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Effects of neurotropic drugs and cerebellar impulses on cortically stimulated phasic and tonic muscular activity
EXPERIMENTAL
2, 429-441
NEUROLOGY
Effects
of
Impulses
Neurotropic on
Drugs
Cortically
Tonic
lnstitutul
(1960)
and
Stimulated
Muscular ZUCKERMANN
de Neurologie,
I. P. Pavlov, March
Phasic
and
Activity
EMIL
Received
Cerebellar
Bucuresti,
Romania
16, 1960
Investigations were performed on awake cats and rats to study: the type of muscular response to trains of stimuli applied on the motor area at a frequency of 1 to 30 per second; and the influence exerted on this response by iterative stimulation of the paleocerebellum as well as by certain drugs depressing the central nervous system (chloralcse, Luminal, chlorpromazine). Cortical stimulation at 1 to 6 per second induced only clonus synchronous with stimulation rhythm, without changing myographic background level. The electromyogram after four to ten stimuli revealed a low-voltage asynchronous tonic activity following each clonic discharge during a period of 100 to 5OOmsec. At frequencies between 6 and 15 per second, this tonic activity changed the background trace of the myogram, clonus being superimposed on a sustained muscular contraction. The drugs in subanesthetic dcses strongly depress tonic without affecting clonic activity. Muscular response consequently maintained its clonic aspect, synchronous with stimulation rhythm up to a frequency of 25 or 30 per second. Iterative stimulation of the paleocerebellum with subliminal stimuli, when simultaneous with cortical stimulation, induced a strong inhibition of clonic, as well as tonic components of muscular response. When cortical stimulation was applied 1 to 50 set after arresting cerebellar stimulation an important facilitation of muscular response occurred, particularly affecting its tonic component. Introduction
Early experiments on motor cortex excitability revealed that lowfrequency stimulation (below 1S-20/set) induces muscular jerks synchronous with stimulation rhythms (23) instead of smooth, organized movements specific to higher stimulation frequencies. Later, the phenomenon was observed by other investigators, without being subjected, however, to a systematic analysis (3, 7, 12, 17). Study of activity of final motor neurons elicited by such cortical stimuli could yield valuable in429
430
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formation concerning the mechanism by which the motor cortex affects the final common path. It was assumed that simple neuronal processes, causing smooth movements at high frequencies, can be detected by gradually increasing lowfrequency stimulation. The indicators used to analyze the phenomenon were the myogram and the electromyogram (EMG) of cortically activated muscles. Certain disadvantages of this method have been pointed out (8, 9, IO). It does not permit study of final neurons in the subliminal fringe, and conveys only indirect information on the type of alpha neurons involved in muscular activity. Its use is justified, however, in the initial stage of investigation by an important advantage, namely, that of permitting a general evaluation of efferent spinal neuron activity. The present article reports the first stages of investigation: myographic and electromyographic effects caused by trains of rectangular cortical stimuli (0.1 msec) of liminal intensity and frequencies between 1 and 30 per second; modification of these effects by subanesthetic doses of drugs depressing activity of the central nervous system (Luminal, chloralose, chlorpromazine) ; and modification of muscular effects by iterative paleocerebellar stimulation before, during, or after onset of cortical shocks. The latter two conditions were investigated in the hope of obtaining additional data concerning physiological significance of muscular response to cortical stimulation. Various components of this response were expected to appear unequally sensitive to neurotropic drugs. Considering the importance of the cerebellum in adjusting muscular posture and synergy, it was thought that interference of cortical and cerebellar impulses should cause changes in the usual response to cortical stimuli. In this way, details of the mechanism of spinal-neuron activation by cortical stimulation, as well as new data referring to the long-debated question of corticocerebellar relations, could be obtained. Method Condition A. Myographic and EMG responses to cortical liminal stimuli were studied in 2.5 cats and 30 rats. Stimulation frequencies ranged from 1 to 30 per second (0.1 m&pulse). After obtaining constant responses, drugs were administered intraperitoneally in 3-ml volumes (Luminal, 15-30 mg/kg body weight in 12 cats and 12 rats; chloralose, 15-40 mg/kg in 6 cats and 8 rats; chlorpromazine, S-10 mg/kg in 7 cats
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and 10 rats). Responses to identical cortical stimuli were again studied at intervals of 30 and 60 min. Condition B. After establishing the myographic and EMG responses of 25 cats to cortical stimuli at a frequency of 1 to 6 per second, the effect of an iterative cerebellar stimulation on these responses was observed. The intensity of this stimulation was subliminal, from the motor point of view. Cortical and cerebellar stimulating and recording electrodes were copper wires 150 u in diameter, insulated except at the tips. They were implanted in the cortical motor area and in the medial homolateral vermis at the boundary between culmen and simplex. Interfocal distance was 1 mm in all cases. Recording electrodes for the EMG, of silver wire insulated except the 2-mm tip, were implanted in the muscle. Distances between electrodes were 6 to 8 mm in the cat and 2 to 4 mm in the rat. Five cats (A) had additional stimulating electrodes implanted into the mesencephalic reticular formation; in 15 cats (B) , respiration and carotid blood pressure were also simultaneously recorded. A preparatory operation (under ether) was always performed several hours before the main experiment. At the beginning of the experiment, the awake animal was immobilized in a special frame. The EEG from the sigmoid gyrus, as well as the EMG of the extensor carpi and flexor digitorum muscles were recorded on a Schwartzer encephalograph. The EMG was taken either bilaterally or from the limb contralateral to the stimulated cortex. The myogram was recorded by means of an isotonic system, either on a kymograph or encephalograph, through a piezoelectrical crystal. Cerebellar stimulation was performed (B) with sine waves at frequencies of 20, 100, 200, and 500 cycles per second, the intensity being subliminal in all respects (somatomotor, respiratory, and vasomotor) . Results
and
Comments
Muscular Activity Elicited by Trains of Low-Frequency Cortical Stimuli. As seen in Fig. 1A and B, muscular activity elicited by cortical stimulation at 1 to 4 per second displayed characteristic jerks, appearing with a small latency after each shock. Amplitude and duration of jerks were constant throughout cortical stimulation. The myogram revealed no change in muscular tension in the interval between jerks. The EMG showed that the first 4 to 10 cortical shocks induced an ample synchronous volley of muscular discharges at the moment of the jerk, and no other
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activity in the interval between jerks. The next shocks, however, caused an additional muscular activity (we shall conventionally call it tonic), displaying asynchronous discharges of small amplitude and low frequency; these discharges followed each clonic burst and lasted 100 to 500 msec. Appearance of tonic activity had no influence on amplitude or duration of clonus. Amplitude and duration of tonic activity increased with the
A2
-:
FIG. 1. Myographic and electromyographic effects induced by low-frequency stimulation of motor cortex: A, stimulation frequency l/set, rectangular pulses of 0.1 msec and liminal intensity (left sigmoid gyrus, cat 12) ; I, myogram of right forelimb (flexion of wrist) ; 2, EMG of right flexor digitorum; 3, EMG of right extensor carpi; calibrations, 1 set and 2OOpv. B, stimulation frequency 3 set, rectangular pulses of 0.1 msec and liminal intensity (left sigmoid gyrus, cat 12) ; same legend as in A. C, stimulation frequency IOsec, rectangular pulses of 0.1 msec and liminal intensity (left motor cortex, rat 17) ; myogram of right forelimb (flexion of wrist). D, stimulation frequency 2Osec, rectangular pulses of 0.1 msec and liminal intensity (left motor cortex, rat) ; same legend as in C; see text for details.
number of shocks in a volley. It is remarkable that the number of stimuli necessary to cause this tonic activity decreased with increase in frequency; e.g., in cat 18, at a frequency of 1 per second, tonic activity appeared after eight stimuli; at 2 per second, after six stimuli; and at 3 per second, after four stimuli. At frequencies higher than 4 per second, tonic activity between clonuses increased, causing obvious mechanical changes in the interval between. When the frequency reached about 7 to 9 per second in the cat and 12 to 14 per second in the rat, the response was a characteristically irregular
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tonic contraction, on which clonuses were superimposed, still faithfully following the stimulation rhythm (Fig. 1C). Higher frequencies ( lS/sec in cats; 20/set in rats) led to sustained tonic movements lasting throughout stimulation and sometimes interrupted by phasic movements; however, these were not synchronous with stimulation rhythms (Fig. 1D). At stimulation intensities immediately below those causing appearance of clonus, six to twelve stimuli induced a low-voltage tonic asynchronous muscular activity, similar to that usually appearing after each clonus (Fig. SB). We are thus facing two types of cortically-induced muscular activity: tonic and clonic, It is possible that the tonic activity represents merely an afterdischarge of neurons, intensely and synchronously active during the clonus. But this is improbable, because the increase in tonic activity due to stimulation by a larger number of impulses was not accompanied by a corresponding increase in clonic activity. On the other hand, a purely tonic response could be obtained; this suggests that tonic activity may have been due to activation of other spinal neurons than those causing clonic activity. In any case, appearance of tonic, in contrast to clonic activity always required a summation of cortical stimuli (3-10 shocks). In these conditions, a continuous flow of asynchronous impulses reached the level of final neurons, assuring their sustained activity even in the interval separating cortical stimuli. This multiplication of impulses was probably achieved by activation of certain cortical and/or subcortical reverberating chains. The level at which the summation process determining tonic activity occurs cannot be ascertained on the basis of present data. Paton and Amassian’s (16) investigations showed activation of cortical reverberating chains, and repetitive discharges in the pyramidal tract appeared following the first shock in a volley. Perhaps in our cases, summation processes occurred at a medullary level, where iterative activity appeared only after the fourth to tenth shocks in the train. The process could also be assumed to occur only after activation of certain subcortical extrapyramidal neurons by addition of pyramidal impulses reaching them through collateral pyramidal fibers. The change of muscular response from a clonic to a smooth sustained one, required a larger increase of stimulation frequency in the rat than in the cat. This difference was probably due to a larger number of interneurons in the cortical reverberating chains and, consequently, to an
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increase in number of impulses reaching spinal neurons after each cortical stimulus in the latter species. Eflect of Drugs Depressing the Central Nervous System. All three drugs in subanesthetic doses had similar effects on the phenomena. An appreciable decrease or even complete disappearance of tonic activity between clonuses was observed when using frequencies of 1 to 4 per second; however, the amplitude and duration of clonus did not diminish by more than 5 or 10 per cent (Fig. 2A and B) . The type of movement, consisting
C
Irn-
FIG. 2. Effect of 20mg/kg body weight Luminal on myographic and electromyographic effects induced by low-frequency stimulation of motor cortex: A, stimulation frequency l/set, rectangular pulses of 0.1 msec and liminal intensity (left sigmoid gyrus, cat 12, as in Fig. 1; 30min after i.p. administration of Luminal) ; same legend as Fig. IA. B, stimulation frequency 3 set, rectangular pulses of 0.1 msec and liminal intensity (left sigmoid gyrus, cat 12, as in Fig. 1; 45 min after i.p. administration of Luminal; same legend as Fig. IA. C, Stimulation frequency 20/set, rectangular pulses of 0.1 msec and liminal intensity (left motor cortex, rat I?), as in Fig. 1C; 45 min after i.p. administration of Luminal; same legend as Fig. 1C; see text for details.
of clonus without tonic modifications, changed to a smooth organized one at appreciably higher frequencies than before administering the drug. A clonus responsesynchronous with the stimulation rhythm was obtained even at 20 to 30 per second in the rat (Fig. ZC), although the normal responseat this frequency had been an organized movement displaying no discernible clonus (Fig. 1D). The amplitude of clonus was unchanged or diminished, at most, by 5 or 10 per cent. The above data may indicate that the sensitivity of neurons causing tonic activity is qualitatively different from that of neurons determining clonic activity. The lower sensitivity of clonic activity to depressive
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drugs is probably due to the fact that it occurs by paucisynaptic or even monosynaptic pathways, as appears from certain investigations by Bernhard and Bohm ( 1). High sensitivity of tonic activity may be explained by occurrence through polysynaptic pathways (6), rather than by a higher sensitivity of tonic spinal neurons. This assumption is based on two series of data: those of Granit (9)) showing that in the decerebrate or spinal animal large doses of anesthetic or chlorpromazine lead to a “tonic change” of spinal neurons; and of Schlag (19), showing that drugs depress pyramidal I-waves without affecting D-waves. Further experimental evidence in favor of our assumption was obtained on five cats whose cortex and mesencephalic reticular formation were stimulated with identical parameters. Threshold intensities were established, giving similar responses when stimulating the two structures. Drugs were then administrated as in other animals. The tonic component was always considerably more depressed in the case of cortical than in reticular stimulation, which proves that the inhibitory process did not occur at spinal level, but rather at the level of the directly stimulated structure. Eflects of Cerebellar Stimulation on Muscular Response Elicited by Low Frequency Cortical Stimulation. Coincidence of cerebellar (sine waves of 100-500 c/set) and cortical stimulation (l-l/set) always caused an important inhibition of clonic, as well as tonic responses (Fig. 3). The effect was the same, whether the two stimuli started simultaneously or one preceded the other by 0.5 to 10 sec. The inhibitory effect disappeared immediately after discontinuing cerebellar stimulation. When the cortical stimulus was applied after cessation of the cerebellar one, a marked facilitatory effect appeared (Figs. 3 and 4), exhibiting the following peculiarities: Although amplitude and duration of clonus remained unchanged for the first ten to twelve shocks in the cortical train, tonic activity was precocious, displaying longer duration and higher intensity following each cortical shock (Fig. 4A and B). Thus, before stimulating the cerebellum of animal 38, tonic activity appeared after eight stimuli, its duration (between stimuli 8 and 11) was 150 to 200 msec following each clonus, and the average amplitude reached 150 pv. When the same cortical train was started 5 set after cessation of cerebellar stimulation (500 c/set) , tonic activity appeared after four stimuli, its duration (between stimuli 4 and 7) was 400 msec, and its average amplitude, 300 pv.
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When a train of cortical stimuli following a previous cerebellar stimulation was prolonged, it was found that after 10 to 15 stimuli, the marked increase in a tonic background was accompanied by an increase in amplitude and duration of clonus. This phenomenon was obvious when cortical stimulation, without any previous cerebellar one, appeared to be subliminal (from the clonic point of view), inducing, however, tonic activity following a long latency. When preceded by cerebellar stimulation, the cortical one, having the same parameters as before, caused tonic activity
4
t
t
4
FIG. 3. Strong inhibition due to cerebellar stimulation of both phasic and tonic component of muscular response elicited by cortical shocks. After cessation of cerebellar stimulation, the tonic component of muscular response is facilitated; the clonic one is not affected. From top to bottom: 1, myogram flexions of the right wrist; 2, electromyogram of flexor digitorum; 3, electromyogram of extensor carpi; 4, electromyogram of biceps brachii, all from right foreleg; 5, electrocorticogram of left gyrus sigmoideus; 6, electrocorticogram of left gyrus ectosylvius; white arrows indicate period of cortical stimulation. Stimulation rhythm revealed by artifacts on sigmoid lead; black arrows, start and cessation of cerebellar stimulation; calibrations, 1 set, 1OOpv ECG, and 2oOpv EMG.
to appear after an appreciably smaller number of shocks; after a few other shocks, inducing a marked increase in tonic background, small clonusesappeared, subsequently exhibiting progressive increasesin amplitude (Fig. 5A and B). Facilitation reached a maximum in the period immediately following the end of cerebellar stimulation, then decreasedlinearly, and disappeared after 30 to 50 set, at most. Facilitation duration and intensity increased as the frequency of cerebellar stimulation was raised progressively from 20 to 300 cycles per second,and then remained constant up to 500 cycles per second. Facilitation was the more obvious the higher the frequency of
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4. Cerebellar stimulation enhancing tonic effects induced by low-frequency stimulation of motor cortex (cat 31): A, stimulation frequency 1 set, rectangular pulses of 0.1 msec and liminal intensity (left sigmoid gyrus, cat 31) ; I, myogram of left limb (flections of wrist), 2, EMG of right flexor digitorum, 3, EMG of right extensor carpi, 4, ECG of left sigmoid gyrus. B, same cortical stimulation as in A, following a few seconds after paleo,cerebellar iterative stimulation (SO0 c/set waves lasting 5 set at an intensity 10 per cent below motor threshold) ; same legend as A. Cerebellar stimulation marked by first two arrows, beginning of cortical stimulation marked by third arrow; see text for details; calibrations, 1 set and 200 uv. FIG.
FIG. 5. Cerebellar stimulation enhancing tonic effects induced by low-frequency stimulation of motor cortex (cat 31): A, between white arrows cerebellar stimulation (500 c/set, 5 set, intensity 10 per cent below motor threshold) ; between black arro,ws, cortical stimulation as in Fig. 4A, but at an intensity 15 per cent below clonic threshold. B, between black arrows, cortical stimulation as in A, without any previous cerebellar stimulation; same legend as in Fig. 4; see text for details; calibrations, 1 set and 2OOpv.
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cortical shocks. This phenomenon could be observed accurately at frequencies below 4 per second; for example, in cat 29, almost no facilitation could be observed for a train of 1 per second starting 25 set after a cerebellar stimulation of 100 per second. With the same conditions, a train of 2 per second was intensely facilitated from the seventh stimulus onward, while for one of 3 per second, facilitation appeared at the fourth stimulus. Facilitation occurred in circumstances in which cerebellar stimulation induced no somatic, respiratory, or vasomotor changes. It also appeared during desynchronization of cortical activity induced by cerebellar stimulation, as well as long after cortical activity resumed its normal relaxed aspect. Facilitation also occurred when cortical stimulation was started some time after cessation of cerebellar stimulation (Fig. 4), as well as in the case when the two stimulations coincided for a certain period (Fig. 3) ; in the latter case, facilitation immediately followed cessation of cerebellar stimulation. Interaction between cerebral cortex and cerebellum did not seem to be reciprocal, In eight animals it was found that cortical stimulation at a frequency of 20 to 500 cycles per second had no modifying influence on muscular response to a train of rectangular stimuli applied at 1 to 4 per second on the paleocerebellum. Stimulating the cortex and cerebellum by the same leads, but reversing stimulation parameters, produced cerebellar facilitation of muscular response to cortical stimuli, just as in all other animals. The data argue in favor of different origins of clonic and tonic activities. Only by accepting the proposition that they are induced by different neurons, is it possible to understand why tonic activity is much more precocious and intensely facilitated than clonic activity. Facilitation is probably due to an intense presynaptic bombardment, the phenomenon being similar to post-tetanic potentiation (PTP; 4, 24). This interpretation is also supported by the fact that high frequencies of cerebellar stimulation were more effective than low frequencies. The greater sensitivity to PTP of small alpha neurons, as compared to large alpha, suggests that tonic activity in these experiments may have been due to excitation of the latter (5, 9). Enhancement of cortical by cerebellar impulses can only be explained by accepting the existence of at least one spinal interneuron common to cortico- and cerebellospinal pathways, the axonal terminations of this neuron providing the level at which the phenomenon of PTP occurs. The occlusion of cortical by interfering cerebellar stimuli
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also points to the existence of common interneurons. Terminal endings of neurons activated by the cerebellum are probably arranged in a pattern that assures inhibition of cortical, when coinciding with cerebellar impulses, in spite of the fact that, from the motor point of view, the intensity of cerebellar stimuli is subliminal, while that of cortical ones is supraliminal. The special arrangement of cerebellar terminations, with respect to cortical ones, is also illustrated by the fact that the muscular response to low frequency cerebellar stimulation is not facilitated by iterative cortical stimulation, which probably causes an important flow of presynaptic impulses at the level of interneurons. Our data obtained on awake animals confirm certain observations by other investigators on animals under narcosis, concerning cerebellocortical interference in the process of adjusting motor activity (13, 14, 15, 18, 21, 22). Schulmann and Delgado’s (20) conclusions concerning the absence of such interference in the awake animal should be limited to the neocerebellum, stimulated by these authors. In our experiments, cortical stimulation elicited a specific type of muscular response, which could by no means be imitated by the cerebellar stimulus, having fundamentally different parameters; the assumption by Schulmann and Delgado that cerebellar facilitation of cortical movement may actually be a muscular movement directly induced by cerebellar impulses, may therefore be discarded. Cerebellocortical interference in the awake animal appears to differ somewhat in form from that in an animal under narcosis. The interval of 3 to 20 set, by which cerebellar stimuli must precede cortical ones in the narcotized animal, is unnecessary in the awake animal. Also, inhibition occurs in the awake animal only at coinciding cerebellar and cortical stimuli, facilitation appearing after cessation of cerebellar stimuli; in the animal under narcosis, the aftereffect seems to be of the same type as the effect during coincidence of stimuli, i.e., usually inhibitory, but in some cases facilitatory. Interference of cerebellar and cortical impulses, probably occurring at spinal levels, as far as adjustment of motor activity is concerned, is achieved by special mechanisms independent of other efferent cerebellar actions. For instance, the period of cortical desynchronisation induced by cerebellar stimulation is shorter than facilitation of cortical motor impulses. Kreindler and Zuckermann ( 11) have shown that cerebellar stimulation by a sine wave of higher intensity, but otherwise identical with
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that used in the present investigation, induces a three-phase motor effect in the awake animal: activation during stimulation, inhibition for 5 to 10 set, and finally, prolonged rebound. But cerebellar facilitation of cortical movements reachesits maximum immediately after discontinuing stimulation, and then decreaseslinearly without exhibiting any phenomenon indicating transition from motor inhibition to rebound period. Vegetative changes cannot be involved in the explanation of occlusion or activation of cortical impulses, becauseneither respiratory, nor cardiac, nor vasomotor modifications occurred in the process. This comparison between various effects of cerebellar stimulation proves that facilitation of cortical stimuli occurs at the lowest stimulation intensity and exhibits the longest duration. Under physiological conditions, this cerebellar activation induced by afferent or even cortical stimuli changes the receptivity of spinal neurons to pyramidal stimuli, thus ensuring optimal conditions for the development of volitional activity, as already assumedby Bremer ( 2 ) . References
I. 2. 3.
4. 5. 6. 7.
8. 9.
10.
BERNHARD, C. S., and E. BOHM, Monosynaptic corticospinal activation of fore limb motoneurones in monkey. Acta Physiol. Stand. 31: 104-112, 1954. BUMMER, F., Le cervelet. In “Trait6 de physiologie normale et pathologique,” Tome X: “Physiologie nerveuse,” pp, 39-135; G. H. Roger, and L. Binet, (eds.), Paris, Masson, 1935. CURE, C., and T. RASSMUSSEN,Effects of altering the parameters of electrical stimulating currents upon motor responses from the precentral gyrus of macaca mulata. Brain 77: 18-33, 1954. Eccnes, J. C., “The neurophysiological basis of mind; the principles of neurophysiology,” p. 314; Oxford, Clarendon Press, 1953. ECCLES, J. C., R. M. E~CLES, and A. LUNDBERG, The action potentials of the alpha motoneurones supplying fast and slow muscles. J. Physiol. (London) 142: 275-291, 1958. FRENCH, J. D., M. VE~ZEANO, and H. W. MAGOUN, A neural basis of the anesthetic state. A.M.A. Arch. Neurol. Psych&. 69: 519, 1953. GLUSMAN, M., J. RANSOHOFF,J. L. POOL, H. GRUNDFEST, and F. A. METZER, Electrical excitability of the human motor cortex. I. The parameters of the electrical stimulus. J. Newosurg. 9: 461-471, 1952. GRANIT, R., and C. JOB, Electromyographic and monosynaptic definition of reflex excitability during muscle stretch. /. Neurophysiol. 16: 409-420, 1952. GRANIT, R., H. D. HENATSCE, and G. STED, Tonic and phasic ventral horn cells differentiated by posttetanic potentiation in cat extensor. Actu Physiol. Zkuad. 31: 114-126, 1956. GRANIT, R., C. S. PHILIPS, S. SKOGLUND, and G. Srxo, Differentiation of tonic from phasic alpha ventral cells by stretch, pinna and crossed extensor reflexes. J. Neurophysiol.
20: 470-481,
1957.
MUSCULAR ACTIVITY 11.
12.
13.
14. 15. 16. 17. 18. 19.
20. 21. 22. 23. 24.
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KREINDLER, A., and E. ZUCKEIMAN, Effets corticaux somato-moteurs, vasomoteurs et respiratoires de la stimulation paleocerebelleuse chez le chat en etat de veille. (In press.) MIHAILOVICI, L., and J. M. R. DELGADO, Electrical stimulation of monkey brain with various frequencies and pulse duration. J. Neurophysiol. 19: 21-36, 1956. MORWZI, G., Sui rapporti fra cervelletto e corteccia cerebrale. III. Meccanismi e localizzazioni delle azioni inibitrici e dinamogene de1 cervelletto. Arch. fisiol. 41: 183-206, 1941. MORIJZZI, G., “Problems in cerebellar physiology.” Springfield, Illinois, Thomas, 19.50. NULSEN, F. E., S. P. W. BLACK, and C. S. DRAKE, Inhibition and facilitation of motor activity by the anterior cerebellum. Federutton Proc. 7: 86-87, 1948. PATON, H. D., and V. E. AMASSIAN, Single and multiple unit analysis of cortical stage of pyramidal tract activation. J. Neurophysiol. 17: 345-362, 1954. ROSENBLUETH, A., and W. B. CANNON, Cortical responses to electric stimulation. Am. J. Physiol. 136: 660-740, 1942. ROSSI, G., Sugli effetti consecutivi alla stimolazione contemporanea della corteccia cerebrale e di quella cerebellare. Arch. fiziol. 10: 389-399, 1912. SCHLAC, J. D. A., Effect of drugs on pyramidal discharges evoked by cortical stimulation. Electroemephalo. and Clin Neurophysiol. 8: 421-425, 1956. SCHOOLMAN, A., and J. M. R. DELGAW, Cerebro-cerebellar relations in the awake cat. J. Neurophysiol. 21: 1-16, 1958. SNIDER, R. S., W. S. MCCULLOCH, and H. W. MAGOUN, A cerebello-bulboreticular pathway for suppression. J. Neurophysiol. 12: 325-334, 1949. SNIDE, R. S., and H. W. MAGOUN, Facilitation produced by cerebellar stimulation. 1. Neurophysiol. 12: 335-345, 1949. WEDENSKI, N. E., On the muscular rhythm elicited by cerebra-cortical stimulation. Fiziol. Sist. Nerv., But., 1955. WILSON, V. J., Posttetanic potentiation of polysynaptic reflexes of the spinal cord. J. Gen. Physiol. 39: 197-206, 1955.
2, 429-441
NEUROLOGY
Effects
of
Impulses
Neurotropic on
Drugs
Cortically
Tonic
lnstitutul
(1960)
and
Stimulated
Muscular ZUCKERMANN
de Neurologie,
I. P. Pavlov, March
Phasic
and
Activity
EMIL
Received
Cerebellar
Bucuresti,
Romania
16, 1960
Investigations were performed on awake cats and rats to study: the type of muscular response to trains of stimuli applied on the motor area at a frequency of 1 to 30 per second; and the influence exerted on this response by iterative stimulation of the paleocerebellum as well as by certain drugs depressing the central nervous system (chloralcse, Luminal, chlorpromazine). Cortical stimulation at 1 to 6 per second induced only clonus synchronous with stimulation rhythm, without changing myographic background level. The electromyogram after four to ten stimuli revealed a low-voltage asynchronous tonic activity following each clonic discharge during a period of 100 to 5OOmsec. At frequencies between 6 and 15 per second, this tonic activity changed the background trace of the myogram, clonus being superimposed on a sustained muscular contraction. The drugs in subanesthetic dcses strongly depress tonic without affecting clonic activity. Muscular response consequently maintained its clonic aspect, synchronous with stimulation rhythm up to a frequency of 25 or 30 per second. Iterative stimulation of the paleocerebellum with subliminal stimuli, when simultaneous with cortical stimulation, induced a strong inhibition of clonic, as well as tonic components of muscular response. When cortical stimulation was applied 1 to 50 set after arresting cerebellar stimulation an important facilitation of muscular response occurred, particularly affecting its tonic component. Introduction
Early experiments on motor cortex excitability revealed that lowfrequency stimulation (below 1S-20/set) induces muscular jerks synchronous with stimulation rhythms (23) instead of smooth, organized movements specific to higher stimulation frequencies. Later, the phenomenon was observed by other investigators, without being subjected, however, to a systematic analysis (3, 7, 12, 17). Study of activity of final motor neurons elicited by such cortical stimuli could yield valuable in429
430
ZUCKERMANN
formation concerning the mechanism by which the motor cortex affects the final common path. It was assumed that simple neuronal processes, causing smooth movements at high frequencies, can be detected by gradually increasing lowfrequency stimulation. The indicators used to analyze the phenomenon were the myogram and the electromyogram (EMG) of cortically activated muscles. Certain disadvantages of this method have been pointed out (8, 9, IO). It does not permit study of final neurons in the subliminal fringe, and conveys only indirect information on the type of alpha neurons involved in muscular activity. Its use is justified, however, in the initial stage of investigation by an important advantage, namely, that of permitting a general evaluation of efferent spinal neuron activity. The present article reports the first stages of investigation: myographic and electromyographic effects caused by trains of rectangular cortical stimuli (0.1 msec) of liminal intensity and frequencies between 1 and 30 per second; modification of these effects by subanesthetic doses of drugs depressing activity of the central nervous system (Luminal, chloralose, chlorpromazine) ; and modification of muscular effects by iterative paleocerebellar stimulation before, during, or after onset of cortical shocks. The latter two conditions were investigated in the hope of obtaining additional data concerning physiological significance of muscular response to cortical stimulation. Various components of this response were expected to appear unequally sensitive to neurotropic drugs. Considering the importance of the cerebellum in adjusting muscular posture and synergy, it was thought that interference of cortical and cerebellar impulses should cause changes in the usual response to cortical stimuli. In this way, details of the mechanism of spinal-neuron activation by cortical stimulation, as well as new data referring to the long-debated question of corticocerebellar relations, could be obtained. Method Condition A. Myographic and EMG responses to cortical liminal stimuli were studied in 2.5 cats and 30 rats. Stimulation frequencies ranged from 1 to 30 per second (0.1 m&pulse). After obtaining constant responses, drugs were administered intraperitoneally in 3-ml volumes (Luminal, 15-30 mg/kg body weight in 12 cats and 12 rats; chloralose, 15-40 mg/kg in 6 cats and 8 rats; chlorpromazine, S-10 mg/kg in 7 cats
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and 10 rats). Responses to identical cortical stimuli were again studied at intervals of 30 and 60 min. Condition B. After establishing the myographic and EMG responses of 25 cats to cortical stimuli at a frequency of 1 to 6 per second, the effect of an iterative cerebellar stimulation on these responses was observed. The intensity of this stimulation was subliminal, from the motor point of view. Cortical and cerebellar stimulating and recording electrodes were copper wires 150 u in diameter, insulated except at the tips. They were implanted in the cortical motor area and in the medial homolateral vermis at the boundary between culmen and simplex. Interfocal distance was 1 mm in all cases. Recording electrodes for the EMG, of silver wire insulated except the 2-mm tip, were implanted in the muscle. Distances between electrodes were 6 to 8 mm in the cat and 2 to 4 mm in the rat. Five cats (A) had additional stimulating electrodes implanted into the mesencephalic reticular formation; in 15 cats (B) , respiration and carotid blood pressure were also simultaneously recorded. A preparatory operation (under ether) was always performed several hours before the main experiment. At the beginning of the experiment, the awake animal was immobilized in a special frame. The EEG from the sigmoid gyrus, as well as the EMG of the extensor carpi and flexor digitorum muscles were recorded on a Schwartzer encephalograph. The EMG was taken either bilaterally or from the limb contralateral to the stimulated cortex. The myogram was recorded by means of an isotonic system, either on a kymograph or encephalograph, through a piezoelectrical crystal. Cerebellar stimulation was performed (B) with sine waves at frequencies of 20, 100, 200, and 500 cycles per second, the intensity being subliminal in all respects (somatomotor, respiratory, and vasomotor) . Results
and
Comments
Muscular Activity Elicited by Trains of Low-Frequency Cortical Stimuli. As seen in Fig. 1A and B, muscular activity elicited by cortical stimulation at 1 to 4 per second displayed characteristic jerks, appearing with a small latency after each shock. Amplitude and duration of jerks were constant throughout cortical stimulation. The myogram revealed no change in muscular tension in the interval between jerks. The EMG showed that the first 4 to 10 cortical shocks induced an ample synchronous volley of muscular discharges at the moment of the jerk, and no other
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activity in the interval between jerks. The next shocks, however, caused an additional muscular activity (we shall conventionally call it tonic), displaying asynchronous discharges of small amplitude and low frequency; these discharges followed each clonic burst and lasted 100 to 500 msec. Appearance of tonic activity had no influence on amplitude or duration of clonus. Amplitude and duration of tonic activity increased with the
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FIG. 1. Myographic and electromyographic effects induced by low-frequency stimulation of motor cortex: A, stimulation frequency l/set, rectangular pulses of 0.1 msec and liminal intensity (left sigmoid gyrus, cat 12) ; I, myogram of right forelimb (flexion of wrist) ; 2, EMG of right flexor digitorum; 3, EMG of right extensor carpi; calibrations, 1 set and 2OOpv. B, stimulation frequency 3 set, rectangular pulses of 0.1 msec and liminal intensity (left sigmoid gyrus, cat 12) ; same legend as in A. C, stimulation frequency IOsec, rectangular pulses of 0.1 msec and liminal intensity (left motor cortex, rat 17) ; myogram of right forelimb (flexion of wrist). D, stimulation frequency 2Osec, rectangular pulses of 0.1 msec and liminal intensity (left motor cortex, rat) ; same legend as in C; see text for details.
number of shocks in a volley. It is remarkable that the number of stimuli necessary to cause this tonic activity decreased with increase in frequency; e.g., in cat 18, at a frequency of 1 per second, tonic activity appeared after eight stimuli; at 2 per second, after six stimuli; and at 3 per second, after four stimuli. At frequencies higher than 4 per second, tonic activity between clonuses increased, causing obvious mechanical changes in the interval between. When the frequency reached about 7 to 9 per second in the cat and 12 to 14 per second in the rat, the response was a characteristically irregular
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tonic contraction, on which clonuses were superimposed, still faithfully following the stimulation rhythm (Fig. 1C). Higher frequencies ( lS/sec in cats; 20/set in rats) led to sustained tonic movements lasting throughout stimulation and sometimes interrupted by phasic movements; however, these were not synchronous with stimulation rhythms (Fig. 1D). At stimulation intensities immediately below those causing appearance of clonus, six to twelve stimuli induced a low-voltage tonic asynchronous muscular activity, similar to that usually appearing after each clonus (Fig. SB). We are thus facing two types of cortically-induced muscular activity: tonic and clonic, It is possible that the tonic activity represents merely an afterdischarge of neurons, intensely and synchronously active during the clonus. But this is improbable, because the increase in tonic activity due to stimulation by a larger number of impulses was not accompanied by a corresponding increase in clonic activity. On the other hand, a purely tonic response could be obtained; this suggests that tonic activity may have been due to activation of other spinal neurons than those causing clonic activity. In any case, appearance of tonic, in contrast to clonic activity always required a summation of cortical stimuli (3-10 shocks). In these conditions, a continuous flow of asynchronous impulses reached the level of final neurons, assuring their sustained activity even in the interval separating cortical stimuli. This multiplication of impulses was probably achieved by activation of certain cortical and/or subcortical reverberating chains. The level at which the summation process determining tonic activity occurs cannot be ascertained on the basis of present data. Paton and Amassian’s (16) investigations showed activation of cortical reverberating chains, and repetitive discharges in the pyramidal tract appeared following the first shock in a volley. Perhaps in our cases, summation processes occurred at a medullary level, where iterative activity appeared only after the fourth to tenth shocks in the train. The process could also be assumed to occur only after activation of certain subcortical extrapyramidal neurons by addition of pyramidal impulses reaching them through collateral pyramidal fibers. The change of muscular response from a clonic to a smooth sustained one, required a larger increase of stimulation frequency in the rat than in the cat. This difference was probably due to a larger number of interneurons in the cortical reverberating chains and, consequently, to an
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increase in number of impulses reaching spinal neurons after each cortical stimulus in the latter species. Eflect of Drugs Depressing the Central Nervous System. All three drugs in subanesthetic doses had similar effects on the phenomena. An appreciable decrease or even complete disappearance of tonic activity between clonuses was observed when using frequencies of 1 to 4 per second; however, the amplitude and duration of clonus did not diminish by more than 5 or 10 per cent (Fig. 2A and B) . The type of movement, consisting
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FIG. 2. Effect of 20mg/kg body weight Luminal on myographic and electromyographic effects induced by low-frequency stimulation of motor cortex: A, stimulation frequency l/set, rectangular pulses of 0.1 msec and liminal intensity (left sigmoid gyrus, cat 12, as in Fig. 1; 30min after i.p. administration of Luminal) ; same legend as Fig. IA. B, stimulation frequency 3 set, rectangular pulses of 0.1 msec and liminal intensity (left sigmoid gyrus, cat 12, as in Fig. 1; 45 min after i.p. administration of Luminal; same legend as Fig. IA. C, Stimulation frequency 20/set, rectangular pulses of 0.1 msec and liminal intensity (left motor cortex, rat I?), as in Fig. 1C; 45 min after i.p. administration of Luminal; same legend as Fig. 1C; see text for details.
of clonus without tonic modifications, changed to a smooth organized one at appreciably higher frequencies than before administering the drug. A clonus responsesynchronous with the stimulation rhythm was obtained even at 20 to 30 per second in the rat (Fig. ZC), although the normal responseat this frequency had been an organized movement displaying no discernible clonus (Fig. 1D). The amplitude of clonus was unchanged or diminished, at most, by 5 or 10 per cent. The above data may indicate that the sensitivity of neurons causing tonic activity is qualitatively different from that of neurons determining clonic activity. The lower sensitivity of clonic activity to depressive
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drugs is probably due to the fact that it occurs by paucisynaptic or even monosynaptic pathways, as appears from certain investigations by Bernhard and Bohm ( 1). High sensitivity of tonic activity may be explained by occurrence through polysynaptic pathways (6), rather than by a higher sensitivity of tonic spinal neurons. This assumption is based on two series of data: those of Granit (9)) showing that in the decerebrate or spinal animal large doses of anesthetic or chlorpromazine lead to a “tonic change” of spinal neurons; and of Schlag (19), showing that drugs depress pyramidal I-waves without affecting D-waves. Further experimental evidence in favor of our assumption was obtained on five cats whose cortex and mesencephalic reticular formation were stimulated with identical parameters. Threshold intensities were established, giving similar responses when stimulating the two structures. Drugs were then administrated as in other animals. The tonic component was always considerably more depressed in the case of cortical than in reticular stimulation, which proves that the inhibitory process did not occur at spinal level, but rather at the level of the directly stimulated structure. Eflects of Cerebellar Stimulation on Muscular Response Elicited by Low Frequency Cortical Stimulation. Coincidence of cerebellar (sine waves of 100-500 c/set) and cortical stimulation (l-l/set) always caused an important inhibition of clonic, as well as tonic responses (Fig. 3). The effect was the same, whether the two stimuli started simultaneously or one preceded the other by 0.5 to 10 sec. The inhibitory effect disappeared immediately after discontinuing cerebellar stimulation. When the cortical stimulus was applied after cessation of the cerebellar one, a marked facilitatory effect appeared (Figs. 3 and 4), exhibiting the following peculiarities: Although amplitude and duration of clonus remained unchanged for the first ten to twelve shocks in the cortical train, tonic activity was precocious, displaying longer duration and higher intensity following each cortical shock (Fig. 4A and B). Thus, before stimulating the cerebellum of animal 38, tonic activity appeared after eight stimuli, its duration (between stimuli 8 and 11) was 150 to 200 msec following each clonus, and the average amplitude reached 150 pv. When the same cortical train was started 5 set after cessation of cerebellar stimulation (500 c/set) , tonic activity appeared after four stimuli, its duration (between stimuli 4 and 7) was 400 msec, and its average amplitude, 300 pv.
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When a train of cortical stimuli following a previous cerebellar stimulation was prolonged, it was found that after 10 to 15 stimuli, the marked increase in a tonic background was accompanied by an increase in amplitude and duration of clonus. This phenomenon was obvious when cortical stimulation, without any previous cerebellar one, appeared to be subliminal (from the clonic point of view), inducing, however, tonic activity following a long latency. When preceded by cerebellar stimulation, the cortical one, having the same parameters as before, caused tonic activity
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FIG. 3. Strong inhibition due to cerebellar stimulation of both phasic and tonic component of muscular response elicited by cortical shocks. After cessation of cerebellar stimulation, the tonic component of muscular response is facilitated; the clonic one is not affected. From top to bottom: 1, myogram flexions of the right wrist; 2, electromyogram of flexor digitorum; 3, electromyogram of extensor carpi; 4, electromyogram of biceps brachii, all from right foreleg; 5, electrocorticogram of left gyrus sigmoideus; 6, electrocorticogram of left gyrus ectosylvius; white arrows indicate period of cortical stimulation. Stimulation rhythm revealed by artifacts on sigmoid lead; black arrows, start and cessation of cerebellar stimulation; calibrations, 1 set, 1OOpv ECG, and 2oOpv EMG.
to appear after an appreciably smaller number of shocks; after a few other shocks, inducing a marked increase in tonic background, small clonusesappeared, subsequently exhibiting progressive increasesin amplitude (Fig. 5A and B). Facilitation reached a maximum in the period immediately following the end of cerebellar stimulation, then decreasedlinearly, and disappeared after 30 to 50 set, at most. Facilitation duration and intensity increased as the frequency of cerebellar stimulation was raised progressively from 20 to 300 cycles per second,and then remained constant up to 500 cycles per second. Facilitation was the more obvious the higher the frequency of
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4. Cerebellar stimulation enhancing tonic effects induced by low-frequency stimulation of motor cortex (cat 31): A, stimulation frequency 1 set, rectangular pulses of 0.1 msec and liminal intensity (left sigmoid gyrus, cat 31) ; I, myogram of left limb (flections of wrist), 2, EMG of right flexor digitorum, 3, EMG of right extensor carpi, 4, ECG of left sigmoid gyrus. B, same cortical stimulation as in A, following a few seconds after paleo,cerebellar iterative stimulation (SO0 c/set waves lasting 5 set at an intensity 10 per cent below motor threshold) ; same legend as A. Cerebellar stimulation marked by first two arrows, beginning of cortical stimulation marked by third arrow; see text for details; calibrations, 1 set and 200 uv. FIG.
FIG. 5. Cerebellar stimulation enhancing tonic effects induced by low-frequency stimulation of motor cortex (cat 31): A, between white arrows cerebellar stimulation (500 c/set, 5 set, intensity 10 per cent below motor threshold) ; between black arro,ws, cortical stimulation as in Fig. 4A, but at an intensity 15 per cent below clonic threshold. B, between black arrows, cortical stimulation as in A, without any previous cerebellar stimulation; same legend as in Fig. 4; see text for details; calibrations, 1 set and 2OOpv.
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cortical shocks. This phenomenon could be observed accurately at frequencies below 4 per second; for example, in cat 29, almost no facilitation could be observed for a train of 1 per second starting 25 set after a cerebellar stimulation of 100 per second. With the same conditions, a train of 2 per second was intensely facilitated from the seventh stimulus onward, while for one of 3 per second, facilitation appeared at the fourth stimulus. Facilitation occurred in circumstances in which cerebellar stimulation induced no somatic, respiratory, or vasomotor changes. It also appeared during desynchronization of cortical activity induced by cerebellar stimulation, as well as long after cortical activity resumed its normal relaxed aspect. Facilitation also occurred when cortical stimulation was started some time after cessation of cerebellar stimulation (Fig. 4), as well as in the case when the two stimulations coincided for a certain period (Fig. 3) ; in the latter case, facilitation immediately followed cessation of cerebellar stimulation. Interaction between cerebral cortex and cerebellum did not seem to be reciprocal, In eight animals it was found that cortical stimulation at a frequency of 20 to 500 cycles per second had no modifying influence on muscular response to a train of rectangular stimuli applied at 1 to 4 per second on the paleocerebellum. Stimulating the cortex and cerebellum by the same leads, but reversing stimulation parameters, produced cerebellar facilitation of muscular response to cortical stimuli, just as in all other animals. The data argue in favor of different origins of clonic and tonic activities. Only by accepting the proposition that they are induced by different neurons, is it possible to understand why tonic activity is much more precocious and intensely facilitated than clonic activity. Facilitation is probably due to an intense presynaptic bombardment, the phenomenon being similar to post-tetanic potentiation (PTP; 4, 24). This interpretation is also supported by the fact that high frequencies of cerebellar stimulation were more effective than low frequencies. The greater sensitivity to PTP of small alpha neurons, as compared to large alpha, suggests that tonic activity in these experiments may have been due to excitation of the latter (5, 9). Enhancement of cortical by cerebellar impulses can only be explained by accepting the existence of at least one spinal interneuron common to cortico- and cerebellospinal pathways, the axonal terminations of this neuron providing the level at which the phenomenon of PTP occurs. The occlusion of cortical by interfering cerebellar stimuli
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also points to the existence of common interneurons. Terminal endings of neurons activated by the cerebellum are probably arranged in a pattern that assures inhibition of cortical, when coinciding with cerebellar impulses, in spite of the fact that, from the motor point of view, the intensity of cerebellar stimuli is subliminal, while that of cortical ones is supraliminal. The special arrangement of cerebellar terminations, with respect to cortical ones, is also illustrated by the fact that the muscular response to low frequency cerebellar stimulation is not facilitated by iterative cortical stimulation, which probably causes an important flow of presynaptic impulses at the level of interneurons. Our data obtained on awake animals confirm certain observations by other investigators on animals under narcosis, concerning cerebellocortical interference in the process of adjusting motor activity (13, 14, 15, 18, 21, 22). Schulmann and Delgado’s (20) conclusions concerning the absence of such interference in the awake animal should be limited to the neocerebellum, stimulated by these authors. In our experiments, cortical stimulation elicited a specific type of muscular response, which could by no means be imitated by the cerebellar stimulus, having fundamentally different parameters; the assumption by Schulmann and Delgado that cerebellar facilitation of cortical movement may actually be a muscular movement directly induced by cerebellar impulses, may therefore be discarded. Cerebellocortical interference in the awake animal appears to differ somewhat in form from that in an animal under narcosis. The interval of 3 to 20 set, by which cerebellar stimuli must precede cortical ones in the narcotized animal, is unnecessary in the awake animal. Also, inhibition occurs in the awake animal only at coinciding cerebellar and cortical stimuli, facilitation appearing after cessation of cerebellar stimuli; in the animal under narcosis, the aftereffect seems to be of the same type as the effect during coincidence of stimuli, i.e., usually inhibitory, but in some cases facilitatory. Interference of cerebellar and cortical impulses, probably occurring at spinal levels, as far as adjustment of motor activity is concerned, is achieved by special mechanisms independent of other efferent cerebellar actions. For instance, the period of cortical desynchronisation induced by cerebellar stimulation is shorter than facilitation of cortical motor impulses. Kreindler and Zuckermann ( 11) have shown that cerebellar stimulation by a sine wave of higher intensity, but otherwise identical with
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that used in the present investigation, induces a three-phase motor effect in the awake animal: activation during stimulation, inhibition for 5 to 10 set, and finally, prolonged rebound. But cerebellar facilitation of cortical movements reachesits maximum immediately after discontinuing stimulation, and then decreaseslinearly without exhibiting any phenomenon indicating transition from motor inhibition to rebound period. Vegetative changes cannot be involved in the explanation of occlusion or activation of cortical impulses, becauseneither respiratory, nor cardiac, nor vasomotor modifications occurred in the process. This comparison between various effects of cerebellar stimulation proves that facilitation of cortical stimuli occurs at the lowest stimulation intensity and exhibits the longest duration. Under physiological conditions, this cerebellar activation induced by afferent or even cortical stimuli changes the receptivity of spinal neurons to pyramidal stimuli, thus ensuring optimal conditions for the development of volitional activity, as already assumedby Bremer ( 2 ) . References
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