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Presynaptic depolarization produced by and in identified cutaneous afferent fibres in the rabbit
SHORT COMMUNICATIONS
187
Presynaptic depolarization produced by and in identified cutaneous afferent fibres in the rabbit J~img e t a l 4 have presented ewdence to suggest that m the cat's spinal cord there are two specific systems which generate presynaptlc depolarization m mechanoreceptive cutaneous afferent fibres These are (1) a 'tomc' system actwated mainly from slowly adapting receptors and which mainly depolarizes axons innervating slowly adapting receptors, and (2) a 'phasic' system activated by and mainly depolarizing axons innervating rapidly adapting receptors There was some overlap of the systems Thus the 'tonic' system seemed to depolarize some axons of rapidly adapting hair follicle receptors and the 'phasic' system to depolarize axons of slowly adapting foot pad receptors An opportunity to test these ideas m a d~fferent way and on a different species has arisen In the rabbit, electrical stimulation o f the sural (SU) nerve, m the pophteal fossa, at stimulus strengths up to 1 3 times threshold (1 3 T) excites a pure population of axons of slowly adapting afferent umts which are almost exclusively type 1 slowly adapting umts 1 At stimulus strengths of 1 3-1 5 T type T hair follicle umts are recruited into the volley and above 1 5 T type G hair follicle units are also excited 1 The cutaneous branches of the posterior tlblal nerve (medml plantar, MP) contain no type 1 slowly adapting umts or type T hmr follicle units Over 92 ~ of the myehnated axons having conduction velocities greater than the Adt range innervate type G hair folhcle receptors 1 Electrical stimulation of the MP nerve at strengths below those which excite A6 fibres will excite a population of axons which innervates predominantly rapidly adapting receptors The experiments were performed on rabbits anaesthetized with either a mixture of 109/o urethane and 1 ~o chloralose (w/v in normal sahne), 7 ml/kg, or 2 5 ~ thlopentone sodium, 30 rng/kg initially, both given intravenously Anaesthesia was maretamed with rejections of either 25 ~ urethane or 2 5 ~ th~opentone respectively, as reqmred Arterial blood pressure was monitored and maintained above 70 mm Hg by intravenous dextrans if necessary The SU and MP nerves were exposed through small skin incisions and placed on stimulating and/or recording salver-salver chloride electrodes and covered with low melting-point paraffin wax Ingolng afferent volleys from the SU and MP nerves were monitored with a monopolar silver ball electrode at the dorsal root entrance zones, dorsal root potentials (DRPs) were recorded from the central parts of cut filaments of either the L7 or $1 rootlets and the excltabdlty of the terminal axons of either the SU or MP nerves was tested by Wall's method 6 using a tungsten mlcroelectrode as the stimulating cathode in the dorsal horn All electrical stlmuh were 0 2 msec square waves Conditioning and testing stlmuh to peripheral nerves consisted of 5 shocks at 200 c/sec Electrical stimulation of the SU and MP nerves produced DRPs and the corresponding P waves on the cord dorsum at lumbosacral levels As m the cat2, 5, the amplitude of the DRPs increased with increasing stimulus strengths There were differences, however, between the DRPs ehclted from the two nerves (Fig 1A) DRPs from the SU nerve exhibited a very steep relationship between amplitude and stimulus Brain Research, 38 (1972) 187-192
188
SHORT COMMUNICATIONS 100 SC 6C
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6
4
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Shmulus strength In multiples of threshold for the whole nerve
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BoS
.~
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~
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-.~-123MP~123MP
20 100
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100
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' 360
CondptKx~ng- testing =nterval (ms)
Fig 1 A, Growth of DRPs with increasing strength of stamulatlon of periphelal cutaneous nerves DRP amphtude, expressed as a percentage of the maximum amphtude, is plotted against the sttmulus strength m mult]ples of nerve threshold for SU and MP nerves The relationship for the SU nelve is much steeper than for the MP nerve and most of the DRP produced by SU stamulataon is due to actw~ty to type 1 afferent umts which are exclusively excited by stimulus strengths below 1 3 T The open and filled symbols,ndicat¢ different experiments B, C, Interactions of DRPs produced by actwlty in ~dentlfied afferent umts The araphtudes of the test DRPs, expressed as a percentage of the control value, are plotted agaanst the conchtmmng-testmg intervals for the nerves at the strengths indicated The SU stimulus produces a pure type 1 volley and the MP stimulus a type G volley DRPs produced by the separate inputs interact showing that some afferent fibres are depolarized by both inputs
strength Between 65 and 95 70 of maximum DRP amphtude had been reached by stimulus strengths of 1 3 T and maximum amplitude was reached at less than 4 T In other words type 1 slowly adapting afferent units were responsible for a very large proportion of the DRP produced by stimulation of the SU nerve In contrast the growth of DRPs produced by MP stzmulatlon was more gradual, 50 70 of maximal amplitude being attained at about 1 5 T (type G hmr folhcle units) and maximum amphtude was not reached until the stimulus strength was about 10 0 T, and therefore excited all of the group II axons For both nerves, further increase of stimulus strength after maximum DRP amplitudes had been attained produced a second peak on the dechnlng phase of the DRP and prolonged the time course beyond 100 msec DRPs produced by selective electrical stimulation of axons of type 1 slowly adapting umts and type G hair follicle units were interacted In Fig 1B, C the amplitudes of test DRPs are plotted against the conditioning-testing intervals If the systems which generate presynaptlc depolarization m response to inputs from these two types of mechanoreceptors are completely independent m terms of both input and Brain Research, 38 (1972) 187-192
SHORT COMMUNICATIONS
1110
189
A - o - SU ~ MP .-e- MP ~ MP
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180 "-e" MP ~ SU - 0 - SU-- SU
160
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Fig 2 ExcRabdRy changes m cutaneous afferent fibre terminals produced by actwlty m identified afferent fibres A and C show the excltabshty changes produced m the terminals of type G umts by volleys m either the SU or MP nerve No changes are produced by actwlty m type 1 umts, which are excited by SU stlmuh of less than 1 3 T Increasing the SU stimulus to excRe type T and G umts (above I 3 T) produces increases m the exc~tabdRy of the type G terminals Condmonmg w~th volleys in type G umts of the MP nerve (1 0 T upwards) leads to increases m excltabdRy B and D show the excltabflRy changes m the fastest SU axons (mnelvatlng maanly type 1 receptors) after condmonmg with volleys m eRher SU or MP nerves The most marked effects from MP occur at about 3 T or more and there ss httle action at less than 1 5 T (except for the filled orcles in D) Actwlty m type 1 umts (less than 1 3 T SU m B) ISvery effectwe in increasing the excltablhty of the fastest SU axons A and B are from the same experiment C shows the sudden increase m excltabdlty of type G axons produced at a SU stimulus whmh actwates type T umts D shows the extremes of exotab)hty changes recorded, m different experiments, m the fastest SU axons m lesponse to condRmnmg with type G volleys
output, then there should be complete independence o f the D R P s It can be seen, however, that the a m p h t u d e o f a test D R P is reduced at intervals less than 100 mse¢, the depression being m a x i m a l at 10-30 msec in different experiments If there are two systems for generating presynaptlc depolarization then at some stage the outputs o f the systems must converge to produce depolarization in s o m e afferent fibres m response to activation o f either type 1 or type G afferent umts Greater reductions in the test D R P a m p h t u d e (up to 70 ~o) were produced if type G units were used for c o n d i t i o n i n g D R P s o f type 1 umts (not illustrated) This suggests that activity in type G units m a y lead to depolarization o f the axons o f type 1 umts but not w c e versa W h e n the c o n d i t i o n i n g and testing volleys were in the same afferent fibres then there was m u c h more severe depression o f the test D R P s , as expected z If the S U stimulus was increased to excite axons o f types T or type T plus G units in addition to type 1 Brain Research, 38 (1972) 187-192
190
S H O R T ( OMMUNICATION,',,
then the size of a test D R P produced by activation of type G umts m the MP nerve was further depressed and the t~me course of the depression was prolonged but never to the extents shown when conditioning and testing volleys excited the same fibres The D R P experiments show that type 1 and type G inputs can actwate presynapt~c depolarizing systems and that there is lnteractton at some point In order to examine which afferent fibres are depolarized other methods are required We used the techmque of testing the excltabdlty of the afferent fibre terminals b The excitability of the terminals of the fastest MP axons (type G) was examined after condltiomng with volleys in either the SU or MP nerves Fig 2A, C show the excltabd~ty changes of the type G terminals plotted against the strength of the condltlonlng stimulus The excltabdlty was measured as the height of the first peak of the ant~drom~c compound action potentml produced m the MP nerve by stimulation m the dorsal horn at the site of maximum recorded negatw~ty corresponding to the peak of the P wave of the cord dorsum It was ~mportant to check that the latency of the first peak of the antldromlc MP potential ln&cated axonal conduction velocities of about 60 m/sec m order to confirm that there were no muscle afferent fibres m the MP nerve When con&tlomng with SU volleys, stlmuh below I 3 T, t e confined to type I umts, never altered the exc~tabfllty of the terminals of type G axons Act~wty m type 1 umts does not lead to presynaptlc depolarization of the axons of type G umts Excitability increases m the axons of type G umts were produced by SU volleys at 1 3 T or more m all experiments, m&catmg that input from type T umts can lead to depolarization of type G axons As the SU stimulus was increased to 4 0 T then greater increases m excltabdlty were produced showing that type G umts m the SU nerve were also effective m exciting the presynaptlc depolarizing system When con&tzonmg with MP volleys excitability increases of type G axons began at 1 0 T, 1 e w~th stimulation of type G axons (Fig 2A) The excltabdlty of the terminals of the fastest SU axons (mainly type 1) 1 was examined after con&tlonmg with either MP or SU volleys With MP volleys very httle action was usually observed at stimulus strengths below 1 5 T (Figs 2B, D, 3B), but at greater strengths progressively more increase m excltablhty occurred (F~gs 2B, D, 3B) which may have been due to the small number of axons innervating other than type G hair folhcle receptors It is not possible on the basts of the present experiments to be certain that the small increase m excltablhty of SU terminals produced by actwlty m type G umts was due to depolarization of type 1 axons On the one hand, when con&tlonlng with SU volleys (Figs 2B, 3C) much of the excltabdlty increase produced m the fastest SU terminals was due to actlwty m type 1 umts (stimulus strengths of 1 3 T and below), but, on the other hand, the D R P experiments showed some involvement of type G inputs m reducing D R P s produced by type 1 units (see above) The answer to th~s problem must await &rect mtra-axonal recording of primary afferent depolarization m ~dent~fied axons m response to stimulation of ~dent~fied cutaneous afferent umts The t~me courses of the excitability changes are shown m F~g 3 F~g 3A gives an example of the lack of action of type 1 umts (1 2 T SU) on type G axons and the effects of raising the con&t~onmg stimulus to include type T umts (1 33 T SU), which Brain Research, 38 (1972) 187-192
191
SHORT COMMUNICATIONS
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- •
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Fig 3 Time courses of exotab]hty changes produced by and m Identified cutaneous axons A and B are f r o m the same experiment The curves in C are all f r o m the same experiment The curves m D are f r o m different experiments F o r further description see the text
produced a clear increase m excltablhty of the type G axons but with a short time course (less than 50 msec) This short t~me course was characteristic of such actions and raising a SU c o n d m o m n g stimulus to 6 5 T only lengthened the time course to about 80 msec Longer time courses (up to 100 msec) were observed for the actmns of type G M P volleys on the fastest SU axons (Fig 3B) The longest t~me courses, as with D R P interactions, were seen when the c o n d m o n m g and testing were performed on the same nerve (Fig 3C, D) For the SU nerve, time courses in excess of 300 msec were seen even at strengths of condxtlonlng stimulation which only activated type 1 axons For the MP nerve the time course was about 200 msec The blphaslc nature of the excltabdlty changes seen m Fig 3C, D was only observed when the same nerve was used for both conditioning and testing The second peak may be due to recovery from postactwatlon depression produced by the conditioning volley The experiments reported m the present paper have shown that, m the rabbit, actwlty m type 1 slowly adapting cutaneous afferent units does not lead to presynaptlc depolarization of axons of type G hair folhcle afferent umts, whereas there is an extremely powerful action on axons of type 1 umts themselves Type G hair folhcle Brain Research, 38 (1972) 187-192
192
SHORI COMMUNICAIIONS
afferent units hkewlse have a powerful presynaptlc i n h i b i t o r y action on themselves These observations confirm the suggestion 4 that there are at least two systems in the spinal cord which generate presynaptlc depolarization m m e c h a n o r e c e p t w e afferent nerve fibres T h a t there ~s some interaction between the two systems was indicated in the D R P experiments This interaction may be due to e~ther (1) actw~ty m type G units leading to depolarization of type I axons, or (2) activity m type G a n d type 1 units leading to depolarization of some other p r i m a r y afferent fibres (perhaps g r o u p lb muscle afferent fibres3), or to both of these posslblhtles occurring s~multaneously This work was supported by a g r a n t from the Me&cal Research Council
Department o] Phystology, Royal (Dtck) School of Vetermary Studies, Umverslty of Edinburgh, Edmburgh EH9 1QH (Great Brttam)
A G BROWN*
R E HAYDEN**
1 BROWN,A G , AND HARDEN, R E , The distribution of cutaneous receptors m the rabbWs hind hmb and differential electrical stimulation of their axons, J Physzol (£ond). 213 (1971) 495-506 2 ECCLES,J C , KOSTYUK,P G , A~D SCHM1DT,R F , Central pathways responsible for depolarization of primary afferent fibres, J Phystol ([ond), 161 (1962) 237-257 3 ECCLES,J C , SCHMIDT,R F , ANDWILLIS,W D , Depolarization of central terminals of Group Ib afferent fibers of muscle, J Neurophyswl, 26 (1963) 1-27 4 JANIG, W , SCH~IDT, R F , ANDZIMMERMANN,M , Two spectfic feedback pathways to the central afferent terminals of phasic and tomc mechanoreceptors, Exp Brain Res, 6 (1968) 116-129 5 SCHMIDT, R F , SENGES, J , AND ZIMMERMANN, M , Excltabdlty measurements at the central terminals of smgle mechano-receptol afferents dunng slow potentml changes, Exp Bram Re~, 3 (1967) 220-233 6 WALL,P D , Excitability changes m afferent fibre terminations and their relation to slow potentmls, J Phystol (Lond), 142 (1958) 1-21 (Accepted December 22nd, 1971)
* Belt Memorial Research Fellow ** Commonwealth Umversltles' Scholar
Bram Research, 38 (1972) 187-192
187
Presynaptic depolarization produced by and in identified cutaneous afferent fibres in the rabbit J~img e t a l 4 have presented ewdence to suggest that m the cat's spinal cord there are two specific systems which generate presynaptlc depolarization m mechanoreceptive cutaneous afferent fibres These are (1) a 'tomc' system actwated mainly from slowly adapting receptors and which mainly depolarizes axons innervating slowly adapting receptors, and (2) a 'phasic' system activated by and mainly depolarizing axons innervating rapidly adapting receptors There was some overlap of the systems Thus the 'tonic' system seemed to depolarize some axons of rapidly adapting hair follicle receptors and the 'phasic' system to depolarize axons of slowly adapting foot pad receptors An opportunity to test these ideas m a d~fferent way and on a different species has arisen In the rabbit, electrical stimulation o f the sural (SU) nerve, m the pophteal fossa, at stimulus strengths up to 1 3 times threshold (1 3 T) excites a pure population of axons of slowly adapting afferent umts which are almost exclusively type 1 slowly adapting umts 1 At stimulus strengths of 1 3-1 5 T type T hair follicle umts are recruited into the volley and above 1 5 T type G hair follicle units are also excited 1 The cutaneous branches of the posterior tlblal nerve (medml plantar, MP) contain no type 1 slowly adapting umts or type T hmr follicle units Over 92 ~ of the myehnated axons having conduction velocities greater than the Adt range innervate type G hair folhcle receptors 1 Electrical stimulation of the MP nerve at strengths below those which excite A6 fibres will excite a population of axons which innervates predominantly rapidly adapting receptors The experiments were performed on rabbits anaesthetized with either a mixture of 109/o urethane and 1 ~o chloralose (w/v in normal sahne), 7 ml/kg, or 2 5 ~ thlopentone sodium, 30 rng/kg initially, both given intravenously Anaesthesia was maretamed with rejections of either 25 ~ urethane or 2 5 ~ th~opentone respectively, as reqmred Arterial blood pressure was monitored and maintained above 70 mm Hg by intravenous dextrans if necessary The SU and MP nerves were exposed through small skin incisions and placed on stimulating and/or recording salver-salver chloride electrodes and covered with low melting-point paraffin wax Ingolng afferent volleys from the SU and MP nerves were monitored with a monopolar silver ball electrode at the dorsal root entrance zones, dorsal root potentials (DRPs) were recorded from the central parts of cut filaments of either the L7 or $1 rootlets and the excltabdlty of the terminal axons of either the SU or MP nerves was tested by Wall's method 6 using a tungsten mlcroelectrode as the stimulating cathode in the dorsal horn All electrical stlmuh were 0 2 msec square waves Conditioning and testing stlmuh to peripheral nerves consisted of 5 shocks at 200 c/sec Electrical stimulation of the SU and MP nerves produced DRPs and the corresponding P waves on the cord dorsum at lumbosacral levels As m the cat2, 5, the amplitude of the DRPs increased with increasing stimulus strengths There were differences, however, between the DRPs ehclted from the two nerves (Fig 1A) DRPs from the SU nerve exhibited a very steep relationship between amplitude and stimulus Brain Research, 38 (1972) 187-192
188
SHORT COMMUNICATIONS 100 SC 6C
°Mp
4(
2C 2
6
4
¢o
8
Shmulus strength In multiples of threshold for the whole nerve
"" 100
BoS
.~
oC
~ r''''~-
O
80 60 40
A S ~
~
-°"lOgSU~129MP
-.~-123MP~123MP
20 100
200
100
200
' 360
CondptKx~ng- testing =nterval (ms)
Fig 1 A, Growth of DRPs with increasing strength of stamulatlon of periphelal cutaneous nerves DRP amphtude, expressed as a percentage of the maximum amphtude, is plotted against the sttmulus strength m mult]ples of nerve threshold for SU and MP nerves The relationship for the SU nelve is much steeper than for the MP nerve and most of the DRP produced by SU stamulataon is due to actw~ty to type 1 afferent umts which are exclusively excited by stimulus strengths below 1 3 T The open and filled symbols,ndicat¢ different experiments B, C, Interactions of DRPs produced by actwlty in ~dentlfied afferent umts The araphtudes of the test DRPs, expressed as a percentage of the control value, are plotted agaanst the conchtmmng-testmg intervals for the nerves at the strengths indicated The SU stimulus produces a pure type 1 volley and the MP stimulus a type G volley DRPs produced by the separate inputs interact showing that some afferent fibres are depolarized by both inputs
strength Between 65 and 95 70 of maximum DRP amphtude had been reached by stimulus strengths of 1 3 T and maximum amplitude was reached at less than 4 T In other words type 1 slowly adapting afferent units were responsible for a very large proportion of the DRP produced by stimulation of the SU nerve In contrast the growth of DRPs produced by MP stzmulatlon was more gradual, 50 70 of maximal amplitude being attained at about 1 5 T (type G hmr folhcle units) and maximum amphtude was not reached until the stimulus strength was about 10 0 T, and therefore excited all of the group II axons For both nerves, further increase of stimulus strength after maximum DRP amplitudes had been attained produced a second peak on the dechnlng phase of the DRP and prolonged the time course beyond 100 msec DRPs produced by selective electrical stimulation of axons of type 1 slowly adapting umts and type G hair follicle units were interacted In Fig 1B, C the amplitudes of test DRPs are plotted against the conditioning-testing intervals If the systems which generate presynaptlc depolarization m response to inputs from these two types of mechanoreceptors are completely independent m terms of both input and Brain Research, 38 (1972) 187-192
SHORT COMMUNICATIONS
1110
189
A - o - SU ~ MP .-e- MP ~ MP
•
180 "-e" MP ~ SU - 0 - SU-- SU
160
160
140
140
120
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Fig 2 ExcRabdRy changes m cutaneous afferent fibre terminals produced by actwlty m identified afferent fibres A and C show the excltabshty changes produced m the terminals of type G umts by volleys m either the SU or MP nerve No changes are produced by actwlty m type 1 umts, which are excited by SU stlmuh of less than 1 3 T Increasing the SU stimulus to excRe type T and G umts (above I 3 T) produces increases m the exc~tabdRy of the type G terminals Condmonmg w~th volleys in type G umts of the MP nerve (1 0 T upwards) leads to increases m excltabdRy B and D show the excltabflRy changes m the fastest SU axons (mnelvatlng maanly type 1 receptors) after condmonmg with volleys m eRher SU or MP nerves The most marked effects from MP occur at about 3 T or more and there ss httle action at less than 1 5 T (except for the filled orcles in D) Actwlty m type 1 umts (less than 1 3 T SU m B) ISvery effectwe in increasing the excltablhty of the fastest SU axons A and B are from the same experiment C shows the sudden increase m excltabdlty of type G axons produced at a SU stimulus whmh actwates type T umts D shows the extremes of exotab)hty changes recorded, m different experiments, m the fastest SU axons m lesponse to condRmnmg with type G volleys
output, then there should be complete independence o f the D R P s It can be seen, however, that the a m p h t u d e o f a test D R P is reduced at intervals less than 100 mse¢, the depression being m a x i m a l at 10-30 msec in different experiments If there are two systems for generating presynaptlc depolarization then at some stage the outputs o f the systems must converge to produce depolarization in s o m e afferent fibres m response to activation o f either type 1 or type G afferent umts Greater reductions in the test D R P a m p h t u d e (up to 70 ~o) were produced if type G units were used for c o n d i t i o n i n g D R P s o f type 1 umts (not illustrated) This suggests that activity in type G units m a y lead to depolarization o f the axons o f type 1 umts but not w c e versa W h e n the c o n d i t i o n i n g and testing volleys were in the same afferent fibres then there was m u c h more severe depression o f the test D R P s , as expected z If the S U stimulus was increased to excite axons o f types T or type T plus G units in addition to type 1 Brain Research, 38 (1972) 187-192
190
S H O R T ( OMMUNICATION,',,
then the size of a test D R P produced by activation of type G umts m the MP nerve was further depressed and the t~me course of the depression was prolonged but never to the extents shown when conditioning and testing volleys excited the same fibres The D R P experiments show that type 1 and type G inputs can actwate presynapt~c depolarizing systems and that there is lnteractton at some point In order to examine which afferent fibres are depolarized other methods are required We used the techmque of testing the excltabdlty of the afferent fibre terminals b The excitability of the terminals of the fastest MP axons (type G) was examined after condltiomng with volleys in either the SU or MP nerves Fig 2A, C show the excltabd~ty changes of the type G terminals plotted against the strength of the condltlonlng stimulus The excltabdlty was measured as the height of the first peak of the ant~drom~c compound action potentml produced m the MP nerve by stimulation m the dorsal horn at the site of maximum recorded negatw~ty corresponding to the peak of the P wave of the cord dorsum It was ~mportant to check that the latency of the first peak of the antldromlc MP potential ln&cated axonal conduction velocities of about 60 m/sec m order to confirm that there were no muscle afferent fibres m the MP nerve When con&tlomng with SU volleys, stlmuh below I 3 T, t e confined to type I umts, never altered the exc~tabfllty of the terminals of type G axons Act~wty m type 1 umts does not lead to presynaptlc depolarization of the axons of type G umts Excitability increases m the axons of type G umts were produced by SU volleys at 1 3 T or more m all experiments, m&catmg that input from type T umts can lead to depolarization of type G axons As the SU stimulus was increased to 4 0 T then greater increases m excltabdlty were produced showing that type G umts m the SU nerve were also effective m exciting the presynaptlc depolarizing system When con&tzonmg with MP volleys excitability increases of type G axons began at 1 0 T, 1 e w~th stimulation of type G axons (Fig 2A) The excltabdlty of the terminals of the fastest SU axons (mainly type 1) 1 was examined after con&tlonmg with either MP or SU volleys With MP volleys very httle action was usually observed at stimulus strengths below 1 5 T (Figs 2B, D, 3B), but at greater strengths progressively more increase m excltablhty occurred (F~gs 2B, D, 3B) which may have been due to the small number of axons innervating other than type G hair folhcle receptors It is not possible on the basts of the present experiments to be certain that the small increase m excltablhty of SU terminals produced by actwlty m type G umts was due to depolarization of type 1 axons On the one hand, when con&tlonlng with SU volleys (Figs 2B, 3C) much of the excltabdlty increase produced m the fastest SU terminals was due to actlwty m type 1 umts (stimulus strengths of 1 3 T and below), but, on the other hand, the D R P experiments showed some involvement of type G inputs m reducing D R P s produced by type 1 units (see above) The answer to th~s problem must await &rect mtra-axonal recording of primary afferent depolarization m ~dent~fied axons m response to stimulation of ~dent~fied cutaneous afferent umts The t~me courses of the excitability changes are shown m F~g 3 F~g 3A gives an example of the lack of action of type 1 umts (1 2 T SU) on type G axons and the effects of raising the con&t~onmg stimulus to include type T umts (1 33 T SU), which Brain Research, 38 (1972) 187-192
191
SHORT COMMUNICATIONS
A
160~
- •
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4
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~
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~V~.e,,~- e
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50
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Cond)tK~nmg
150
200
testing interval (ms)
Fig 3 Time courses of exotab]hty changes produced by and m Identified cutaneous axons A and B are f r o m the same experiment The curves in C are all f r o m the same experiment The curves m D are f r o m different experiments F o r further description see the text
produced a clear increase m excltablhty of the type G axons but with a short time course (less than 50 msec) This short t~me course was characteristic of such actions and raising a SU c o n d m o m n g stimulus to 6 5 T only lengthened the time course to about 80 msec Longer time courses (up to 100 msec) were observed for the actmns of type G M P volleys on the fastest SU axons (Fig 3B) The longest t~me courses, as with D R P interactions, were seen when the c o n d m o n m g and testing were performed on the same nerve (Fig 3C, D) For the SU nerve, time courses in excess of 300 msec were seen even at strengths of condxtlonlng stimulation which only activated type 1 axons For the MP nerve the time course was about 200 msec The blphaslc nature of the excltabdlty changes seen m Fig 3C, D was only observed when the same nerve was used for both conditioning and testing The second peak may be due to recovery from postactwatlon depression produced by the conditioning volley The experiments reported m the present paper have shown that, m the rabbit, actwlty m type 1 slowly adapting cutaneous afferent units does not lead to presynaptlc depolarization of axons of type G hair folhcle afferent umts, whereas there is an extremely powerful action on axons of type 1 umts themselves Type G hair folhcle Brain Research, 38 (1972) 187-192
192
SHORI COMMUNICAIIONS
afferent units hkewlse have a powerful presynaptlc i n h i b i t o r y action on themselves These observations confirm the suggestion 4 that there are at least two systems in the spinal cord which generate presynaptlc depolarization m m e c h a n o r e c e p t w e afferent nerve fibres T h a t there ~s some interaction between the two systems was indicated in the D R P experiments This interaction may be due to e~ther (1) actw~ty m type G units leading to depolarization of type I axons, or (2) activity m type G a n d type 1 units leading to depolarization of some other p r i m a r y afferent fibres (perhaps g r o u p lb muscle afferent fibres3), or to both of these posslblhtles occurring s~multaneously This work was supported by a g r a n t from the Me&cal Research Council
Department o] Phystology, Royal (Dtck) School of Vetermary Studies, Umverslty of Edinburgh, Edmburgh EH9 1QH (Great Brttam)
A G BROWN*
R E HAYDEN**
1 BROWN,A G , AND HARDEN, R E , The distribution of cutaneous receptors m the rabbWs hind hmb and differential electrical stimulation of their axons, J Physzol (£ond). 213 (1971) 495-506 2 ECCLES,J C , KOSTYUK,P G , A~D SCHM1DT,R F , Central pathways responsible for depolarization of primary afferent fibres, J Phystol ([ond), 161 (1962) 237-257 3 ECCLES,J C , SCHMIDT,R F , ANDWILLIS,W D , Depolarization of central terminals of Group Ib afferent fibers of muscle, J Neurophyswl, 26 (1963) 1-27 4 JANIG, W , SCH~IDT, R F , ANDZIMMERMANN,M , Two spectfic feedback pathways to the central afferent terminals of phasic and tomc mechanoreceptors, Exp Brain Res, 6 (1968) 116-129 5 SCHMIDT, R F , SENGES, J , AND ZIMMERMANN, M , Excltabdlty measurements at the central terminals of smgle mechano-receptol afferents dunng slow potentml changes, Exp Bram Re~, 3 (1967) 220-233 6 WALL,P D , Excitability changes m afferent fibre terminations and their relation to slow potentmls, J Phystol (Lond), 142 (1958) 1-21 (Accepted December 22nd, 1971)
* Belt Memorial Research Fellow ** Commonwealth Umversltles' Scholar
Bram Research, 38 (1972) 187-192