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Repetitive antidromic discharges in fast cutaneous nerve fibres
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Repetitive antidromic discharges in fast cutaneous nerve fibres The experiments described in this p a p e r were p e r f o r m e d to o b t a i n m o r e inf o r m a t i o n on the p o s t e r i o r r o o t reflex elicited by stimulation o f afferent fibres near or in the spinal cord. Spinal cats with the c o r d transected at CI were used. A f t e r l u m b o s a c r a l laminectomy, the L7, Sl and 2 ventral nerve roots were cut, and the L7 a n d S1 p o s t e r i o r nerve roots p r e p a r e d for stimulation or recording. C u t a n e o u s nerves such as the s a p h e n o u s and the sural, were p r e p a r e d for stimulation or recording. Pools filled with mineral oil m a i n t a i n e d at 37°C were f o r m e d over the exposed spinal c o r d a n d the nerves in the legs. M i c r o e l e c t r o d e stimulation o f intraspinal afferent endings was carried out using micropipettes filled with 4 M NaC1 with a resistance o f l - 2 M~2. S t i m u l a t i o n or r e c o r d i n g electrodes were m a d e o f silver or p l a t i n u m wire. C o n d e n s e r c o u p l e d pre-amplifiers were used to amplify the nerve action potentials and these were fed into a d o u b l e b e a m c a t h o d e ray tube. I f the central endings o f afferent fibres are stimulated in a spinal cat according
_m
1
Fig. 1. Sural nerve action potentials recorded at the level of the popliteal space (upper tracings), and of the ankle (lower tracings) following stimulation by microelectrode of afferent fibres on the surface of the dorsal columns (A); or in depth in the dorsal columns and dorsal horn: 300 /~ deep (B); 600 # deep (C); 900 # deep (D) and 1200 kt deep (E). Time marker in E = 2 msec. Brain Research, 11 (1968)268-272
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to the method introduced by Wall 2, the activity evoked in peripheral cutaneous nerve fibres, such as those of the sural, consists of a brief spike, corresponding to the discharge in fibres conducting at 50-60 m/sec, followed after an interval of 2-3 msec, during which there is little or no activity, by a second phase of discharges. This second phase of activity consists of a prolonged discharge lasting some 10-15 msec, which rapidly reaches a peak and then declines in a series of small, repetitive, diminishing waves (Fig. 1). We wished to have further information about the nature of this second phase of activity. In agreement with other workers 2, we found that these two phases of activity were evoked wherever the afferent fibres are stimulated, whether by microelectrode in the region of the afferent terminals in the dorsal horn, or in the posterior columns, or by macroelectrodes placed on the L7 posterior nerve root (Figs. 1 and 2A, C). In all these cases the two phases of activity retain their character and time relations, whatever the position of the stimulating electrodes along the course of the afferent fibres inside or outside the spinal cord. The latency of the second phase of the evoked potentials is consistent either with activity set up directly by the stimulus in fibres conducting at or below 35 m/sec, or alternatively with activity due to an indirect mechanism of stimulation of fibres of unknown conduction velocity, or with a combination of the two mechanisms. The evidence is that this second phase is due to a dorsal root reflex produced by antidromic discharges conducted almost completely in fast cutaneous fibres, as shown by the following results: (1) The second phase of activity is present in the sural action potential, pro-
Fig. 2. Sural nerve action potentials recorded at the level of the popliteal space (upper tracings) and of the ankle (lower tracings) following stimulation of L7 posterior nerve root with single shock;
2.3 V (A), 2 V (B), 3 V (C); repeat 2 V stimulation after section of L7 root central to stimulating electrodes(D). Brain Research, 11 (1968) 268-272
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duced by stimulation of the L7 posterior nerve root, even when the stimulus is barely threshold for the fastest fibres of the sural nerve. I n some cases the second c o m p o n e n t is present even in the absence of the first phase of activity, that is when the stimulus is subthreshold for even the fastest fibres of the sural nerve, b u t p r e s u m a b l y threshold for other fibres of the posterior root (Fig. 2B). (2) The two phases of activity, set up by stimulation of the L7 posterior nerve
Fig. 3. Sural nerve action potentials recorded at the level of the popliteal space (upper tracings), and at the ankle (lower tracings in A, B, D, E). A-C and F: stimulation of L7 posterior nerve root; D and E: microelectrode stimulation of the central endings of the sural fibres in the dorsal horn. Lower tracings in A and D: monopolar recordings of sural potentials at the ankle. After A and D the sural nerve was crushed between the two electrodes at the popliteal level; B, E: monopolar records of the sural potentials at the popliteal space. After B, E, L7 posterior nerve root was cut proximal to the stimulating electrodes. C: sural action potential at popliteal space following L7 posterior root section. F: as in C, but stimulus strength increased x 3, and amplification increased x 10. Time marker in C and F 2 msec. Conduction distance between the two recording sites ~ 5.5 cm. Fast spike in F retouched. Brain Research, 11 (1968) 268-272
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root, are affected in a different way by section of the root between the stimulating electrodes and the zone of entry into the cord. The first phase of activity is unaffected by the root section, while the second component disappears (Figs. 2D and 3C). Alternatively instead of cutting the root, conduction in fibres proximal to the stimulating electrodes was blocked by cooling the root at 4-6°C, with the result of delaying and eventually blocking the second component, without affecting the first. (3) A comparison can be made between the latencies of the first and second phases evoked either by stimulating of the L7 posterior nerve root or by microelectrode stimulation of the dorsal horn endings of the sural nerve fibres. The latency of the first phase elicited by microelectrode stimulation of the intraspinal sural endings is 0.3-0.5 msec longer than that obtained by stimulating of the L7 posterior nerve root, as it would be expected considering the longer conduction distance. On the other hand, analysis of the results obtained in several preparations shows that the latency of the second phase following root stimulation may be either 0.3-0.7 msec longer, equal (Fig. 3), or 0.3-0.5 msec shorter than that following microelectrode stimulation. This wide range of latencies of the second phase of the sural potential indicates that the mechanism which produces this dorsal root reflex has an inherent variability. This may be due to alternative reflex pathways and varying efficiency of synaptic transmission, depending on whether the mechanism is activated by root or intraspinal afferent endings stimulation. There can be therefore no doubt that the second phase of the evoked sural potential is due to a dorsal root reflex. Experiments were then performed to establish the velocity of conduction of the fibres involved in the reflex. It has been found that the reflex discharge in afferent fibres is conducted in large cutaneous fibres conducting at 50-60 m/sec. This is shown by the following results: (1) Monopolar records of the activity set up by stimulation of the root or central terminals in the dorsal horn, taken at two different points of the sural nerve, allow the calculation of the conduction velocity of the fibres whose activity is represented by the second phase. It has been found that the velocity of these fibres is 50-60 m/sec (Figs. 3A, B, D, E). (2) Weak stimulation of the afferent terminals within the cord produced in some experiments a small first spike, followed by small repetitive waves. The difference in latency of these small waves at two different levels of the sural nerve was used to calculate the conduction velocity, which varied in different experiments and for different small waves between 45 and 60 m/sec. The contribution of more slowly conducting fibres to the second phase of activity must be very limited, if it exists at all, because no repetitive activity was ever found in such fibres when weak stimulus strengths were employed. (3) The action potential of the sural nerve, elicited by strong dorsal root stimulation, following root section at the zone of entry of the root into the cord, shows that the amplitude of the action potentials of slowly conducting fibres is small compared with the elevation of the second phase of activity recorded with the root intact (Fig. 3F). The results are a confirmation that the second phase of the action potentia Brain Research, 11 (1968)268-272
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elicited by stimulation of the afferent fibres in or near the spinal cord is a dorsal root reflex, as proposed by Wall 2. Our measurements show that this reflex has a long central delay, 2.5 msec or more in duration 1, and that this delay is different according to whether the afferent fibres are stimulated at their central terminals in the dorsal horn or in the posterior nerve root. The first phase of the sural action potentials has latencies 0.3-0.5 msec longer when the central endings are stimulated in the dorsal horn than when the afferent fibres are stimulated in the posterior nerve root, a difference easily explained by the longer conduction distance. One would expect the second, reflex phase of the sural potential to have a shorter latency by approximately the same interval (0.3-0.5 msec) when elicited by microelectrode stimulation than when elicited by posterior root stimulation. This has not been a consistent finding, and in fact in some experiments the latency following microelectrode stimulation was longer rather than shorter than that following root stimulation. This discrepancy cannot be explained by a difference in central processes due to the different number of afferent fibres stimulated in the two instances, because the first spike was always of comparable magnitude no matter what the position of the stimulating electrode was. We therefore think that there may be a difference in length or efficiency between the reflex pathways used in the two situations. Our results show that the dorsal root reflex is conducted in the fast sural fibres. There has been no evidence of involvement of the slower cutaneous fibres, and even if there were activity in such afferent fibres, it would contribute very little to the recorded potential: the potential due to direct stimulation of the slow fibres is very small in relation to the size of the second phase of activity of reflex origin. We should like to acknowledge the support of the Medical Research Council in these experiments. Physiological Laboratory, University of Liverpool, Liverpool (Great Britain)
1. CALMA A. A. QUAYLE
1 FRANK, K., AND FUORTES,M. G. F., Potentials recorded from the spinal cord with microelectrodes J. Physiol. (Lond.), 130 (1955) 625-654. 2 WALL, P. D., Excitability changes in afferent fibres terminations and their relation to slow potentials, J. Physiol. (Lond.), 142 (1958) 1-21. (Accepted July 29th, 1968)
Brain Research, 11 (1968) 268-272
SHORT
COMMUNICATIONS
Repetitive antidromic discharges in fast cutaneous nerve fibres The experiments described in this p a p e r were p e r f o r m e d to o b t a i n m o r e inf o r m a t i o n on the p o s t e r i o r r o o t reflex elicited by stimulation o f afferent fibres near or in the spinal cord. Spinal cats with the c o r d transected at CI were used. A f t e r l u m b o s a c r a l laminectomy, the L7, Sl and 2 ventral nerve roots were cut, and the L7 a n d S1 p o s t e r i o r nerve roots p r e p a r e d for stimulation or recording. C u t a n e o u s nerves such as the s a p h e n o u s and the sural, were p r e p a r e d for stimulation or recording. Pools filled with mineral oil m a i n t a i n e d at 37°C were f o r m e d over the exposed spinal c o r d a n d the nerves in the legs. M i c r o e l e c t r o d e stimulation o f intraspinal afferent endings was carried out using micropipettes filled with 4 M NaC1 with a resistance o f l - 2 M~2. S t i m u l a t i o n or r e c o r d i n g electrodes were m a d e o f silver or p l a t i n u m wire. C o n d e n s e r c o u p l e d pre-amplifiers were used to amplify the nerve action potentials and these were fed into a d o u b l e b e a m c a t h o d e ray tube. I f the central endings o f afferent fibres are stimulated in a spinal cat according
_m
1
Fig. 1. Sural nerve action potentials recorded at the level of the popliteal space (upper tracings), and of the ankle (lower tracings) following stimulation by microelectrode of afferent fibres on the surface of the dorsal columns (A); or in depth in the dorsal columns and dorsal horn: 300 /~ deep (B); 600 # deep (C); 900 # deep (D) and 1200 kt deep (E). Time marker in E = 2 msec. Brain Research, 11 (1968)268-272
SHORT COMMUNICATIONS
269
to the method introduced by Wall 2, the activity evoked in peripheral cutaneous nerve fibres, such as those of the sural, consists of a brief spike, corresponding to the discharge in fibres conducting at 50-60 m/sec, followed after an interval of 2-3 msec, during which there is little or no activity, by a second phase of discharges. This second phase of activity consists of a prolonged discharge lasting some 10-15 msec, which rapidly reaches a peak and then declines in a series of small, repetitive, diminishing waves (Fig. 1). We wished to have further information about the nature of this second phase of activity. In agreement with other workers 2, we found that these two phases of activity were evoked wherever the afferent fibres are stimulated, whether by microelectrode in the region of the afferent terminals in the dorsal horn, or in the posterior columns, or by macroelectrodes placed on the L7 posterior nerve root (Figs. 1 and 2A, C). In all these cases the two phases of activity retain their character and time relations, whatever the position of the stimulating electrodes along the course of the afferent fibres inside or outside the spinal cord. The latency of the second phase of the evoked potentials is consistent either with activity set up directly by the stimulus in fibres conducting at or below 35 m/sec, or alternatively with activity due to an indirect mechanism of stimulation of fibres of unknown conduction velocity, or with a combination of the two mechanisms. The evidence is that this second phase is due to a dorsal root reflex produced by antidromic discharges conducted almost completely in fast cutaneous fibres, as shown by the following results: (1) The second phase of activity is present in the sural action potential, pro-
Fig. 2. Sural nerve action potentials recorded at the level of the popliteal space (upper tracings) and of the ankle (lower tracings) following stimulation of L7 posterior nerve root with single shock;
2.3 V (A), 2 V (B), 3 V (C); repeat 2 V stimulation after section of L7 root central to stimulating electrodes(D). Brain Research, 11 (1968) 268-272
270
SHORT COMMUNICATIONS
duced by stimulation of the L7 posterior nerve root, even when the stimulus is barely threshold for the fastest fibres of the sural nerve. I n some cases the second c o m p o n e n t is present even in the absence of the first phase of activity, that is when the stimulus is subthreshold for even the fastest fibres of the sural nerve, b u t p r e s u m a b l y threshold for other fibres of the posterior root (Fig. 2B). (2) The two phases of activity, set up by stimulation of the L7 posterior nerve
Fig. 3. Sural nerve action potentials recorded at the level of the popliteal space (upper tracings), and at the ankle (lower tracings in A, B, D, E). A-C and F: stimulation of L7 posterior nerve root; D and E: microelectrode stimulation of the central endings of the sural fibres in the dorsal horn. Lower tracings in A and D: monopolar recordings of sural potentials at the ankle. After A and D the sural nerve was crushed between the two electrodes at the popliteal level; B, E: monopolar records of the sural potentials at the popliteal space. After B, E, L7 posterior nerve root was cut proximal to the stimulating electrodes. C: sural action potential at popliteal space following L7 posterior root section. F: as in C, but stimulus strength increased x 3, and amplification increased x 10. Time marker in C and F 2 msec. Conduction distance between the two recording sites ~ 5.5 cm. Fast spike in F retouched. Brain Research, 11 (1968) 268-272
SHORT COMMUNICATIONS
271
root, are affected in a different way by section of the root between the stimulating electrodes and the zone of entry into the cord. The first phase of activity is unaffected by the root section, while the second component disappears (Figs. 2D and 3C). Alternatively instead of cutting the root, conduction in fibres proximal to the stimulating electrodes was blocked by cooling the root at 4-6°C, with the result of delaying and eventually blocking the second component, without affecting the first. (3) A comparison can be made between the latencies of the first and second phases evoked either by stimulating of the L7 posterior nerve root or by microelectrode stimulation of the dorsal horn endings of the sural nerve fibres. The latency of the first phase elicited by microelectrode stimulation of the intraspinal sural endings is 0.3-0.5 msec longer than that obtained by stimulating of the L7 posterior nerve root, as it would be expected considering the longer conduction distance. On the other hand, analysis of the results obtained in several preparations shows that the latency of the second phase following root stimulation may be either 0.3-0.7 msec longer, equal (Fig. 3), or 0.3-0.5 msec shorter than that following microelectrode stimulation. This wide range of latencies of the second phase of the sural potential indicates that the mechanism which produces this dorsal root reflex has an inherent variability. This may be due to alternative reflex pathways and varying efficiency of synaptic transmission, depending on whether the mechanism is activated by root or intraspinal afferent endings stimulation. There can be therefore no doubt that the second phase of the evoked sural potential is due to a dorsal root reflex. Experiments were then performed to establish the velocity of conduction of the fibres involved in the reflex. It has been found that the reflex discharge in afferent fibres is conducted in large cutaneous fibres conducting at 50-60 m/sec. This is shown by the following results: (1) Monopolar records of the activity set up by stimulation of the root or central terminals in the dorsal horn, taken at two different points of the sural nerve, allow the calculation of the conduction velocity of the fibres whose activity is represented by the second phase. It has been found that the velocity of these fibres is 50-60 m/sec (Figs. 3A, B, D, E). (2) Weak stimulation of the afferent terminals within the cord produced in some experiments a small first spike, followed by small repetitive waves. The difference in latency of these small waves at two different levels of the sural nerve was used to calculate the conduction velocity, which varied in different experiments and for different small waves between 45 and 60 m/sec. The contribution of more slowly conducting fibres to the second phase of activity must be very limited, if it exists at all, because no repetitive activity was ever found in such fibres when weak stimulus strengths were employed. (3) The action potential of the sural nerve, elicited by strong dorsal root stimulation, following root section at the zone of entry of the root into the cord, shows that the amplitude of the action potentials of slowly conducting fibres is small compared with the elevation of the second phase of activity recorded with the root intact (Fig. 3F). The results are a confirmation that the second phase of the action potentia Brain Research, 11 (1968)268-272
272
SHORT COMMUNICATIONS
elicited by stimulation of the afferent fibres in or near the spinal cord is a dorsal root reflex, as proposed by Wall 2. Our measurements show that this reflex has a long central delay, 2.5 msec or more in duration 1, and that this delay is different according to whether the afferent fibres are stimulated at their central terminals in the dorsal horn or in the posterior nerve root. The first phase of the sural action potentials has latencies 0.3-0.5 msec longer when the central endings are stimulated in the dorsal horn than when the afferent fibres are stimulated in the posterior nerve root, a difference easily explained by the longer conduction distance. One would expect the second, reflex phase of the sural potential to have a shorter latency by approximately the same interval (0.3-0.5 msec) when elicited by microelectrode stimulation than when elicited by posterior root stimulation. This has not been a consistent finding, and in fact in some experiments the latency following microelectrode stimulation was longer rather than shorter than that following root stimulation. This discrepancy cannot be explained by a difference in central processes due to the different number of afferent fibres stimulated in the two instances, because the first spike was always of comparable magnitude no matter what the position of the stimulating electrode was. We therefore think that there may be a difference in length or efficiency between the reflex pathways used in the two situations. Our results show that the dorsal root reflex is conducted in the fast sural fibres. There has been no evidence of involvement of the slower cutaneous fibres, and even if there were activity in such afferent fibres, it would contribute very little to the recorded potential: the potential due to direct stimulation of the slow fibres is very small in relation to the size of the second phase of activity of reflex origin. We should like to acknowledge the support of the Medical Research Council in these experiments. Physiological Laboratory, University of Liverpool, Liverpool (Great Britain)
1. CALMA A. A. QUAYLE
1 FRANK, K., AND FUORTES,M. G. F., Potentials recorded from the spinal cord with microelectrodes J. Physiol. (Lond.), 130 (1955) 625-654. 2 WALL, P. D., Excitability changes in afferent fibres terminations and their relation to slow potentials, J. Physiol. (Lond.), 142 (1958) 1-21. (Accepted July 29th, 1968)
Brain Research, 11 (1968) 268-272