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Flexion reflex elicited by ventral root afferents in the cat
Neurosciem'e Letters, 62 (1985) 353-358
353
Elsevier Scientific Publishers Ireland Lid.
NSL 03689
FLEXION REFLEX ELICITED BY V E N T R A L R O O T A F F E R E N T S IN THE CAT
H O N G KEE SHIN, JUN KIM and JIN MO C H U N G *
Marine Biomedical Institute and Department o l Physiolo&v and Biophysics, University (!l Texas Medical Branch, Galveston, T X 77550 (U.S.A.) (Received August 27th, 1985: Revised version received September 12th, 1985: Accepted September 19th, 1985}
Key aor&v
ventral root afferent flexion reflex cord compound action potential
tmmyelinated fiber
single unit activity
cat
spinal
To investigate the physiological role of ventral root afferent fibers, the ventral root was stimulated in spinal cats in an anempt to elicit the flexion reflex, a common nociceptive spinal reflex. The flexion reflex was elicited by stimulating the distal stump of the cut L7 or S I ventral root and recorded from the proximal stump of the cut SI ventral root. In 12 of the 14 cats studied, the flexion reflex was recorded as a compound action potential. The finding was confirmed by recording single-unit activity from 5 units in two cats. These results suggest that the ventral root afferent fibers exert a variety of physiological effects that include eliciting such spinal reflexes as the flexion reflex. The responsible fibers in the ventral root travel distally Iowards the dorsal root ganglion to enter the spinal cord through the dorsal root.
The fact that the spinal root in mammals contains a significant number of unmyelinated primary afferent fibers has become well-recognized in recent years [1, 6, 7]. Although there is evidence to suggest that some of these afferent fibers enter the spinal cord directly through the ventral root [9, 11, 12], most seem to travel distally towards the dorsal root ganglion and ultimately enter the cord through the dorsal root [13, 141. We have been investigating the physiological role of these ventral root afferent fibers. Stimulation of the distal stump of the cut ventral root in the cat excites dorsal horn cells carrying nociceptive information [3, 4]. The same stimulation also produces an increase in systemic arterial blood pressure [2]. The results of these studies suggest that the ventral root afferent fibers may play a wide variety of physiological roles. Therefore, the present study was undertaken to see if the ventral root afferent fibers elicit the spinal flexion reflex, which is one of the important central actions of primary afferent fibers. The experiments were conducted on 14 spinal cats (2.0 3.5 kg). Nine were anesthetized with c~-chloralose (60 mg/kg, i.v.) and 5 were decerebrated at the mid-collicular *Author for correspondence and reprint requests at: Marine Biomedical Institute, University of Texas Medical Branch, 200 University Boulevard, Galveston, TX 77550, U.S.A. 0304-394085:$ 03.30 © 1985 Elsevier Scientilic Publishers Ireland Ltd.
354
level under initial anesthesia with a mixture of nitrous oxide, oxygen and halothane. Animals were paralysed by gallamine triethiodide (Flaxedil, 4 mg/kg/h) infused through a cannula placed in the cephalic vein and artificially ventilated. The end-tidal CO2 level was maintained between 3.5 and 4.5~. Rectal temperature was kept near 3 7 C with an electrical heating blanket. After the lumbosacral spinal cord was exposed by a laminectomy, the cord was pinched with a pair of forceps at LI L2 level. The L7 ventral root was cut near the spinal cord. The S I ventral root was also cut midway between the spinal cord and the dorsal root ganglion. A mineral oil pool was made around the exposed spinal cord and the temperature of the pool was maintained near body temperature with a heating coil. The distal stump of the cut L7 or S 1 ventral root was placed on tripolar stimulating electrodes. The electrode nearest to the dorsal root ganglion was grounded to prevent current spread to adjacent neural tissues. Furthermore, the cord dorsum potential was monitored to check tbr current spread. The distal stump of the cut ventral root was stimulated electrically with a single pulse or a train of three pulses (50 Hz internal frequency), applied once every 10 s at a suprathreshold intensity for C fibers (0.5 ms, l0 mA). The flexion reflex was recorded from the proximal stump of the cut SI ventral root with bipolar recording electrodes. After the flexion reflex was recorded in the form of compound action potentials, the root was divided until single-unit activity could be recorded. The recorded activity was amplified and displayed on an oscilloscope. Single-unit activity was used to trigger a window discriminator from which output was fed into a computer to compile peristimulus time histograms. Both sides of 14 cats were tested. From at least one side of 12 cats, the flexion reflex could be recorded as a compound action potential from a fascicle of the proximal stump of the cut S I ventral root following stimulation of the distal stump of the cut S1 or L7 ventral root. Fig. I A - C shows 3 examples of such responses. The pattern of recorded compound action potentials varied widely from one fascicle to another and from one animal to another. As shown in Fig. I D, the flexion reflex elicited by L7 ventral root stimulation did not change appreciably after sectioning the sciatic nerve, suggesting that the reflex was not elicited indirectly by activation of receptors in the periphery due to excitation of m o t o r fibers following ventral root stimulation. The response however, disappeared after sectioning the L7 dorsal root. The same resuits were obtained from both sides in two cats. Furthermore, the reflex elicited by stimulation of the distal stump of the cut S 1 ventral root did not change after sectioning the S1 spinal nerve about I cm distal to the dorsal root ganglion. These results suggest that the fibers in the ventral root responsible for eliciting the flexion reflex travel distally toward the dorsal root ganglion and then enter the spinal cord through the dorsal root. Therefore. the conduction distance of the reflex was assumed to be the sum of the length of the ventral root from the stimulating electrodes to the dorsal root ganglion, the length of the dorsal root and the length of the ventral root from the spinal cord to the recording electrodes. The maximum conduction velocity of afferent fibers responsible for this flexion reflex was estimated from the conduction distance divided by the latency to the beginning of the reflex after subtraction of the
355
D ~
A
CONTROL
+ B
C
DR CUT
Fig. I. Examples of the flexion reflex recorded as a compound action potential. All recordings were made I¥om the proximal slump of the cut SI ventral root while stimulating the distal slump of the cut S1 or [,7 ventral root with a single square wave pulse (0.5 ms, 10 mA) at the time shown by the arrow. A D arc from 4 different experiments. A and B are single-sweep recordings of the flexion reflex elicited by stimulation of the SI and L7 ventral root, respectively. C is the averaged response from 10 consecutive stimulations of the SI ventral root repeated once every 10 s. In D, the control response elicited by stimulation of the L7 ventral root did not change after sectioning the sciatic nerve (PN CUT), but disappeared after the L7 dorsal root was cut (DR CUT). All time scale bars indicate 30 ms. Conduction distances in A D were 69, 77, 49 and 53 mm, respectively.
m i n i m u m c o n d u c t i o n t i m e (to a c c o u n t for the c e n t r a l d e l a y a n d the m o t o r fiber c o n d u c t i o n t i m e ) w h i c h was a s s u m e d to be the m i n i m u m l a t e n c y o f the reflex elicited by s t i m u l a t i o n o f the d o r s a l r o o t close to the spinal c o r d ( a b o u t 3 ms). T h e m a x i m u m c o n d u c t i o n v e l o c i t i e s c a l c u l a t e d this w a y f o l l o w i n g s t i m u l a t i o n o f the S I v e n t r a l r o o t in 19 cases r a n g e d f r o m 0.9 to 7.0 m / s [3.2_+2.2 m / s ( m e a n + S.D.)]. T h e p r o x i m a l s t u m p o f the S I v e n t r a l r o o t was d i v i d e d f u r t h e r until s i n g l e - u n i t a c t i v i t y c o u l d be r e c o r d e d . R e c o r d i n g s w e r e m a d e f r o m 5 single m o t o r a x o n s in t w o cats. Fig. 2 s h o w s a n e x a m p l e o f such an e x p e r i m e n t . P r e s t i m u l u s - t i m e h i s t o g r a m s w e r e c o m p i l e d f r o m the m o t o r a x o n d i s c h a r g e s in r e s p o n s e to 10 c o n s e c u t i v e stimuli a p p l i e d to the SI d o r s a l r o o t , S1 v e n t r a l r o o t a n d L7 v e n t r a l r o o t . S i n g l e - s w e e p r e c o r d i n g s o f the a c t i o n p o t e n t i a l are s h o w n in the insets o f c o r r e s p o n d i n g histog r a m s . A l t h o u g h the flexion reflex p r o d u c e d by v e n t r a l r o o t s t i m u l a t i o n was n o t as p o w e r f u l as t h a t p r o d u c e d b y d o r s a l r o o t s t i m u l a t i o n , definite reflex a c t i v i t y c a n be
356
SINGLE
STIMULATION
TRAIN
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S1 DORSAL
I
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v1 30
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Fig. 2. An example of the flexion reflex recorded as single-unit activity. Peristimulus-time histograms were compiled from the unit activity recorded from a fascicle divided from the proximal stump of the cut S1 ventral root after 10 consecutive stimulations of the S1 dorsal root, the distal stump of the cut S I ventral root and the distal stump of the cut L7 ventral root. Left and right panels show responses to stimulation with single shocks and a train of 3 pulses (internal frequency, 50 Hz), respectively. Note that some of the stimulus artefact triggered the window discriminator during SI ventral root stimulation. Bin width was set at 8 ms in all histograms. Insets show single sweeps of oscilloscope tracings of action potentials tbllowing each stimulus (time scale, 500 ms). Insets on the left panel also show the cord dorsum potentials produced by each stimulus (time scale, 50 ms).
seen following ventral root stimulation. The response was particularly apparent when the activity was temporally summed by stimulation with a train of 3 pulses. To insure against current being spread to adjacent tissue during ventral root stimulation, we always monitored the cord dorsum potential. Monitoring the cord dorsum potential was found to be a reliable method to check current spread in a previous study [4]. As the insets in Fig. 2 show, the cord dorsum potential could be recorded during dorsal root stimulation whereas it was absent during ventral root stimulation. Furthermore, previous studies [2, 4] already demonstrated that physiological effects produced by stimulating the distal stump of the cut ventral root were interrupted by pinching the root distal to the stimulating site and were restored by stimulation distal to the pinched site.
357 The present study together with previous studies [2-4] conducted in our laboratory have demonstrated that stimulation o f the afferent fibers in the ventral root o f the cat (1) activate the dorsal horn cells carrying nociceptive information, (2) raise systemic blood pressure and (3) produce a spinal flexion reflex. These physiological actions o f the ventral root afferent fibers, however, are exerted by the fibers traveling distally towards the dorsal root ganglion to enter the spinal cord through the dorsal root, not by the fibers that enter the spinal cord directly through the ventral root. Attempts to produce physiological effects by stimulating the proximal stump of the cut ventral root have failed [2, 4]. In addition, electrical stimulation o f the central stump o f the cut ventral root failed to elicit the flexion reflex, although some increase in discharges of m o t o r nerve fibers has been observed after algesic chemical injection into the systemic circulation in dorsal rhizotomized cats [15, 16]. The existence of ventral root afferent fibers that enter the spinal cord through the dorsal root has been supported further by showing the presence o f a large n u m b e r o f fibers in continuity between the dorsal and ventral root [10]. Although the fastest fibers in the ventral root eliciting the flexion reflex m a y be A6 fibers, the overall estimated conduction velocity suggests that most of them are unmyelinated. This is consistent with the results o f previous studies [2-4, 10]. The functional significance o f ventral root afferent fibers is not yet clear. One o f their potential functions would be nociception since the primary afferent fibers in the ventral root are reported to be largely somatic and visceral nociceptors [5, 8]. The results of the present study show clearly that stimulation of the distal stump o f the cut ventral root elicits the flexion reflex in much the same way as does activation o f the afferent fibers in the dorsal root. Therefore, our results support the idea that ventral root afl'erents can produce a wide variety of physiological action including a nociceptive spinal reflex. This work was supported by N I H G r a n t s NS21266, NS18830 and NS11255. We thank Heidi Freeborn for the art work and p h o t o g r a p h y . H.K.S. was supported in part by H a n y a n g University, College o f Medicine, Seoul, Korea. J.K. was supported in part by an alumni fund from Seoul National University College o f Medicine, Seoul, Korea. J.M.C. is the recipient o f a PHS Research Career Development Award NS00995. 1 Applebaum. Mi., Clifton, G.L., Coggeshall, R.E., Coulter, J.D., Vance. W.H. and Willis, W.D., Unrnyelinated fibres in the sacral 3 and caudal 1 ventral roots of the cat, J. Physiol. (London), 256 (1976) 557 572. 2 Chung, J.M., Kim, J. and Shin, H.K., Blood pressure response evoked by ventral root afferent fibres in the cat, J. Physiol. (London), in press. 3 Chung, J.M., Lee, K.H., Endo, K. and Coggeshal[, R.E., Activation of central neurons by ventral root afferents, Science, 222 (1983) 934~935. 4 ('hung, J.M., Lee, K.H., Kim, J. and Coggeshall, R.E., Activation of dorsal horn cells by ventral root stimulation in the cat, J. Neurophysiol., 54 (1985) 261 272. 5 Clifton, G.L., Coggeshall, R.E., Vance, W.H. and Willis, W.D., Receptive fields of unmyelinated ventral root afferent fibres in the cat, J. Physiol. (London) 256 (1976) 573 600. 6 Coggeshall, R.E., Appelbaum, M.L., Fazen, M., Stubbs. T.B. and Sykes, M.T., Unmyelinated axons
358
7 8 9 10 11 12 13
14 15
16
in human ventral roots, a possible explanation for the failure of dorsal rhizotomy to relieve p~lirl. Brain, 98 (1975) 157 166. Coggeshall, R.E., Coulter, J.D. and Willis, W.D., Unmyelinated axons in the ventral roots of the cat lumbosacrat enlargement, J. Comp. Neurol., 153 (1974) 39-58. Coggeshall, R.E. and Ito, H., Sensory fibres in ventral roots L7 and SI in the cat, J. Physiol. (London), 267 (1977) 215--235. Hosobuchi, Y., The majority of unmyelinated afferent axons in human ventral roots probably conduct pain, Pain, 8 (1980) 167 180. Kim, J, and Chung, J.M., Electrophysiological evidence for the presence of fibers in continuity between dorsal and ventral roots in the cat, Brain Res., 338 (1985) 355-359. Light, A.R. and Metz, C.B., The morphology of spinal cord efferent and afferent neurons contributing to the ventral roots of the cat, J. Comp. Neurol., 179 (1978) 501-516. Maynard, C.W., Leonard, R:B., Coulter, J.D. and Coggeshall, R.E., Central connections of ventral root afferents as demonstrated by the H R P method, J. Comp. Neurol., 172 (1977) 601-608. Risling, M,, Dalsgaard, C.J., Cukierman, A. and Cuello, A.C., Electron microscopic and immunohistochemical evidence that unmyelinated ventral root axons make U-turns or enter the spinal pia mater, J. Comp. Neurol., 225 (1984) 53-63. Risling, M. and Hildebrand, C., Occurrence of unmyelinated axon profiles at distal, middle and proximal levels in the ventral root L7 of cats and kittens, J. Neurol., Sci., 56 (1982) 219-231. Voorhoeve, P.E. and Nauta, J., Do nociceptive ventral root afferents exert central somatic effects'? In J.J. Bonica, U. Lindblom and A. Iggo (Eds.), Proceedings of the Third World Congress on Pain (Advances in Pain Research and Therapy, Vol, 5), Raven Press, New York, 1983, pp. 105- 110. Voorhoeve, P.E. and Zwaagstra, B., Central effects by ventral root nociceptive afferents, Exp. Brain Res., Suppl. 9 (1984) 116- 125.
353
Elsevier Scientific Publishers Ireland Lid.
NSL 03689
FLEXION REFLEX ELICITED BY V E N T R A L R O O T A F F E R E N T S IN THE CAT
H O N G KEE SHIN, JUN KIM and JIN MO C H U N G *
Marine Biomedical Institute and Department o l Physiolo&v and Biophysics, University (!l Texas Medical Branch, Galveston, T X 77550 (U.S.A.) (Received August 27th, 1985: Revised version received September 12th, 1985: Accepted September 19th, 1985}
Key aor&v
ventral root afferent flexion reflex cord compound action potential
tmmyelinated fiber
single unit activity
cat
spinal
To investigate the physiological role of ventral root afferent fibers, the ventral root was stimulated in spinal cats in an anempt to elicit the flexion reflex, a common nociceptive spinal reflex. The flexion reflex was elicited by stimulating the distal stump of the cut L7 or S I ventral root and recorded from the proximal stump of the cut SI ventral root. In 12 of the 14 cats studied, the flexion reflex was recorded as a compound action potential. The finding was confirmed by recording single-unit activity from 5 units in two cats. These results suggest that the ventral root afferent fibers exert a variety of physiological effects that include eliciting such spinal reflexes as the flexion reflex. The responsible fibers in the ventral root travel distally Iowards the dorsal root ganglion to enter the spinal cord through the dorsal root.
The fact that the spinal root in mammals contains a significant number of unmyelinated primary afferent fibers has become well-recognized in recent years [1, 6, 7]. Although there is evidence to suggest that some of these afferent fibers enter the spinal cord directly through the ventral root [9, 11, 12], most seem to travel distally towards the dorsal root ganglion and ultimately enter the cord through the dorsal root [13, 141. We have been investigating the physiological role of these ventral root afferent fibers. Stimulation of the distal stump of the cut ventral root in the cat excites dorsal horn cells carrying nociceptive information [3, 4]. The same stimulation also produces an increase in systemic arterial blood pressure [2]. The results of these studies suggest that the ventral root afferent fibers may play a wide variety of physiological roles. Therefore, the present study was undertaken to see if the ventral root afferent fibers elicit the spinal flexion reflex, which is one of the important central actions of primary afferent fibers. The experiments were conducted on 14 spinal cats (2.0 3.5 kg). Nine were anesthetized with c~-chloralose (60 mg/kg, i.v.) and 5 were decerebrated at the mid-collicular *Author for correspondence and reprint requests at: Marine Biomedical Institute, University of Texas Medical Branch, 200 University Boulevard, Galveston, TX 77550, U.S.A. 0304-394085:$ 03.30 © 1985 Elsevier Scientilic Publishers Ireland Ltd.
354
level under initial anesthesia with a mixture of nitrous oxide, oxygen and halothane. Animals were paralysed by gallamine triethiodide (Flaxedil, 4 mg/kg/h) infused through a cannula placed in the cephalic vein and artificially ventilated. The end-tidal CO2 level was maintained between 3.5 and 4.5~. Rectal temperature was kept near 3 7 C with an electrical heating blanket. After the lumbosacral spinal cord was exposed by a laminectomy, the cord was pinched with a pair of forceps at LI L2 level. The L7 ventral root was cut near the spinal cord. The S I ventral root was also cut midway between the spinal cord and the dorsal root ganglion. A mineral oil pool was made around the exposed spinal cord and the temperature of the pool was maintained near body temperature with a heating coil. The distal stump of the cut L7 or S 1 ventral root was placed on tripolar stimulating electrodes. The electrode nearest to the dorsal root ganglion was grounded to prevent current spread to adjacent neural tissues. Furthermore, the cord dorsum potential was monitored to check tbr current spread. The distal stump of the cut ventral root was stimulated electrically with a single pulse or a train of three pulses (50 Hz internal frequency), applied once every 10 s at a suprathreshold intensity for C fibers (0.5 ms, l0 mA). The flexion reflex was recorded from the proximal stump of the cut SI ventral root with bipolar recording electrodes. After the flexion reflex was recorded in the form of compound action potentials, the root was divided until single-unit activity could be recorded. The recorded activity was amplified and displayed on an oscilloscope. Single-unit activity was used to trigger a window discriminator from which output was fed into a computer to compile peristimulus time histograms. Both sides of 14 cats were tested. From at least one side of 12 cats, the flexion reflex could be recorded as a compound action potential from a fascicle of the proximal stump of the cut S I ventral root following stimulation of the distal stump of the cut S1 or L7 ventral root. Fig. I A - C shows 3 examples of such responses. The pattern of recorded compound action potentials varied widely from one fascicle to another and from one animal to another. As shown in Fig. I D, the flexion reflex elicited by L7 ventral root stimulation did not change appreciably after sectioning the sciatic nerve, suggesting that the reflex was not elicited indirectly by activation of receptors in the periphery due to excitation of m o t o r fibers following ventral root stimulation. The response however, disappeared after sectioning the L7 dorsal root. The same resuits were obtained from both sides in two cats. Furthermore, the reflex elicited by stimulation of the distal stump of the cut S 1 ventral root did not change after sectioning the S1 spinal nerve about I cm distal to the dorsal root ganglion. These results suggest that the fibers in the ventral root responsible for eliciting the flexion reflex travel distally toward the dorsal root ganglion and then enter the spinal cord through the dorsal root. Therefore. the conduction distance of the reflex was assumed to be the sum of the length of the ventral root from the stimulating electrodes to the dorsal root ganglion, the length of the dorsal root and the length of the ventral root from the spinal cord to the recording electrodes. The maximum conduction velocity of afferent fibers responsible for this flexion reflex was estimated from the conduction distance divided by the latency to the beginning of the reflex after subtraction of the
355
D ~
A
CONTROL
+ B
C
DR CUT
Fig. I. Examples of the flexion reflex recorded as a compound action potential. All recordings were made I¥om the proximal slump of the cut SI ventral root while stimulating the distal slump of the cut S1 or [,7 ventral root with a single square wave pulse (0.5 ms, 10 mA) at the time shown by the arrow. A D arc from 4 different experiments. A and B are single-sweep recordings of the flexion reflex elicited by stimulation of the SI and L7 ventral root, respectively. C is the averaged response from 10 consecutive stimulations of the SI ventral root repeated once every 10 s. In D, the control response elicited by stimulation of the L7 ventral root did not change after sectioning the sciatic nerve (PN CUT), but disappeared after the L7 dorsal root was cut (DR CUT). All time scale bars indicate 30 ms. Conduction distances in A D were 69, 77, 49 and 53 mm, respectively.
m i n i m u m c o n d u c t i o n t i m e (to a c c o u n t for the c e n t r a l d e l a y a n d the m o t o r fiber c o n d u c t i o n t i m e ) w h i c h was a s s u m e d to be the m i n i m u m l a t e n c y o f the reflex elicited by s t i m u l a t i o n o f the d o r s a l r o o t close to the spinal c o r d ( a b o u t 3 ms). T h e m a x i m u m c o n d u c t i o n v e l o c i t i e s c a l c u l a t e d this w a y f o l l o w i n g s t i m u l a t i o n o f the S I v e n t r a l r o o t in 19 cases r a n g e d f r o m 0.9 to 7.0 m / s [3.2_+2.2 m / s ( m e a n + S.D.)]. T h e p r o x i m a l s t u m p o f the S I v e n t r a l r o o t was d i v i d e d f u r t h e r until s i n g l e - u n i t a c t i v i t y c o u l d be r e c o r d e d . R e c o r d i n g s w e r e m a d e f r o m 5 single m o t o r a x o n s in t w o cats. Fig. 2 s h o w s a n e x a m p l e o f such an e x p e r i m e n t . P r e s t i m u l u s - t i m e h i s t o g r a m s w e r e c o m p i l e d f r o m the m o t o r a x o n d i s c h a r g e s in r e s p o n s e to 10 c o n s e c u t i v e stimuli a p p l i e d to the SI d o r s a l r o o t , S1 v e n t r a l r o o t a n d L7 v e n t r a l r o o t . S i n g l e - s w e e p r e c o r d i n g s o f the a c t i o n p o t e n t i a l are s h o w n in the insets o f c o r r e s p o n d i n g histog r a m s . A l t h o u g h the flexion reflex p r o d u c e d by v e n t r a l r o o t s t i m u l a t i o n was n o t as p o w e r f u l as t h a t p r o d u c e d b y d o r s a l r o o t s t i m u l a t i o n , definite reflex a c t i v i t y c a n be
356
SINGLE
STIMULATION
TRAIN
STIMULATION
S1 DORSAL
I
E 40
v1 30
3e ~
20
2e
E N T
101 e
s
i
o
~h I ....
'
'=~-
N 10
, 1. . . .
T
e
Time In Millimeconds
Time In Mi i i i$eoonds
E25!~'
S 1 VENTRAL
20
i
V
ii
ii i
NEl ,TIl S
0
T 100 200 300 Time In Milliseconds
400
See
~ 20.
T~-4
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I', f
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15
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*"
o
I
.
,
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+
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20
..
.
T
50o
s
500
r
15 1
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400
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L 7 VENTRAL ROOT E
E
v
V
l
-J.t
O'
0
1~e ~'~: ~ ~ Time In Milliseoond~
--
III
I
~ee
•
=,
~00
+4,+
Fig. 2. An example of the flexion reflex recorded as single-unit activity. Peristimulus-time histograms were compiled from the unit activity recorded from a fascicle divided from the proximal stump of the cut S1 ventral root after 10 consecutive stimulations of the S1 dorsal root, the distal stump of the cut S I ventral root and the distal stump of the cut L7 ventral root. Left and right panels show responses to stimulation with single shocks and a train of 3 pulses (internal frequency, 50 Hz), respectively. Note that some of the stimulus artefact triggered the window discriminator during SI ventral root stimulation. Bin width was set at 8 ms in all histograms. Insets show single sweeps of oscilloscope tracings of action potentials tbllowing each stimulus (time scale, 500 ms). Insets on the left panel also show the cord dorsum potentials produced by each stimulus (time scale, 50 ms).
seen following ventral root stimulation. The response was particularly apparent when the activity was temporally summed by stimulation with a train of 3 pulses. To insure against current being spread to adjacent tissue during ventral root stimulation, we always monitored the cord dorsum potential. Monitoring the cord dorsum potential was found to be a reliable method to check current spread in a previous study [4]. As the insets in Fig. 2 show, the cord dorsum potential could be recorded during dorsal root stimulation whereas it was absent during ventral root stimulation. Furthermore, previous studies [2, 4] already demonstrated that physiological effects produced by stimulating the distal stump of the cut ventral root were interrupted by pinching the root distal to the stimulating site and were restored by stimulation distal to the pinched site.
357 The present study together with previous studies [2-4] conducted in our laboratory have demonstrated that stimulation o f the afferent fibers in the ventral root o f the cat (1) activate the dorsal horn cells carrying nociceptive information, (2) raise systemic blood pressure and (3) produce a spinal flexion reflex. These physiological actions o f the ventral root afferent fibers, however, are exerted by the fibers traveling distally towards the dorsal root ganglion to enter the spinal cord through the dorsal root, not by the fibers that enter the spinal cord directly through the ventral root. Attempts to produce physiological effects by stimulating the proximal stump of the cut ventral root have failed [2, 4]. In addition, electrical stimulation o f the central stump o f the cut ventral root failed to elicit the flexion reflex, although some increase in discharges of m o t o r nerve fibers has been observed after algesic chemical injection into the systemic circulation in dorsal rhizotomized cats [15, 16]. The existence of ventral root afferent fibers that enter the spinal cord through the dorsal root has been supported further by showing the presence o f a large n u m b e r o f fibers in continuity between the dorsal and ventral root [10]. Although the fastest fibers in the ventral root eliciting the flexion reflex m a y be A6 fibers, the overall estimated conduction velocity suggests that most of them are unmyelinated. This is consistent with the results o f previous studies [2-4, 10]. The functional significance o f ventral root afferent fibers is not yet clear. One o f their potential functions would be nociception since the primary afferent fibers in the ventral root are reported to be largely somatic and visceral nociceptors [5, 8]. The results of the present study show clearly that stimulation of the distal stump o f the cut ventral root elicits the flexion reflex in much the same way as does activation o f the afferent fibers in the dorsal root. Therefore, our results support the idea that ventral root afl'erents can produce a wide variety of physiological action including a nociceptive spinal reflex. This work was supported by N I H G r a n t s NS21266, NS18830 and NS11255. We thank Heidi Freeborn for the art work and p h o t o g r a p h y . H.K.S. was supported in part by H a n y a n g University, College o f Medicine, Seoul, Korea. J.K. was supported in part by an alumni fund from Seoul National University College o f Medicine, Seoul, Korea. J.M.C. is the recipient o f a PHS Research Career Development Award NS00995. 1 Applebaum. Mi., Clifton, G.L., Coggeshall, R.E., Coulter, J.D., Vance. W.H. and Willis, W.D., Unrnyelinated fibres in the sacral 3 and caudal 1 ventral roots of the cat, J. Physiol. (London), 256 (1976) 557 572. 2 Chung, J.M., Kim, J. and Shin, H.K., Blood pressure response evoked by ventral root afferent fibres in the cat, J. Physiol. (London), in press. 3 Chung, J.M., Lee, K.H., Endo, K. and Coggeshal[, R.E., Activation of central neurons by ventral root afferents, Science, 222 (1983) 934~935. 4 ('hung, J.M., Lee, K.H., Kim, J. and Coggeshall, R.E., Activation of dorsal horn cells by ventral root stimulation in the cat, J. Neurophysiol., 54 (1985) 261 272. 5 Clifton, G.L., Coggeshall, R.E., Vance, W.H. and Willis, W.D., Receptive fields of unmyelinated ventral root afferent fibres in the cat, J. Physiol. (London) 256 (1976) 573 600. 6 Coggeshall, R.E., Appelbaum, M.L., Fazen, M., Stubbs. T.B. and Sykes, M.T., Unmyelinated axons
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