralateral side. The maximum increase of this depolarization results from 5 volleys and amounts to 207~.
The interaction of DRPs evoked by stimuli at 4T (i.e. producing potentials of maximum amplitude) exhibited only large and nondiscriminative occlusion. The results obtained below were obtained with DRPs whose amplitude was 50 ~ of maximum. From Fig. 7 it may be seen that interaction of the ipsilateral testing potential (D) with conditioning DRP evoked from L6 dorsal root (B) produces depolarization (F) which is larger than single potentials but smaller than their sum. This indicates occlusion of the DRPs. Similar effects were obtained following interaction of the testing potential with conditioning DRPs set by volleys in Ca2 (C) or in the S1 dorsal roots (A) but resulting depolarization (E and G, respectively) disclose quantitative differences in the size of occlusion. The mean values of interaction between ipsilateral DRPs obtained in experiments in 8 cats are positive indicating that all interactions produce occlusion of depolarizations. When conditioning volley is applied to L6 dorsal root the mean value of interaction is 55.4 + 8.1 ~. Conditioning stimulation of Ca2 dorsal root produces smaller occlusion and the value of interaction amounts to 31.0 _+ 7.3 ~. As it was expected the largest occlusion reaching 86.3 _+ 12.0~o occurs when two volleys are set in the same S1 dorsal root. Occlusion between contralateral DRPs is smaller than that found on ipsilateral side of the cord. Preceding the testing DRP by conditioning volley in L6 dorsal root yields the mean interaction value of 38.3 _+ 11.7 ~. The analogous value for interaction between the DRP from Ca2 dorsal root and the testing depolarization amounts to 28.5 + 9.9~. Similar to ipsilateral potentials the greatest value of interaction reaching 54.9 _+ 11.0 ~ is
Interaction of DRPs. Since the DRPs produced by volleys spreading caudally and cranially in the spinal cord were recorded from the same dorsal root, it was of interest to find out if they result from depolarization of the same primary afferent terminals. A preliminary answer to this question can be obtained by studying interaction of the DRPs. The testing DRP produced in $2 dorsal root by stimulation of the neighbouring S1 dorsal root was preceded at fixed interval by conditioning volley in L6 or Ca3 dorsal roots. For comparison the conditioning stimuli were also applied to S1 dorsal root. When two pathways producing the DRPs have some elements in common an occlusion or facilitation of depolarizations may be expected. Summation of the DRPs should occur when both volleys activate independent pathways. The size of interaction was calculated by subtracting the size of DRP produced by two stimuli from the sum of depolarizations evoked separately by conditioning and testing volleys. The amplitudes of the DRPs were expressed in percentages the size of the mean testing depolarization being 100~. The larger are calculated values the greater interactions between potentials. The values close to zero would indicate summation of the DRPs.
206 attained when both stimulations are applied to the same S1 dorsal root. Discussion
In their original observations Barron and Matthews (1) noted that in decerebrate cats DRPs could be recorded 5 - 6 segments from the point of entry of afferent volley. They also found that the decrease of potentials was greater with depolarization traveling in the caudal direction. The caudal spread of DRPs confined to 6 segments of the cord was observed by Devor et al. [7] in unanaesthetized spinal cats. In our experiments at the distance of 6 segments from the stimulating electrodes the DRPs spreading cranially were decreased almost to zero. The length of 6 segments recurrent in all these findings suggests the range of pathways which under a variety of experimental conditions are able to transmit DRPs evoked by single volleys along the spinal cord. There are no data on the number of synapses and diameter of axons in~volvedin the spread of DRPs. For that reason the calculation of conduction velocity from latencies of the DRPs gives only approximate characteristic of pathways subserving the spread of depolarization. These calculations showed that DRPs spreading caudally could more rapidly exert their influence in caudal segments than depolarizations conducted cranially in upper parts of the cord. In the lower lumbar cord the effectiveness of prolonged stimulation in maintaining depolarization of the dorsal roots was decreased with an increase in the stimulation frequency. It was hypothetized that this reduction is caused by build-up of homosynaptic depression. Since this effect was particularly striking on the contralateral side of the cord it was inferred that homosynaptic depression is enhanced by additional synapses transmitting DRPs to the other side of the cord [15]. Rapid reduction of effectiveness with increase of the distance fromthe stimulating electrodes observed by us at fixed stimulation frequency demonstrates that this factor is even more potent during spread of depolarization along the spinal cord. Temporal facilitation in pathways producing DRPs is attributed to activation of additional synapses depolarizing the primary afferent fibres [9]. It was larger on contralateral side of the lower lumbar cord [15]. We present evidence that similar pattern is exhibited by DRPs in $2 segment and by depolarizations spreading caudally. It is conceivable that sudden increase of contralateral DRPs spreading in this direction results from facilitation by volleys traversing the cord from ipsilateral side in a few segments below the stimulated dorsal root. The same temporal facilitation of the ipsiand contralateral DRPs spreading cranially indicates lesser availability of synapses activated by volleys in the
PflfigersArch. 381 (1979) Ca2 dorsal root. This finding may be accounted for by structure of the substantia gelatinosa which at this level forms single sheet without special synaptic relays transmitting depolarization accross the cord [26, 29]. The interaction between the DRPs depends on the type of stimulated fibres, strength of stimulation and the kind of animal. Occlusion prevails in cats while in frogs facilitation is more frequent [4, 20]. In our experiments occlusion of depolarization was the only observed form of interaction. Smaller interaction of potentials spreading cranially showed that longranging pathway which are carrying them are to greater extent separate from local neuronal pathways generating the DRPs than are pathways carrying potentials in the caudal direction. Some properties of the DRPs spreading along the cord show correlation with neuronal arrangement and connections of the substantia gelatinosa as described by Szent~gothai [27]. Lateral part of Lissauer's tract and lateral fasciculus proprius are built up of axons of the substantia gelatinosa neurones whose collaterals after course of 5 - 6 segments terminate in the same structure. Since the maximum craniad spread of the DRPs produced by single volleys occurs at the same length of the cord it is possible that it is mediated by this longitudinal system. Recent experiments of Cervero et al. [5] show that the DRPs evoked by stimulation of the L4 (or L5) dorsal root and recorded in the neighbouring segment are reduced to about 50 % after section of Lissauer's tract at the junction between these segments. The section of the dorsal column and of dorsolateral funiculus produces larger decrease of depolarization. These findings suggest that the short range spread of the DRPs depends on several propriospinal pathways. It cannot be excluded that the spread of the DRPs over short and long distances along the cord is subserved by different fibre systems. The relationship between longranging afferents studied by Wall and Werman between L1 and S1 [21] and the spread of DRPs cannot be established at present. These afferents do not produce dorsal root reflexes and thus do not seem to have effective connections with pathways generating depolarization of distant presynaptic terminals. On the other hand Szent~tgothai describes collaterals given off by large calibered afferents which enter the substantia gelantinosa at the length of 4 segments. At the same length of the cord the index of effectiveness of prolonged stimulation for DRPs spreading caudally is maintained above zero and after passing approximately the same number of segments sudden incrases in the latency of the DRPs are observed. These similarities suggest that although the area in which collaterals of one large calibered afferent arborize is not critical for the maximum spread of depolarization, some properties of the DRPs related to the number of activated
K. Lupa et al. : Spread of Dorsal Root Potentials Along Spinal Cord
synapses heavily depend on the range of arborization. Correlations found by us are only qualitative and are limited to a few properties of the DRPs. Nevertheless, they suggest that the spread of primary afferent depolarization occurs in the substantia gelatinosa and via its long range connecting system. The amplitude of the DRPs, the speed with which they reach distant segments of the cord or effectiveness of stimulation in maintaining depolarization of the dorsal roots reflects the extent of control of the sensory input by presynaptic inhibition. Taking this into consideration our findings show the preponderance of depolarization generated at the L6 level and spreading to lower sacral and caudal segments over that produced in caudal parts of the cord and transmitted to the lower lumbar segments. A functional significance of this differentiation may be visualised when we consider that afferents entering the lumbar cord mainly originate from receptors of hind limb and give rise to important nociceptive and postural reflexes while sensory fibres reaching lower sacral and caudals segments innervate organs of the midline [2, 13, 22-24, 28] which are rarely activated in emergency situations. This differentiation would permit to block impulses entering lower segments of the cord and by contrast enhance the role of impulses from hind limbs.
References 1. Barron, D. H., Matthews, B. H. C.: The interpretation of potential changes in the spinal cord. J. Physiol. (Lond.) 92, 276 321 (1938) 2. Bradley, W. E., Teague, C. T. : Spinal cord organization of micturition reflex afferents. Exp. Neurol. 22, 504-516 (1968) 3. Bruggencate, G., Lux, H. D., LieN, L. : Possible relationships between extracellular potassium activity and presynaptic inhibition in the spinal cord of the cat. Pflfigers Arch. 349, 301 317 (1974) 4. Carpenter, D. O., Rudomin, P. : The organization of primary afferent depolarization in the isolated spinal cord of the frog. J. Physiol. (Lond.) 229, 471-493 (1973) 5. Cervero, F., Iggo, A., Molony, V. : The tract of Lissauer and the dorsal root potential. J. Physiol_ (Lond.) 282, 295-305 (1978) 6. Devanandan, M. S., Holmqvist, B., Yokota, T. : Presynaptic depolarization of group I muscle afferents by contralateral afferent volleys. Acta Physiol. Cand. 63, 4 6 - 5 4 (1965) 7. Devor, M., Merrill, E. G., Wall, P. D. : Dorsal horn cells that respond to stimulation of distant dorsal roots. J. Physiol. (Lond.) 270, 519-531 (1977) 8. Eccles, J. C., Kostyuk, P. G., Schmidt, R. F. : Central pathways responsible for depolarization of primary afferent fibres. J. Physiol. (Lond.) 161, 237-257 (i962) 9. Eccles, J. C., Schmidt, R. F., Willis, W. D. : The location and the mode of action of the presynaptic inhibitory pathways on to group I afferent fibres from muscle. J. NeurophysioI. 26, 506-522 (1963) 10. Eccles, J. C., Schmidt, R. F., Willis, W. D. : Depolarization of the central terminals of cutaneous afferent fibrs. J. Neurophysiol. 26, 646-661 (1963)
207 11. Eccles, R. M., Holmqvist, B., Voorhoeve, P. E.: Presynaptic inhibition from contralateral cutaneous afferent fibres. Acta Physiol. Scan& 62, 464-473 (1964) 12. Eccles, R. M., Holmqvist, B., Voorhoeve, P. E. : Presynaptic depolarization of cutaneous afferents by volleys in coutralateral muscle afferents. Acta Physiol. Scand. 62, 4 7 4 - 4 8 4 (1964) 13. Fletcher, T. F., Bradley, W. E. : Afferent nerve endings in the urinary bladder of the cat. Am. J. Anat. 128, 147-157 (1970) 14. Hentall, I. : Delayed off-responses in the substantia gelantinosa. In: Abstr. VIIth Ann. Meet. Soc. Neurosci., vol. 3, p. 502. Anaheim, California 1977 t5. Holobut, W., Niechaj, A. : The dorsal root potentials produced on both sides of the spinal cord by long-lasting stimulation of the cutaneous afferents. J. Physiol. (Lond.) 230, 1 5 - 2 7 (1973) 16. Hotobut, W., Niechaj, A. : Properties of the dorsal root potentials produced on both sides of the spinal cord by stimulation of the cutaneous afferents. Acta Neurobiol. Exp. (Warsz.) 34, 629-643 (1974) 17. Kriz, N., Sykovfi, E., Ujec, E., Vyklick~r, L.: Changes of extracellular potassium concentration induced by neuronal activity in the spinal cord of the cat. J. Physiol. (Lond.) 238, 1 15 (1974) 18. Kriz, N., Sykovfi, E., Vykliek), L.: Extracellular potassium changes in the spinal cord of the cat and their relation to slow potentials, active transport and impulse transmission. J. Physiol. (Lond.) 249, 167-182 (1975) 19. Lloyd, D. P. C. : Electrotonns in dorsal nerve roots. Cold Spring Harbor Syrup. Quant. Biol. 17, 203-219 (1952) 20. Lloyd, D. P. C., McIntyre, A. K. : On the origins of dorsal root potentials. J. Gen. Physiol. 32, 409-443 (1949) 21. Lupa, K., Niechaj, A. : Bilateral dorsal root potentials in the lower sacral spinal cord. Pfl(igers Arch. 369, 187-192 (1977) 22. Oliver, J. E., Bradley, W. E., Fletcher, T. F.: Spinal cord representation of the micturition reflex. J. Comp. Neurol. 137, 3 2 9 - 346 (1969) 23. Oliver, J. E., Bradley, W. E., Fletscher, T. F. : Spinal cord distribution of the somatic innervation of the external urethral sphincter of the cat. J. Neurol. Sci. 10, 1 1 - 2 3 (1970) 24. Reid, K. H. : Dermatomes and skin innervation density in the cat's tail. Exp. Neurol. 26, 1 - 1 6 (1970) 25. R~thelyi, M., Szentfigothai, J. : Distribution and connections of afferent fibres in the spinal cord. In: Handbook of sensory physiology, vol. 2, Somatosensory system (A. Iggo, ed.), pp. 207-252. Berlin-Heidelberg-New York: Springer t973 26. Rexed, B. : The cytoarchitectonic organization of the spinal cord in the cat. J. Comp. Neurol. 96, 415-495 (1952) 27. Szentggothai, J. : Neuronal and synaptic arrangement in the substantia gelatinosa Rolandi. J. Comp. Neurol. 122, 2 1 9 - 239 (1964) 28. Todd, J. K. : Afferent impulses in the pudendal nerves of the cat. Quart. J. Exp. Physiol. 49, 258-267 (1964) 29. Wall, P. D. : The origin of a spinal-cord slow potential. J. Physiol. (Lond.) 164, 508--526 (1962) 30. Wall, P. D. : Presynaptic control of impulses at the first central synapse in the cutaneous pathway. In: Physiology of spinal neurones, vol. 12, Progress in brain research (J. C. Eccles and J. P. Schad6, eds.), pp. 98-112. Amsterdam, London-New York: Elsevier 1964 31. Wall, P. D., Werman, R.: The physiology and anatomy of long ranging afferent fibres within the spinal cord. J. Physiol. (Lond.) 255, 321 --334 (1976) 32. Wilcox, G. L., Luttges, M. W., Mayer, D. J. : Unit activity of substantia gelatinosa neurons: relation to the dorsal root potential. In: Abstr. VIth Ann. Meet. Soc. Neurosci., vol. 2, p.956. Toronto 1976
Received February 6~Accepted June 2, 1979
PflfigersArch. 381,201-207 (1979)
EuropeanJournal of Physiology
9 by Springer-Verlag 1979
Spread of the Dorsal Root Potentials in Lower Lumbar, Sacral and Upper Caudal Spinal Cord K. Lupa, G. W6jcik, M. OZ6g, and A. Niechaj Department of Human Physiology,MedicalSchool, Dymitrowa11, PL-20-080 Lublin, Poland
Abstract. Spread of the dorsal root potentials (DRPs)
along lower lumbar, sacral, and upper caudal segments of the cord has been studied in spinal cats. Ipsilateral DRPs produced by stimulation of L5 dorsal root with single volleys and recorded in consecutively more and more caudal segments gradually decrease and after passing 6 segments attain 47 % of amplitude observed in L6 dorsal root. DRPs spreading cranially from Ca2 dorsal root along 6 segments decrease virtually to zero. Depolarizations spreading caudally show greater conduction velocity, are maintained during repetitive stimulation in larger number of segments and display larger occlusion than DRPs spreading cranially. These findings show the preponderance of depolarization spreading from lower lumbar cord to sacral and caudal segments over that produced in caudal parts of the cord and spreading cranially. The extent of cranial spread of DRPs, appearance of sudden increases in latency of DRPs and changes in effectiveness of stimulation in maintaining prolonged depolarization show correlations with neuronal arrangement of the substantia gelatinosa. This suggests that substantia gelatinosa subserves the spread of DRPs along the spinal cord. Key words: Spread of dorsal root potentials Presynaptic inhibition - Sacral spinal cord - Caudal spinal cord.
Introduction
The dorsal root potentials (DRPs) reflect depolarization of presynaptic terminals of primary afferent fibres and are considered to indicate generation of presynaptic inhibition. The exact mechanism of presynaptic inhibition has not yet been elucidated. According to Eccles et al. [8] the terminal depolarization depends on activity in polysynaptic pathways.
The stimulation of the cutaneous afferent fibres activates the chain of at least two interneurones/of type C and D/situated at the base of the dorsal horn whose activity depolarizes the primary afferent terminals. Wall maintains that the depolarization is produced by activity in the cells of the substantia gelatinosa [29, 30] and units whose response pattern corresponds to the time cours of presynaptic inhibition have been identified [14, 32]. Following suggestion of Barron et al. [1] recent studies demonstrated that increase in concentration of extracellular potassium ions evoked by stimulation of afferent nerves may produce depolarization of primary afferent terminals [3, 17, 18]. However, the lack of correlation between some traits of this depolarization and extracellular potassium level shows that the latter cannot be the only factor responsible for presynaptic inhibiton. The depolarization of the dorsal roots has been most commonly measured in or very close to the fibres which transmit afferent volleys into the spinal cord. The DRPs evoked by these volleys were also detected in contralateral dorsal roots [1, 6, 11, 12, 15, 16, 19]. The spread of the DRPs along the spinal cord was studied to much lesser extent and only concerned ipsilateral side [1, 7]. In our previous experiments stimulation of the L6 dorsal root or of the cutaneous nerve entering the cord at the lumbar level was found to produce bilateral DRPs in $3 segment. The depolarizations were smaller but in many respects were similar to the DRPs evoked by stimulation of the neighbouring dorsal root [21]. It has been suggested that transmission of depolarization to the neighbouring dorsal root occurs via Lissauer tract which is propriospinal tract of the substantia gelatinosa [29]. Neuronal arrangement of the substantia gelatinosa may provide basis for spread of the primary afferent depolarization over considerably longer distances along the spinal cord [25, 27]. In the present experiments we have investigated the spread of the DRPs in lower lumbar, sacral and upper
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caudal segments of the cat spinal cord and attempted to c o r r e l a t e its p r o p e r t i e s w i t h t h e k n o w n p a t t e r n o f connections of the substantia gelatinosa.
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Methods Fourty-eight cats weighing 1.9-3.9kg were anaesthetized with tiamylal sodium (Surital, Parke-Davis, 30 mg/kg body weight, i.p.), supplemented every 2 h with 15 mg/kg, i.v. The spinal cord was transected at Thl 3. Following laminectomy all dorsal roots from L5 to Ca2 were cut on both sides of the cord at the points where they enter the dura. Then the distances between the points of entry of the most caudal rootlets of each dorsal root into the cord were measured on the right side. Preparations were de-efferented by bilateral section of the ventral roots from L6 through $3. In each dorsal root the most caudal rootlets of an average diameter of 0 . 5 - 0 . 7 r a m were separated. When the whole dorsal root had diameter less than 0.8 mm (as it occurred in some smaller preparations with Cal and Ca2 dorsal roots) its teasing into smaller strands was not attempted. For measuring the size of depolarization, its latency and effectiveness during prolonged stimulation we recorded bilateral DRPS at various distances between stimulating and recording electrodes. The stimulating electrodes were placed at dorsal roots of the most cranial and most caudal segments of the studied part of the cord. The recording electrodes were mounted on dorsal rootlets of the segment close to the cranial stimulating electrodes and after studying the DRPs elicited by volleys from both pairs of electrodes they were moved into caudal direction allowing to record depolarization from all segments lying between the points of entry of afferent volleys. When testing temporal facilitation and interaction of the DRPs the distance between stimulating and recording electrodes was fixed. The DRPs were recorded from $2 dorsal root. They were produced by stimulation of L6 and Ca2 dorsal roots, i.e. three segments cranially or caudally from the recording electrodes. Hooked platinum wire electrodes were used for stimulation and recording. To record the DRPs one electrode was placed close but not touching the cord and the other on the cut end. The inter-electrode distance was 15 mm. The DRPs were produced by single square wave pulses of 0.1 ms width or by repetitive stimulation at 250 c/s lasting 500 ms. The strength of stimulation was expressed in multiples of threshold (T). The strength of a single puIse applied to L5 dorsal root which produced liminal DRP in the L6 dorsal root was considered to be the threshold. Except where otherwise stated the intensity of stimulation was 4T. Single DRPs were evoked every 7 - 1 0 s. The interaction of the DRPs was produced every 1 2 - 1 5 s. The amplifiers Used for recording the DRPs were a.c. coupled with I s time constant. Potentials evoked by prolonged stimulation were recorded with a d.c. amplifier. The animals were immobilized with gallamine triethiodide (Flaxedil, Specia, 10 mg/kg, i.v.) and kept on artificial respiration. The temperature of the oil covering the cord and of the animals was kept at 3 6 - 3 8 ~ by radiant heat.
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Fig. 1. Dorsal root potentials produced by stimulation of L5 dorsal root (A) and Ca2 dorsal root (B) with single volleys and recorded bilaterally from dorsal roots of four segments. Symbol above records show the level of recording. Upper traces of each record represent ipsilateral potentials and lower traces contralateral potentials
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Fig. 2. Amplitudes of dorsal root potentials evoked by stimulation of L5 dorsal root (A) and Ca2 dorsal root (B) with single volleys and' recorded in consecutive segments of the cord. The magnitudes of DRPs in percentages in relation to control values taken as 100 are plotted against the distances between the points of entry of dorsal roots into the cord. Mean amplitudes of DRPs recorded in roots adjacent to stimulated dorsal roots were considered as contol values. They were calculated separately for ipsi- and contralateral potentials. Open circles represent ipsilateral DRPs, filled circles contralateral ones. Each point is arithmetic mean of eleven measurements
Results
DRPs Recorded at Various Distances from the Point of Entry of Afferent Volley into the Spinal Cord. D o r s a l roots L5 and Ca2 were stimulated one after the other, a n d b i l a t e r a l D R P s w e r e r e c o r d e d c o n s e c u t i v e l y a t all segments in between these roots. The sample DRPs evoked by stimulation of L5 dorsal root with single v o l l e y s a n d r e c o r d e d a t f o u r d i f f e r e n t levels a r e s h o w n i n Fig. I A . T h e c h a n g e s i n t h e size o f p o t e n t i a l s r e c o r d e d f r o m all s e g m e n t s a r e r e p r e s e n t e d i n Fig. 2 A .
It may be seen that with an increase in distance between stimulating and recording electrodes ipsilateral DRPs d e c r e a s e a n d i n C a 1 s e g m e n t t h e y a t t a i n a b o u t 47 % o f the level seen in L6 dorsal root, Quite unexpectedly contralateral depolarizations increase to more than 150% in segments just caudal to the stimulating electrode and than slowly diminish attaining at the most distant segment the value close to that observed in L6 dorsal root.
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Fig. 4. Changesin index of effectivenessof stimulation in maintaining prolonged depolarization of ipsilateral dorsal roots in consecutive segments of the cord. Index of effectivenessin percentagesis plotted against distances between points of entry of dorsal roots into the cord. Open circlesrepresent values calculated for DRPs produced by stimulation of L5 dorsal root and filledcirclesthose for DRPs evoked from Ca2 dorsal root. Each point is arithmetic mean of nine measurements
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(0.5 s at 250 c/s) of L5 dorsal root (A) and Ca2 dorsal root (B) and recorded in six segments of the cord. The amplitudes of DRPs in percentages in relation to control values taken as 100 are plotted against the distances between the points of entry of dorsal roots into the cord. Controls werecalculated as in Fig. 2. Open circlesrepresent ipsi- DRPs and filled circles contralateral DRPs. Each point is arithmetic mean of eleven measurements
decrease more slowly than in the case shown in Fig. 2B and in L6 dorsal root they reach 39.5 % and 56.6 % of their control on ipsi- and contralateral side. In none experiment did the D R P s decreased to zero in the most distant segments.
Effectiveness of Stimulation in Maintaining Prolonged Depolarization of Dorsal Roots. Depolarization of the The D R P s evoked by stimulation of the Ca2 dorsal root are shown in Fig. 1B and 2B. We will describe them beginning from the segment neighbouring to the stimulating electrodes (i.e. from right to left). When recording electrodes are moved to more distant segments the decrease in size of ipsi- D R P s is much greater than in the case illustrated in Fig. 1A and 2A and in L6 dorsal root the D R P s attain 17.6 % of the initial level Similar reduction is observed on the other side of the cord and in L6 segment potentials are reduced to 4.6 %. Because the course of changes of the D R P s on both sides of the cord is similar it is approximated by one curve. It should be noted that in 27.3 % of preparations stimulation of the Ca2 dorsal root did not produce any depolarization in L6 dorsal roots and in 18.2%of animals there were no bilateral D R P s in L7 dorsal roots. In all preparations bilateral D R P s evoked from L5 dorsal root were invariably observed at all segmental levels. The changes in the mean size of the DRPs produced by long-lasting repetitive stimulation are shown in Fig. 3. When L5 dorsal root is stimulated (Fig. 3 A) ipsiD R P s decrease more slowly then when evoked by single volleys and in the Cal segment they reach 84.1% of the control. The rise of contralateral D R P s in proximal segments is also less pronounced and attains 127 % of control. In almost all distant segments the level of depolarization is above the control value. The DRPs evoked by stimulation of Ca2 dorsal root (Fig. 3B)
dorsal roots maintained throughout the period of tetanization shows the potency of presynaptic inhibition in conditions of prolonged influx of sensory impulses. It has been demonstrated that ipsilateral potentials are maintained during all tetanus but contralateral D R P s are not sustained and after reaching maximum they rapidly decrease to zero [9, 10, 15]. When D R P produced by stimulation of the L6 dorsal root is recorded in ipsi- $3 dorsal root it falls to zero shorthly after the onset of tetanus being very similar to contralateral potential observed in lumbar segments [21]. This finding suggests that the effectiveness of stimulation in maintaining prolonged D R P is related to the distance between stimulating and recording electrodes. In this series of experiments we have determined the index of effectiveness for D R P s recorded in all segments between L5 and Ca2 dorsal root. It was calculated as the ratio of the level of depolarization measured at the end of stimulation to the m a x i m u m level of potential attained at its beginning and was expressed in percentages. Since it was found that the index calculated for all contralateral DRPs is zero, Fig. 4 shows only the changes in effectiveness of ipsilateral depolarization. The index of effectiveness for the DRPs produced by stimulation of L5 dorsal root (open circles) in the neighbouring L6 segment amounts to 15.5 ~ . It decreases when recording electrodes are moved to consecutive segments and this reduction is proportional to the distance between both pairs of
204 electrodes. Four segments away from stimulating electrodes ($2) the index amounts to 4.7 ~ and in the next segments it drops to zero. The decrease of the index calculated for DRPs evoked from Ca2 dorsal root is much more rapid (filled circles). After spreading two segments index drops to 4.6 ~ and at the $2 segmental level it is close to 0 ~.
Latencies ofDRPs. A significant increase in the time to peak and reduction in amplitude of the DRPs recorded at more distant segments made it difficult to obtain a complete set of the latency measurements in a single preparation. Figure 5 shows the latencies of the DRPs elicited by stimulation of the L5 dorsal root and spreading in the caudal direction. In all segments latencies of ipsi- potentials (open circles) are shorter than latencies of contralateral DRPs (filled circles). It should be noted that latencies of ipsi- DRPs recorded from two segments close to the stimulating electrode (L6 and L7) are almost the same amounting to 2.23 and 2.31 ms. Nearly identical latencies are also displayed by contralateral DRPs recorded from L7 and $I dorsal roots. In this experiment latencies of the DRPs evoked by stimulation of the Ca2 dorsal root and spreading cranially could only be measured in 4 segments of the cord (Fig. 5 B). It may be seen that in the Ca1 dorsal roots latencies are the same on both sides of the cord. Then the increase in the latency of contralateral DRPs becomes irregular and much more rapid than of the ipsi- potentials. Of particular interest is a large increase in the latency of contralateral DRPs occurring between $2 and $1 dorsal root where at a distance of about 5"ram it rises by 7.7 ms. The conduction velocities of the DRPs spreading caudally between L5 and Ca1 segments were calculated in 9 preparations. The mean conduction velocity ofipsiDRPs amounting to 4.12 _+ 0.31 m/s (2 _+ S.D.) is significantly higher than that of the contralateral potentials which is 2.23 _+ 0.27 m/s (P < 0.05). The mean conduction velocities of DRPs spreading cranially between Ca2 and $1 segments calculated in 7 preparations amounted to 2.71 + 0.48 m/s for ipsi- and 1.64 +_ 0.29 m/s for contralateral potentials. The difference between these values was statistically nonsignificant (P > 0.05). The sudden increases in latencies of the DRPs spreading cranially similar to those shown in Fig. 5 B were seen in 6 out of 7 preparations. They were observed on both sides of the cord and usually occurred 3 - 4 segments from the point of entry of an afferent v~?lley. The increases in latencies of the DRPs traveling cdudally were observed in 4 out of 9 preparations. They were seen on both sides of the cord after depolarization spreading 3 or 4 segments.
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Fig. 5. Latenciesof dorsal root potentialsevokedby stimulationof L5 dorsal root (A) and Ca2 dorsal root (B) and measuredin consecutive segments of the cord. Abscissa - distancesbetweenpoints of entry of dorsal roots into the cord. Ordinate - latencyin milliseconds.Open circles show ipsi- and filled circlescontralateral DRP. Each point is arithmetic mean of six to eight measurements
Temporal Facilitation of DRPs. In these experiments we have stimulated L6 or Ca2 dorsal roots with increasing number of volleys and recorded the resulting DRPs from $2 dorsal roots. The temporal facilitation of potentials spreading along three segments of the cord was compared with that of the DRPs elicited by stimulation of adjacent dorsal root. All depolarizations were evoked by submaximal stimulation. Figure 6A (triangles) shows that when the number of volleys in the neighbouring dorsal root is increased the ipsi-DRP rises fairly fast reaching 141 ~ of the control value following two volleys and 186 ~ after 5 or 6 volleys. Longer trains of volleys do not significantly affect the amplitude of depolarization. The increase of depolarization produced by stimulation of L6 dorsal root (open circles) is slower its plateau amounting to about 165 ~ being attained after 7 volleys. Temporal facilitation of the DRPs produced by volleys ascending from Ca2 dorsal root (filled circles) is simila/" to that resulting from stimulation of the S1 dorsal root. Two volleys enhance depolarization to 134 ~o and its maximals increase at 5 volleys is 193~. On the contralateral side of the cord (Fig. 6B) the increase of the DRPs evoked by growing number of afferent volleys is in general more rapid and peak depolarization higher than on the ipsilateral side. Two volleys applied to SI dorsal root (triangles) increase the DRPs to 184 ~ and after 5 volleys potentials reach the peak amounting to 294 ~ . Facilitation of the DRPs elicited from L6 dorsal root has similar time course but the maximum increase is lower and reaches 266 ~ of control. The volleys
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Fig. 6. A and B. Temporal facilitation of dorsal root potentials recorded in $2 dorsal roots and evoked by stimulation of $1 dorsal ~ root (triangles), L6 dorsal root (open circles), and Ca2 dorsal root (filled circles). Ipsilateral potentials are shown in A and contralateral potentials in B. Abscissa - number of volleys. Ordinate - amplitude of potentials in percentages in relation to the size of DRPs produced by single volleys taken as 100. Each point is arithmetic mean of nine to eleven measurements
Fig. 7A--G. Interaction of dorsal root potentials recorded from $2 dorsal root. Interval between conditioning and testing volleys was 20ms. Upper row of records represents conditioning potentials evoked by stimulation of S1 dorsal root (A), L6 dorsal root (B), Ca2 dorsal root (C), and testing potentials produced by stimulation of $1 dorsal root (D). Lower row of records (E, F, G)~hows interaction of conditioning potentials with testing depolarization. Upper traces in each record show ipsilateral and lower traces contralateral DRPs
ascending to $2 segment from Ca2 dorsal root produce relatively small augmentation of the contralateral DRP (filled circles). Following 2 volleys depolarization reaches 133 ~ which is value similar to that observed on the contralateral side. The maximum increase of this depolarization results from 5 volleys and amounts to 207~.
The interaction of DRPs evoked by stimuli at 4T (i.e. producing potentials of maximum amplitude) exhibited only large and nondiscriminative occlusion. The results obtained below were obtained with DRPs whose amplitude was 50 ~ of maximum. From Fig. 7 it may be seen that interaction of the ipsilateral testing potential (D) with conditioning DRP evoked from L6 dorsal root (B) produces depolarization (F) which is larger than single potentials but smaller than their sum. This indicates occlusion of the DRPs. Similar effects were obtained following interaction of the testing potential with conditioning DRPs set by volleys in Ca2 (C) or in the S1 dorsal roots (A) but resulting depolarization (E and G, respectively) disclose quantitative differences in the size of occlusion. The mean values of interaction between ipsilateral DRPs obtained in experiments in 8 cats are positive indicating that all interactions produce occlusion of depolarizations. When conditioning volley is applied to L6 dorsal root the mean value of interaction is 55.4 + 8.1 ~. Conditioning stimulation of Ca2 dorsal root produces smaller occlusion and the value of interaction amounts to 31.0 _+ 7.3 ~. As it was expected the largest occlusion reaching 86.3 _+ 12.0~o occurs when two volleys are set in the same S1 dorsal root. Occlusion between contralateral DRPs is smaller than that found on ipsilateral side of the cord. Preceding the testing DRP by conditioning volley in L6 dorsal root yields the mean interaction value of 38.3 _+ 11.7 ~. The analogous value for interaction between the DRP from Ca2 dorsal root and the testing depolarization amounts to 28.5 + 9.9~. Similar to ipsilateral potentials the greatest value of interaction reaching 54.9 _+ 11.0 ~ is
Interaction of DRPs. Since the DRPs produced by volleys spreading caudally and cranially in the spinal cord were recorded from the same dorsal root, it was of interest to find out if they result from depolarization of the same primary afferent terminals. A preliminary answer to this question can be obtained by studying interaction of the DRPs. The testing DRP produced in $2 dorsal root by stimulation of the neighbouring S1 dorsal root was preceded at fixed interval by conditioning volley in L6 or Ca3 dorsal roots. For comparison the conditioning stimuli were also applied to S1 dorsal root. When two pathways producing the DRPs have some elements in common an occlusion or facilitation of depolarizations may be expected. Summation of the DRPs should occur when both volleys activate independent pathways. The size of interaction was calculated by subtracting the size of DRP produced by two stimuli from the sum of depolarizations evoked separately by conditioning and testing volleys. The amplitudes of the DRPs were expressed in percentages the size of the mean testing depolarization being 100~. The larger are calculated values the greater interactions between potentials. The values close to zero would indicate summation of the DRPs.
206 attained when both stimulations are applied to the same S1 dorsal root. Discussion
In their original observations Barron and Matthews (1) noted that in decerebrate cats DRPs could be recorded 5 - 6 segments from the point of entry of afferent volley. They also found that the decrease of potentials was greater with depolarization traveling in the caudal direction. The caudal spread of DRPs confined to 6 segments of the cord was observed by Devor et al. [7] in unanaesthetized spinal cats. In our experiments at the distance of 6 segments from the stimulating electrodes the DRPs spreading cranially were decreased almost to zero. The length of 6 segments recurrent in all these findings suggests the range of pathways which under a variety of experimental conditions are able to transmit DRPs evoked by single volleys along the spinal cord. There are no data on the number of synapses and diameter of axons in~volvedin the spread of DRPs. For that reason the calculation of conduction velocity from latencies of the DRPs gives only approximate characteristic of pathways subserving the spread of depolarization. These calculations showed that DRPs spreading caudally could more rapidly exert their influence in caudal segments than depolarizations conducted cranially in upper parts of the cord. In the lower lumbar cord the effectiveness of prolonged stimulation in maintaining depolarization of the dorsal roots was decreased with an increase in the stimulation frequency. It was hypothetized that this reduction is caused by build-up of homosynaptic depression. Since this effect was particularly striking on the contralateral side of the cord it was inferred that homosynaptic depression is enhanced by additional synapses transmitting DRPs to the other side of the cord [15]. Rapid reduction of effectiveness with increase of the distance fromthe stimulating electrodes observed by us at fixed stimulation frequency demonstrates that this factor is even more potent during spread of depolarization along the spinal cord. Temporal facilitation in pathways producing DRPs is attributed to activation of additional synapses depolarizing the primary afferent fibres [9]. It was larger on contralateral side of the lower lumbar cord [15]. We present evidence that similar pattern is exhibited by DRPs in $2 segment and by depolarizations spreading caudally. It is conceivable that sudden increase of contralateral DRPs spreading in this direction results from facilitation by volleys traversing the cord from ipsilateral side in a few segments below the stimulated dorsal root. The same temporal facilitation of the ipsiand contralateral DRPs spreading cranially indicates lesser availability of synapses activated by volleys in the
PflfigersArch. 381 (1979) Ca2 dorsal root. This finding may be accounted for by structure of the substantia gelatinosa which at this level forms single sheet without special synaptic relays transmitting depolarization accross the cord [26, 29]. The interaction between the DRPs depends on the type of stimulated fibres, strength of stimulation and the kind of animal. Occlusion prevails in cats while in frogs facilitation is more frequent [4, 20]. In our experiments occlusion of depolarization was the only observed form of interaction. Smaller interaction of potentials spreading cranially showed that longranging pathway which are carrying them are to greater extent separate from local neuronal pathways generating the DRPs than are pathways carrying potentials in the caudal direction. Some properties of the DRPs spreading along the cord show correlation with neuronal arrangement and connections of the substantia gelatinosa as described by Szent~gothai [27]. Lateral part of Lissauer's tract and lateral fasciculus proprius are built up of axons of the substantia gelatinosa neurones whose collaterals after course of 5 - 6 segments terminate in the same structure. Since the maximum craniad spread of the DRPs produced by single volleys occurs at the same length of the cord it is possible that it is mediated by this longitudinal system. Recent experiments of Cervero et al. [5] show that the DRPs evoked by stimulation of the L4 (or L5) dorsal root and recorded in the neighbouring segment are reduced to about 50 % after section of Lissauer's tract at the junction between these segments. The section of the dorsal column and of dorsolateral funiculus produces larger decrease of depolarization. These findings suggest that the short range spread of the DRPs depends on several propriospinal pathways. It cannot be excluded that the spread of the DRPs over short and long distances along the cord is subserved by different fibre systems. The relationship between longranging afferents studied by Wall and Werman between L1 and S1 [21] and the spread of DRPs cannot be established at present. These afferents do not produce dorsal root reflexes and thus do not seem to have effective connections with pathways generating depolarization of distant presynaptic terminals. On the other hand Szent~tgothai describes collaterals given off by large calibered afferents which enter the substantia gelantinosa at the length of 4 segments. At the same length of the cord the index of effectiveness of prolonged stimulation for DRPs spreading caudally is maintained above zero and after passing approximately the same number of segments sudden incrases in the latency of the DRPs are observed. These similarities suggest that although the area in which collaterals of one large calibered afferent arborize is not critical for the maximum spread of depolarization, some properties of the DRPs related to the number of activated
K. Lupa et al. : Spread of Dorsal Root Potentials Along Spinal Cord
synapses heavily depend on the range of arborization. Correlations found by us are only qualitative and are limited to a few properties of the DRPs. Nevertheless, they suggest that the spread of primary afferent depolarization occurs in the substantia gelatinosa and via its long range connecting system. The amplitude of the DRPs, the speed with which they reach distant segments of the cord or effectiveness of stimulation in maintaining depolarization of the dorsal roots reflects the extent of control of the sensory input by presynaptic inhibition. Taking this into consideration our findings show the preponderance of depolarization generated at the L6 level and spreading to lower sacral and caudal segments over that produced in caudal parts of the cord and transmitted to the lower lumbar segments. A functional significance of this differentiation may be visualised when we consider that afferents entering the lumbar cord mainly originate from receptors of hind limb and give rise to important nociceptive and postural reflexes while sensory fibres reaching lower sacral and caudals segments innervate organs of the midline [2, 13, 22-24, 28] which are rarely activated in emergency situations. This differentiation would permit to block impulses entering lower segments of the cord and by contrast enhance the role of impulses from hind limbs.
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Received February 6~Accepted June 2, 1979
