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Responses of spinal cord neurons to graded noxious and non-noxious stimuli
Brain Research, 64 (1973) 425-429
425
© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
Responses of spinal cord neurons to graded noxious and non-noxious stimuli
DONALD D. PRICE AND ANDREW C. BROWE Department o f Physiology, Medical College o f Virginia, Virginia Commonwealth University, Richmond, Ira. 23298 (U.S.A.)
(Accepted September 18th, 1973)
Although the responses of dorsal horn cells to cutaneous nerve stimulation and to some adequate stimuli have been well characterized, little is known concerning the responses of these cells to graded noxious stimulation a,~,8,9. It is important to consider the nociceptive responses of cells in all spinal cord laminae since it has been recently shown that cells in laminae I, IV-VI1 project to the contralateral thalamus in primates v and that many cells in laminae IV-VI project in the spinocervical tract in cats 2. Therefore, the present study was undertaken to characterize responses of spinal cord cells to graded noxious and non-noxious radiant heat, since these stimuli can be easily quantified and repeated. Data were collected from unanestbetized spinal cats (C1) initially prepared under sodium methohexital anesthesia, paralyzed with gallamine triethiodide, and maintained under adequate physiological conditions 9. A laminectomy exposed the lumbosacral enlargement. After the right hind leg was shaved and painted with India ink, 3-4 small thermistors were affixed with celloidin to various regions of the leg as required. Tungsten microelectrodes (5-20 M ~ ) were driven into the L7 or SI dorsal horn until postsynaptic unit activity was identified by the usual criteria 9,1°. Recording sites were measured along electrode tracts that were marked at the end by small lesions. These were histologically confirmed by directly photographing them in 50 # m unstained sections. Ninety-two spinal cord neurons in laminae IV-VII were characterized in terms of their responses to radiant heat, electrical stimulation of receptive fields, and 3 types of mechanical stimuli: (l) t o u c h - - hair movement and light stroking of skin, (2) p r e s s u r e - - compressing skin with flattened forceps, and (3) p i n c h - - with small serrated forceps. A Hardy-Wolf-Goodell dolorimeter calibrated in mcal/sq, cm/sec was used to elevate the temperature of 2 sq. cm areas of skin at various rates and to different extents. Skin temperatures at the thermistors were monitored via a Yellow Springs telethermometer onto a polygraph and oscilloscope. Electrical pulses were delivered via very small needle electrodes (2-5 mm separation) to the most excitatory portion of receptive fields. A shock intensity (35 V, 3 msec duration) was used that was sufficient to evoke both brief latency bursts of spikes and several hundred
426
SHORT COMMUNICATIONS
millisecond prolonged discharges in some dorsal horn cells. The latter was undoubtedly an effect of Ad and/or C fiber stimulationS, 9 and its presence or absence was noted for most neurons from which we recorded. Preparation of leg pockets and recording from nerves was not used during unit recording because such preparation increases the spontaneous discharge of dorsal horn cells and somewhat confounds their responses to physiological stimuli. This analysis yielded 5 classes of units distinguished by the range of mechanical stimuli over which they increased their firing frequency: (I) touch, (II) touch-pressure, (III) touch-pressure-pinch, (IV) pressure-pinch, and (V) pinch only. Class I can be subdivided into units that responded only to hair movement, units that responded only to light touch on non-hairy skin, and units that responded to both stimuli 1. The responses of these cells adapted with maintained compression of skin. Many of the touch sensitive cells responded optimally when the stimulus moved across the footpads and adapted rapidly to a static punctate stimulus. Class II cells responded with a higher frequency to steady compression of the skin than to hair movement or light touch and were therefore activated by slow adapting touch and/or pressure receptors. Cells in classes I and II were usually insensitive to heat or responded to skin temperatures below 43 °C, a temperature that is threshold for pain in normal humans (Table I). An example of class II cell's response to warming is shown in Fig. 1. Nearly all of class I and about one-half of class II units gave only brief discharges in response to electrical shocks to their receptive fields. The remainder responded with prolonged discharges (Table I). Seventy percent of class I and II cells were found in lamina IV. Cells in classes III-IV can be subdivided into those responding to warming, those responding differentially to warming and noxious skin temperatures, those responding only to noxious skin temperatures (43-50 °C), and those unresponsive to heat (Table I). One class V cell responded to noxious heat. Most cells in classes III-V responded to single electrical shocks to their receptive fields with brief latency bursts of spikes and with long latency ( > 200 msec) prolonged discharges that increased with each successive stimulus when intershock intervals were less than 3 secs,9. Therefore, it is likely that these units are activated by nociceptors supplied by Ad and C fibers, especially the C fiber polymodal noci-
TABLE I R E S P O N S E S OF S P I N A L N E U R O N S T O H E A T A N D T O E L E C T R I C A L S T I M U L A T I O N O F R E C E P T I V E F I E L D S
Unit category
I. II. III. IV. V.
Touch Touch-pressure Touch-pressure-pinch Pressure-pinch Pinch
No. o f units in category
% With threshold response between:
% Responding to shocks' to receptive field with:
35-42°C
43-50°C
1nit. burst only Prolonged discharge
19 15 31 22 5
0 33 19 14 0
5 13 50 41 20
95 53 6 9 20
5 47 94 91 80
427
SHORT COMMUNICATIONS
I00 90 80 7O
~~ ~ ~
2~ MCAL/CM2/SEC
6O
_~ 5o 4O
.~ s2Oo IO
34
40
45
SKIN TEMPERATURECa
50
oo1- T © ® 90, 80 70 60 j i 7 5 50
250 225 MCAL ~
40
so
~c.
IO I
"r
i
i
i
i
i
i
394041424344454647 SKINTEMPERATUREC°
I00 / I I I I I Nj 0 4 8 12 16 20 24 28 RESPONSELATENCY,SECONDS
Fig. 1. Representative responses of dorsal horn cells to radiant heat. A: firing frequencies of 6 units are plotted as a function of skin temperature, with the irradiation intensity held constant at 200 mcal/sq, cm/sec. The dorsal horn lamina in which each of these units were located is indicated at the right of each curve. The responses of 3 units to warming (34-42 ~C) declined as skin temperatures approached noxious levels. Note that the thresholds and slopes of the 3 units responding to noxious skin temperatures (43-50 °C) are different from each other. B: responses of a lamina V neuron to increases in skin temperature using 3 irradiation intensities. The thresholds and slopes are similar in all 3 cases. C: the relationship between irradiation intensity and the duration of heat stimulation required to excite 2 dorsal horn cells. Each point on both curves corresponds to a skin temperature close to 43 °C.
ceptors 3. H o w e v e r , class I I I a n d class IV units also a p p e a r to be activated by low t h r e s h o l d receptors supplied by large d i a m e t e r fibers. Some o f these cells were l o c a t e d in l a m i n a e IV a n d V I I b u t m o s t were in layers V - V I . T e m p e r a t u r e response ranges varied a m o n g these cells f r o m 2 to 8 C °. Cells with n a r r o w a n d wide s t i m u l u s - r e s p o n s e ranges are shown in Fig. 1. Responses o f cells r e s p o n d i n g to n a r r o w ranges o f skin t e m p e r a t u r e declined o r p l a t e a u e d when these ranges were exceeded. In m o s t cases wherein a cell r e s p o n d e d vigorously to n o x i o u s skin t e m p e r a t u r e s , its response to this stimulus was greater t h a n t h a t which c o u l d be elicited by i n n o c u o u s stimuli such as touch, pressure or warming. N o cells were f o u n d t h a t r e s p o n d e d to heat a n d n o t m e c h a n i c a l stimuli, a result also confirmed for l a m i n a I cells 3. This was true even when heat was used as a searching stimulus to find units. H e a t sensitive cells whose thresholds were within the nociceptive range (43 °C o r m o r e ) r e s p o n d e d at a given skin t e m p e r a t u r e i n d e p e n d e n t o f the rate o f h e a t transfer used to arrive at that t e m p e r a t u r e . This finding is illustrated by the skin t e m p e r a t u r e -
428
SHORT COMMUNICATIONS
39 4 0 41 42 43 4 4 45 46 47 48 4 9 50 THRESHOLD SKIN TEMR C°
Fig. 2. Histograms of response thresholds of 40 units to increases in skin temperature. In most cases the threshold is the average of 3 or more responses. However, response thresholds that were greater than 45 °C were determined from one trial since repeated exposure to these temperatures resulted in skin damage and lowering of response thresholds. firing frequency curves and the strength-duration curves of Fig. 1B and C. O f particular interest is that these irradiation intensity-duration curves are similar to those for pain threshold in normal humans and for the flexion reflex in spinal humans and cats 5. Skin temperature response thresholds that were clearly identified in 40 units are tabulated in Fig. 2. These thresholds were distributed continuously over a 36-50 °C range with 2 modes, one in the warming and the other in the nociceptive range. Therefore, as skin temperatures increase from 35 to 50 °C, there is in the dorsal horn a progressive recruitment of higher threshold units as well as increased firing in some lower threshold units. Fifty dorsal horn cells were antidromically tested for projection into D L C 4-5 segments above L7. A unit was considered to have responded antidromically to D L C stimulation if (a) a constant latency ( < 0.2 msec variation) single action potential followed a single D L C stimulus; (b) if unit responses followed stimulation up to 200/sec; (c) the waveform and amplitude of the action potential was identical to orthodromically elicited action potentials in the same unit. Twenty-two cells were considered antidromically activated. These included 10 cells in classes I - I I , 8 ceils in class III, and 4 cells in class IV. Some of these responded to warming and others responded to noxious heat in a characteristic manner described above. These results indicated a potential mechanism whereby a population of dorsal horn cells could signal the difference between an innocuous and a noxious stimulus by an increase in the number and types of units recruited and by an increased firing in wide dynamic range cells. Thus a noxious mechanical stimulus would excite cells responding exclusively as well as those cells responding differentially to this stimulus. A noxious heat stimulus would excite unique fractions of cells within classes I I I - V and might possibly excite some as yet unidentified cells responsive only to heat (Table I). Since there is evidence that cells in these various classes project into sensory tracts1,2,4,7, s those variables pertaining to the overall output of the dorsal horn are likely to be important in the coding of noxious events. This work was supported by a grant from the National Institute of Neurological Diseases and Stroke (NS 10251-01). We are grateful to Mr. Ed. Waldrip for his technical assistance,
SHORT COMMUNICATIONS
429
1 BROWN,A. G., AND FRANZ, D. N., Responses of spinocervical tract neurons to natural stimulation of identified cutaneous receptors, Exp. Brain Res., 7 (1969) 231-249. 2 BRYAN, R. N., TREVINO, D. L., COULTER, J. D., AND WILLIS, W. D., Location and somatotopic organization of cells of origin of the spino-cervical tract, Exp. Brain Res., 17 (1973) 177-189. 3 CHRISTENSEN,B. R., AND PERL, E. R., Spinal neurons specifically excited by noxious or thermal stimuli, J. Neurophysiol., 33 (1970) 293-307. 4 FETZ, E. E., Pyramidal tract effects on interneurons in the cat lumbar dorsal horn, J. Neurophysiol., 31 (1968) 69-80. 5 HARDY, J. D., Threshold of pain and reflex contraction as related to noxious stimuli, J. appl. Physiol., 5 (1953) 725-739. 6 PRICE, D. D., AND WAGMAN, 1. H., The physiological roles of A and C fiber input to the dorsal horn of M. mulatta, Exp. Neurol., 29 (1970) 383-399. 7 TAUB, A., AND BISHOP, P. O., The spinocervical tract: dorsal column linkage, conduction velocity, primary afferent spectrum, Exp. Neurol., 13 (1965) 1-21. 8 TREVINO,D. C., COULTER,J. D., AND WILLIS, W. D., Excitation of cells of origin of the spinothalamic tract in the monkey, Fed. Proc., 31 (1972) 940. 9 WAGMAN,I. H., AND PRICE, D. O., Responses of dorsal horn cells of M. mulatta to cutaneous and sural nerve A and C fiber stimuli, J. Neurophysiol., 32 (1969) 803-817. 10 WALL, P. D., The laminar organization of dorsal horn and effects of descending impulses, J. Physiol. (Lond.), 188 (1967) 403-424.
425
© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
Responses of spinal cord neurons to graded noxious and non-noxious stimuli
DONALD D. PRICE AND ANDREW C. BROWE Department o f Physiology, Medical College o f Virginia, Virginia Commonwealth University, Richmond, Ira. 23298 (U.S.A.)
(Accepted September 18th, 1973)
Although the responses of dorsal horn cells to cutaneous nerve stimulation and to some adequate stimuli have been well characterized, little is known concerning the responses of these cells to graded noxious stimulation a,~,8,9. It is important to consider the nociceptive responses of cells in all spinal cord laminae since it has been recently shown that cells in laminae I, IV-VI1 project to the contralateral thalamus in primates v and that many cells in laminae IV-VI project in the spinocervical tract in cats 2. Therefore, the present study was undertaken to characterize responses of spinal cord cells to graded noxious and non-noxious radiant heat, since these stimuli can be easily quantified and repeated. Data were collected from unanestbetized spinal cats (C1) initially prepared under sodium methohexital anesthesia, paralyzed with gallamine triethiodide, and maintained under adequate physiological conditions 9. A laminectomy exposed the lumbosacral enlargement. After the right hind leg was shaved and painted with India ink, 3-4 small thermistors were affixed with celloidin to various regions of the leg as required. Tungsten microelectrodes (5-20 M ~ ) were driven into the L7 or SI dorsal horn until postsynaptic unit activity was identified by the usual criteria 9,1°. Recording sites were measured along electrode tracts that were marked at the end by small lesions. These were histologically confirmed by directly photographing them in 50 # m unstained sections. Ninety-two spinal cord neurons in laminae IV-VII were characterized in terms of their responses to radiant heat, electrical stimulation of receptive fields, and 3 types of mechanical stimuli: (l) t o u c h - - hair movement and light stroking of skin, (2) p r e s s u r e - - compressing skin with flattened forceps, and (3) p i n c h - - with small serrated forceps. A Hardy-Wolf-Goodell dolorimeter calibrated in mcal/sq, cm/sec was used to elevate the temperature of 2 sq. cm areas of skin at various rates and to different extents. Skin temperatures at the thermistors were monitored via a Yellow Springs telethermometer onto a polygraph and oscilloscope. Electrical pulses were delivered via very small needle electrodes (2-5 mm separation) to the most excitatory portion of receptive fields. A shock intensity (35 V, 3 msec duration) was used that was sufficient to evoke both brief latency bursts of spikes and several hundred
426
SHORT COMMUNICATIONS
millisecond prolonged discharges in some dorsal horn cells. The latter was undoubtedly an effect of Ad and/or C fiber stimulationS, 9 and its presence or absence was noted for most neurons from which we recorded. Preparation of leg pockets and recording from nerves was not used during unit recording because such preparation increases the spontaneous discharge of dorsal horn cells and somewhat confounds their responses to physiological stimuli. This analysis yielded 5 classes of units distinguished by the range of mechanical stimuli over which they increased their firing frequency: (I) touch, (II) touch-pressure, (III) touch-pressure-pinch, (IV) pressure-pinch, and (V) pinch only. Class I can be subdivided into units that responded only to hair movement, units that responded only to light touch on non-hairy skin, and units that responded to both stimuli 1. The responses of these cells adapted with maintained compression of skin. Many of the touch sensitive cells responded optimally when the stimulus moved across the footpads and adapted rapidly to a static punctate stimulus. Class II cells responded with a higher frequency to steady compression of the skin than to hair movement or light touch and were therefore activated by slow adapting touch and/or pressure receptors. Cells in classes I and II were usually insensitive to heat or responded to skin temperatures below 43 °C, a temperature that is threshold for pain in normal humans (Table I). An example of class II cell's response to warming is shown in Fig. 1. Nearly all of class I and about one-half of class II units gave only brief discharges in response to electrical shocks to their receptive fields. The remainder responded with prolonged discharges (Table I). Seventy percent of class I and II cells were found in lamina IV. Cells in classes III-IV can be subdivided into those responding to warming, those responding differentially to warming and noxious skin temperatures, those responding only to noxious skin temperatures (43-50 °C), and those unresponsive to heat (Table I). One class V cell responded to noxious heat. Most cells in classes III-V responded to single electrical shocks to their receptive fields with brief latency bursts of spikes and with long latency ( > 200 msec) prolonged discharges that increased with each successive stimulus when intershock intervals were less than 3 secs,9. Therefore, it is likely that these units are activated by nociceptors supplied by Ad and C fibers, especially the C fiber polymodal noci-
TABLE I R E S P O N S E S OF S P I N A L N E U R O N S T O H E A T A N D T O E L E C T R I C A L S T I M U L A T I O N O F R E C E P T I V E F I E L D S
Unit category
I. II. III. IV. V.
Touch Touch-pressure Touch-pressure-pinch Pressure-pinch Pinch
No. o f units in category
% With threshold response between:
% Responding to shocks' to receptive field with:
35-42°C
43-50°C
1nit. burst only Prolonged discharge
19 15 31 22 5
0 33 19 14 0
5 13 50 41 20
95 53 6 9 20
5 47 94 91 80
427
SHORT COMMUNICATIONS
I00 90 80 7O
~~ ~ ~
2~ MCAL/CM2/SEC
6O
_~ 5o 4O
.~ s2Oo IO
34
40
45
SKIN TEMPERATURECa
50
oo1- T © ® 90, 80 70 60 j i 7 5 50
250 225 MCAL ~
40
so
~c.
IO I
"r
i
i
i
i
i
i
394041424344454647 SKINTEMPERATUREC°
I00 / I I I I I Nj 0 4 8 12 16 20 24 28 RESPONSELATENCY,SECONDS
Fig. 1. Representative responses of dorsal horn cells to radiant heat. A: firing frequencies of 6 units are plotted as a function of skin temperature, with the irradiation intensity held constant at 200 mcal/sq, cm/sec. The dorsal horn lamina in which each of these units were located is indicated at the right of each curve. The responses of 3 units to warming (34-42 ~C) declined as skin temperatures approached noxious levels. Note that the thresholds and slopes of the 3 units responding to noxious skin temperatures (43-50 °C) are different from each other. B: responses of a lamina V neuron to increases in skin temperature using 3 irradiation intensities. The thresholds and slopes are similar in all 3 cases. C: the relationship between irradiation intensity and the duration of heat stimulation required to excite 2 dorsal horn cells. Each point on both curves corresponds to a skin temperature close to 43 °C.
ceptors 3. H o w e v e r , class I I I a n d class IV units also a p p e a r to be activated by low t h r e s h o l d receptors supplied by large d i a m e t e r fibers. Some o f these cells were l o c a t e d in l a m i n a e IV a n d V I I b u t m o s t were in layers V - V I . T e m p e r a t u r e response ranges varied a m o n g these cells f r o m 2 to 8 C °. Cells with n a r r o w a n d wide s t i m u l u s - r e s p o n s e ranges are shown in Fig. 1. Responses o f cells r e s p o n d i n g to n a r r o w ranges o f skin t e m p e r a t u r e declined o r p l a t e a u e d when these ranges were exceeded. In m o s t cases wherein a cell r e s p o n d e d vigorously to n o x i o u s skin t e m p e r a t u r e s , its response to this stimulus was greater t h a n t h a t which c o u l d be elicited by i n n o c u o u s stimuli such as touch, pressure or warming. N o cells were f o u n d t h a t r e s p o n d e d to heat a n d n o t m e c h a n i c a l stimuli, a result also confirmed for l a m i n a I cells 3. This was true even when heat was used as a searching stimulus to find units. H e a t sensitive cells whose thresholds were within the nociceptive range (43 °C o r m o r e ) r e s p o n d e d at a given skin t e m p e r a t u r e i n d e p e n d e n t o f the rate o f h e a t transfer used to arrive at that t e m p e r a t u r e . This finding is illustrated by the skin t e m p e r a t u r e -
428
SHORT COMMUNICATIONS
39 4 0 41 42 43 4 4 45 46 47 48 4 9 50 THRESHOLD SKIN TEMR C°
Fig. 2. Histograms of response thresholds of 40 units to increases in skin temperature. In most cases the threshold is the average of 3 or more responses. However, response thresholds that were greater than 45 °C were determined from one trial since repeated exposure to these temperatures resulted in skin damage and lowering of response thresholds. firing frequency curves and the strength-duration curves of Fig. 1B and C. O f particular interest is that these irradiation intensity-duration curves are similar to those for pain threshold in normal humans and for the flexion reflex in spinal humans and cats 5. Skin temperature response thresholds that were clearly identified in 40 units are tabulated in Fig. 2. These thresholds were distributed continuously over a 36-50 °C range with 2 modes, one in the warming and the other in the nociceptive range. Therefore, as skin temperatures increase from 35 to 50 °C, there is in the dorsal horn a progressive recruitment of higher threshold units as well as increased firing in some lower threshold units. Fifty dorsal horn cells were antidromically tested for projection into D L C 4-5 segments above L7. A unit was considered to have responded antidromically to D L C stimulation if (a) a constant latency ( < 0.2 msec variation) single action potential followed a single D L C stimulus; (b) if unit responses followed stimulation up to 200/sec; (c) the waveform and amplitude of the action potential was identical to orthodromically elicited action potentials in the same unit. Twenty-two cells were considered antidromically activated. These included 10 cells in classes I - I I , 8 ceils in class III, and 4 cells in class IV. Some of these responded to warming and others responded to noxious heat in a characteristic manner described above. These results indicated a potential mechanism whereby a population of dorsal horn cells could signal the difference between an innocuous and a noxious stimulus by an increase in the number and types of units recruited and by an increased firing in wide dynamic range cells. Thus a noxious mechanical stimulus would excite cells responding exclusively as well as those cells responding differentially to this stimulus. A noxious heat stimulus would excite unique fractions of cells within classes I I I - V and might possibly excite some as yet unidentified cells responsive only to heat (Table I). Since there is evidence that cells in these various classes project into sensory tracts1,2,4,7, s those variables pertaining to the overall output of the dorsal horn are likely to be important in the coding of noxious events. This work was supported by a grant from the National Institute of Neurological Diseases and Stroke (NS 10251-01). We are grateful to Mr. Ed. Waldrip for his technical assistance,
SHORT COMMUNICATIONS
429
1 BROWN,A. G., AND FRANZ, D. N., Responses of spinocervical tract neurons to natural stimulation of identified cutaneous receptors, Exp. Brain Res., 7 (1969) 231-249. 2 BRYAN, R. N., TREVINO, D. L., COULTER, J. D., AND WILLIS, W. D., Location and somatotopic organization of cells of origin of the spino-cervical tract, Exp. Brain Res., 17 (1973) 177-189. 3 CHRISTENSEN,B. R., AND PERL, E. R., Spinal neurons specifically excited by noxious or thermal stimuli, J. Neurophysiol., 33 (1970) 293-307. 4 FETZ, E. E., Pyramidal tract effects on interneurons in the cat lumbar dorsal horn, J. Neurophysiol., 31 (1968) 69-80. 5 HARDY, J. D., Threshold of pain and reflex contraction as related to noxious stimuli, J. appl. Physiol., 5 (1953) 725-739. 6 PRICE, D. D., AND WAGMAN, 1. H., The physiological roles of A and C fiber input to the dorsal horn of M. mulatta, Exp. Neurol., 29 (1970) 383-399. 7 TAUB, A., AND BISHOP, P. O., The spinocervical tract: dorsal column linkage, conduction velocity, primary afferent spectrum, Exp. Neurol., 13 (1965) 1-21. 8 TREVINO,D. C., COULTER,J. D., AND WILLIS, W. D., Excitation of cells of origin of the spinothalamic tract in the monkey, Fed. Proc., 31 (1972) 940. 9 WAGMAN,I. H., AND PRICE, D. O., Responses of dorsal horn cells of M. mulatta to cutaneous and sural nerve A and C fiber stimuli, J. Neurophysiol., 32 (1969) 803-817. 10 WALL, P. D., The laminar organization of dorsal horn and effects of descending impulses, J. Physiol. (Lond.), 188 (1967) 403-424.