Bram Research, 507 (1989) 69-84

69

Elsevier BRES 15122

Physiological characteristics of responses of wide dynamic range spinal neurones to cutaneously applied vibration in the cat M W Salter ~ 2

*

a n d J L H e n r y ~-3

Departments of ;Physiology, -'Research m Anaesthesia and ~Ps~chtatrv, McGtll University, Montreal Que (( anada) (Accepted 13 June 1989)

Ke~ words Spinal dorsal horn, Afferent inhibition Vibration, Somatosensorv system, W~de dynamic range neuron

Extracellular single-unit recordings were made from w~de dynamic range neurones in the lumbar dorsal horn of anaesthetized or decerebrated cats V,bratmn applied to the skin at a frequency of 80 Hz could evoke 3 distract types of response - - excitation depression or a blphaslc response consisting of exotatlon followed by depression By applwng wbrat,on at different sites a given neurone was found to show more than one type of response Parametric studies of the depressant and blphas~c responses were made because previous studies indicated that adenosine mediates the depression in these types of response Thus, amphtude- and frequency-response relatlonsh,ps were determined at md,vxdual stimulation sites amphtude was vaned from 0 001 to 1 0 mm (frequency, 80 Hz) and the frequencies studied were 10, 20 40, 80, 120 and 240 Hz (amphtude, 0 15 mm) Vibration at amplitudes greater than 0 15 mm caused a decrease in the rate of discharge during the period of stimulation the magmtude of this decrease varying directly with amplitude, at amplitudes of I) 15 mm and less v,bratlon had no stat~stlcaUy sigmficant effect With regard to the frequency-response relationship a decrease m discharge rate occurred at frequencms of 120 and 240 Hz with the more pronounced effect at 240 Hz, excitation occurred at 40 Hz and there was no statlst~call) significant effect at other frequencies Amphtude- and frequency-response relauonshlps for the depressant and the blphaslc responses were analyzed separately In the case of depressant responses the magmtude was monotomcally related to the amphtude of snmulatlon and depression occurred only at frequencies of 80 Hz or greater, with higher frequenoes being more effective The blphaslc responses appeared to consist of 2 subtypes termed b~phas~c-1 and blphaslc-2 responses For blphaslc-1 responses, the amplitude- and frequency-response curves were similar to those of depressant responses Blphaslc-2 responses differed m that the response was b~phaslc when the stimulation frequency was 80 Hz or greater and the amphtude was 0 3 mm or more yet, at lower frequenoes and/or amplitudes vibration evoked excitation The s,mllarmes m the amphtude- and frequency-response relationships of depressant and b~phaslc-1 responses raise the posslbd~tv that these responses might be mediated b} a single class of primary afferent Both depressant and blphaslc responses were evoked when stlmulauon parameters (2/*m 240 Hz) were used which selectively activate Paclman corpuscle afferents '9 2~, Depression with 240-Hz stimulation was attenuated by administration of caffeine (60 mg/kg l v ) suggesting that the depressant and blphaslc-1 responses may be medmted b} afferents from Paclman corpuscles INTRODUCTION

n e u r o n e s 42 a~ a n d t h e p u r p o s e of t h e p r e s e n t r e p o r t is to describe the physiological properties of these responses

V ~ b r a t l o n - l n d u c e d r e s p o n s e s o f n e u r o n e s in t h e p e riphery, the dorsal column nuclei and the cerebral cortex have been

characterized

in d e t a i l a n d s u m m a r i z e d

in

In p a r t i c u l a r ,

we

report

the effects ot systematically

varying the amplitude and frequency of the vibrational stimulation

D i f f e r e n t t y p e s of p r i m a r y a f f e r e n t r e s p o n d

r e w e w s b y D a n a n - S m l t h 12 a n d M o u n t c a s t l e ~4 F e w s t u d -

p r e f e r e n t i a l l y t o m e c h a n i c a l s t i m u l i of specific a m p l i t u d e s

les, h o w e v e r , h a v e m v e s t ~ g a t e d t h e r e s p o n s e s ot n e u r o n e s m t h e d o r s a l h o r n 21 '~2 T h e r e f o r e , we h a v e m a d e a

a n d f r e q u e n c i e s ( f o r r e v i e w s s e e B u r g e s s a n d Perl 6 a n d

detaded investigation of the responses of these neurones

a large range of amplitudes and ~requencles evidence

to v i b r a t i o n a l s t i m u l a t i o n

Wide dynamic range (WDR)

D a r l a n - S m i t h i 2 ) , a n d it was a n t i c i p a t e d t h a t b y s t u d y i n g might be obtained

on

the type

or types

of p r i m a r y

n e u r o n e s w e r e t h e f o c u s o f this s t u d y b e c a u s e t h e y h a v e

afferent mediating the responses

b e e n l m p h c a t e d as m e d i a t i n g a n u m b e r o f r e s p o n s e s t o

tigation tocuses on depressant and btphaslc responses,

peripheral stimulation including responses to noxious c u t a n e o u s s t i m u h in experimental a n i m a l s 14 ~8 ~9 S4 a n d

because

also p o s s i b l y in h u m a n s 32

be the chemical

We

have

reported

elsewhere

neuropharmacoioglcal

investigations on vibration-reduced responses of WDR

only

noclcepttve

The parametric inves-

neurones

exhibit

these

re-

s p o n s e s 4~ a n d b e c a u s e e v i d e n c e exists t h a t a d e n o s i n e m a y mediator

at t h e

s p i n a l level o l t h e

depression and the depressant component ot the blphaslc r e s p o n s e 42

* Present address Playfair Neurosclence Umt, The Toronto Hospital, 399 Bathurst St , Toronto Ont Canada MST 2S8 J L Henry Department of Physiology McIntvre Me&cal Soences Bulld,ng 3655 Drummond Street Montreal Que Canada H3G IY6

Correspondence

70

MAI ERIALS AND ME'IHODS

Animal preparation E x p e r i m e n t s were done on 59 adult cats 57 were anaesthenzed, 55 with s o d m m pentobarb~tal (40 mg/kg, ~ p supplemental doses 5 mg/kg l ~ every 3 h) and 2 w~th a-chloralose (60 mg/kg, i v ) and 2 were anaemlcally decerebrated u n d e r anaesthesm induced b~ halothane ( S o m n o t h a n e , Hoechst) which was subsequently discontinued T h r o u g h o u t the experiments the carotid arterial blood pressure was continuously &splayed on a Grass P5 polygraph The m e a n arterial pressure was maintained above 80 m m Hg w~th intravenous infusion of 1 0 ~ dextran (Macrodex, Pharmacta) or n o r a d r e n a h n c b~tartrate (Levophed, Winthrop, 0 002% m normal sahne) if necessar) Spinal s e g m e n t s L5 to L7 were surgically exposed for recording and were covered w~th warm mineral od to prevent drying and coohng In all e x p e r i m e n t s the spinal cord was transected at the hrst l u m b a r level to remove descending influences and to chromate thc poss~bd~t) that the v~branon-mduced responses of lumbar dorsal horn n e u r o n e s m~ght be m e d m t e d wa suprasplnal structures Prior to the transection 0 1 ml of 1 0% hdocame hydrochlonde were rejected into the L1 s e g m e n t to mmtm~ze spinal shock A f t e r bdateral p n e u m o t h o r a x , the a m m a l s were paralyzed w~th p a n c u r o n l u m bromide (Pavulon O r g a n o n , l mg/kg , v repeated w h e n reqmred) and ~entllated amficmlly End-tidal CO~ concentration was maintained between 3 5% and 5 0% Rectal temperature was m a m t a m e d at 38 °C w~th a servo-controlled mfrared bulb Peno&cally spinal cord c~rculat~on was m o m t o r e d wsuallv using a &ssectmg stereomlcroscope

Recording and data acqutsttton Single u m t spikes were recorded extracellularly w~th multlbarrelied glass mlcroplpettes (overall n p d m m e t e r 4 - 8 k~m) A solution of 2 7 M NaCI was placed m the central recording barrel 0 m p e d a n c e 4 - 8 MI2 m e a s u r e d at 100 Hz) The raw data were amphfied, &splayed on oscdloscopes and recorded on magnetic cassette tape and on film T h e rate of &scharge was continuously &splayed on a Grass P5 polygraph Intervals between spikes and between s n m u h were c o m p u t e d to __.0 5 ms using an I B M personal c o m p u t e r and hardware and software developed m our laboratory 4z These intervals were stored on hard d~sk In a few cases attempts were m a d e to determine the central delay for the response to w b r a n o n In these cases, a salver ball electrode was placed on the dorsal root entry zone and the cord dorsum potentml ~8 was m o m t o r e d The central delay was calculated as the ttme between the afferent volley m the cord d o r s u m potentml and the start of the response to v~bratmn The approximate conduction velocity for the afferents which apparently m e d m t e d the earhest parts of the responses was calculated as 300 m m / ( l a t e n c y - 2 ms) Th~s &stance was the average from the toe to the L7 s e g m e n t m e a s u r e d by laymg a thread along the path of the peripheral nerve m several ammals Two m d h s e c o n d s were subtracted to account for synapnc delay (1 ms) and for the apparent delay (1 ms) caused by peripheral a c n v a n o n of the primary afferents which ,s described m the text O n e ol the outer barrels of the m~crop~pette contained a solunon of s o d i u m k-glutamate (1 M, p H 7 4 S~gma) Only umts exc,ted by iontophoretlc application of glutamate were stu&ed to e h m m a t e recordings from fibres T h e exc~tabd~ty of selected neurones was increased b) a p p h c a n o n of glutamate to m v e s n g a t e the effect of the level of excitability on the response to v,bratton Leakage of glutamate between apphcat~ons was minimized by a retaining current (10 nA) T h e neurones were classified funcnonally on the basis of responses to natural non-v~braaonal st~muh40 Each neurone included m the present study was considered to be a W D R on the basis that ~t was excited both by noxious and by non-noxious c u t a n e o u s s n m u l a n o n Importantly the response to v~branon was not used as a criterion m the classff~canon For each neurone the excitatory receptive fields for hair m o v e m e n t , hght touch and

I)O~(IOUS pinch ~cre r~prescnt~d on a schematic diagram ol Ih~ hmdhmb In some cases b~polar needle e]cctrodes ,~cre m~t.rted into the receptive field and the response to electrical stimulation o! thc receptive field was determined In 4 experiments the superflcml peroneal or t~b~al nerves were exposed at the ankle and ~er~ stimulated electrically using hook electrodes

Vtbrattonal ~ttrnulatton Vlbranon was generated by a Ieedback-controlled mechamcal stimulator ") to which was attached the stimulator probe The probe consisted of a plexlglass rod 5 c m m length and 10 m m m &amet~r squared off at the ends To test the response of a gwen u m t to ~lbranon the probe was p o s m o n e d to contact the skin so that a constant force of up to 500 mN was apphed during the periods between v~brat~on Care was taken to avoid p o s m o n m g the probe directly over m a j o r peripheral ner,,es The parameters of the vibrational stimulation which were investigated systemahcally were the location of the s n m u l a t o r probe (relatwe to the excitatory receptwe fields for non-vibrational stlmuh) and the a m p h t u d e - and f r e q u e n c y - r e s p o n s e relanonshlps V~branon was apphed for periods of 2 5 - 3 5 s, at regular intervals every 20-25 s, tram d u r a n o n and r e p e t m o n rate were constant for a given umt T h e m m a l morion of the probe was always towards the h m b To ensure that the probe & s p l a c e m e n t was stable, it was m o m t o r e d during the testing wa an output from the control u m t of the m e c h a m c a l s n m u l a t o r W h e n the effects of varying the location ot the s n m u l a t o r probe were m v e s n g a t e d , the m e a n s of driving the s n m u l a t o r was ~denncal to that used previously 42 43 the m e c h a m c a l stimulator was driven by a Grass $88 stimulator which produced rectangular voltage pulses (6 ms d u r a n o n ) at a frequency of 80 Hz Th~s was the m a x i m u m frequency at which the probe returned completely to the rest p o s m o n after each pulse Using these parameters the peak &splacement v a n e d hnearly w~th the input voltage (0 1 m m / V to a m a x i m u m of 1 0 m m ) The transducer of the m e c h a m c a l s n m u t a t o r acted as a low-pass filter (-3 dB at 85 Hz) Thus, the d~splacement of the probe was roughly c u r v d m e a r (see for example Fig 3) It is ~mportant to note that for the remainder of this paper 'receptwe field' wdl be used only with respect to non-vibrational s n m u h Th~s specific use of 'recepnve field' is made because of the well estabhshed use of this term w~th respect to non-vJbranonal stlmuh s'~ ~s and because the non-vibrational recephve fields were used as references when assessing the effects of varying the p o s m o n of the s n m u l a t o r probe The effect of vlbratmn when the s n m u l a t o r probe was placed ms,de the receptive field for non-wbratlonal s n m u h was compared with the effect when the probe was placed outside this receptwe field W h e n the probe was place reside a recepnve field containing foot or toe pads, the p o s m o n was often adjusted so that the probe contacted both hairy and glabrous skin A m p h t u d e - r e s p o n s e relatmnshlps were e x a m i n e d using 80 H z as the frequency of stlmulanon Stlmulanon a m p h t u d e s of 0 05-1 0 m m were routinely uUhzed although m some cases amplitudes as small as 0 001 m m were used W h e n the f r e q u e n c y - r e s p o n s e relanonsh~ps were investigated the snmulatlon consisted of continuous smuso~ds and the s n m u l a t o r was driven by the computer Effects of s n m u l a n o n at frequencies of 10 20, 40 80 120 and 240 Hz were stu&ed routinely and m some cases l and 5 Hz were also used A s high frequency inputs were attenuated by the transducer of the mechamcal s n m u l a t o r , tt was necessary to increase the a m p h t u d e of the voltage smusolds when trequencles of greater than 80 Hz were examined m order to maintain constant peak-to-peak a m p h t u d e of the displacement In a d d m o n , at frequencies greater than 80 Hz the s n m u l a t o r probe faded to return to the control p o s m o n between the cycles causing an olfset m the basehne p o s m o n during the s n m u l a t m n The greatest offset (0 13 m m ) occurred with stlmulatmn at 240 Hz Thus when the f r e q u e n c y - r e s p o n s e relanonshlps were investigated a step &splacemcnt was made concurrently w~th the v~branon so that a constant basehne offset of 0 13 m m occurred ,11 all frequencws

71

Data analysts

never observed

The net effect of vibration was determined by measuring the number of spikes during the period of each stimulation and comparing this number with the number of spikes during the control period The control period which had the same duration as the period of the stimulation was the period immediately preceding each stimulation The rationale for choosing the control period preceding each stimulation was to control for possible variation m the magnitude of the response depending on the rate of firing The percentage change in the number ot spikes during the period of vibration compared with the control period was calculated for each stimulation The average percentage change was determined using at least 5 consecuttve stimulations Statistical sigmficance was determined using the paired t-test or the sign test, as appropriate Means of percentage change for groups of umts were compared using a two-tailed t-test or the Mann-Whitney U-test, as appropriate Effects were considered statistically significant for P < 0 0g Numerical ~alues are reported as the mean +_1 S E M

d i s c h a r g e d e c r e a s e d to b e l o w t h e c o n t r o l level i m m e d i -

In f a c t , w i t h 6 n e u r o n e s

the rate ot

ately after the pertod of vibrational stimulation

This

p o s t - v t b r a t i o n a l d e p r e s s i o n r e c o v e r e d g r a d u a l l y a n d for m o s t n e u r o n e s full r e c o v e r y o c c u r r e d w i t h i n less t h a n 1 s

Depressant response D u r i n g t h e d e p r e s s a n t r e s p o n s e (Fig

2 B ) , t h e d i s c h a r g e r a t e r a p i d l y r e a c h e d its l o w e s t

level a n d t h e n e i t h e r r e m a i n e d m a x i m a l l y d e p r e s s e d o r

2O

b,o[

RESULTS R e c o r d i n g s f r o m 197 W D R

neurones are included m

t h e s e r e s u l t s N o d i f f e r e n c e w a s a p p a r e n t in t h e e f f e c t s of vibration on neurones

recorded

in a n a e s t h e t i z e d c o m -

pared with unanaesthetmzed animals and hence data from t h e 3 p r e p a r a t i o n s are c o n s i d e r e d t o g e t h e r

B

I Three types of response to vtbratlon Vibration

at a f r e q u e n c y o f 80 H z a n d a m p l i t u d e s

g r e a t e r t h a n 0 2 m m was u s e d to classify t h e r e s p o n s e s b e c a u s e t h e s e w e r e t h e p a r a m e t e r s u s e d in o u r n e u r o p h a r m a c o l o g i c a l s t u d i e s 42 4a P S T H s w e r e u s e d to e x a m lne

the

response

to

vibration

in d e t a t l

because

the

n e u r o n e s o f t e n f i r e d i r r e g u l a r l y in t h e a b s e n c e o f stimulation

F r o m a n a l y s i s of P S T H s it was c l e a r t h a t t h e r e

w e r e 3 d i s t i n c t t y p e s o f r e s p o n s e to v i b r a t i o n at 80 H z

C

excitation, depression and a blphaslc response (excitation followed

by depression)

By applying the vibrational

s t i m u l a t i o n at d i f f e r e n t loci o n t h e l p s i l a t e r a l h i n d h m b a given neurone could be induced to show more than one type ot response stimulation

However,

when

the parameters

and the location of the probe

were

of

held

c o n s t a n t o n l y o n e t y p e of r e s p o n s e was o b s e r v e d

Excttatorv response V i b r a t i o n - i n d u c e d e x c i t a t i o n (Fig 1 A ) a p p e a r e d to c o n s i s t o f 3 p a r t s burst which was followed

t h e r e was a n initial

by a period of diminished

e x c i t a t i o n l a s t i n g u p t o 100 m s (in o n e - t h t r d o f cases t h e l o w e s t d t s c h a r g e r a t e d u r i n g this p e r i o d was less t h a n t h e c o n t r o l l e v e l ) , d u r i n g t h e r e m a i n i n g p e r i o d t h e r a t e of d t s c h a r g e p e a k e d a n d t h e n d e c r e a s e d slowly o r s t a y e d at t h e p e a k level t h r o u g h o u t t h e r e m a i n d e r o f t h e v i b r a t i o n The level ot diminished excitation or depression was not r e l a t e d t o t h e n u m b e r o f s p i k e s d u r i n g t h e initial b u r s t I n a n u m b e r o f c a s e s t h e s p i k e d i s c h a r g e was e n t r a i n e d to t h e s t i m u l a t i o n d u r i n g t h e t h i r d p a r t o f t h e r e s p o n s e (not shown) U p o n t e r m i n a t i o n o f v i b r a t i o n a n a f t e r d l s c h a r g e was

Fig 1 Excitatory, depressant and blphaslc responses ot 3 different WDR neurones Each record shows a PSTH compiled from 10 consecutive applications of vibration, the period o! vibration is indicated by the line below each record For each PSTH the bm width ~s 20 ms but notice that the vertical scale for A is dltferenI from that in B and C The receptive fields for noxious and innocuous mechanical stimuli for each of the neurones are represented in the diagrams to the right of each PSTH For A and B the neurones were excited by low intensity mechanical stimuli (hair movement and light touch) applied to the blackened areas and by noxious pinching in both the blackened and hatched areas For the neurone illustrated in C the receptive fields for touch and pinch were coincident and these are shown bv the blackened area The PSTH in A illustrates the excitator2¢ response and the stimulator probe was positioned inside the 1o,~ threshold receptive field as indicated by the arrow in the dmgram to the right, the arrow indicates the orientation of the long axis of the probe as well as its position For B the depressant response, and C, the blphaslc response, the stimulator probe was placed outside the receptive fields at the position indicated by the x The x in this and subsequent figures also signifies that the long axis of the probe was perpendicular to the plantar aspect of the foot For B and C the amplitude of stimulation was 0 '~ mm and it was 0 3 mm m the case of A

72 i n c r e a s e d shghtly d u r i n g the r e m a i n d e r of w b r a t l o n two-thirds

of cases,

the

rate

of discharge

In

remamed

A1

B1

C1

A2

B2

C2

d e p r e s s e d a l t e r the p e r i o d ol s t i m u l a t i o n and g r a d u a l l y r e t u r n e d to the c o n t r o l level, the d e p r e s s i o n typically lasted less t h a n 2 s after the v i b r a t i o n e n d e d , a l t h o u g h as l o n g as 9 s was r e q u i r e d for c o m p l e t e r e c o v e r y

In the

r e m a i n i n g o n e - t h i r d of the cases, w b r a t ~ o n - m d u c e d dep r e s s i o n was f o l l o w e d by an excitation which d e c a y e d to the c o n t r o l level w i t h m 500 ms

Blphastc response This type of r e s p o n s e consisted ot a

short-latency

A

excitation

~"

lollowed

by

depression

4

a

iI

a

B

a

b

/__

±~ +

.

~ +

Fig 3 Comparison of the latencies of excitatory and blphaslc responses Film records as shown from 3 different WDR neurones (A.B,C) Each record is a single oscilloscope sweep, starting at the beginning of the vibrational stimulation which continued beyond the end of the traces A 1 and BI illustrate responses to single electrical stlmuh dehvered at the time indicated by the arrow above each trace A 2 B e, C~ and C~ show responses to vibration The displacement output of the mechanical stimulator ts displayed below the corresponding trace, an upward deflection indicates movement of the probe towards the foot Calibration bars horizontal 4 ms, vertical 150/~V for recording traces and 0 5 mm for mechanical displacement For the neurone m A both the elecmcal and mechanical stimuli were applied to the receptive field for lowintensity mechanical stimulation The electrical stimulus (0 5 mA) was 1 1 × threshold and the mechanical stimulus (0 4 ram) was 1 1 × threshold The remainder of the response to v~bratlon indicated that this was an excitatory rather than biphaslc response (not shown) In B the electrical stimulus (3 0/~A, 1 5 × threshold) was dehvered to the common peroneal nerve and the mechanical stimulus was apphed 2 cm distal to the ankle, outside the excitatory receptwe fields The burst m B 2 was followed by depression and hence this was considered to be a biphaslc response For C vibration evoked excitation when applied reside the receptive field C~ When apphed outside the receptive field Cz, vlbranon evoked a biphasic response with the latency of the excitatory component being slmdar to that of the excitatory response shown above

t h r o u g h o u t the r e m a i n d e r of the p e r i o d of v i b r a t i o n a l stimulation

(Fig

1C)

With

different

neurones

and

d~fferent p a r a m e t e r s of s t i m u l a t i o n the e x c i t a t o r y c o m p o n e n t r a n g e d f r o m a single spike to a burst (for e x a m p l e

÷J

\

Fig 3Cz) w h i c h lasted up to 20 ms 4-

+ + ÷ 44- + Fig 2 Response to vibration depends upon the site of stimulation A each film record shows a single osedloscope sweep, wbratton was applied dunng the period indicated by the bar above each record The upper record was taken with the probe in posiuon 'a' and the lower record in position 'b' When the stimulator probe was placed outside the low threshold receptive field vibration caused a burst (more clearly seen In Fig 3C2) followed by a depression of firing The receptwe fields for meehameal st~muh are illustrated as m Fig 1A The stamulatlon amphtude was 0 5 mm Calibration bars for the film records vertical 250 /aV, horizontal 3 s B for 3 different neurones the diagrams illustrate the receptive fields and indicate the responses to vibration at various stimulaUon sites The following scheme is used to show the recepUve fields excitation by hair movement, blackened area, excitation by light touch, stippled areas, excitation by noxious pinch, hatched, stippled and blackened areas, and inhibition by noxious pinch, dashed areas The arrows show the site of snmulat~on and the symbols +, - or _+ correspond to excitation, depression and b~phasic response, respectively Note that for these dmgrams only the arrows do not indicate the orientation of the probe In all cases the stimulation amplitude was 0 5 mm

D u r i n g the e n s u i n g

d e p r e s s a n t c o m p o n e n t of the blphaslc r e s p o n s e , as was the case for the d e p r e s s i o n , remained shghtly

at the

the discharge rate e i t h e r

lowest level o r g r a d u a l l y i n c r e a s e d

F o l l o w i n g the p e r i o d o f v i b r a t i o n the rate ot

discharge g r a d u a l l y r e c o v e r e d o r t h e r e was p o s t - v i b r a t i o n excitation

Effect of varying the site of apphcatton of vtbratmg probe T h e m o s t i m p o r t a n t f a c t o r in d e t e r m m m g the t y p e of r e s p o n s e to v i b r a t i o n a p p e a r e d to b e the l o c a t i o n of the p r o b e r e l a t i v e to the e x c i t a t o r y r e c e p t i v e fields for low t h r e s h o l d stimuli W i t h the p r o b e p o s i t i o n e d Inside the low t h r e s h o l d r e c e p t i v e field the r e s p o n s e o b s e r v e d m o s t f r e q u e n t l y was e x c i t a t i o n (Fig

2Aa) w h e r e a s v i b r a t i o n

characteristically c a u s e d e i t h e r d e p r e s s i o n o r t h e blphaslc r e s p o n s e (Fig 2Ab) w h e n the p r o b e was p l a c e d o u t s i d e this r e c e p t i v e field

73 By examining a n u m b e r ot sites of stimulation (Fig 2B) it was found that excitation was also e v o k e d when the vibration was applied n e a r the low threshold receptive fields On the other hand. depressant or biphaslc responses were elicited from sites at a distance from these receptive fields The exact distance that the p r o b e had to be placed outside the receptive field varied considerably from n e u r o n e to n e u r o n e Nonetheless. greater than 90¢~ of the neurones exhibited depressant or biphaslc responses from at least one site of stimulation Fhese two types of response could be e v o k e d even if the stimulation site was inside an excitatory receptive field for noxious stimuli (Fig 2B,) Latency of responses The vibration-induced excitation began 8-15 ms (mean 9 6 + 0 8 ms. n = 10) after the onset of the stimulation (conduction velocity for earliest response 50 m s -1) In the cases m e a s u r e d , the central delay was 1 2-1 5 ms (n = 3) The initial burst of the excitatory response occurred at a longer latency than the latency of the excitation due to electrical stimulation of the receptive field (for example c o m p a r e Fig 3 A l and 3A2) This finding is consistent with activation of peripheral receptors by vibration The latency for the excitatory c o m p o n e n t of the blphasic response was similar to the latency ot the excltatton (tor e x a m p l e compare Fig 3C 1 and 3C2) In addition, the latency of the excitatory c o m p o n e n t of the blphaslc response to vibration was longer than the latency of the excitation caused by electrical stimulation of p e r i p h e r a l nerves (for example c o m p a r e Fig 3B~ and 3B2) Depression was o b s e r v e d as early as 10 ms (conduction velocity 37 5 in s ~) after the onset of vibration (Fig 4). the shortest central delay which could be d e t e r m i n e d with accuracy was 2 - 3 ms Thus. the m i n i m u m values for latency and for central dela~ were both greater with depression than with excitation or with the excitatory c o m p o n e n t of the blphaslc response A f t e r the beginning of the depression the rate of discharge decreased abruptly to its lowest level the decline from the control to the lowest level ol discharge sometimes occurred in less than 1 ms

Control e:~pertments In each of 4 experiments it was found that responses to vibration were abolished by transection of one or more peripheral nerves at the level of the ankle ipsllateral to the site of recording Repetitive electrical stimulation of the central, cut end of the nerves m i m i c k e d the ettect of vibration Thus. the effects of vibration were not due to m o v e m e n t of the electrode at the site ot recording Vibration causes depression of glutamate-evolved acttvt 0' The n e u r o n e shown in Fig 5 was silent in the absence of overt stimulation and lontophoretlc application ot

8

10 ms

b'n-~ O

I

Fig 4 Latency ol the onset oi ~lbratlon-lnduc~d depression For a neurone which was depressed by ~lbranon the PSTH illustrates the periods just before and after the start o! snmulaton ~IhlsPSFH was compiled from 200 sequennal response~ and the rate o! discharge is shown as the number of spikes per l-ms bm ~Ih~ snmulator probe was placed at the posLnon indicated b~ the arrow in the diagram to the right The displacement output from the mechanical stimulator is dlustrated belo~ the PSTH The amphtude of snmulanon was 0 4 mm The receptive fields for noxious pinch and tor innocuous mechamcal stlmuh are represented in the diagram using the ~ame scheme as m Fig 1A

glutamate was used to evoke an on-going discharge In this case two amplitudes (0 2 and 0 5 mm) were examined and the rate of discharge was systematically varied by adjusting the ejection current for glutamate A t both a m p h t u d e s vibration caused depression of the glutamatee v o k e d discharge and for each a m p h t u d e there was a statistically significant correlation between the n u m b e r ot spikes during the control p e r i o d and the n u m b e r during the vibration The slopes ot the regression lines were significantly different consistent with the visual impression from the PSTHs that the 0 5 mm a m p h t u d e was more effective In all cases tested (n -- 54). cutaneously applied vibration blocked the discharge either e v o k e d or increased by glutamate This b l o c k a d e of activity was observed both during the d e p r e s s a n t response and during the depressant c o m p o n e n t ot the blphaslc response

H Parametric data lor responses evoked when the stimulator probe was positioned outside the receptive fzeld Jot low threshold non-vibrational sttmuh ~ompamon o] effects of vartou~ amphtudes o] sttrnulatton The effect ol varying stimulation amplitude was investigated systematically with 24 neurones. 7 of these exhibited vibration-induced depression and 17 showed the blphaslc response W h e n examining the a m p l i t u d e response relationship the frequenc~ ot stimulation was 8(1 Hz Results tor one n e u r o n e are illustrated in Fig 6 The film records in Fig 6 illustrate that the n u m b e r ol spikes during the p e r i o d of stimulation decreased progressively when the amplitude was increased from (I 1 to 0 5 mm The a m p h t u d e - r e s p o n s e curve m the graph in Fig 6B shows that the average decrease In the n u m b e r of spikes during the period of stimulation was greatest with an amplitude of 0 5 mm and that the magnitude ot the decrease was m a i n t a i n e d when the stlmuhitlon a m p h t u d e

74

20-

2s

02mm

t

0.2rnm

1(

(

sptkes during 10 wbratlon

NKmm

,is spikes during period before wbrat,on Fig 5 Vibration-reduced depression of discharge evoked by glutamate For this neurone activity was evoked by lontophoreuc election of glutamate and the level of firing was t~trated by varying the ejectton current In the graph the number of spikes during wbrat~on is plotted versus the number during the control period for 20 stimulations using a stimulation amphtude of either 0 2 (triangles) or 0 5 (circles) mm Least-squares regression lines are shown (0 2 mm, r = 0 63, 0 5 mm, r = 0 78) Note that at both amplitudes wbrat~on caused depression at all firing levels, although the larger amphtude ol stimulation produced significantly greater depression (analysis of covanance F t 3b " ~ 6 0, P = 0 02) PSTHs corresponding to the 2 stimulaton amplitudes are shown at the right, the penod of vibration is indicated by the bar below each PSTH The receptive fields for noxious and innocuous mechanical stimulation are illustrated as in Fig 1A

w a s i n c r e a s e d u p to 1 0 m m

A s i l l u s t r a t e d by t h e r e s e t

in t h e u p p e r r i g h t , t h e r e s p o n s e t o w b r a t l o n was b t p h a s l c

t e r m s o f t h e p e r c e n t a g e c h a n g e (Ftg n u m b e r o f s p i k e s (Fig

7A) and the total

7B) dunng the vibration

From

24

t h e p o i n t s o n t h e y-ax~s m t h e g r a p h m F i g 7B ~t c a n b e

n e u r o n e s a r e i l l u s t r a t e d in Fig 7, t h e d a t a a r e s h o w n m

calculated that the basehne rate of firing of the different

lndwldual

amphtude-response

A 0.1

curves

B I!I

from

the

O

¸ ii I -21

°2 i 0.3

I

-4

-4(; ¢D c a: - 6 0 (J

-8(

0.5 -100 0

"

0'.2

0'.4

0~6

0~8

1~0

Amplitude (mm) Fig 6 Comparat2ve effects of vartous amphtudes of stimulation for an mdwldual neurone A each film record shows a single response to vibration (80 Hz) applied dunng the period indicated by the bar above the upper trace The stimulation amplitude, in ram, is mdlcated to the left of each trace Cahbration bars vertical, 300/~V, horizontal, 3 s B graph dlustratmg average percentage change as a function of amplitude of sUmulatlon Each point represents the percentage change m the number of spikes during the entire period of stimulation, averaged for 10 trials Insets (lower left) in the diagram the receptive fields for noxious and innocuous mechanical st~muh are dlustrated using the same scheme as Fig 1A, (upper right) PSTH constructed using 10 trials with the stimulation amphtude of 0 3 mm, the period of v l b r a t l ~ is indicated by the bar below the histogram The vertical scale is 10 spikes bin J and the time bar represents 2 s The bm width for the PSTH is 10 ms

75

in the a m p l i t u d e - r e s p o n s e curves were not correlated

Cornpartson of effect~ of vartous ]requencles of sttmulatton F r e q u e n c y - r e s p o n s e curves were constructed tor

with differences in the baseline rates of discharge

22 neurones For the n e u r o n e shown in Fig 8 t r e q u e n c y -

n e u r o n e s varied from 2 5 to 105 spikes

s 1 Differences

A v e r a g e s of the individual a m p l i t u d e - r e s p o n s e curves for p e r c e n t a g e change and for total n u m b e r of spikes are plotted in Fig

7C

These average curves indicate that

response

curves

were

made

using

stimulation, 0 15 and 0 60 mm

2 amplitudes

of

The smaller stimulation

amplitude was the m a x i m u m amplitude which could be

vibration caused a decrease in the n u m b e r of spikes

generated

which was graded with respect to stimulation amplitudes

amplitude was chosen because this was m the range

greater than 0 05 m m

where, on the basis of the population response

The increase in the average

n u m b e r of spikes observed using 0 05 mm stimulation

at

a frequency

of 240

Hz

The

greater the

percentage change using a trequency of 80 Hz was

failed to be statistically significant (t = 1 5, P > 0 10)

expected to be most negative

This increase was due to excitation of a subpopulatlon of

vibration tailed to have a statistical b significant effect at

neurones which exhibited the blphaslc response as will be

frequencies of 1, 5, 10 20 and 40 Hz

With the greater

amphtude

decrease

described in section 1II By interpolating between points it was calculated that,

At both

a statistically significant

amplitudes,

m the

n u m b e r ot spikes during the stimulation was observed at

on average, the n u m b e r of spikes during the vibration

80 Hz and at the additional frequency examined in this

was decreased 50% when the stimulation amphtude was

case, 60 Hz With the smaller amplitude, the percentage

0 45 mm, the modal value was 0 40 mm

change at 80 Hz was less negative than [or the greater

With respect to the excitatory response to vibration,

amplitude

and

maximum

depression

w~th 0 15 mm

the responses were graded and the magnitude of the

stimulation occurred only with frequencies of 120 and 240

excitation increased with increasing stimulation ampli-

Hz

tude (tor example see Fig 12)

Individual f r e q u e n c y - r e s p o n s e cur~es for responses of 22 neurones are shown in Fig

A loo

B

If

50~l '

amphtude was 0 15 mm

response had a unique f r e q u e n c y - r e s p o n s e relationship 300 200

'

H o w e v e r , in all cases the percentage change was most negative for frequencies of 120 or 240 Hz and was much

100

less negative (and

- , 6'

9A, m each case the

The curves illustrate that each

spikes during vibration

in some cases p o s m v e )

frequencies, especially 40 Hz or below

at lower

For the average

of the whole sample (Fig 9B) only the points for 120 and

50

240 Hz were significantly less than zero, with the greater

1.o

Amplitude (ram)

Amphtude (ram)

i

B'--~ C

-50

-100 ]

spikes -20 durmg vJbrat;on

0

02

04 06 A m p h t u d e (ram)

0

0

F~g 7 Amplitude-response curves for snmulatlon at 80 Hz Indlwdual cur~es for responses of 24 neurones are Illustrated m the graph in A which shows the percentage change and m the graph in B which shows the a,,erage number of spikes during the period ot vlbranon In A and B the right-hand end of each line indicates the maximum amplitude tested Averages ot the lndlwdual curves for percentage change (circles) and for number of spikes (triangles) are represented in C Fhe upper limit of the graph in C is 0 8 mm because only 60Q of the neurones were tested using greatcr amplitudes and this proportion was considered too small to allow rehable calculation of the average

1

5

10 20 40 Frequency (Hz)

80 120 240

Fig 8 Comparative effects ot various trequencles ol stimulation tot one neurone A a semllogarlthmlc graph =s shown m which percentage change is plotted ~s trequenc~ ot stimulation for amplitudes of (I 6 mm (dots) and I) 15 mm (triangles) The points lndmate the average percentage change calculated using 5 trials For each point the average percentage change v,as compared with 0uslngthet-test ** P < 0 0 1 *** P

Physiological characteristics of responses of wide dynamic range spinal neurones to cutaneously applied vibration in the cat.

Extracellular single-unit recordings were made from wide dynamic range neurones in the lumbar dorsal horn of anaesthetized or decerebrated cats. Vibra...
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