Brain Research, 97 (1975) 33-46 © Elsevier Scientific Publishing Company, Amsterdam - - Printed in The Netherlands
P A T T E R N S OF C O R T I C A L M O T O N E U R O N E POOLS
PROJECTION
TO
HINDLIMB
33
MUSCLE
F L O Y D J. T H O M P S O N * AND J U L I O J. F E R N A N D E Z * *
The Rockefeller University, New York, N. Y. 10021 (U.S.A.) (Accepted April 14th, 1975)
SUMMARY
This study was undertaken to examine the organization of the hindlimb area of the motor cortex. Two specific questions were posed. The first was: are the cortical neurones which control the excitability of a given motoneurone pool localized in a small zone of cortex or are they diffuse? The second question was: does microstimulation in the hindlimb area of the motor cortex activate spinal motoneurones in a reciprocal fashion, i.e., is cortically elicited facilitation of a muscle accompanied by inhibition of the antagonist ? lntracortical microstimulation was used to condition monosynaptic reflexes of the cat hindlimb to study the organization of a cortical projection to lumbar motoneurone pools. Cortical neurones which produced facilitation or inhibition of a given monosynaptic reflex were localized within a small zone of the cortex. Facilitatory and inhibitory effective zones were found to have similar shape and size. Intracortical microstimulation elicited facilitation or inhibition of individual monosynaptic reflexes without eliciting reciprocal effects on the antagonists. The pyramidal tract was shown to play an important role in the mediation of cortically elicited facilitation as well as inhibition of the monosynaptic reflexes in the lumbar cord.
INTRODUCTION
Studies on the cortical control of the lumbar spinal cord in the cat showed that a predominant pattern of facilitation of flexor motoneurones and inhibition of
* Present address: Department of Neuroscience, University of Florida, College of Medicine, Gainesville, Fla. 32610, U.S.A. ** Present address: Catedra de Fisiologia, Universidad del Zulia, Facultad de Medicina, Maracaibo, Venezuela.
34 extensor motoneurones followed stmulation of the surface of the motor cortex l, +~'.:-~ These results were compatible with Sherrington's observation 90 which suggested thai the motor cortex controls coordinated movement through reciprocal mechanisms of the spinal cord. More recently, however, details of the cortical efferent system controlling the contraction of individual forelimb muscles in cats and monkeys were presented by Asanuma and coworkers:' 7 These investigators developed a technique for delivering current pulses to the depth of the cortex through a microelectrode. Using this method, they showed that single distal forelimb muscles could be activated by microstimulation of a small number of cortical neurones. They observed that cortical neurones which controlled the excitability of a given motoneurone pool were located within a small zone of cortex. The present study was undertaken to examine the organization of the hindtimb area of the motor cortex using microstimulation techniques. Since spinal reflex mechanisms are more easily investigated in the lumbar cord than in the cervical cord, the relationship between cortically elicited facilitation and inhibition and spinal reflex mechanisms could also be investigated. Two specific questions were posed. The first was: are the cortical neurones which control the excitability of a given motoneurone pool localized in a small zone of cortex or are they diffuse? The second question was: does microstimulation in the hindlimb area of the motor cortex activate spinal motoneurones in a reciprocal fashion, i.e., is cortically elicited facilitation of a muscle accompanied by inhibition of the antagonist? Answers to these questions were sought by studying the monosynaptic reflexes recorded from 8 branches of the sciatic nerve conditioned by microstimulation in the cortex. The results will demonstrate that discrete cortical zones exist which control the excitability of individual motoneurone pools in the absence of reciprocal effects (on other motoneurone pools tested). METHODS
Preparation Experiments were carried out on 24 adults cats of either sex weighing between 2.0 and 3.5 kg. Surgical procedures were done under pentobarbital sodium (Nembutal) anaesthesia (35 mg/kg i.p.). The left pericruciate cortex was exposed and a closed chamber of 3 cm internal diameter was mounted over the craniotomy opening with dental impression wax, and the chamber was filled with warm mineral oil t0. The cisterna magna was opened to drain cerebrospinal fluid and prevent swelling of the brain. The exposed cortex was then photographed. These photographs were used to map the location of electrode tracks made during the experiment. Spinal cord segments of L5-$2 were exposed by a routine method and dorsal roots L5-$2 on the right side were divided and the proximal ends were mounted on bipolar electrodes for stimulation. Monophasic recordings of compound action potentials were taken from 8
35 branches of the sciatic nerve sectioned at their points of entry to the muscles. The sciatic nerve branches used were: deep peroneal (D. per., flexor) superficial peroneal (S. Per., flexor) posterior tibial (P. Tib., extensor) deep posterior (D. Post., extensor) plantaris (PI., extensor) lateral gastrocnemius (L. Gast., extensor) medial gastrocnemius (M. Gast., extensor) posterior biceps semitendinosus (PBSt., flexor). The dorsal roots and nerve branches were immersed in mineral oil pools. The animal was securely held by the combination of head holder, vertebral spinous clamp, and right leg clamp. All wound margins and pressure points were infiltrated with a long lasting anaesthetic (Zyljection, Abbott). Light anaesthesia was maintained with supplemental doses of Nembutal (5 rag) intravenously at 2-h intervals throughout the experiment. After completion of surgery, the animals were paralyzed with gallamine triethiodide (Flaxedil, 5 mg/kg, i.v.) and artificial ventilation started. Body temperature was monitored with a rectal probe and kept at 35-38 °C with a heating pad.
Stimulation and recording The monosynaptic reflexes were elicited by stimulation of appropriate dorsal roots with single shocks of 0.2 msec duration and were recorded from the individual sciatic nerve branches. The amplitude of the reflex was adjusted to approximately one-third maximal amplitude to insure the availability of additional motoneurones by facilitation. Cortical areas which produce low threshold effects on lumbar spinal reflexes were localized by stimulating the cortical surface at various points and testing the spinal reflexes for facilitation or inhibition. Trains of 10 anodal pulses were delivered through a ball electrode. The duration of the train was 30 msec and the individual pulses were 0.2 msec in duration. The cortically elicited facilitation or inhibition of the test reflexes appeared on the test reflexes after a latency of 10-12 msec and was maximal 30-40 msec following the start of the cortical stimulus train of pulses. This pattern was the same for both surface and depth stimulation with the minimal current intensity stimulation parameters to be described below. The conduction-test interval was, therefore, fixed at 35 msec. Since stimulation of the cortical surface was done to delineate low threshold areas, the current intensity was initially fixed at 1 mA. When an effect on the reflexes was observed, the stimulus intensity was reduced to determine the threshold of the effect. Records of these points were made on the photographs previously taken of the cortical surface. A tungsten microelectrode 21 was then inserted into the cortex at the point of lowest threshold. These microelectrodes were composed of tungsten wires that were
36 insulated with glass except for the tips. The lengths and diameters ol tile exposed tip~ ranged between 10 and 18 #m. The electrode was advanced into the cortex in 0.2 mm steps. At each step, trains of 10 cathodal pulses (0.2 msec duration, 300 Hz, 3 ~cc intervals) were delivered to examine the effect on the spinal reflexes. A maxinlum intensity of 20/xA was used throughout the penetrations. When an effect appeared on a given reflex with 20/~A, the threshold current was determined. The spread ol a cortical area which produced an effect was mapped by making additional penetration,~ around the effective area. The determination of cortically elicited fitcilitation or inhibition of the test reflexes was determined by visually discriminating the amplitude of the control test reflex and the conditioned test reflex displayed on ~ln oscilloscope. Whenever the amplitude of the monosynaptic reflex was unstable, and when threshold determinations required precise comparison of control and conditioned test reltex amplitude, a Nicolet signal averaging computer was used to compare the control and conditioned test reflexes. At the end of each experiment, several lesions were made in the cortex by passing a negative current of 20 #A tbr 5 sec. These lesions were used as reference points during histological examination to reconstruct the electrode tracks. The animal was then killed with an overdose of pentobarbital and the portion of the brain where the penetrations were made was removed and fixed in 101'i, formalin. Frozen sections of 50/tin thickness were made and stained by the KliJver-Barrera method.
Roslro/
f.
"%
i J
P.C Dimple
Fig. 1. Shaded region shows the location of the cortical area within which the lowest threshold points were found for producing facilitation or inhibition of the spinal reflexes by surface stimulation. DOtdash line encloses the explored cortical area.
37 TABLE I D I S T R I B U T I O N OF EFFECTIVE FOCI IN E A C H NERVE T E S T E D ; S U R F A C E S T I M U L A T I O N
Surface focus listed is the lowest threshold point for producing a given effect in any given experiment. No. of exp. the = number of experiments in which the effect was examined; % appear. -- No. of loci/ No. of exp, x 100%; ~ facil. -- No. of facil, foci/No, of foci for that reflex x 100~; ~ inhib. No. of inhib, foci/No, of loci for that reflex x 100%. Nerve
No. o f exp.
Facilitation No. o f foci
Inhibition % appear,
% fiwil,
No. o f foci
% appear,
"/ o inhib.
D. Per. S. Per. P. Tib. D. Post. PI.
7 8 13 15 14
2 3 8 7 0
28.6 37.5 61.5 46.6 0
100 75.0 72.7 46.6 0
0 1 3 8 10
0 12.5 23.0 53.4 71.4
0 25.0 27.0 53.4 100
M. Gast. L. Gast. PBSt Total Flex. Ext.
9 10 8 84 23 61
2 3 5 30 10 20
22.2 30.0 62.5 35.7 43.5 32.8
25.0 30.0 83.4 45.4 83.4 37.0
6 7 I 36 2 34
66.6 70.0 12.5 42.8 8.7 55.7
75.0 0 16.6 54.6 16.6 63.0
RESULTS
T h e results will be presented in two parts. The first p a r t will present the d a t a regarding the general features o f the effective cortical areas. The second will present d a t a concerning the r e l a t i o n s h i p between cortically elicited facilitation a n d inhibition o f flexor and extensor reflexes.
(I) General features of the effective cortical areas (A ) Preliminary surface mapping Localization of low threshold points. S t i m u l a t i o n o f the cortical surface p r o d u c e d facilitation o r inhibition o f the l u m b a r spinal reflexes, In all cats, the lowest t h r e s h o l d points a p p e a r e d on the b a n k o f the p o s t e r i o r sigmoid gyrus, 1.5-4.5 m m from the midline o f the brain. This is shown in Fig. 1. The s h a d e d region represents the cortical area in which the lowest t h r e s h o l d p o i n t s p r o d u c e d facilitation or inhibition o f one or m o r e spinal reflexes with c u r r e n t intensities o f 200-400 # A in cats. In one cat, the t h r e s h o l d was 600 # A . The 1.5-2.0 m m p o r t i o n o f the p o s t e r i o r sigmoid gyrus immediately a d j a c e n t to the midline, i.e. lateral to the midline, was covered by bone. This was left intact to p r e v e n t i n t e r r u p t i o n o f the superior sagittal sinus. S t i m u l a t i o n o f the cortex at p o i n t s outside the region indicated on the p o s t e r i o r sigmoid gyrus (Fig. 1) elicited facilitation or inhibition o f the spinal reflexes, but required higher current intensities to p r o d u c e the effects. T h e total cortical area e x p l o r e d is enclosed within the d o t - d a s h line in Fig. 1. S t i m u l a t i o n o f the cortical surface within the s h a d e d area could p r o d u c e
38
20
18
16
14
12
I0
B
0 8
=. 6
E
2
o
. . . .
,;
. . . .
~? ,
.
..~;
D,,p~t,/mml
A Fig. 2. A: threshold current intensities for facilitation of the reflex in D. Post. are plotted against depth in the cortex. B: the vertical line marks the path of a reconstructed microelectrode track. The horizontal bar marks 0.2 mm intervals. Microstimulation at 3 points marked with circles produced facilitation of the reflex in D. Post.
facilitation and/or inhibition of several spinal reflexes. The number of spinal reflexes on which effects were elicited was reduced by decreasing the stimulus intensity. Facilitation or inhibition of a single reflex could be produced by reducing the intensity of stimulation in 11 of 16 cats. In 5 cats, a minimum of 2 reflexes was facilitated or inhibited from the lowest threshold point, and could not be separated by reducing the stimulus intensity. The effects on spinal reflexes produced by surface stimulation are summarized in Table 1.
(B) Intracortical microstimulation After determining the low threshold area on the surface of the cortex which produced facilitation or inhibition of the spinal reflexes a tungsten microelectrode was inserted into the centre of the area. The effects of microstimulation in the depth of the cortex were examined by using stimulation current of 20 #A or less. The threshold for facilitation or inhibition of a given reflex usually decreased sharply within a specific cortical region which corresponded to a consecutive sequence of stimulated points. An example of these findings is illustrated in Fig. 2A and B. The microelectrode track through the posterior sigmoid gyrus was reconstructed from the histology in Fig. 2B. The horizontal bars mark the 0.2 mm points where the effects of stimulation were tested on the reflexes. Mierostimulation at the 3 points marked by circles produced facilitation of the reflexes in D. Post. The changes in threshold are shown in Fig. 2A. Additional penetrations were made in this cortical area to determine the distribution of low threshold points for facilitation of the same reflex. Microstimulation with 20 #A or less produced facilitation in restricted regions in 9 of 14 additional
39
A13413
o o~
Post
l
A Fig. 3. Distribution of low threshold points within the cortex. A : several penetrations reconstructed in a sagittal section of the posterior sigmoid gyrus. Circles mark points at which microstimulation with 20/~A or less produced facilitation of the reflex in D. Post, Solid circles mark points which produced facilitation with less than 10/tA. The penetration identification numbers appear at the surface. B: schematic drawing of postcruciate gyrus which shows the location of the surface points of the penetrations relative to the midline to the sulcus, and to each other.
penetrations. The distribution of low threshold points is shown in Fig. 3A and B. Out of 14 penetrations, 4 passed through points which facilitated the reflex in D. Post. with microstimulation of less than 10 #A, and the minimum threshold points are indicated by filled circles in these penetrations. As shown in Fig. 3A, the low threshold points for a given reflex were located in a small area in the cortex. The edge of the zone was irregularly shaped, but the lowest threshold points (filled circles Fig. 3A), TABLE II DISTRIBUTION OF EFFECTIVE FOCI IN EACH NERVE TESTED; DEPTH STIMULATION Each focus is the lowest threshold point for producing a given effect in any given experiment by ICMS. No. of exp., ~ appear., ~ facil., and ~ inhib, were calculated by the same method described in the legend to Table I.
Nerve
D. Per. S. Per. P. Tib. D. Post PI. M, Gast. L. Gast. PBSt Total Flex. Ext.
No. o f exp.
7 8 13 15 14 9 10 8 84 23 61
Facilitation
Inhibition
No. of
%
No. of
%
%
foci
appear,
facil,
%
foei
appear,
inhib.
2 2 4 6 3 2 2 3 24 7 17
28.6 25.0 30.8 40.0 21,4 22.2 20.0 37.5 28.6 30.4 27.9
66.7 66.7 100 75.0 27.3 50.0 100 50.0 58.6 63.6 56.6
1 I 0 2 8 2 0 3 17 4 13
14,3 12.5 0 13.3 57.1 22.2 0 37.5 20.2 17.4 21.3
33.3 33.3 0 25.0 72.7 50.0 0 50.0 41.4 36.4 43.4
40 .... J J
..... . .
j/j
J
J
x 8~ 4
1
2
Fig. 4. Three-dimensional reconstruction of low threshold area within a block of cortex from the bank of the posterior sigmoid gyrus. Thickened vertical bars enclose effective points encountered in the reconstructed penetrations. Dotted line encloses the volume of cortex whichrepresents the approximal spatial extent of the inhibitory effective zone for D. Post. found in the centre of the zone, formed a narrow band oriented parallel to the cortical radial fibres. For convenience, low threshold points within the cortex which produced facilitation or inhibition of a given reflex will be referred to as effective points for that reflex, and the collection of such effective points will be referred to as the effective zone for that reflex. Altogether, 41 effective zones were mapped out from 16 cats. Of these, 24 were facilitatory and 17 were inhibitory. Table II summarizes these results. Facilitatory zones were encountered for all reflexes. Inhibitory zones were encountered for all of the reflexes except P. Tib. and L. Gust. To construct ~i three-dimensional configuration of a given effective zone, many penetrations were made into a zone and the effective volume was reconstructed by histological examination. This was done for 6 different effective zones in 3 different cats, and an example is shown in Fig. 4. The numbers on the surface indicate the points of the penetrations and the thin vertical lines mark the electrode tracks. The thickened vertical lines show effective areas encountered in the penetrations. Microstimulation in these areas with 20 /~A or less produced inhibition of the reflex in D. Post. As shown, the inhibitory area was encountered in penetrations l, 3, 4, 7, 8, and 5 but not in penetrations 2 and 6. The dotted lines in Fig. 4 enclose the effective volume of the cortex revealed by the penetrations as the inhibitory zone for D. Post. The narrow edge of the wedge extended to the edge of the grey matter and the other end of the wedge was located at 0.5 mm depth beneath the surface of the cortex. Microstimulation in the superficial cortical layer with 20/~A or less did not produce effects on the reflexes in this or any of the other experiments. The approximate size of this
41 PENETRATION
No
5
n
Tibialis [Focilitotioqt)
Deep Posterior (Inhibition) I0
2.
3.0
B A
Fig. 5. Overlap of two effective zones. The drawing in B shows the surface points of 6 penetrations which have been schematically reconstructed in A. The thickened vertical bars enclose effective points for inhibition of the reflex in D. Post. (black bar) and facilitation of the reflex in P. Tib. (white bar).
effective zone is 1.0 m m × 1.3 mm at the wider top, becoming smaller at the deeper end. Altogether, 5 effective zones were explored and the size of the other effective zone was similar to the one shown in Fig. 4. The size of their larger ends ranged from 0.8 to 1.8 sq.mm and averaged 1.2 sq.mm. Their lengths along the radial fibres ranged from 1.4 to 2.0 mm and averaged 1.6 mm. Considerable spatial overlap of one effective zone onto the other was observed in these effective zones, an example of which is shown in Fig. 5. Details of this figure and the interaction of overlapping effective zones will be presented in part II of the Results.
(lI) Relationship between cortically elicited facilitation and inhibition of.flexor and extensor reflexes It has been shown that the fringes of the effective zones overlapped with other effective zones. More specifically, microstimulation with 20 # A or less frequently influenced more than one reflex. An example of overlapping effective zones is illustrated in Fig. 5. The drawing in Fig. 5B shows the surface points where penetrations were made and the electrode tracks are reconstructed in Fig. 5A. The filled bars represent areas for inhibition of the reflex in D. Post., the open bars for facilitation of the reflex in P. Tib. ; in penetrations 3, 5, 7, and 8, these bars overlapped. The overlap was minimal in penetrations 5, 7, and 8, i.e., at the effective point, and more extensive in penetration 3. However, within the overlapping areas, the threshold current for producing the effects on the two reflexes was different. The lowest threshold point (i.e. the focus) for producing inhibition in D. Post. was encountered in penetration
42 TABLE III DISTRIBUTION
{}F O V E R L A P P I N { i
P A I R S OF EFFE{7[IVE Z O N E S
Each overlapping pair was tabulated according to the flexor (Flex} or extensor (Ext) muscle c!assiti cation of the individual reflexes, and according to the combination of facilitation (F) and inhibition (I) observed in each pair. F ~F Flex ! flex Flex text Ext i ext
1 12 7
I
I 8 3
IF~ {} 3 3
No. 1 at the depth of 2.4 mm and for facilitation in P. Tib. was in penetration No. 5 at 1.4 mm depth. There was no overlap in penetrations 1 and 4. These observations indicate that although the fringes of these two effective zones overlapped, the centres of the zones were separate. Spatial overlap of this sort was observed in 33 of the 41 effective zones examined. A table was made to explore the possibility that these overlaps coincided with a reciprocal pattern of facilitation and inhibition. The overlap was tabulated according to the functional classification of the muscles involved and also according to the combination of the effects observed. For example, the overlap shown in Fig. 5 for the 2 reflexes in extensor nerves was classified as Ext + Ext F -~ 1. As shown in Table I11, there was no systematic pattern of agonist facilitation combined with antagonist inhibition. On the contrary, the F ,-.' 1 combinations for the antagonists were outnumbered by the F ~- F combinations. This indicated that microstimulation produced effects on a given spinal reflex independently of systematic reciprocal effects on the reflex of the antagonist.
Pyramidal tract section The role of the pyramidal tract in mediating the cortically elicited facilitation and inhibition of lumbar spinal reflexes was examined in 3 cats. This was tested by comparing the threshold for cortically elicited effects on lumbar spinal reflexes before and after transection of the ipsilateral medullary pyramid. The pyramidal tract was sectioned progressively by making small cuts in the exposed ventral surface while delivering microstimulation. Fig. 6A shows the relationship between stimulus intensity and the amplitude of cortically elicited inhibition in M. Gast. before and after pyramidal transection. Before transection, microstimulation with 20 #A intensity completely inhibited the reflex in M. Gast. After making a small cut, the threshold value increased, a n d the amount of inhibition with the same intensity decreased approximately 50 ~ of the presection control. After pyramidal transection, the threshold value for inhibition increased to 20 #A, and at this intensity, the amount of inhibition was only 107/;, of the control reflex. Facilitation of L. Gast. and D. Per. were tested in the other two experiments.
43 100
'
•
o ~.~
~ ....
o
........
o
"o
\\ 80-
"A . . . . .
Ax \ \
•
o After Section
• After Portlol Section \
• Before Section
60'
40'
~ 20
..........".).i\. ........... ; ~ i
;o ;2 ,; ~ ;8 io
; ;
Stim. Intenslt y / ~ A /
A
Fig. 6. Effects of pyramidal section on cortically elicited inhibition. A: Relationship between ICMS intensity and amplitude of cortically elicited inhibition on the reflex in M. Gast. before, after partial transection, and after transection of the ipsilateral medullary pyramid. B : drawing of the histological section of the brain stem at the level of the transection.
In the former, the threshold value tripled and in the latter, the value doubled following transection of the pyramid. These results indicate that the pyramidal tract plays an important role in the mediation of cortical effects elicited by microstimulation on the lumbar reflexes. DISCUSSION
The existence of a radial arrangement of neurones within the depth of the motor cortex which controls the contraction of individual forelimb muscles was shown by Asanuma and Sakata 6. The present experiments have shown the existence of a similar organization in the hindlimb area of the motor cortex. In addition, it was shown that there exist discrete inhibitory zones. The shape and size of an inhibitory zone mapped out within the depth of the cortex was similar to that of the facilitatory zones. It is well known that the use of the train stimulus parameters favours temporal summation. For this reason, low intensity currents can be used instead of large current single shock stimuli. It is a natural question to ask whether the summation produced by train stimulation occurred at the level of the cortex or at the level of the spinal cord. This experiment has shown that the low threshold points for eliciting effects on individual spinal test reflexes were highly localized within the cortex, but cannot conclusively answer questions regarding spinal cord organization. Neuronal recruitment could be taking place in the cortex and/or in the spinal cord. Anderson e t al. 3,4 and also Carpenter e t al. s,9 have reported cortically elicited primary afferent depolarization. Depolarization of this nature has been shown by Eccles e t al. 13 to have a close parallelism to presynaptic inhibition. However, Andersen e t al. 3,¢ and Carpenter e t aL 8,9 reported that cortical stimulation did not depolarize group Ia afferents. Since the monosynaptic test reflex could be completely suppressed
44 by microstimulation (control in Fig. 6), it is unlikely that this inhibition was produced by cortically elicited primary afferent depolarization of the group la afl'erents. The lowest threshold points for eliciting facilitation or inhibition of the spinal reflexes were found on the postcruciate gyrus. Although effects could also be produced by stimulation of certain points on the medial portion of the precruciate gyrus m some experiments, the threshold intensities required were always higher than those for points on the postcruciate gyrus. The bank of the postcruciate gyrus I-2 mm from the cruciate sulcus provided lower threshold points than more caudal portions of the gyrus. Low threshold zones were delimited deep in the bank of the postcruciate gyrus by microstimulation. Localization of the hindlimb area of the motor cortex to the banks of the cruciate sulcus was originally reported by Stout 22 and confirmed by Delgado tl and Delgado and Livingston to. The present experiment is in agreement with the previous lindings and, in addition, localized the lowest threshold point~ Io the postcruciate gyrus for lumbar spmal reflexes. Hongo and ,lankowska a~ recently studied cortically elicited effects that were transmitted to lumbar spinal reflexes by extrapyramidal pathways in pyramidotomized cats. The threshold intensities for producing the effects were comparable to those observed for intact preparations. However, the lowest threshold cortical area for producing the effects in pyramidotomized cats was located posterior to the postcruciate dimple and did not overlap the low threshold area observed in this experiment which was on the bank of the postcruciate gyrus. In the present experiment, the pyramidal tract was shown to be important in the mediation of both thcilitation and inhibition elicited by intracortical microstimulation. Tower ez reported that inhibition elicited by stimulation of the cortical su,Tace was preserved and facilitation abolished following section of the pyramidal tract. However, in the experiment shown in Fig. 6. the powerful inhibition of M. Gast. elicited by intracortical microstimulation was almost completely abolished by pyramidal section. The tindings in the present experiment show that the pyramidal tract is also important in the mediation of inhibition of lumbar spinal reflexes elicited by intracortical microstimulation. It has been shown by Lundberg and coworkers t~' t0 that stimulation of the cortical surface activates spinal interneurones which are intercalated along the segmental reflex pathways to motoneurones. Activation of these interneurones produced a pattern of flexor facilitation combined with extensor inhibition.On the other hand, the effects elicited by microstimulation were transmitted to individual motoneurone pools without systematically producing reciprocal effects on the antagonists. This suggests that the facilitation or inhibition produced by microstimulation is transmitted by spinal interneurones which are not involved in the segmental reflex pathways. On the basis of recording from spinal interneurones which were activated exclusively by cortical stimulation, Vasilenko and Kostyuk 25 proposed that 'the pyramidal system is linked in the spinal cord with a specialized internuncial apparatus, to a large measure independent of the internuncials responsible for segmental motor reflexes'. The findings of the present experiments are compatible with this proposal. The present experiments revealed the existence of st cortical efferent system which controls the excitability of individual motoneurone pools in the lumbar cord in
45 a d d i t i o n to t h e o n e r e p o r t e d by L u n d b e r g a n d c o w o r k e r s 15-19 a n d A g n e w a n d c o w o r k e r s t , '~. ACKNOWLEDGEMENTS
T h e a u t h o r s w i s h to t h a n k Dr. H. A s a n u m a f o r helpful d i s c u s s i o n a n d v a l u a b l e criticism t h r o u g h o u t t h e e x p e r i m e n t s , and also to t h a n k D r . C. G h e z for his useful c o m m e n t s o n the m a n u s c r i p t . T h i s r e s e a r c h was s u p p o r t e d by N I H G r a n t NS-10705.
REFERENCES 1 AGNEW, R. F., AND PRESTON, J. B., Motor cortex - - pyramidal effects on single ankle flexor and extensor motoneurones of the cat, Exp. Neurol., 12 (1965) 384-398. 2 AGNEW, R. F., PRESTON,J. B., AND WHITLOCK, D. G., Patterns of motor cortex effects on ankle flexor and extensor motoneurones in the 'pyramidal' cat preparation, Exp. Neurol., 8 (1963) 248-263. 3 ANDERSON,P., ECULES,J. C., AND S~ARS, T. A., Presynaptic inhibitory action of cerebral cortex on the spinal cord, Nature (Lond.), 194 (1962) 740-741. 4 ANDERSON, P., ECCLES, J. C., AND SEARS, T. A., Cortically evoked depolarization of primary afferent fibers in the spinal cord, J. Neurophysiol., 27 (1964) 62-77. 5 ASANUMA, H., AND ROSI~N, 1., Topographical organization of cortical efferent zones projecting to distal forelimb muscles in the monkey, Exp. Brain Res., 14 (1972) 243-256. 6 ASANUMA, 1-1., AND SAKATA, H., Functional organization of a cortical efferent system examined with focal depth stimulation in cats, J. Neurophysiol., 30 (1967) 34-54. 7 ASANUMA, H., AND WARD, J. E., Patterns of contraction of distal forelimb muscles produced by intracortical stimulation in cats, Brain Research, 27 (1971) 97-109. 8 CARPENTER, D., LUNDBERG, A., AND NORRSELL, U., Effects from the pyramidal tract on primary afferents and on spinal reflex actions to primary afferents, Experientia (Basel), 18 (1962) 337-338. 9 CARPENTER,D., LUNDBERG,A., AND NORRSELL, U., Primary afferent depolarization evor, ed from the sensorimotor cortex, Acta physiol, scand., 59 (1963) 126-142. 10 DAVIES, P. W., Chamber for microelectrode studies in the cerebral cortex, Science, 124 (1956) 179-180. I1 D[LGADO, J. M. R., Hidden motor cortex of the cat, Amer. J. PhysioL, 170 (1952) 673-681. 12 DELGAOO, J. M. R., AND LIVINGSTON, R. B., Motor representations in the frontal sulci of the cat, Yah, J. Biol. Med., 28 (1955-56) 245-252. 13 ECCLES, J. C., ECCLES, R. M., AND MAGNI, F., Central inhibitory action attributable to presynaptic depolarization produced by muscle afferent volleys, J. Physiol. (Lond.), 159 (1961) 147-166. 14 HONGO, T., AND JANKOWSKA, E., Effects from the sensorimotor cortex on the spinal cord in cats with transected pyramids, Exp. Brain Res., 3 (1967) 117-134. 15 LUNDBERG, A., Supraspinal control of transmission in reflex paths to motoneurones and primary afferents. In J. C. ECCLES AND J. P. SCHAD~ (Eds.), Physiology of Spinal Neurons, Progress in Brain Research, Vol. 12, Elsevier, Amsterdam, 1964, pp. 197-221. 16 LUNDBERG,A., The supraspinal control of transmission of spinal reflex pathways. In L. WIDEN (Ed.), Recent Advances hi Clinical Neurophysiology, Electroenceph. elin. Neurophysiol., 25, Suppl. (1967) 35 46. 17 LUNDBERG, A., NORRSELL, U., AND VOORHOEVE, P., Pyramidal effects on lumbosacral interneurones activated by somatic afferents, Acta physiol, scand., 56 (1962) 221~229. 18 LUNDBERG, A., AND VOORHOEVE, P. E., Pyramidal activation of interneurones of various spinal reflex arcs in the cat, Experientia (Basel), 17 (1961) 46. 19 LUNDBERG, A., AND VOORHOEVE, P. E., Effects from the pyramidal tract on spinal reflex arcs, Acta physiol, scand., 56 (1962) 201-219. 20 SHERRINGTON,C, S., hltegrative Action of the Nervous System, Yale University Press, New Haven, Conn., 1906 (1916 IV. printing).
46 21 STONEY,S. D., JR., 'I'HOMPSON,W. D., AND ASANUMA,H., Excitation of pyramidal tract cells by intracortical microstimulation: Effective extent of stimulating current, .L Neurophy,~h,L, I I (1963) 652-669. 22 STOUT, J. D., On the motor functions of the cerebral cortex of the cat, Psychobh)logv, i (1'.~71) 177--229. 23 TOWER, S., The dissociation of cortical excitation from cortical inhibition by pyramidal section and the syndrome of that lesion in the cat, Brain, 58 (1935) 238-255. 24 UNEt~URA, K., AND PRESTON, J. B., Comparison of motor cortex influences upon various hindlimb motoneurones in pyramidal cats and primates, d. Neurophysiol., 28 (1965) 398-412. 25 VASII~ENKO,D. A., AND KOSTYUK, P. G., Functional properties of interneurones activated monosynaptically by the pyramidal tract, Neurosci. Transl., 1-4 (1967-68) 66-72.
P A T T E R N S OF C O R T I C A L M O T O N E U R O N E POOLS
PROJECTION
TO
HINDLIMB
33
MUSCLE
F L O Y D J. T H O M P S O N * AND J U L I O J. F E R N A N D E Z * *
The Rockefeller University, New York, N. Y. 10021 (U.S.A.) (Accepted April 14th, 1975)
SUMMARY
This study was undertaken to examine the organization of the hindlimb area of the motor cortex. Two specific questions were posed. The first was: are the cortical neurones which control the excitability of a given motoneurone pool localized in a small zone of cortex or are they diffuse? The second question was: does microstimulation in the hindlimb area of the motor cortex activate spinal motoneurones in a reciprocal fashion, i.e., is cortically elicited facilitation of a muscle accompanied by inhibition of the antagonist ? lntracortical microstimulation was used to condition monosynaptic reflexes of the cat hindlimb to study the organization of a cortical projection to lumbar motoneurone pools. Cortical neurones which produced facilitation or inhibition of a given monosynaptic reflex were localized within a small zone of the cortex. Facilitatory and inhibitory effective zones were found to have similar shape and size. Intracortical microstimulation elicited facilitation or inhibition of individual monosynaptic reflexes without eliciting reciprocal effects on the antagonists. The pyramidal tract was shown to play an important role in the mediation of cortically elicited facilitation as well as inhibition of the monosynaptic reflexes in the lumbar cord.
INTRODUCTION
Studies on the cortical control of the lumbar spinal cord in the cat showed that a predominant pattern of facilitation of flexor motoneurones and inhibition of
* Present address: Department of Neuroscience, University of Florida, College of Medicine, Gainesville, Fla. 32610, U.S.A. ** Present address: Catedra de Fisiologia, Universidad del Zulia, Facultad de Medicina, Maracaibo, Venezuela.
34 extensor motoneurones followed stmulation of the surface of the motor cortex l, +~'.:-~ These results were compatible with Sherrington's observation 90 which suggested thai the motor cortex controls coordinated movement through reciprocal mechanisms of the spinal cord. More recently, however, details of the cortical efferent system controlling the contraction of individual forelimb muscles in cats and monkeys were presented by Asanuma and coworkers:' 7 These investigators developed a technique for delivering current pulses to the depth of the cortex through a microelectrode. Using this method, they showed that single distal forelimb muscles could be activated by microstimulation of a small number of cortical neurones. They observed that cortical neurones which controlled the excitability of a given motoneurone pool were located within a small zone of cortex. The present study was undertaken to examine the organization of the hindtimb area of the motor cortex using microstimulation techniques. Since spinal reflex mechanisms are more easily investigated in the lumbar cord than in the cervical cord, the relationship between cortically elicited facilitation and inhibition and spinal reflex mechanisms could also be investigated. Two specific questions were posed. The first was: are the cortical neurones which control the excitability of a given motoneurone pool localized in a small zone of cortex or are they diffuse? The second question was: does microstimulation in the hindlimb area of the motor cortex activate spinal motoneurones in a reciprocal fashion, i.e., is cortically elicited facilitation of a muscle accompanied by inhibition of the antagonist? Answers to these questions were sought by studying the monosynaptic reflexes recorded from 8 branches of the sciatic nerve conditioned by microstimulation in the cortex. The results will demonstrate that discrete cortical zones exist which control the excitability of individual motoneurone pools in the absence of reciprocal effects (on other motoneurone pools tested). METHODS
Preparation Experiments were carried out on 24 adults cats of either sex weighing between 2.0 and 3.5 kg. Surgical procedures were done under pentobarbital sodium (Nembutal) anaesthesia (35 mg/kg i.p.). The left pericruciate cortex was exposed and a closed chamber of 3 cm internal diameter was mounted over the craniotomy opening with dental impression wax, and the chamber was filled with warm mineral oil t0. The cisterna magna was opened to drain cerebrospinal fluid and prevent swelling of the brain. The exposed cortex was then photographed. These photographs were used to map the location of electrode tracks made during the experiment. Spinal cord segments of L5-$2 were exposed by a routine method and dorsal roots L5-$2 on the right side were divided and the proximal ends were mounted on bipolar electrodes for stimulation. Monophasic recordings of compound action potentials were taken from 8
35 branches of the sciatic nerve sectioned at their points of entry to the muscles. The sciatic nerve branches used were: deep peroneal (D. per., flexor) superficial peroneal (S. Per., flexor) posterior tibial (P. Tib., extensor) deep posterior (D. Post., extensor) plantaris (PI., extensor) lateral gastrocnemius (L. Gast., extensor) medial gastrocnemius (M. Gast., extensor) posterior biceps semitendinosus (PBSt., flexor). The dorsal roots and nerve branches were immersed in mineral oil pools. The animal was securely held by the combination of head holder, vertebral spinous clamp, and right leg clamp. All wound margins and pressure points were infiltrated with a long lasting anaesthetic (Zyljection, Abbott). Light anaesthesia was maintained with supplemental doses of Nembutal (5 rag) intravenously at 2-h intervals throughout the experiment. After completion of surgery, the animals were paralyzed with gallamine triethiodide (Flaxedil, 5 mg/kg, i.v.) and artificial ventilation started. Body temperature was monitored with a rectal probe and kept at 35-38 °C with a heating pad.
Stimulation and recording The monosynaptic reflexes were elicited by stimulation of appropriate dorsal roots with single shocks of 0.2 msec duration and were recorded from the individual sciatic nerve branches. The amplitude of the reflex was adjusted to approximately one-third maximal amplitude to insure the availability of additional motoneurones by facilitation. Cortical areas which produce low threshold effects on lumbar spinal reflexes were localized by stimulating the cortical surface at various points and testing the spinal reflexes for facilitation or inhibition. Trains of 10 anodal pulses were delivered through a ball electrode. The duration of the train was 30 msec and the individual pulses were 0.2 msec in duration. The cortically elicited facilitation or inhibition of the test reflexes appeared on the test reflexes after a latency of 10-12 msec and was maximal 30-40 msec following the start of the cortical stimulus train of pulses. This pattern was the same for both surface and depth stimulation with the minimal current intensity stimulation parameters to be described below. The conduction-test interval was, therefore, fixed at 35 msec. Since stimulation of the cortical surface was done to delineate low threshold areas, the current intensity was initially fixed at 1 mA. When an effect on the reflexes was observed, the stimulus intensity was reduced to determine the threshold of the effect. Records of these points were made on the photographs previously taken of the cortical surface. A tungsten microelectrode 21 was then inserted into the cortex at the point of lowest threshold. These microelectrodes were composed of tungsten wires that were
36 insulated with glass except for the tips. The lengths and diameters ol tile exposed tip~ ranged between 10 and 18 #m. The electrode was advanced into the cortex in 0.2 mm steps. At each step, trains of 10 cathodal pulses (0.2 msec duration, 300 Hz, 3 ~cc intervals) were delivered to examine the effect on the spinal reflexes. A maxinlum intensity of 20/xA was used throughout the penetrations. When an effect appeared on a given reflex with 20/~A, the threshold current was determined. The spread ol a cortical area which produced an effect was mapped by making additional penetration,~ around the effective area. The determination of cortically elicited fitcilitation or inhibition of the test reflexes was determined by visually discriminating the amplitude of the control test reflex and the conditioned test reflex displayed on ~ln oscilloscope. Whenever the amplitude of the monosynaptic reflex was unstable, and when threshold determinations required precise comparison of control and conditioned test reltex amplitude, a Nicolet signal averaging computer was used to compare the control and conditioned test reflexes. At the end of each experiment, several lesions were made in the cortex by passing a negative current of 20 #A tbr 5 sec. These lesions were used as reference points during histological examination to reconstruct the electrode tracks. The animal was then killed with an overdose of pentobarbital and the portion of the brain where the penetrations were made was removed and fixed in 101'i, formalin. Frozen sections of 50/tin thickness were made and stained by the KliJver-Barrera method.
Roslro/
f.
"%
i J
P.C Dimple
Fig. 1. Shaded region shows the location of the cortical area within which the lowest threshold points were found for producing facilitation or inhibition of the spinal reflexes by surface stimulation. DOtdash line encloses the explored cortical area.
37 TABLE I D I S T R I B U T I O N OF EFFECTIVE FOCI IN E A C H NERVE T E S T E D ; S U R F A C E S T I M U L A T I O N
Surface focus listed is the lowest threshold point for producing a given effect in any given experiment. No. of exp. the = number of experiments in which the effect was examined; % appear. -- No. of loci/ No. of exp, x 100%; ~ facil. -- No. of facil, foci/No, of foci for that reflex x 100~; ~ inhib. No. of inhib, foci/No, of loci for that reflex x 100%. Nerve
No. o f exp.
Facilitation No. o f foci
Inhibition % appear,
% fiwil,
No. o f foci
% appear,
"/ o inhib.
D. Per. S. Per. P. Tib. D. Post. PI.
7 8 13 15 14
2 3 8 7 0
28.6 37.5 61.5 46.6 0
100 75.0 72.7 46.6 0
0 1 3 8 10
0 12.5 23.0 53.4 71.4
0 25.0 27.0 53.4 100
M. Gast. L. Gast. PBSt Total Flex. Ext.
9 10 8 84 23 61
2 3 5 30 10 20
22.2 30.0 62.5 35.7 43.5 32.8
25.0 30.0 83.4 45.4 83.4 37.0
6 7 I 36 2 34
66.6 70.0 12.5 42.8 8.7 55.7
75.0 0 16.6 54.6 16.6 63.0
RESULTS
T h e results will be presented in two parts. The first p a r t will present the d a t a regarding the general features o f the effective cortical areas. The second will present d a t a concerning the r e l a t i o n s h i p between cortically elicited facilitation a n d inhibition o f flexor and extensor reflexes.
(I) General features of the effective cortical areas (A ) Preliminary surface mapping Localization of low threshold points. S t i m u l a t i o n o f the cortical surface p r o d u c e d facilitation o r inhibition o f the l u m b a r spinal reflexes, In all cats, the lowest t h r e s h o l d points a p p e a r e d on the b a n k o f the p o s t e r i o r sigmoid gyrus, 1.5-4.5 m m from the midline o f the brain. This is shown in Fig. 1. The s h a d e d region represents the cortical area in which the lowest t h r e s h o l d p o i n t s p r o d u c e d facilitation or inhibition o f one or m o r e spinal reflexes with c u r r e n t intensities o f 200-400 # A in cats. In one cat, the t h r e s h o l d was 600 # A . The 1.5-2.0 m m p o r t i o n o f the p o s t e r i o r sigmoid gyrus immediately a d j a c e n t to the midline, i.e. lateral to the midline, was covered by bone. This was left intact to p r e v e n t i n t e r r u p t i o n o f the superior sagittal sinus. S t i m u l a t i o n o f the cortex at p o i n t s outside the region indicated on the p o s t e r i o r sigmoid gyrus (Fig. 1) elicited facilitation or inhibition o f the spinal reflexes, but required higher current intensities to p r o d u c e the effects. T h e total cortical area e x p l o r e d is enclosed within the d o t - d a s h line in Fig. 1. S t i m u l a t i o n o f the cortical surface within the s h a d e d area could p r o d u c e
38
20
18
16
14
12
I0
B
0 8
=. 6
E
2
o
. . . .
,;
. . . .
~? ,
.
..~;
D,,p~t,/mml
A Fig. 2. A: threshold current intensities for facilitation of the reflex in D. Post. are plotted against depth in the cortex. B: the vertical line marks the path of a reconstructed microelectrode track. The horizontal bar marks 0.2 mm intervals. Microstimulation at 3 points marked with circles produced facilitation of the reflex in D. Post.
facilitation and/or inhibition of several spinal reflexes. The number of spinal reflexes on which effects were elicited was reduced by decreasing the stimulus intensity. Facilitation or inhibition of a single reflex could be produced by reducing the intensity of stimulation in 11 of 16 cats. In 5 cats, a minimum of 2 reflexes was facilitated or inhibited from the lowest threshold point, and could not be separated by reducing the stimulus intensity. The effects on spinal reflexes produced by surface stimulation are summarized in Table 1.
(B) Intracortical microstimulation After determining the low threshold area on the surface of the cortex which produced facilitation or inhibition of the spinal reflexes a tungsten microelectrode was inserted into the centre of the area. The effects of microstimulation in the depth of the cortex were examined by using stimulation current of 20 #A or less. The threshold for facilitation or inhibition of a given reflex usually decreased sharply within a specific cortical region which corresponded to a consecutive sequence of stimulated points. An example of these findings is illustrated in Fig. 2A and B. The microelectrode track through the posterior sigmoid gyrus was reconstructed from the histology in Fig. 2B. The horizontal bars mark the 0.2 mm points where the effects of stimulation were tested on the reflexes. Mierostimulation at the 3 points marked by circles produced facilitation of the reflexes in D. Post. The changes in threshold are shown in Fig. 2A. Additional penetrations were made in this cortical area to determine the distribution of low threshold points for facilitation of the same reflex. Microstimulation with 20 #A or less produced facilitation in restricted regions in 9 of 14 additional
39
A13413
o o~
Post
l
A Fig. 3. Distribution of low threshold points within the cortex. A : several penetrations reconstructed in a sagittal section of the posterior sigmoid gyrus. Circles mark points at which microstimulation with 20/~A or less produced facilitation of the reflex in D. Post, Solid circles mark points which produced facilitation with less than 10/tA. The penetration identification numbers appear at the surface. B: schematic drawing of postcruciate gyrus which shows the location of the surface points of the penetrations relative to the midline to the sulcus, and to each other.
penetrations. The distribution of low threshold points is shown in Fig. 3A and B. Out of 14 penetrations, 4 passed through points which facilitated the reflex in D. Post. with microstimulation of less than 10 #A, and the minimum threshold points are indicated by filled circles in these penetrations. As shown in Fig. 3A, the low threshold points for a given reflex were located in a small area in the cortex. The edge of the zone was irregularly shaped, but the lowest threshold points (filled circles Fig. 3A), TABLE II DISTRIBUTION OF EFFECTIVE FOCI IN EACH NERVE TESTED; DEPTH STIMULATION Each focus is the lowest threshold point for producing a given effect in any given experiment by ICMS. No. of exp., ~ appear., ~ facil., and ~ inhib, were calculated by the same method described in the legend to Table I.
Nerve
D. Per. S. Per. P. Tib. D. Post PI. M, Gast. L. Gast. PBSt Total Flex. Ext.
No. o f exp.
7 8 13 15 14 9 10 8 84 23 61
Facilitation
Inhibition
No. of
%
No. of
%
%
foci
appear,
facil,
%
foei
appear,
inhib.
2 2 4 6 3 2 2 3 24 7 17
28.6 25.0 30.8 40.0 21,4 22.2 20.0 37.5 28.6 30.4 27.9
66.7 66.7 100 75.0 27.3 50.0 100 50.0 58.6 63.6 56.6
1 I 0 2 8 2 0 3 17 4 13
14,3 12.5 0 13.3 57.1 22.2 0 37.5 20.2 17.4 21.3
33.3 33.3 0 25.0 72.7 50.0 0 50.0 41.4 36.4 43.4
40 .... J J
..... . .
j/j
J
J
x 8~ 4
1
2
Fig. 4. Three-dimensional reconstruction of low threshold area within a block of cortex from the bank of the posterior sigmoid gyrus. Thickened vertical bars enclose effective points encountered in the reconstructed penetrations. Dotted line encloses the volume of cortex whichrepresents the approximal spatial extent of the inhibitory effective zone for D. Post. found in the centre of the zone, formed a narrow band oriented parallel to the cortical radial fibres. For convenience, low threshold points within the cortex which produced facilitation or inhibition of a given reflex will be referred to as effective points for that reflex, and the collection of such effective points will be referred to as the effective zone for that reflex. Altogether, 41 effective zones were mapped out from 16 cats. Of these, 24 were facilitatory and 17 were inhibitory. Table II summarizes these results. Facilitatory zones were encountered for all reflexes. Inhibitory zones were encountered for all of the reflexes except P. Tib. and L. Gust. To construct ~i three-dimensional configuration of a given effective zone, many penetrations were made into a zone and the effective volume was reconstructed by histological examination. This was done for 6 different effective zones in 3 different cats, and an example is shown in Fig. 4. The numbers on the surface indicate the points of the penetrations and the thin vertical lines mark the electrode tracks. The thickened vertical lines show effective areas encountered in the penetrations. Microstimulation in these areas with 20 /~A or less produced inhibition of the reflex in D. Post. As shown, the inhibitory area was encountered in penetrations l, 3, 4, 7, 8, and 5 but not in penetrations 2 and 6. The dotted lines in Fig. 4 enclose the effective volume of the cortex revealed by the penetrations as the inhibitory zone for D. Post. The narrow edge of the wedge extended to the edge of the grey matter and the other end of the wedge was located at 0.5 mm depth beneath the surface of the cortex. Microstimulation in the superficial cortical layer with 20/~A or less did not produce effects on the reflexes in this or any of the other experiments. The approximate size of this
41 PENETRATION
No
5
n
Tibialis [Focilitotioqt)
Deep Posterior (Inhibition) I0
2.
3.0
B A
Fig. 5. Overlap of two effective zones. The drawing in B shows the surface points of 6 penetrations which have been schematically reconstructed in A. The thickened vertical bars enclose effective points for inhibition of the reflex in D. Post. (black bar) and facilitation of the reflex in P. Tib. (white bar).
effective zone is 1.0 m m × 1.3 mm at the wider top, becoming smaller at the deeper end. Altogether, 5 effective zones were explored and the size of the other effective zone was similar to the one shown in Fig. 4. The size of their larger ends ranged from 0.8 to 1.8 sq.mm and averaged 1.2 sq.mm. Their lengths along the radial fibres ranged from 1.4 to 2.0 mm and averaged 1.6 mm. Considerable spatial overlap of one effective zone onto the other was observed in these effective zones, an example of which is shown in Fig. 5. Details of this figure and the interaction of overlapping effective zones will be presented in part II of the Results.
(lI) Relationship between cortically elicited facilitation and inhibition of.flexor and extensor reflexes It has been shown that the fringes of the effective zones overlapped with other effective zones. More specifically, microstimulation with 20 # A or less frequently influenced more than one reflex. An example of overlapping effective zones is illustrated in Fig. 5. The drawing in Fig. 5B shows the surface points where penetrations were made and the electrode tracks are reconstructed in Fig. 5A. The filled bars represent areas for inhibition of the reflex in D. Post., the open bars for facilitation of the reflex in P. Tib. ; in penetrations 3, 5, 7, and 8, these bars overlapped. The overlap was minimal in penetrations 5, 7, and 8, i.e., at the effective point, and more extensive in penetration 3. However, within the overlapping areas, the threshold current for producing the effects on the two reflexes was different. The lowest threshold point (i.e. the focus) for producing inhibition in D. Post. was encountered in penetration
42 TABLE III DISTRIBUTION
{}F O V E R L A P P I N { i
P A I R S OF EFFE{7[IVE Z O N E S
Each overlapping pair was tabulated according to the flexor (Flex} or extensor (Ext) muscle c!assiti cation of the individual reflexes, and according to the combination of facilitation (F) and inhibition (I) observed in each pair. F ~F Flex ! flex Flex text Ext i ext
1 12 7
I
I 8 3
IF~ {} 3 3
No. 1 at the depth of 2.4 mm and for facilitation in P. Tib. was in penetration No. 5 at 1.4 mm depth. There was no overlap in penetrations 1 and 4. These observations indicate that although the fringes of these two effective zones overlapped, the centres of the zones were separate. Spatial overlap of this sort was observed in 33 of the 41 effective zones examined. A table was made to explore the possibility that these overlaps coincided with a reciprocal pattern of facilitation and inhibition. The overlap was tabulated according to the functional classification of the muscles involved and also according to the combination of the effects observed. For example, the overlap shown in Fig. 5 for the 2 reflexes in extensor nerves was classified as Ext + Ext F -~ 1. As shown in Table I11, there was no systematic pattern of agonist facilitation combined with antagonist inhibition. On the contrary, the F ,-.' 1 combinations for the antagonists were outnumbered by the F ~- F combinations. This indicated that microstimulation produced effects on a given spinal reflex independently of systematic reciprocal effects on the reflex of the antagonist.
Pyramidal tract section The role of the pyramidal tract in mediating the cortically elicited facilitation and inhibition of lumbar spinal reflexes was examined in 3 cats. This was tested by comparing the threshold for cortically elicited effects on lumbar spinal reflexes before and after transection of the ipsilateral medullary pyramid. The pyramidal tract was sectioned progressively by making small cuts in the exposed ventral surface while delivering microstimulation. Fig. 6A shows the relationship between stimulus intensity and the amplitude of cortically elicited inhibition in M. Gast. before and after pyramidal transection. Before transection, microstimulation with 20 #A intensity completely inhibited the reflex in M. Gast. After making a small cut, the threshold value increased, a n d the amount of inhibition with the same intensity decreased approximately 50 ~ of the presection control. After pyramidal transection, the threshold value for inhibition increased to 20 #A, and at this intensity, the amount of inhibition was only 107/;, of the control reflex. Facilitation of L. Gast. and D. Per. were tested in the other two experiments.
43 100
'
•
o ~.~
~ ....
o
........
o
"o
\\ 80-
"A . . . . .
Ax \ \
•
o After Section
• After Portlol Section \
• Before Section
60'
40'
~ 20
..........".).i\. ........... ; ~ i
;o ;2 ,; ~ ;8 io
; ;
Stim. Intenslt y / ~ A /
A
Fig. 6. Effects of pyramidal section on cortically elicited inhibition. A: Relationship between ICMS intensity and amplitude of cortically elicited inhibition on the reflex in M. Gast. before, after partial transection, and after transection of the ipsilateral medullary pyramid. B : drawing of the histological section of the brain stem at the level of the transection.
In the former, the threshold value tripled and in the latter, the value doubled following transection of the pyramid. These results indicate that the pyramidal tract plays an important role in the mediation of cortical effects elicited by microstimulation on the lumbar reflexes. DISCUSSION
The existence of a radial arrangement of neurones within the depth of the motor cortex which controls the contraction of individual forelimb muscles was shown by Asanuma and Sakata 6. The present experiments have shown the existence of a similar organization in the hindlimb area of the motor cortex. In addition, it was shown that there exist discrete inhibitory zones. The shape and size of an inhibitory zone mapped out within the depth of the cortex was similar to that of the facilitatory zones. It is well known that the use of the train stimulus parameters favours temporal summation. For this reason, low intensity currents can be used instead of large current single shock stimuli. It is a natural question to ask whether the summation produced by train stimulation occurred at the level of the cortex or at the level of the spinal cord. This experiment has shown that the low threshold points for eliciting effects on individual spinal test reflexes were highly localized within the cortex, but cannot conclusively answer questions regarding spinal cord organization. Neuronal recruitment could be taking place in the cortex and/or in the spinal cord. Anderson e t al. 3,4 and also Carpenter e t al. s,9 have reported cortically elicited primary afferent depolarization. Depolarization of this nature has been shown by Eccles e t al. 13 to have a close parallelism to presynaptic inhibition. However, Andersen e t al. 3,¢ and Carpenter e t aL 8,9 reported that cortical stimulation did not depolarize group Ia afferents. Since the monosynaptic test reflex could be completely suppressed
44 by microstimulation (control in Fig. 6), it is unlikely that this inhibition was produced by cortically elicited primary afferent depolarization of the group la afl'erents. The lowest threshold points for eliciting facilitation or inhibition of the spinal reflexes were found on the postcruciate gyrus. Although effects could also be produced by stimulation of certain points on the medial portion of the precruciate gyrus m some experiments, the threshold intensities required were always higher than those for points on the postcruciate gyrus. The bank of the postcruciate gyrus I-2 mm from the cruciate sulcus provided lower threshold points than more caudal portions of the gyrus. Low threshold zones were delimited deep in the bank of the postcruciate gyrus by microstimulation. Localization of the hindlimb area of the motor cortex to the banks of the cruciate sulcus was originally reported by Stout 22 and confirmed by Delgado tl and Delgado and Livingston to. The present experiment is in agreement with the previous lindings and, in addition, localized the lowest threshold point~ Io the postcruciate gyrus for lumbar spmal reflexes. Hongo and ,lankowska a~ recently studied cortically elicited effects that were transmitted to lumbar spinal reflexes by extrapyramidal pathways in pyramidotomized cats. The threshold intensities for producing the effects were comparable to those observed for intact preparations. However, the lowest threshold cortical area for producing the effects in pyramidotomized cats was located posterior to the postcruciate dimple and did not overlap the low threshold area observed in this experiment which was on the bank of the postcruciate gyrus. In the present experiment, the pyramidal tract was shown to be important in the mediation of both thcilitation and inhibition elicited by intracortical microstimulation. Tower ez reported that inhibition elicited by stimulation of the cortical su,Tace was preserved and facilitation abolished following section of the pyramidal tract. However, in the experiment shown in Fig. 6. the powerful inhibition of M. Gast. elicited by intracortical microstimulation was almost completely abolished by pyramidal section. The tindings in the present experiment show that the pyramidal tract is also important in the mediation of inhibition of lumbar spinal reflexes elicited by intracortical microstimulation. It has been shown by Lundberg and coworkers t~' t0 that stimulation of the cortical surface activates spinal interneurones which are intercalated along the segmental reflex pathways to motoneurones. Activation of these interneurones produced a pattern of flexor facilitation combined with extensor inhibition.On the other hand, the effects elicited by microstimulation were transmitted to individual motoneurone pools without systematically producing reciprocal effects on the antagonists. This suggests that the facilitation or inhibition produced by microstimulation is transmitted by spinal interneurones which are not involved in the segmental reflex pathways. On the basis of recording from spinal interneurones which were activated exclusively by cortical stimulation, Vasilenko and Kostyuk 25 proposed that 'the pyramidal system is linked in the spinal cord with a specialized internuncial apparatus, to a large measure independent of the internuncials responsible for segmental motor reflexes'. The findings of the present experiments are compatible with this proposal. The present experiments revealed the existence of st cortical efferent system which controls the excitability of individual motoneurone pools in the lumbar cord in
45 a d d i t i o n to t h e o n e r e p o r t e d by L u n d b e r g a n d c o w o r k e r s 15-19 a n d A g n e w a n d c o w o r k e r s t , '~. ACKNOWLEDGEMENTS
T h e a u t h o r s w i s h to t h a n k Dr. H. A s a n u m a f o r helpful d i s c u s s i o n a n d v a l u a b l e criticism t h r o u g h o u t t h e e x p e r i m e n t s , and also to t h a n k D r . C. G h e z for his useful c o m m e n t s o n the m a n u s c r i p t . T h i s r e s e a r c h was s u p p o r t e d by N I H G r a n t NS-10705.
REFERENCES 1 AGNEW, R. F., AND PRESTON, J. B., Motor cortex - - pyramidal effects on single ankle flexor and extensor motoneurones of the cat, Exp. Neurol., 12 (1965) 384-398. 2 AGNEW, R. F., PRESTON,J. B., AND WHITLOCK, D. G., Patterns of motor cortex effects on ankle flexor and extensor motoneurones in the 'pyramidal' cat preparation, Exp. Neurol., 8 (1963) 248-263. 3 ANDERSON,P., ECULES,J. C., AND S~ARS, T. A., Presynaptic inhibitory action of cerebral cortex on the spinal cord, Nature (Lond.), 194 (1962) 740-741. 4 ANDERSON, P., ECCLES, J. C., AND SEARS, T. A., Cortically evoked depolarization of primary afferent fibers in the spinal cord, J. Neurophysiol., 27 (1964) 62-77. 5 ASANUMA, H., AND ROSI~N, 1., Topographical organization of cortical efferent zones projecting to distal forelimb muscles in the monkey, Exp. Brain Res., 14 (1972) 243-256. 6 ASANUMA, 1-1., AND SAKATA, H., Functional organization of a cortical efferent system examined with focal depth stimulation in cats, J. Neurophysiol., 30 (1967) 34-54. 7 ASANUMA, H., AND WARD, J. E., Patterns of contraction of distal forelimb muscles produced by intracortical stimulation in cats, Brain Research, 27 (1971) 97-109. 8 CARPENTER, D., LUNDBERG, A., AND NORRSELL, U., Effects from the pyramidal tract on primary afferents and on spinal reflex actions to primary afferents, Experientia (Basel), 18 (1962) 337-338. 9 CARPENTER,D., LUNDBERG,A., AND NORRSELL, U., Primary afferent depolarization evor, ed from the sensorimotor cortex, Acta physiol, scand., 59 (1963) 126-142. 10 DAVIES, P. W., Chamber for microelectrode studies in the cerebral cortex, Science, 124 (1956) 179-180. I1 D[LGADO, J. M. R., Hidden motor cortex of the cat, Amer. J. PhysioL, 170 (1952) 673-681. 12 DELGAOO, J. M. R., AND LIVINGSTON, R. B., Motor representations in the frontal sulci of the cat, Yah, J. Biol. Med., 28 (1955-56) 245-252. 13 ECCLES, J. C., ECCLES, R. M., AND MAGNI, F., Central inhibitory action attributable to presynaptic depolarization produced by muscle afferent volleys, J. Physiol. (Lond.), 159 (1961) 147-166. 14 HONGO, T., AND JANKOWSKA, E., Effects from the sensorimotor cortex on the spinal cord in cats with transected pyramids, Exp. Brain Res., 3 (1967) 117-134. 15 LUNDBERG, A., Supraspinal control of transmission in reflex paths to motoneurones and primary afferents. In J. C. ECCLES AND J. P. SCHAD~ (Eds.), Physiology of Spinal Neurons, Progress in Brain Research, Vol. 12, Elsevier, Amsterdam, 1964, pp. 197-221. 16 LUNDBERG,A., The supraspinal control of transmission of spinal reflex pathways. In L. WIDEN (Ed.), Recent Advances hi Clinical Neurophysiology, Electroenceph. elin. Neurophysiol., 25, Suppl. (1967) 35 46. 17 LUNDBERG, A., NORRSELL, U., AND VOORHOEVE, P., Pyramidal effects on lumbosacral interneurones activated by somatic afferents, Acta physiol, scand., 56 (1962) 221~229. 18 LUNDBERG, A., AND VOORHOEVE, P. E., Pyramidal activation of interneurones of various spinal reflex arcs in the cat, Experientia (Basel), 17 (1961) 46. 19 LUNDBERG, A., AND VOORHOEVE, P. E., Effects from the pyramidal tract on spinal reflex arcs, Acta physiol, scand., 56 (1962) 201-219. 20 SHERRINGTON,C, S., hltegrative Action of the Nervous System, Yale University Press, New Haven, Conn., 1906 (1916 IV. printing).
46 21 STONEY,S. D., JR., 'I'HOMPSON,W. D., AND ASANUMA,H., Excitation of pyramidal tract cells by intracortical microstimulation: Effective extent of stimulating current, .L Neurophy,~h,L, I I (1963) 652-669. 22 STOUT, J. D., On the motor functions of the cerebral cortex of the cat, Psychobh)logv, i (1'.~71) 177--229. 23 TOWER, S., The dissociation of cortical excitation from cortical inhibition by pyramidal section and the syndrome of that lesion in the cat, Brain, 58 (1935) 238-255. 24 UNEt~URA, K., AND PRESTON, J. B., Comparison of motor cortex influences upon various hindlimb motoneurones in pyramidal cats and primates, d. Neurophysiol., 28 (1965) 398-412. 25 VASII~ENKO,D. A., AND KOSTYUK, P. G., Functional properties of interneurones activated monosynaptically by the pyramidal tract, Neurosci. Transl., 1-4 (1967-68) 66-72.
