Cardiac responses to stimulation of discrete loci within canine sympathetic ganglia following hexamethonium

Journal of the Autonomt~ Nervous System, 11 (1984) 243-255 Elsewer

243

JAN 00386

Cardiac responses to stimulation of discrete loci within canine sympathetic ganglia following hexamethonium J C Brandys 1, D A Hopkins 2 and J A Armour 3 Departments of Surger~ ! Anatomy : and Phvstolog) and Btoph~st~s ~, Fatuity of Medt~me Dalhouwe Unn,ersttv, Hahfa-t, N S B3H 4H7 (Canada) (Received November 24th 1983) (Revised version received June 8th 1984) (Accepted June 17th 1984)

Key words sympathetic - cardiac - stellate ganghon - middle cervical ganghon medmstlnal ganglion

Abstract In dogs wtuch had been administered hexamethomum, locahzed stamulatlon with bipolar electrodes m the middle cervical and stellate ganglia as well as ganglia m the caudal pole nerve and ventral ansa e h o t e d alterations m cardlodynamlcs The middle cervical and stellate gangha were divided into 30 discrete regions m order to provide a standardized map of these gangha Of all the responses ehclted, 5% consisted of increased heart rate while 38% consisted of augmented motroplsm without accompanying chronotroplc changes Both rate and force were augmented In 57% of the responses The sites m which stimulation consistently ehclted chronotropic responses were located In right-sided ganglia Loci which consistently generated force changes on stimulation were in the middle cervical and stellate ganglia bilaterally Cardiac chronotroplsm and motroplsm were augmented by stimulation of ganglia located in the ventral subclavian ansae or the left caudal pole nerve No cardiac responses were elicited by stimulation within the superior cervical gangha It is concluded that locahzed stimulation of specific sites m the major thoracic ganglia or smaller medlastlnal gangha can alter cardiac chronotroplsm a n d / o r motroptsm

Correspondence J Armour, Department of Physiology and Biophysics Faculty of Medicine Dalhousle Umverslty Hahfax, N S B3H 4H7, Canada 0165-1838/84/$03 00 © 1984 Elsevier Soence Pubhshers B V

244 Introduction It has been shown that the efferent axons m each of the major cardlopulmonary nerves subserve different cardmc functions [3,13] The majority of canine sympatheUc postganghon~c neurons winch innervate the heart are located m the m~ddle cerwcal ganglm ( M C G ) [4,9] The stellate gangha (SG) contain fewer cardmc postganghomc neurons and very few are present m the superior cerwcal gangha The efferent postganghomc sympathetic neurons, winch project axons m cardtopulmonary nerves, are distributed throughout the M C G and m the cramal pole of the SG SUmulauon of discrete regions of the M C G or the cranml pole of the SG generates compound action potentmls m the majority of the lpsdateral cardlopulmonary nerves after hexamethomum admlmstratlon [11] Thus, neuroanatom~cal and neurophyslologlcal experiments have confirmed the importance of these gangha m cardmc regulauon and have demonstrated that neurons throughout the M C G and m the cranial pole of the SG innervate the heart However. the manner m which neurons located throughout these ganglm modify cardmc function Is unknown In the present series of experiments, l o o within the MCG, SG, superior cervical gangha and medmstmal gangha were stimulated with bipolar electrodes following the administration of hexamethomum m order to determine whether and how the heart could be affected by activation of neurons within specific 1oo of sympathetic gangha

Materials and Methods Nineteen mongrel dogs of either sex, weighing 14-28 kg, were tranqmlhzed with fentanyl o t r a t e (0.03 m g / k g i m ) and anestheUzed with a-chloralose (100 m g / k g 1 v ) PoslUve pressure ventllaUon was mtUated with a Bird Mark 7 respirator A bdateral thoracotomy was performed via the fourth intercostal space and the maddle cervical and stellate gangha were exposed on the right or left side of the ammal The gangha on the right side of the thorax were investigated m 8 dogs and those on the left m 11 dogs In 4 of these expen_ments, the superior cerxqcal gangha were also exposed bdaterally via a ventral nudhne recision in the neck The pericardium was opened, and a Walton-Brodle straan gauge arch was sutured to each atrium to measure atrial force Four mtramyocarclial pressure transducers (Komgsberg Instruments, Model P16-12) were placed witlun the rmdwall of the right ventncular conus and the ventral, lateral and dorsal walls of the left ventricle A Cordls G M 7 pig-tad catheter was placed m the left ventncular cavaty from the aorta vaa a femoral artery recision and was attached to a Model 80 Trantec Bentley transducer. A lead II E K G and all other measured parameters were monitored on a Beckman Type 504 D Dynograph pen recorder The sympathetic ganglm on the exposed side were decentrahzed by sectlomng the cramal end of the vagosympathetlc trunk as well as all of the connecUons of the stellate ganglion with the central nervous system and the sympathetic chain The stellate ganglion was placed m a mineral off bath Then atropme sulphate (1 m g / k g )

245 was a d m i n i s t e r e d i n t r a v e n o u s l y A n i n t r a v e n o u s bolus of h e x a m e t h o n i u m (1 m g / k g ) was given a n d a c o n t i n u o u s i v d r i p of h e x a m e t h o n l u m (1 m g / m i n ) was b e g u n The efficacy of the ganglionic b l o c k a d e b y the h e x a m e t h o n l u m was tested by s t i m u l a t i o n of the p r e g a n g l l o n i c fibers in the thoracic r a m i before a n d after the a d m i n i s t r a t i o n o f h e x a m e t h o n l u m while r e c o r d i n g the c a r d i o v a s c u l a r responses T h e a m o u n t of h e x a m e t h o n l u m used, always resulted in a significant r e d u c t i o n in systolic b l o o d pressure It has been d e m o n s t r a t e d that it is necessary to a d m i n i s t e r h e x a m e t h o m u m in this dosage m o r d e r to block c o m p l e t e l y nicotinic cholinergic s y n a p t i c transmission [2] S y s t e m a t i c s t i m u l a t i o n of discrete regions within the M C G a n d S G was then c a r r i e d out while m o n i t o r i n g the c a r d i a c responses In 4 animals, s t i m u l a t i o n s were p e r f o r m e d In discrete regions along the left c a u d a l p o l e nerve [4] a n d right subc l a v i a n ansa In 4 animals, discrete regions within the left a n d right s u p e r i o r cervical g a n g l i a were s t i m u l a t e d Stimuli (4 V, 5 ms a n d 10 Hz) were delivered via a fine b i p o l a r tungsten e l e c t r o d e which h a d its tips s e p a r a t e d by less than 1 m m The wtres of the electrode were i n s u l a t e d to within 1 m m of their tips A G r a s s SD-9 s t i m u l a t o r was used, the o u t p u t of which was m o n i t o r e d on a T e l e q u l p m e n t T y p e D4 oscilloscope Sufficient time was allowed to elapse between s t i m u l a t i o n s so that the p r e p a r a t i o n r e t u r n e d to c o n t r o l levels before the next s t i m u l a t i o n A m a p for each ganglion was d e v e l o p e d in o r d e r to s t a n d a r d i z e and record s y s t e m a t i c a l l y the l o c a t i o n s of the stlmulat, on loci within the s y m p a t h e t i c g a n g h a T h e m a p was c o n s t r u c t e d b y dividing the stellate a n d m i d d l e cervical ganglia into 30 areas, the ansae a n d r a m i being used as l a n d m a r k s ( F i g 1) Because the right stellate g a n g h o n is longer a n d t h i n n e r than the left stellate ganglion, which is thicker a n d c o n t a i n s a m e d i a l flexion at its rostral end, these areas were not equal Th,s technique allowed r e p r o d u c i b i l i t y with respect to the areas s t i m u l a t e d from dog to dog In each dog, all 30 loci In b o t h m a j o r ganglia of the o p e r a t e d side were s t i m u l a t e d H e a r t rate, atrial forces of c o n t r a c t i o n , l n t r a m y o c a r d l a l pressures a n d l n t r a v e n t n c u l a r pressures were r e c o r d e d before, d u r i n g a n d after s t i m u l a t i o n s A regional force response was d e f i n e d as a response in which some, b u t not all, of the ventrlcular i n t r a m y o c a r d l a l pressures were a u g m e n t e d In a generalized force response, all of the i n t r a m y o c a r d m l a n d atrial forces were a u g m e n t e d The responses were t a b u l a t e d a n d c o m p a r e d to the c o n t r o l values using an analysxs of v a r i a n c e S u b s e q u e n t to the analysis of variance, S t u d e n t t-tests were p e r f o r m e d c o m p a r i n g the c o n t r o l a n d s t i m u l a t i o n results for all loci within each ganglion for heart rate and right ventrlcular conal force F o r c e changes in the right ventrlcular conus were c h o s e n b e c a u s e it has been d e m o n s t r a t e d that conal force is most readily influenced b y c a r d i a c nerve s t i m u l a t i o n [13] T h e results were a n a l y z e d i n d i v i d u a l l y for each ganglion F o r example, the h e a r t rate response following s t i m u l a t i o n of the 30 loci in the right stellate ganglion h a d 35 degrees of f r e e d o m for all a n i m a l s a n d areas with an S S of 98348 9 a n d an M S of 2809 97 T h e i n t e r v e n t i o n for all areas [30] h a d an S S of 14363 0 a n d an M S of 478 77 The residual, with 344 ° freedom, h a d an S S value of 59139 3 a n d an M S value of 171 92 This ganglion h a d an F value of 2 785 (P<001)

246

In those e x p e r i m e n t s m which the c a u d a l p o l e nerve a n d the ventral ansae were mvesUgated, s t i m u l a t i o n was p e r f o r m e d along the course of these structures W h e n a locus along these structures was f o u n d which, when stimulated, p r o d u c e d cardtac responses, I n d m ink was then placed on that locus The m a r k e d locus and the a d j a c e n t Ussue from which responses were not ehc~ted were r e m o v e d for histological processing T h e tissue was fixed by , m m e r s l o n m 10% f o r m a l m p r i o r to the cutting of 4 0 / ~ m frozen sections The sections were stained with neutral red

Results E l e c t n c a l s t i m u l a t i o n of discrete regaons wlttun the M C G a n d S G as well as along the course o f the ventral s u b c l a v m n ansae o r c a r d m c nerves e v o k e d c a r d i a c responses m all 19 dogs investigated These responses v a n e d a c c o r d i n g to the region b e m g s t i m u l a t e d F t g 1 d l u s t r a t e s the d]vlslons of the M C G a n d SG, i d e n t i f y i n g b y letter a n d n u m b e r the g a n g h o m c regions W h e n the regions of the M C G or S G were s t i m u l a t e d , the m o s t c o m m o n c a r d m c response ehc~ted was an increase m b o t h h e a r t rate a n d force of c o n t r a c t i o n ( T a b l e I) In other instances, ellctted responses consisted o f rate o r force changes N o n e of the s t l m u l a u o n s in a variety of regions of the n g h t or left s u p e r i o r cervlcal g a n g h a p r o d u c e d a n y d e t e c t a b l e c a r d m c responses RIGItT

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Fig 1 A schemaUc summary of the areas winch consistently ehclted slgmhcant cardmc responses m the 19 dogs The areas are marked differently according to the level of slgmficance and whether force or rate changes were produced

247 TABLE 1 A SUMMARY OF RESPONSES ELICITED FOLLOWING STIMULATION OF ALL LOCI IN THE DIFFERENT GANGLIA, CLASSIFIED ACCORDING TO THE TYPES OF RESPONSES ELICITED RATE ONLY, FORCE ONLY (GENERAL OR REGIONAL) OR BOTH RATE AND FORCE THE PERCENTAGE BELOW EACH NUMBER INDICATES THE PERCENTAGE OF EACH TYPE OF RESPONSE ELICITED Regmn being stlmulated

Total number of areas responding to stimulation

Heart rate responses

Generahzed force responses

Regional force responses

Rate and force responses

Right-sided gangha Right Stellate Gangha

96

6 (6%)

9 (9%)

Right Middle Cervical Gangha

38

7 (18%)

1 (2%)

0

30 (80%)

Right Ventral Ansa Gangha

17

1 (6%)

1 (6%)

0

15 (88~.)

151

14 (9%)

11 (7%)

Total Right-S~ded Gangha

5 (5%)

76 (79q~)

5 (3%)

121 (80%)

Left-sided gangha Left Stellate Gangha

69

0

43 (62%)

10 (159~)

16 (23%)

Left M~ddle Cervical Gangha

61

0

25 (41%)

14 (23%)

22 (36~)

Left Caudal Pole Nerve Gangha

3

0

0

1 (33%)

2 (66%)

Total Left-Sided Gangha

133

0 (0%)

68 (51%)

25 (19%)

40 (30%)

Total Right and Left Gangha

284

14 (5%)

79 (28%)

30 (10%)

161 (57%)

It was found that during systematic stimulation of discrete regions within a ganglion, stimulation of regions which were adjacent to one another somettmes p r o d u c e d d i f f e r e n t r e s p o n s e s F o r i n s t a n c e , as s h o w n i n F i g 2. s t i m u l a t i o n o f a r e a A3 of the right SG in one dog elicited no response but stimulation of the i m m e d i a t e l y a d j a c e n t a r e a s A 2 o r B3 i n t h e s a m e a n i m a l a l t e r e d l n o t r o p l s m p a r t i c u l a r l y i n t h e v e n t r a l a n d l a t e r a l w a l l s o f t h e left v e n t r i c l e S t i m u l a t i o n o f s o m e areas in the MCG in this dog altered cardIodynamlcs, but stimulation of areas in the s u p e r i o r c e r v i c a l g a n g l i o n d i d n o t ( F i g 2) S t i m u l a t i o n o f s o m e loci In t h e r i g h t S G o r M C G a u g m e n t e d c h r o n o t r o p l s m o n l y T h i s o c c u r r e d w h e n s t i m u l a t i n g t h e r i g h t - s i d e d g a n g h a i n 9% o f t h e s i t e s ( T a b l e I) A u g m e n t a t i o n o f l n o t r o p l s m m o n e r e g i o n o f t h e h e a r t o c c u r r e d i n 10% o f t h e s~tes

248

( F i g 3, T a b l e I) G e n e r a h z e d p o s m v e m o t r o p l c responses were elicited m 28% of the active loct, b u t the m a j o r i t y of ehclted responses revolved b o t h chronotrop~sm and motroplsm S t l m u l a u o n s a l o n g the course of the left c a u d a l pole nerve were p e r f o r m e d m 4 a m m a l s In each. a locus within 2 cm of the left M C G was f o u n d a l o n g the course of ttus nerve wluch, when s u m u l a t e d , g e n e r a t e d c a r d m c responses These were usually regtonal m n a t u r e (Ftg 4) S t i m u l a t i o n of the nerve on etther s~de of th~s locus p r o d u c e d n o c a r d m c r e s p o n s e S u b s e q u e n t h~stologlcal e x a m i n a t i o n of the c a u d a l p o l e nerve d e m o n s t r a t e d the presence of n e u r o n a l cell b o d i e s m the regions m which s t i m u l a t i o n p r o d u c e d c a r d m c changes, b u t not m the a d j a c e n t regions of this nerve m which st~mulatlon d i d not ehclt c a r d m c responses ( F i g 5B) S t i m u l a t i o n s along the course of the right ventral a n s a were carried out m 6 dogs T h e results were s~mllar to those d e s c r i b e d for the c a u d a l pole nerve m as m u c h as m o t r o p l s m m the ventral p o r t i o n s o f the ventricles was a u g m e n t e d H i s t o l o g i c a l e x a m i n a t i o n of the

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Fig 2 Cardmc responses to stlmulaUon of discrete loca wltlun sympatheuc gang,ha Sumulauon of areas A3, A2, C9 and B3 m the right stellate ganghon (RSG), as well as areas C5, F4, and E3 m the right MCG and a typical area m the right superior cervJc,al ganglion (RSCG) produced a variety of responses m the EKG, right areal force (RAF) and left atrial force (LAF), right ventncular conal mtramyocardlal pressure (RVC-IMP), left ventncular mtramyocarchal pressure m the ventral (LW-IMP), lateral (LVL-IMP) and dorsal (LVD-IMP) walls, as well as left ventncular chamber pressure (IVP)

249 a n s a e a l s o r e v e a l e d t h e p r e s e n c e o f cell b o d i e s o n l y a t t h e s t i m u l a t i o n s , t e w h i c h p r o d u c e d c a r d i a c r e s p o n s e s ( F i g 5 A ) a n d n o t m a r e a s o f t h e v e n t r a l a n s a a d j a c e n t to t h a t stte

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Fig 3 Stimulation of area A7 in the medial edge of the left stellate ganghon augmented both atrial forces shghtly but did not affect heart rate (144 beats/finn) The right venmcular conus IMP increased from 20 to 30 mmHg and the left ventncular ventral IMP increased from 43 to 54 mmHg The left ventncular lateral IMP was unchanged (75 mmHg) and the dorsal IMP decreased (75-70 mmHg) Left ~entrlcular cav,ty pressure increased from 88 to 98 mmHg

250

T o d e t e r m i n e w h e t h e r s t i m u l a t i n g one area wtthm a m a j o r thoracic g a n g h o n c o n s t s t e n t l y p r o d u c e d c a r d t a c responses, the responses to s t i m u l a t i o n in each locus w i t h m a ganglion were a n a l y z e d C e r t a m areas of a g a n g h o n when stimulated consistently p r o d u c e d c a r d i a c responses A r e a s were f o u n d which g e n e r a t e d c h r o n o t r o p i c a n d / o r l n o t r o p l c c a r d i a c changes at the P < 0 0 5 and P < 0 01 levels of slgntficance Thus, an a n y given dog, the areas tdentified in Fig 1 w o u l d be most h k e l y to m l t m t e a cardxac response when s o m u l a t e d R e s p o n s e s were also ehctted when o t h e r areas were s t i m u l a t e d (e g Ftgs 2 a n d 3), however, these dtd not occu~ an enough a n i m a l s to reach statistical stgnificance In addatton, there were areas from which no responses were ehcited m any dog It was f o u n d that statistically s~gmficant m o t r o p i c responses were p r o d u c e d by s t i m u l a t i o n m all m a j o r thoracic g a n g h a Sttmulatton of areas m the right-sided g a n g h a resulted in stat|stlcally significant mcreases in h e a r t rate, while on the left side rate responses, although ehcIted, were not consistent enough to reach staostacal sigmttcance for a n y gjven area W h e n the c a r d i a c responses m e a s u r e d an all 19 e x p e r i m e n t s were g r o u p e d m t o those involving either rate, g e n e r a h z e d force, regional force or rate a n d force, at was f o u n d that s t i m u l a t i o n of loct wtthsn the r~ght stellate g a n g h o n most conststently a u g m e n t e d b o t h rate a n d force (Table I) A b o u t 6% of these responses revolved only heart rate a n d 14% onl3 m o t r o p l s m In the right MCG, 80% of the acttve regtons affected b o t h rate a n d force while 18% affected only rate a n d 2% affected force alone Stamulanon along the right ventral s u b c l a v m n a n s i p r o d u c e d rate a n d force changes i n 8 8 % of the active loci O n the right stde 80% ot

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F i g 4 S t i m u l a t i o n of the c a u d a l p o l e n e r v e m 3 a d j a c e n t a r e a s T h e c r a m a l a r e a ( A ) a n d the c a u d a l a r e a (C) e h c l t e d n o c h a n g e m c a r d l o d y n a m l c s A n m c r e a s e m h e a r t r a t e a n d left v e n t n c u l a r d o r s a l I M P w i t h a d e c r e a s e m I V P o c c u r r e d w h e n the a r e a m b e t w e e n w a s s t i m u l a t e d (B) A r e a B w a s 1 m m f r o m a r e a s A and C

251

the areas w h i c h affected the heart when stimulate& altered both inotroplsm and chronotroplsm, 9% only chronotroplsm and 10% only motroplsm In contrast, stimulation of discrete regions wlthm the left-sided ganglia augmented lnotroplsm in 70% of the active areas, chronotroplsm and inotropism in 30% while no area wa~ stimulated which augmented chronotroplsm alone Of all the responses elicited, 38% affected lnotroplsm, 5% chronotroplsm and 57% both inotroptsm and chronotropJsm Of all the sites mvestlgated in the SGs and MCGs~ only 23% produced cardiac responses when stimulated In two of the animals, sttmulatlon of a discrete region wtthin a thoractc autonomic ganglion elicited an mttlal augmentation of inotroplsm which was followed by a steady dechne in lnotropism (Fig 6), culmmatmg in ventrlcular fibrillation in one instance and cardxac standstill in the other These responses appeared to be the direct consequence of the ganghonlc mterventton because there was no indication of prior change m the preparation

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Fig 5 Histology of the right v e n t r a l ansa (panel A) d e m o n s t r a t i n g the locus w h e r e s t i m u l a t i o n p r o d u c e d a c a r d i a c effect A g r o u p o'f ,aeurons ts indicated in the ansa by the large a r r o w T h e small a r r o w indicates a few n e u r o n s outside the m a r e g r o u p C a l i b r a t i o n b a r = 500 `am Panel B illustrates the hlstolog'~ ol a g r o u p of n e u r o n s in the left c a u d a l pole n e r v e i e the left c a u d a l pole ner',e g a n g h o n This Ussue was t a k e n f r o m the locus of the n e r v e which w h e n s t i m u l a t e d elicited c a r d i a c responses (Fig 4B) C a h b r a t l o n b a r = 100 a m

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IVP (nm~

Fig 6 Mechanical stimulation (arrow) caused by insertion of the electrodes into area B4 m the left MCG resulted in an initial motroplc augmentation m the 4 ventncular areas monitored, followed by a steady decline of IMPs and IVP The gaps m the record represent 30 s of deleted record Fibrillation occurred shortly after the time that the figure ends

Discussion The present experiments have demonstrated that, following h e x a m e t h o n m m adrmmstratlon, stimulation of discrete regions within acutely decentralized canine sympathetic ganglia can elicit cardiac responses Such samulatlons can produce chronotroplc changes without affecting motropism, regional or generalized m o t r o p i c responses without affecting chronotropism, or both chronotroplc and m o t r o p i c cardiac effects It is k n o w n that specific sympathetic cardiac nerves affect specific regions of the heart [3,12,13] The cells of o n g m of the axons m specific c a r d l o p u l m o n a r y nerves are located throughout the lpsllateral M C G and the cranial pole of the lpsdateral SG [4] I n keeping with this anatomy, stimulation with bipolar electrodes of regions throughout the M C G or the cranml pole of the SG following the adrmmstration of h e x a m e t h o n m m results m the generation of c o m p o u n d action potentials m all lpsllateral cardiac nerves [11] Thus, although each c a r d i o p u l m o n a r y nerve has functionally specific efferent axons [4] which regulate specific cardiac regions [3], these efferent axons arise from sympathetic p o s t g a n g h o m c neurons located throughout the lpsllateral M C G and cranial poles of the ipsdateral SG [4] That is, neurons m one locus of a major thoracic sympathetic ganglion project axons into all the major lpsilateral c a r d i o p u l m o n a r y nerves In addition, recent evidence ln&cates

253 that n e u r o n s in such a locus d o not i n n e r v a t e specific c a r d i a c regions [9] T h e present e x p e r i m e n t s d e m o n s t r a t e d that, in an acutely d e c e n t r a l i z e d p r e p a r a tion m which h e x a m e t h o n l u m h a d been a d m i n i s t e r e d , 23% of the s t i m u l a t i o n s resulted in altered c a r d l o d y n a m l c s W h e n the m a p s of the ganglia were analyzed, it was f o u n d that there were areas in which s t i m u l a t i o n never elicited a c h a n g e in c a r d l o d y n a m l c s T h e r e were o t h e r areas in which s t i m u l a t i o n o c c a s i o n a l l y elicited c a r d i a c responses ( F i g s 2 a n d 3) a n d there were areas which consistently p r o d u c e d c a r d i a c responses ( F i g 1) In o t h e r words, there were ganglionic areas in which there was a very high p r o b a b i l i t y of o b t a i n i n g a c a r d i a c r e s p o n s e f r o m d o g to d o g when that area was s t i m u l a t e d The stimulus delivered via the fine b i p o l a r electrodes a p p e a r e d to be fairly well localized b e c a u s e large n u m b e r s of ganglionic areas, when s t i m u l a t e d , p r o d u c e d no d e t e c t a b l e c a r d i a c responses even t h o u g h they were often a d j a c e n t to an a r e a which elicited c a r d i a c responses when s t i m u l a t e d ( F i g 2) F u r t h e r m o r e , some areas p r o d u c e only c h r o n o t r o p i c or i n o t r o p l c responses when s t i m u l a t e d , suggesting that there was little c u r r e n t spread, otherwise m o r e generalized responses w o u l d have o c c u r r e d N o areas were f o u n d in the s u p e r i o r cervical ganglia which, when s t i m u l a t e d , elicited c a r d i a c responses This IS consistent with the fact that very few s y m p a t h e t i c p o s t g a n g h o n i c n e u r o n s in the s u p e r i o r cervical g a n g h a project axons into c a r d l o p u l m o n a r y nerves [4] or the heart [9] T h e n u m b e r of n e u r o n s a c t i v a t e d b y b i p o l a r s t i m u l a t i o n in a discrete ganglionic region is u n k n o w n However, the n u m b e r which IS sufficient to p r o d u c e a c a r d i a c r e s p o n s e m a y be, in some instances, quite small because it was f o u n d that c a r d i a c c h r o n o t r o p i s m a n d / o r i n o t r o p i s m c o u l d be a u g m e n t e d b y s t i m u l a t i o n of discrete areas along the right s u b c l a v i a n a n s a or the left c a u d a l p o l e nerve ( F i g 4) S u b s e q u e n t histological e x a m i n a t i o n of such areas d e m o n s t r a t e d the p r e s e n c e of small clusters of n e u r o n s In these loci, b u t not in the nerve a d j a c e n t to such an area ( F i g 5) T h e fact that s t i m u l a t i o n of a small n u m b e r of n e u r o n s elicited c a r d i a c responses could i n d i c a t e that the stimulus a c t i v a t e d i n t e r n e u r o n s either at the site of s t i m u l a t i o n or d i s t a n t f r o m the site of s t i m u l a t i o n in the same or a n o t h e r ganglion [5,11], which in turn might have a c t i v a t e d a relatively larger n u m b e r of s y m p a t h e t i c p o s t g a n g h o n i c neurons, t h e r e b y altering c a r d l o d y n a m i c s T h e idea that l n t e r n e u r o n s a n d / o r i n t e r g a n g h o n l c n e u r o n s could p l a y a role in these effects is suggested by the o b s e r v a t i o n s that (1) s t i m u l a t i o n of some ganglionic areas which d o not c o n t a i n c a r d i a c efferent p o s t g a n g h o n l c s y m p a t h e t i c n e u r o n s can m o d i f y c a r d i o d y n a m i c s ( F i g 1, right stellate ganglion), (2) s t i m u l a t i o n of s o m e g a n g h o n areas which are k n o w n to c o n t a i n c a r d i a c efferent p o s t g a n g h o m c s y m p a t h e t i c n e u r o n s does not necessarily m o d i f y c a r d l o d y n a m l c s ( F i g 1, left stellate ganglion a n d m i d d l e cerwcal ganglia), a n d (3) s t i m u l a t i o n of a discrete cluster of n e u r o n s a l o n g a nerve elicited c a r d i a c responses in spite of the small n u m b e r s of n e u r o n s p r e s e n t I n t e r n e u r o n s exist m a u t o n o m i c ganglia [7,8,14] These have been p r o p o s e d to a c c o u n t for m u l t l s y n a p t l c t r a n s m i s s i o n within a u t o n o m i c ganglia [7,14], p e r h a p s m o d i f y i n g the activity of the c a r d l o p u l m o n a r y efferent p o s t g a n g h o n l c s y m p a t h e t i c axons [11] T h e g a n g h o m c areas identified in F i g 1 d o not e n c o m p a s s the entire d i s t r i b u t i o n of c a r d i a c s y m p a t h e t i c p o s t g a n g h o n l c efferent n e u r o n s in the m a j o r thoracic ganglia [4,9] A lack of c o r r e s p o n d e n c e b e t w e e n the a n a t o m i c a l d i s t r i b u t i o n of s y m p a t h e t i c

254 efferent p o s t g a n g h o m c n e u r o n s i n n e r v a t i n g the heart has been observed m ~tudtes m which m e t a b o h c activity of acutely decentrahzed thoracic a u t o n o m i c gangha was assessed after s t i m u l a t i o n of a c a r & o p u l m o n a r y nerve [10] With the present state ol knowledge, the basis of these differences is unclear Since synaptlc m e c h a m s m s exist within thoracic sympathetic ganglia which are not affected by h e x a m e t h o m u m [1,2,6], a n d since ~t is possible that these may revolve m t e r n e u r o n s m the present series of experiments such synapttc m e c h a n i s m s might have been actwated Clusters of n e u r o n s along the ventral subclavian ansa as well as the (.audal pole nerve m a y modify card~odynanucs These neurons, which are k n o w n to project axons m c a r & o p u l m o n a r y nerves [4] or to the heart [9], generate c o m p o u n d action potentials m specific c a r d l o p u l m o n a r y nerves when stimulated [11] W h e n regions c o n t a i n i n g such clusters of n e u r o n s were stimulated, the cardiac response was frequently c o n f i n e d to one region of the heart (Fig 4) which ~s consistent w~th the physaologlcal f i n d i n g that such gangha usually project efferent axons into one c a r d l o p u l m o n a r y nerve [11] The c a p a b d l t y of the techmques Utdlzed m th~s study to locate a small cluster of n e u r o n s along the course of a nerve supports the a s s u m p t i o n that the methods used are fmrly specific m identifying loc~ of n e u r o n s The lack of response ellctted by s t i m u l a t i n g elsewhere along an ansa or a c a r d l o p u l m o n a r y nerve p r o b a b l y m & c a t e s that the st~mulatlon parameters employed d~d not activate e n o u g h post- or pre-ganghom(, axons to p r o d u c e s~gmficant car&ac responses In two of the experiments, the a m m a l s died relatively rapidly following either p e n e t r a t i o n by the electrodes into a g a n g h o n or after s t i m u l a t i o n of a g a n g h o m t region In one a m m a l , fibrillation ensued a n d m the other the contractd~ty of the heart was gradually reduced (Ftg 6) These data indicate that mampulat~ons of the sympatheUc gangha can, u n d e r some conditions, result m a rap~d deterioration ol cardiac function

Acknowledgements The authors gratefully acknowledge the techmcal assistance of Rachard Livingston a n d the typing assistance of Susan M c l l q u h a m Supported by the Me&cal Research C o u n c d of C a n a d a a n d the N o v a Scotia Heart F o u n d a t i o n

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