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EEG and multiple-unit activity in limbic and motor systems during movement and immobility
Physiology and Behavior. Vol. 7, pp. 337-343. Pergamon P r e u , 1971. Printed in Great Britain
EEG and Multiple-unit Activity in Limbic and Motor Systems during Movement and Immobility' W. R. K L E M M Department o f Biology, Texas A & M University, College Station, Texas 77843, U.S.A. (Received 2 November 1970)
K~MM, W. R. EEG and multiple-unit activity in iimbic and motor systems during movement and immobility. PHYSIOL. BeHAV. 7 (3) 337-343, 1971.--Electrographic activity was recorded from chronically implanted electrodes generally in the caudate nucleus, septum, hippocampus, and midbrain and medullary levels of the brain stem reticular formation (BSRF) of rabbits in order to assess the effects of movements and immobility. Hippocampal theta rhythms were triggered during movements of any sort or during phasic increases in muscle electrical activity. Coinciding with or preceding the theta onset were phasic increases of multiple-unit activity in the BSRF. Injection of tranquilizer simultaneously reduced movements, abolished theta rhythm, and selectively depressed BSRF units. These specific electrographic correlates of muscle activity were not observed in the caudate, septum, or other telencephalic brain structures from which recordings were derived. Transient theta and increased BSRF activity also occurred during the immobility reflex (animal hypnosis) when the tonic muscle relaxation and immobility were phasically interrupted by mild sensory stimulation. The motorassociated phasic increases in BSRF multiple-unit activity were superimposed upon a tonic unit activity increase in the medullary reticular formation, not seen elsewhere, that was characteristic of the reflex. Hippocampus
Brain stem
Muscleactivity
EEG
inhibition of spinal reflexes and both voluntary and involuntary movements. The IR does seem to be a reflex, albeit complex, because it is a specific, relatively stereotyped, involuntary, and unconditioned response to specific stimuli [6].
BEYOND the simple reflex level, appropriate and adaptive body movements must be influenced by those brain processes which regulate emotion and memory. These processes are generally considered to be major functions of the limbic system, one of the major components of which is the hippocampus. A specific electrographic rhythm of the hippocampus, the synchronized 4-9/sec activity known as theta rhythm, has historically been associated with behavioral arousal. More recent studies, however, have indicated that this association may be misleading and that the more basic correlation may be between theta rhythm and voluntary movements [8, 14, 18]. One of the known motor controlling structures, the brain stern reticular formation (BSRF), is known to be functionally related to the limbic system, via the limbic-midbrain circuit. More specifically, the evidence of BSRF activation of the septum which in turn paces hippocampal theta rhythm [13] raises the possibility of intimate relations between BSRF motor functions and the hippocarnpal functions in emotion and memory. The present report concerns an electrographic study of these relations, correlating muscle activity with electrical activity in the hippocampus and BSRF. This correlation was also studied with a specific model of involuntary immobility, the so-called Immobility Reflex (IR), otherwise known as animal hypnosis. The IR is a unique behavioral state which has special relevance to this study because of the profound
METHODS
Twenty-four New Zealand White rabbits, of both sexes and weighing 5.5-6 pounds, were chronically implanted usually with 4 pairs of nichrome electrodes (<0j3 mm dia.) that were insulated with Formvar varnish except for about ½mm at the tips. The principal placements, which were later verified histologically, and stereotaxic coordinates [12] were in the left hippocampus (P5, L3) (20 rabbits), median septum (A2) (18 rabbits), right caudate nucleus (A2, L2.5) (16 rabbits), left medial medullary reticular formation (MRF) (P20 and P24, L1) (8 rabbits), and right medial midbrain reticular (MBRF) formation (P12, L1) (5 rabbits). Other
electrode placements included neocortex (3 rabbits), corpus callosum (I rabbit),claustnm~ (I rabbit),and various areas adjacent to the M R F (inferior olive, pyramids, fasciculus gracilis, solitary tract nucleus, hypogiossal nucleus--8 rabbits). After injection of penicillin-streptomycin and after allowing several days for recovery, EEGs were obtained before, during, and after the IR. IR was induced in a snug, 3-sided chute which permitted
1This research was supported by a grant from the National Institute of Mental Health, No. MH 13072. A preliminary report involving some of these data, discussed in another context, has already been published [7].
337
338
KLEM M
I R states to persist for 10 min or more without interruption [3], even during mild sensory stimulation such as blowing in the rabbit's face or gently jabbing the limbs and belly with a pencil. Rabbits were placed on their back inside the chute, and their head was held down for about the first 5 sec; I R terminated spontaneously, as indicated by an abrupt righting reflex. When Awake, the rabbits moved freely within an open-topped wooden box (27 × 24 × 38 cm high) (Use of the term Awake in reference to non-IR states is done as a semantic convenience, and is not meant to imply that the I R represents a sleeplike condition. Indeed, much published evidence exists to indicate that brain processes are quite active during the I R [6]). Body movements were indicated by motion artifacts in the E E G and by direct observation of the head and the attached recording cable. Motor activity was monitored in 6 rabbits by recording the
nuchal electromyogram (EMG) and by recording voltages generated by rabbit movements on a conventional jiggle platform. Test sessions lasted at least 30 min, during which rabbits were alternately immobilized and awakened. The EEG was recorded with a Grass Model 78 polygraph, with potentials from the animal coupled to the preamplifier via emitter-follower input. The EEG pen amplifiers had ½-amplitude cutoff settings at 1-90 Hz; signals were also simultaneously supplied to a computer or an electronic integrator. Interval histogram analysis of hippocampal EEG waves was performed by leading the preamplifier signal into an on-line computer (Nuclear Chicago, Model 7100). Analysis was based on the principle of detecting voltage excursions across a pre-set discrimination level, followed by the computation of the interval between excursion points. The
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FIG. 1. Composite illustration of correlations of hippocampal theta rhythm, BSRF multiple-unit activity, and muscle activity. In the Awake state, 2 large phasic increns~ in multiple-unit activity are seen only in the MRF, and each is more or less simultaneous with onset (first arrow) of pronounced hipcoeampal (HIP) theta rhythm which terminates (~¢ond arrow) after multipleunit activity returns to normal. (Note also the seeminely random and abrupt episodes of decaeased multiple.unit activity in the hippocm~us.) Dashed vertical lines serve to emphasize the slightly delayed, time-locked inc'r~_~ in EMG and body movements (cross marks indicate motion artifacts in EEG traces). Records during the IR reveal that the only EEG changes are mild slowing of frequenfies and increase in voltage in camdate (CALl) and septum (SEP), along with reduced hipix3camlml theta. Multiple-unit activity is not changed in any area except in the MRF, where a mild increase is sustained. When air is blown in the rabbit's face (arrow), there is a sudden increase in MRF multiple-unit activity, associated with onset of hippocampal them. Although body movements did not occur, isometric contractions are indicated by the increased EMG activity. Time marks, 1 and 5 sec; EEG and EMG voltage calibrati(ms, 50 ttV. Integrator calibrations, solid lines, from bottom to top, 0, 5, 10 and 20 ~V signals of I K H z applied to preamplifier input; dashed line, noise level of entire system with 10,000 fl across preamplifier input.
HIPPOCAMPUS, BRAIN STEM AND MUSCLE ACTIVITY
339
signals were filtered with a t-amplitide cutoff at 30 Hz. The computer sampled 200 waves in each analysis; analog-todigital conversion discriminators, set on a 0-voltage baseline, were triggered on the positive slope of waves and the computer calculated the interval between successive waves. Computer output was read out through a strip-chart recorder. Multiple-unit recording was accomplished by leading preamplifier output (t-amplitude cutoff at 3 KHz) simultaneously to a multiple-unit filter (Kopf Co.) with a lowfrequency cutoff at 300 Hz. The multiple-unit potentials from the filter were led into an R-C integrator (Grass, Model 7P3A). Units were also observed directly on an oscilloscope and photographed as needed.
AWAKE
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w a yes RESULTS
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Obtaining artifact-free recordings from the unrestrained rabbits usually became progressively easier as they became accustomed to the recording conditions. As expected, LEGs varied with the state of agitation. When rabbits were very relaxed, their L E G frequencies slowed, spindles sometimes occurred in the caudate leads, and there was a concurrent reduction in muscle tone, as indicated by lower E M G voltages, and in body movements. Opposite effects were conspicuous when the rabbits were constantly moving about in the restraining box. The unexpected observation was that activity changes in the hippocampus and BSRF were specifically correlated with transient changes in muscle tone and movements. When phasic movements occurred, as documented by increased E M G activity and jiggle stand recording, the hippocampal activity suddenly converted into a rhythmical series of 5-9/sec waves (theta rhythm) (Fig. l, Awake). As shown in the Awake portion of the figure, there were 2 isolated movements, each associated with a burst of theta activity. Theta activity correlated with movements in all rabbits tested. Theta rhythm often outlasted muscle activity by a few seconds or more, as is illustrated for the movement at the fight hand side of the Awake record in Fig. 1. These hippocampal changes were verified by intervalhistogram analysis (Fig. 2, Awake). Such analyses revealed that theta frequencies were dominant during continuous movements and much reduced during periods of less motor activity. Further quantification of the histograms (Table 1,
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FIG. 2. Hippocampal interval histogram data from several experiments during different phases of IR and Awake states. During the IR, tbeta rhythm (reflected in histogram b) was always induced by mild stimulation (which produced muscular tension but which was below the threshold for disrupting the IR); however, in the unstimulated condition, tbeta rhythm was less conspicuous, as in a. During Awake states, the wave interval distribution was a function of the degree of alertness, being diminished during behavioral immobility and relaxation (as in a) and becoming especially prominent during gross body turning movements within the restraining box (as in b).
Awake), revealed that some theta activity was always present in the hippocampus, but that during continued movement, the percentage increased, with most of the waves in the range of 5-9/see (110-200 msec intervals). Such activity decreased distinctly when overt movements were absent. Multiple-unit activity. The activity as observed from all leads on the oscilloscope (Fig. 3, Awake), appeared as a mixture of individual spikes of approximately ~-1 msec duration. The differing amplitudes reveal that they were generated from several neurons; two or more spikes summated when they occurred at approximately the same time. The integrated display of the multiple-unit activity, in the case of caudate and septal leads, maintained a steady level
TABLE 1 RANGE (IN MSECS)CONTAINING AT LEAST t OF TOTAL HIPPOCAMPAL WAVES
Awake
Rabbit 1.
2. 3.
4. 5. 6. 7.
Moving -131-175, 180-200 150-213 140-187 150-180, 187-213 138-145 113-150
IR
Not Moving
Stimulated
-163-168 180-182 175-177 0 0 0
156-190 175-190, 220 -163-165, 206-208 175-180 136-142 113-131, 156-169
Unstimulated 150-152 0 0 0 185-187 0 0
340
KLI!~MM
around which minor fluctuations occurred, irrespective of the rabbit's behavior (Fig. 1, Awake); septal fluctuations were more apparent than those from caudate leads. However, hippocampal unit activity, which was the smallest of any of the areas, had numerous msec periods of relative electrical silence, as indicated by the downward integrator pen deflections in Fig. 1. These deflections did not seem to correlate with presence or absence of theta activity in the hippocampal EEG, nor with any aspect of behavior. By far the most conspicuous aspect of multiple-unit activity was a phasic increase in B S R F leads immediately preceding the onset of hippocampal theta activity, E M G increases and overt movements (Fig. 1, Awake). As muscle tone and movements subsided, the BSFR multiple-unit activity returned to its steady state; as mentioned, hippocampal theta often persisted after motor activity ceased. Not only movements, but also phasic tensing of muscles without overt movement, were associated with hippocampal theta activity and BSRF multiple-unit increases. The degree to which these phenomena concurred was very high, as indicated by counting episodes of each phenomenon in the rabbits which had E M G electrodes (Table 2, Awake). Specific examples of various movement conditions which were associated with the B S R F unit increases are shown in Fig. 4. The magnitude of these unit increases was on the order of about 10 t~V, based on the arbitrary calibration scheme. Still larger increases were noted in monopolar recording with a reference lead insulated in nasal bone. However, only bipolar data are shown because theoretically records obtained in reference recording could be contaminated by artifact potentials from facial muscle or olfactory bulbs. The correlated increase in multiple-unit activity occurred in all rabbits with M R F electrodes (5 at the inferior olive level and 3 at a level just anterior to the decussation of medullary pyramids). Similar unit activity increases were also observed in the 5 rabbits with M B R F electrodes. Control tests for artifact included vigorous manipulations of the recording cable while it was still connected to an unmoving rabbit; no increases in unit activity occurred, although large, movement-related pen excursions appeared in the EEG channels. Although unit increases associated with muscle activity were observed from structures adjacent to the B S R F (n. gracilis, solitary tract n., hypoglossal n., and pyramidal
AWAKE
IR _
c
FIG. 4. Integrated MRF multiple-unit activity illustrating transient increases during Awake and IR states that were associated with movements. In the upper trace, the rabbit was moving calmly about the restraining box. During the IR, increased activity developed in response to stimulation of the rabbit b y blowing air in its face (arrow), which also elicited mild general movements (a). In (b) the rabbit displayed a few spontaneous limb movements (arrows) which coincided with the multiple-unit increases, In (c) a heavily tranquilized rabbit exhibited paroxysmal jaw chewing movements over about 4 sees (solid line) that we.re associated with the rhythmic bursts of increased multiple-unit activity.
tract), no such increases were observed from the caudate, septum, or hippocampus. Nor were any such increases seen from any of the 5 rabbits whose rostral electrodes were in the neocortex, claustrum, and corpus callosum.
Immobility Reflex EEG. During the I g , and in the absence of stimulation, EEGs were generally less activated than when Awake, with mild slowing of frequencies and increased voltage in caudate and septum (Fig. 1, I g ) . Hippocampal theta rhythm was reduced during the I R (Fig. 2, I g ; Table 1); only 2 rabbits displayed major amounts of theta rhythm. One rabbit had continual theta rhythm for about the first lOsec after induction; in the other, them rhythm occurred more or less continuously in association with phasic recurrences of increased E M G activity and M R F multiple-unit activity. In all rabbits, sensory stimulation during the I R could still
TABLE 2 INCIDENCEOF CORRELATIONSAMONGPHASICMRF UNiT INCReASe, THETARHYTHMAND EMG ACTIVITY Awake Rabbits
Total
MRF Unit Increase
IR
No MRF Unit Increase
Total
MRF Unit Increase
No MRF Unit Increase
Observations Theta Increased EMG Theta Increased EMG Observations Theta Increased EMG Theta Increased EMG 71 33 190 134 90 107
68 21 173 125 83 106
64 20 173 124 80 103
2
3
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7 13 7 0 3
3 10 4 0 1
39 18 . . . 27
Totals
635
576
564
32
21
84
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26
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79
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7
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I B
FIG. 3. Multiple-unit potentials in the M R F in Awake and IR states, as they appear on the oscilloscope screen. The increased activity during IR is illustrated on two time scales, full scale of 20 msecs (A) and 100 msecs (B). The large voltages during IR are seen in the fast time scale illustration to result in part from summation of individual action potentials. Voltage calibration: 20 ~zV.
(facing page 340)
HIPPOCAMPUS, BRAIN STEM AND MUSCLE ACTIVITY
341
elicit transient theta rhythm, often associated with flinching, without disrupting the I R state (Fig. 1, IR); still other rabbits exhibited a spontaneously phasic theta that was commonly associated with an abortive attempt at performing the righting reflex, followed by a transient limb extension. All rabbits revealed distinct theta rhythm during the righting reflex which terminated the state. Multiple-unit activity. During the IR, no change in EEG, and either no change or a decrease in tonic multiple-unit activity, was evident in the 9 rabbits with electrodes in the n. gracilis, solitary tract n., hypoglossal n., inferior olive, and pyramidal tract. No change in EEG or multiple-unit activity occurred during the IR in the 5 rabbits whose rostral electrodes were in the neocortex, claustrum, and corpus callosum. The [R did not result in any detectable change in multipleunit activity in either the caudate, septum, or hippocampus (Fig. 1, IR). Paradoxically, during the IR the medial MRF generally revealed more EEG desynchrony associated with a small, but sustained multiple-unit activity increase, even though no movements occurred, muscle tone was decreased, and the electrodes were the same as those used when Awake. However, phasic increases in MRF activity, superimposed on the tonic increase, could be elicited by sensory stimulation which evoked increased muscle tension or flinching (Fig. 1, IR; Fig. 4, IR). The correlation of unit increases, theta rhythm, and muscle activity was observed in almost all instances (Table 2, IR).
decreased movement, lowered muscle tone, abolished hippocampal theta rhythm, and decreased tonic BSRF multiple-unit activity (Fig. 5, Awake). Although tonically depressed, the BSRF unit activity increased phasically when movements did occur. The tonic depression of BSRF units was not paralleled by unit decreases in rostral brain areas. Induction of I R in a tranquilized rabbit could nonetheless produce a tonic level of MRF multiple-unit activity that was greater than when the same tranquilized animal was Awake (Fig. 5). DISCUSSION
The literature is replete with studies of the significance of hippocampal theta rhythm [7, 10, 13, 14, 18]. Many of these studies, however, are interpreted in various ways that relate to such aspects of behavior as arousal, orientation, psychic tension, learning, goal seeking, drive, and reward. Some workers have recently advanced the hypothesis that theta activity is correlated more specifically with limbic system integration of motivational processes associated with movements, regardless of the nature of the movement or its cause [8, 14]. Since these behavioral states are generally associated with movement, theta could correlate specifically with voluntary movement per se. Such a view is taken by Vanderwolf [18], based on his studies of rats and dogs. He further contended that theta rhythm in these species occurred only during movement; the rhythm was absent in states of alertness in which no movement occurred. The present study does clearly reveal[an association between movement and theta rhythm, in either the Awake or IR state. However, the correlations do not exclusively depend on
Tranquilizer The intravenous injection of a tranquilizer (promazine hydrochloride, 4 mg/kg) in 4 Awake rabbits simultaneously
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FIG. 5. Tranquilizer effects. In the Awake state, drug treatment (D) caused the following distinct changes from the non-drug (ND) condition: caudate EEGs contained higher voltases, slower activity, and occasional spindles (arrow). Hippocampal theta activity was greatly reduced, as was the multiple-unit activity in the MRF (and MBRF, but not elsewhere). Nuchal muscle activity decreased, even during comparable periods when no body movements occurred. The figure also illustrates the typical ND changes during IR of increased MRF activity and reduced muscle tone. When the rabbit was drugged and IR was induced, parallel changes again occurred: relatively increased MRF activity and decreased muscle tone. Calibrations as in Fig. 1.
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342 actual movement. Isometric contractions in which tone increased without overt movement were commonly observed in this study to coexist with increased BSRF unit activity and theta rhythm. Many of the movements and their associated theta rhythm are not readily attributable to motivational factors. Particularly during IR, it is unlikely that a rabbit could anticipate the stimulus and develop theta rhythms as part of a neural processing preparation for such an involuntary movement as the flinch response. Although the two rabbits with much theta activity during the IR could have been processing information to prepare for eventual performance of the righting reflex, it seems more probable that their theta activity simply resulted from a higher level of muscle tone. Previous studies have shown that the early post-induction period is the period of maximum muscle tone and it is also during that period when theta activity is most common [4]. Also consistent with this view of the importance of muscle tone is the observation that tranquilizer completely abolished theta activity at the same time that it also reduced muscle tone and M R F multiple-unit activity, both in Awake and I R states. Thus, the most straight-forward interpretation of the significance of theta rhythm may be that it is a non-specific correlate of muscle activity, either isotonic or isometric, which may be voluntary or involuntary [7]. The EEG findings of no specific correlate with the IR (except in the MRF) extends the number of brain areas which have now been demonstrated to exhibit dissociated activity in the behavioral states of I R and alert wakefulness [4, 6]. For the brain areas tested, this present study also reveals that the dissociation exists also at the level of multipleunit activity. In other words, it appears that the essence of the IR is that it is a motor disconnection syndrome in which some neural clutch in the brain stem disengages motor drive from the spinal motoneurons, allowing the rostral brain cells to nonetheless continue relatively normal discharge [4]. Interpretations of multiple-unit activity have to be qualified to an extent. Lack of clear changes in integrator output from rostral brain areas during muscle activity or during IR does not necessarily indicate absence of changes. Integration is an averaging technique, and significant individual events can go undetected; changes could be occurring during movements in these rostral areas but, compared to M R F increases, are not great enough to be manifest. Moreover, the method does not distinguish excitatory from inhibitory neurons which, if they completely reversed their relative activity, might not produce any net change in integrator output. The few exceptions (Table 2) to the usual correlations among muscle activity, hippocampal theta rhythm, and BSRF multiple-unit increases could be attributed to the fact that only a small part of a given brain area was monitored at any one time and that only one of the body muscles was monitored. Increases in multiple-unit activity, such as those noted in the M R F during muscle activity and the IR or the entire BSRF during muscle activity, could reflect a recruitment of new neurons into firing or an increased discharge frequency of already active neurons, in either case, neurons in the area are clearly activated. The fact that movement-related increases occurred throughout the BSRF is not surprising, in view of anatomical evidence for extensive rostral and caudal
KL E MM projections of BSRF neurons [15]. During muscle activity, these units could be more active because of excitation from proprioceptive feedback or because they are sending descending influences upon lower motoneurons. The tonic increase in M R F multiple units during the IR confirms an earlier study [5] in which IR-associated multipleunit increases, as recorded from movable electrodes, occurred over a several millimeter vertical span of the MRF. Multipleunit increases in this present study were observed from the same M R F area during two behavioral states with opposite degrees of muscle activity, consistent with the common view that at least two functionally different pools of neurons co-exist in the same area. One neuronal pool could cause the Awake phasic muscular activity while the other might tonically inhibit motion during the IR. To speculate further, during the IR, both pools could be partially active concurrently, as when phasic increases are superimposed on the tonic increase during transient muscle activity. If the motor activating pool becomes greatly active, it might over-ride the tonic inhibitory pool, ending the IR by initiation of the righting reflex. The demonstration of tonic unit increases during the IR is all the more convincing, if, as already suggested, motor facilitating units in the area are less active during the IR, when muscle activity is reduced. Decreased motor facilitating activity would subtract from the voltages created by discharge of neurons involved in actively inhibiting movements during the 1R, thus tending to obscure detection of the latter's existence by the multiple-unit recording method. The clear preferential tranquilizer suppression of BSRF units, as opposed to units from rostral structures, was not accompanied by an equally clear cancellation of iR's enhancing effect on MRF multiple-unit activity. Although the drug depressed MRF units, it apparently depressed the IR-associated units less than the others, inasmuch as iR could still be accompanied by increased MRF multiple-unit activity, even in the drugged state. The well-known potentiating effect of tranquilizers on the IR [2, 16] may result from a disproportionate suppression of motor excitatory activity, thus indirectly enhancing the effectiveness of co-existing motor inhibitory influences. The well-known tranquilizing effects of tranquilizers may well be secondary to BSRF depression, if, as suggested previously, limbic activation results from BSRF activity and BSRF activity in turn is especially susceptible to tranquilizers. The decrease in tonic level of BSRF activity produced by tranquilizers could result from the diminished feedback that results from reduced muscle activity. On the other hand, the drug causes the reduced muscle activity, and could very well do so by inhibiting BSRF units whose normal function is to excite lower motoneurons during movement. This latter view is supported by previous experiments that demonstrated that the MRF can initiate both excitatory and inhibitory effects, including organized patterns of body movement [11, 17] by producing EPSPs and IPSPs on spinal motoneurons [1, 9]. Taken together, the cited literature and present findings have led to a theory, explicitly advanced previously [7], that sensory stimuli or higher brain influences stimulate the BSRF to not only activate the hippocampal theta rhythm, via limbic-midbrain paths, but also to produce a net activation of skeletal muscle tone.
HIPPOCAMPUS, BRAIN STEM AND MUSCLE ACTIVITY
343
REFERENCF~ 1. Jankowska, E., S, Lund, A. Lundberg and O. Pompeiano. Postsynaptic inhibition in motoneurons evoked from the lower reticular formation. Experientia 20: 701-702, 1964. 2. Klemm, W. R. Drug potentiation of hypnotic restraint of rabbits, as indicated by behavior and brain electrical activity. Lab. Anim. Care. 15: 163--167, 1965. 3. Klemm, W. R. Method to encourage extensive study of animal hypnotic behavior. J. exp. Analysis Behav. 9: 63-64, 1966. 4. Klemm, W. R. Electroencephalographic-behavioral dissociations during animal hypnosis. Electroenceph. clin. Neurophysiol. 21: 365-372, 1966. 5. Klemrn, W. R. Mechanisms of the immobility reflex ("Animal hypnosis"). II. EEG and multiple unit correlates in the brain stem. Communs behav. Biol. 3: 43-52, 1969. 6. Klemm, W. R. Neurophysiologic studies of the immobility reflex ("Animal hypnosis"). In: Neurosciences Research, Vol. 4, edited by S. Ehrenpreis and O. C. Solnitzky. New York: Academic Press, 1971, pp. 165-212. 7. Klemm, W. R. Correlation of hippocampal theta rhythm, muscle activity, and brain stem reticular formation activity. Communs behav. Biol. 5: 147-151, 1970. 8. Liss~ik, L. and E. Endr'oczi. Involvement of limbic structures in conditioning motivation and recent memory. In: Progress in Brain Research, Vol. 27, edited by W. R. Adey and T. Tokizani. Amsterdam: Elsevier, 1967, pp. 246-253. 9. Llimis, R. and C. A. Terzuolo. Mechanisms of supraspinal actions upon spinal cord activities. Reticular inhibitory mechanisms on alpha-extensor motoneurons, J. Neurophysiol. 27: 579-591, 1964.
10. Lopes da Silva, F. H. and A. Kamp. Hippocampal theta frequency shifts and operant behavior. Electroenceph. clin. Neurophysiol. 26: 133-143, 1969. 11. Magoun, H. W. and R. Rhines. An inhibitory system in the bulbar reticular formation. J. Neurophysiol. 9: 165-171, 1946. 12. McBride, R. L. and W. R. Klemm. Stereotaxic atlas of rabbit brain, based on the rapid method of photography of frozen, unstained sections. Communs. behav. Biol. 2: 179-215, 1968. 13. Petschc, H., G. Gogolak and P. A. van Zwieten. Rhythmicity of septal cell discharges at various levels of reticular excitation. Electroenceph. clin. Neurophysiol. 19: 25-33, 1965. 14. Pickenhain, L. and F. Klingberg. Hippocampal slow activity as a correlate of basic behavioral mechanisms in the rat. In: Progress in Brain Research, Vol. 27, edited by W. R. Adcy and T. Tokizani. Amsterdam: Elsevier, 1967, pp. 218-227. 15. Scheibel, M. E. and A. B. Scheibel. Anatomical basis of attention mechanisms in vertebrate brains. In: The Neurosciences, edited by G. C. Quarton, T. Melnechuk and F. O. Schmitt. New York: Rockefeller Univ. Press, 1967, pp. 577-601. 16. Schaeppi, U. and P. H. Rubin. Effects of chlorpromazine upon induced periods of tonic immobility in rabbits. Int. J. Neuropsychol. 1: 71-75, 1965. 17. Sprague, J. M. and W. W. Chambers. Control of posture by reticular formation and cerebellum in the intact anesthetized and unanesthetized and in the decerebrated cat. Am. J. Physiol. 176: 52--64, 1954. 18. Vanderwolf, C. H. Hippocampal electrical activity and voluntary movement in the rat. Electroenceph. clin. Neurophysiol. 26: 407-418, 1969.
EEG and Multiple-unit Activity in Limbic and Motor Systems during Movement and Immobility' W. R. K L E M M Department o f Biology, Texas A & M University, College Station, Texas 77843, U.S.A. (Received 2 November 1970)
K~MM, W. R. EEG and multiple-unit activity in iimbic and motor systems during movement and immobility. PHYSIOL. BeHAV. 7 (3) 337-343, 1971.--Electrographic activity was recorded from chronically implanted electrodes generally in the caudate nucleus, septum, hippocampus, and midbrain and medullary levels of the brain stem reticular formation (BSRF) of rabbits in order to assess the effects of movements and immobility. Hippocampal theta rhythms were triggered during movements of any sort or during phasic increases in muscle electrical activity. Coinciding with or preceding the theta onset were phasic increases of multiple-unit activity in the BSRF. Injection of tranquilizer simultaneously reduced movements, abolished theta rhythm, and selectively depressed BSRF units. These specific electrographic correlates of muscle activity were not observed in the caudate, septum, or other telencephalic brain structures from which recordings were derived. Transient theta and increased BSRF activity also occurred during the immobility reflex (animal hypnosis) when the tonic muscle relaxation and immobility were phasically interrupted by mild sensory stimulation. The motorassociated phasic increases in BSRF multiple-unit activity were superimposed upon a tonic unit activity increase in the medullary reticular formation, not seen elsewhere, that was characteristic of the reflex. Hippocampus
Brain stem
Muscleactivity
EEG
inhibition of spinal reflexes and both voluntary and involuntary movements. The IR does seem to be a reflex, albeit complex, because it is a specific, relatively stereotyped, involuntary, and unconditioned response to specific stimuli [6].
BEYOND the simple reflex level, appropriate and adaptive body movements must be influenced by those brain processes which regulate emotion and memory. These processes are generally considered to be major functions of the limbic system, one of the major components of which is the hippocampus. A specific electrographic rhythm of the hippocampus, the synchronized 4-9/sec activity known as theta rhythm, has historically been associated with behavioral arousal. More recent studies, however, have indicated that this association may be misleading and that the more basic correlation may be between theta rhythm and voluntary movements [8, 14, 18]. One of the known motor controlling structures, the brain stern reticular formation (BSRF), is known to be functionally related to the limbic system, via the limbic-midbrain circuit. More specifically, the evidence of BSRF activation of the septum which in turn paces hippocampal theta rhythm [13] raises the possibility of intimate relations between BSRF motor functions and the hippocarnpal functions in emotion and memory. The present report concerns an electrographic study of these relations, correlating muscle activity with electrical activity in the hippocampus and BSRF. This correlation was also studied with a specific model of involuntary immobility, the so-called Immobility Reflex (IR), otherwise known as animal hypnosis. The IR is a unique behavioral state which has special relevance to this study because of the profound
METHODS
Twenty-four New Zealand White rabbits, of both sexes and weighing 5.5-6 pounds, were chronically implanted usually with 4 pairs of nichrome electrodes (<0j3 mm dia.) that were insulated with Formvar varnish except for about ½mm at the tips. The principal placements, which were later verified histologically, and stereotaxic coordinates [12] were in the left hippocampus (P5, L3) (20 rabbits), median septum (A2) (18 rabbits), right caudate nucleus (A2, L2.5) (16 rabbits), left medial medullary reticular formation (MRF) (P20 and P24, L1) (8 rabbits), and right medial midbrain reticular (MBRF) formation (P12, L1) (5 rabbits). Other
electrode placements included neocortex (3 rabbits), corpus callosum (I rabbit),claustnm~ (I rabbit),and various areas adjacent to the M R F (inferior olive, pyramids, fasciculus gracilis, solitary tract nucleus, hypogiossal nucleus--8 rabbits). After injection of penicillin-streptomycin and after allowing several days for recovery, EEGs were obtained before, during, and after the IR. IR was induced in a snug, 3-sided chute which permitted
1This research was supported by a grant from the National Institute of Mental Health, No. MH 13072. A preliminary report involving some of these data, discussed in another context, has already been published [7].
337
338
KLEM M
I R states to persist for 10 min or more without interruption [3], even during mild sensory stimulation such as blowing in the rabbit's face or gently jabbing the limbs and belly with a pencil. Rabbits were placed on their back inside the chute, and their head was held down for about the first 5 sec; I R terminated spontaneously, as indicated by an abrupt righting reflex. When Awake, the rabbits moved freely within an open-topped wooden box (27 × 24 × 38 cm high) (Use of the term Awake in reference to non-IR states is done as a semantic convenience, and is not meant to imply that the I R represents a sleeplike condition. Indeed, much published evidence exists to indicate that brain processes are quite active during the I R [6]). Body movements were indicated by motion artifacts in the E E G and by direct observation of the head and the attached recording cable. Motor activity was monitored in 6 rabbits by recording the
nuchal electromyogram (EMG) and by recording voltages generated by rabbit movements on a conventional jiggle platform. Test sessions lasted at least 30 min, during which rabbits were alternately immobilized and awakened. The EEG was recorded with a Grass Model 78 polygraph, with potentials from the animal coupled to the preamplifier via emitter-follower input. The EEG pen amplifiers had ½-amplitude cutoff settings at 1-90 Hz; signals were also simultaneously supplied to a computer or an electronic integrator. Interval histogram analysis of hippocampal EEG waves was performed by leading the preamplifier signal into an on-line computer (Nuclear Chicago, Model 7100). Analysis was based on the principle of detecting voltage excursions across a pre-set discrimination level, followed by the computation of the interval between excursion points. The
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FIG. 1. Composite illustration of correlations of hippocampal theta rhythm, BSRF multiple-unit activity, and muscle activity. In the Awake state, 2 large phasic increns~ in multiple-unit activity are seen only in the MRF, and each is more or less simultaneous with onset (first arrow) of pronounced hipcoeampal (HIP) theta rhythm which terminates (~¢ond arrow) after multipleunit activity returns to normal. (Note also the seeminely random and abrupt episodes of decaeased multiple.unit activity in the hippocm~us.) Dashed vertical lines serve to emphasize the slightly delayed, time-locked inc'r~_~ in EMG and body movements (cross marks indicate motion artifacts in EEG traces). Records during the IR reveal that the only EEG changes are mild slowing of frequenfies and increase in voltage in camdate (CALl) and septum (SEP), along with reduced hipix3camlml theta. Multiple-unit activity is not changed in any area except in the MRF, where a mild increase is sustained. When air is blown in the rabbit's face (arrow), there is a sudden increase in MRF multiple-unit activity, associated with onset of hippocampal them. Although body movements did not occur, isometric contractions are indicated by the increased EMG activity. Time marks, 1 and 5 sec; EEG and EMG voltage calibrati(ms, 50 ttV. Integrator calibrations, solid lines, from bottom to top, 0, 5, 10 and 20 ~V signals of I K H z applied to preamplifier input; dashed line, noise level of entire system with 10,000 fl across preamplifier input.
HIPPOCAMPUS, BRAIN STEM AND MUSCLE ACTIVITY
339
signals were filtered with a t-amplitide cutoff at 30 Hz. The computer sampled 200 waves in each analysis; analog-todigital conversion discriminators, set on a 0-voltage baseline, were triggered on the positive slope of waves and the computer calculated the interval between successive waves. Computer output was read out through a strip-chart recorder. Multiple-unit recording was accomplished by leading preamplifier output (t-amplitude cutoff at 3 KHz) simultaneously to a multiple-unit filter (Kopf Co.) with a lowfrequency cutoff at 300 Hz. The multiple-unit potentials from the filter were led into an R-C integrator (Grass, Model 7P3A). Units were also observed directly on an oscilloscope and photographed as needed.
AWAKE
I00 I
w a yes RESULTS
t
Awake LEG.
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Obtaining artifact-free recordings from the unrestrained rabbits usually became progressively easier as they became accustomed to the recording conditions. As expected, LEGs varied with the state of agitation. When rabbits were very relaxed, their L E G frequencies slowed, spindles sometimes occurred in the caudate leads, and there was a concurrent reduction in muscle tone, as indicated by lower E M G voltages, and in body movements. Opposite effects were conspicuous when the rabbits were constantly moving about in the restraining box. The unexpected observation was that activity changes in the hippocampus and BSRF were specifically correlated with transient changes in muscle tone and movements. When phasic movements occurred, as documented by increased E M G activity and jiggle stand recording, the hippocampal activity suddenly converted into a rhythmical series of 5-9/sec waves (theta rhythm) (Fig. l, Awake). As shown in the Awake portion of the figure, there were 2 isolated movements, each associated with a burst of theta activity. Theta activity correlated with movements in all rabbits tested. Theta rhythm often outlasted muscle activity by a few seconds or more, as is illustrated for the movement at the fight hand side of the Awake record in Fig. 1. These hippocampal changes were verified by intervalhistogram analysis (Fig. 2, Awake). Such analyses revealed that theta frequencies were dominant during continuous movements and much reduced during periods of less motor activity. Further quantification of the histograms (Table 1,
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FIG. 2. Hippocampal interval histogram data from several experiments during different phases of IR and Awake states. During the IR, tbeta rhythm (reflected in histogram b) was always induced by mild stimulation (which produced muscular tension but which was below the threshold for disrupting the IR); however, in the unstimulated condition, tbeta rhythm was less conspicuous, as in a. During Awake states, the wave interval distribution was a function of the degree of alertness, being diminished during behavioral immobility and relaxation (as in a) and becoming especially prominent during gross body turning movements within the restraining box (as in b).
Awake), revealed that some theta activity was always present in the hippocampus, but that during continued movement, the percentage increased, with most of the waves in the range of 5-9/see (110-200 msec intervals). Such activity decreased distinctly when overt movements were absent. Multiple-unit activity. The activity as observed from all leads on the oscilloscope (Fig. 3, Awake), appeared as a mixture of individual spikes of approximately ~-1 msec duration. The differing amplitudes reveal that they were generated from several neurons; two or more spikes summated when they occurred at approximately the same time. The integrated display of the multiple-unit activity, in the case of caudate and septal leads, maintained a steady level
TABLE 1 RANGE (IN MSECS)CONTAINING AT LEAST t OF TOTAL HIPPOCAMPAL WAVES
Awake
Rabbit 1.
2. 3.
4. 5. 6. 7.
Moving -131-175, 180-200 150-213 140-187 150-180, 187-213 138-145 113-150
IR
Not Moving
Stimulated
-163-168 180-182 175-177 0 0 0
156-190 175-190, 220 -163-165, 206-208 175-180 136-142 113-131, 156-169
Unstimulated 150-152 0 0 0 185-187 0 0
340
KLI!~MM
around which minor fluctuations occurred, irrespective of the rabbit's behavior (Fig. 1, Awake); septal fluctuations were more apparent than those from caudate leads. However, hippocampal unit activity, which was the smallest of any of the areas, had numerous msec periods of relative electrical silence, as indicated by the downward integrator pen deflections in Fig. 1. These deflections did not seem to correlate with presence or absence of theta activity in the hippocampal EEG, nor with any aspect of behavior. By far the most conspicuous aspect of multiple-unit activity was a phasic increase in B S R F leads immediately preceding the onset of hippocampal theta activity, E M G increases and overt movements (Fig. 1, Awake). As muscle tone and movements subsided, the BSFR multiple-unit activity returned to its steady state; as mentioned, hippocampal theta often persisted after motor activity ceased. Not only movements, but also phasic tensing of muscles without overt movement, were associated with hippocampal theta activity and BSRF multiple-unit increases. The degree to which these phenomena concurred was very high, as indicated by counting episodes of each phenomenon in the rabbits which had E M G electrodes (Table 2, Awake). Specific examples of various movement conditions which were associated with the B S R F unit increases are shown in Fig. 4. The magnitude of these unit increases was on the order of about 10 t~V, based on the arbitrary calibration scheme. Still larger increases were noted in monopolar recording with a reference lead insulated in nasal bone. However, only bipolar data are shown because theoretically records obtained in reference recording could be contaminated by artifact potentials from facial muscle or olfactory bulbs. The correlated increase in multiple-unit activity occurred in all rabbits with M R F electrodes (5 at the inferior olive level and 3 at a level just anterior to the decussation of medullary pyramids). Similar unit activity increases were also observed in the 5 rabbits with M B R F electrodes. Control tests for artifact included vigorous manipulations of the recording cable while it was still connected to an unmoving rabbit; no increases in unit activity occurred, although large, movement-related pen excursions appeared in the EEG channels. Although unit increases associated with muscle activity were observed from structures adjacent to the B S R F (n. gracilis, solitary tract n., hypoglossal n., and pyramidal
AWAKE
IR _
c
FIG. 4. Integrated MRF multiple-unit activity illustrating transient increases during Awake and IR states that were associated with movements. In the upper trace, the rabbit was moving calmly about the restraining box. During the IR, increased activity developed in response to stimulation of the rabbit b y blowing air in its face (arrow), which also elicited mild general movements (a). In (b) the rabbit displayed a few spontaneous limb movements (arrows) which coincided with the multiple-unit increases, In (c) a heavily tranquilized rabbit exhibited paroxysmal jaw chewing movements over about 4 sees (solid line) that we.re associated with the rhythmic bursts of increased multiple-unit activity.
tract), no such increases were observed from the caudate, septum, or hippocampus. Nor were any such increases seen from any of the 5 rabbits whose rostral electrodes were in the neocortex, claustrum, and corpus callosum.
Immobility Reflex EEG. During the I g , and in the absence of stimulation, EEGs were generally less activated than when Awake, with mild slowing of frequencies and increased voltage in caudate and septum (Fig. 1, I g ) . Hippocampal theta rhythm was reduced during the I R (Fig. 2, I g ; Table 1); only 2 rabbits displayed major amounts of theta rhythm. One rabbit had continual theta rhythm for about the first lOsec after induction; in the other, them rhythm occurred more or less continuously in association with phasic recurrences of increased E M G activity and M R F multiple-unit activity. In all rabbits, sensory stimulation during the I R could still
TABLE 2 INCIDENCEOF CORRELATIONSAMONGPHASICMRF UNiT INCReASe, THETARHYTHMAND EMG ACTIVITY Awake Rabbits
Total
MRF Unit Increase
IR
No MRF Unit Increase
Total
MRF Unit Increase
No MRF Unit Increase
Observations Theta Increased EMG Theta Increased EMG Observations Theta Increased EMG Theta Increased EMG 71 33 190 134 90 107
68 21 173 125 83 106
64 20 173 124 80 103
2
3
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7 13 7 0 3
3 10 4 0 1
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Totals
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26
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79
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6
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8
7
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IR
A
I B
FIG. 3. Multiple-unit potentials in the M R F in Awake and IR states, as they appear on the oscilloscope screen. The increased activity during IR is illustrated on two time scales, full scale of 20 msecs (A) and 100 msecs (B). The large voltages during IR are seen in the fast time scale illustration to result in part from summation of individual action potentials. Voltage calibration: 20 ~zV.
(facing page 340)
HIPPOCAMPUS, BRAIN STEM AND MUSCLE ACTIVITY
341
elicit transient theta rhythm, often associated with flinching, without disrupting the I R state (Fig. 1, IR); still other rabbits exhibited a spontaneously phasic theta that was commonly associated with an abortive attempt at performing the righting reflex, followed by a transient limb extension. All rabbits revealed distinct theta rhythm during the righting reflex which terminated the state. Multiple-unit activity. During the IR, no change in EEG, and either no change or a decrease in tonic multiple-unit activity, was evident in the 9 rabbits with electrodes in the n. gracilis, solitary tract n., hypoglossal n., inferior olive, and pyramidal tract. No change in EEG or multiple-unit activity occurred during the IR in the 5 rabbits whose rostral electrodes were in the neocortex, claustrum, and corpus callosum. The [R did not result in any detectable change in multipleunit activity in either the caudate, septum, or hippocampus (Fig. 1, IR). Paradoxically, during the IR the medial MRF generally revealed more EEG desynchrony associated with a small, but sustained multiple-unit activity increase, even though no movements occurred, muscle tone was decreased, and the electrodes were the same as those used when Awake. However, phasic increases in MRF activity, superimposed on the tonic increase, could be elicited by sensory stimulation which evoked increased muscle tension or flinching (Fig. 1, IR; Fig. 4, IR). The correlation of unit increases, theta rhythm, and muscle activity was observed in almost all instances (Table 2, IR).
decreased movement, lowered muscle tone, abolished hippocampal theta rhythm, and decreased tonic BSRF multiple-unit activity (Fig. 5, Awake). Although tonically depressed, the BSRF unit activity increased phasically when movements did occur. The tonic depression of BSRF units was not paralleled by unit decreases in rostral brain areas. Induction of I R in a tranquilized rabbit could nonetheless produce a tonic level of MRF multiple-unit activity that was greater than when the same tranquilized animal was Awake (Fig. 5). DISCUSSION
The literature is replete with studies of the significance of hippocampal theta rhythm [7, 10, 13, 14, 18]. Many of these studies, however, are interpreted in various ways that relate to such aspects of behavior as arousal, orientation, psychic tension, learning, goal seeking, drive, and reward. Some workers have recently advanced the hypothesis that theta activity is correlated more specifically with limbic system integration of motivational processes associated with movements, regardless of the nature of the movement or its cause [8, 14]. Since these behavioral states are generally associated with movement, theta could correlate specifically with voluntary movement per se. Such a view is taken by Vanderwolf [18], based on his studies of rats and dogs. He further contended that theta rhythm in these species occurred only during movement; the rhythm was absent in states of alertness in which no movement occurred. The present study does clearly reveal[an association between movement and theta rhythm, in either the Awake or IR state. However, the correlations do not exclusively depend on
Tranquilizer The intravenous injection of a tranquilizer (promazine hydrochloride, 4 mg/kg) in 4 Awake rabbits simultaneously
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FIG. 5. Tranquilizer effects. In the Awake state, drug treatment (D) caused the following distinct changes from the non-drug (ND) condition: caudate EEGs contained higher voltases, slower activity, and occasional spindles (arrow). Hippocampal theta activity was greatly reduced, as was the multiple-unit activity in the MRF (and MBRF, but not elsewhere). Nuchal muscle activity decreased, even during comparable periods when no body movements occurred. The figure also illustrates the typical ND changes during IR of increased MRF activity and reduced muscle tone. When the rabbit was drugged and IR was induced, parallel changes again occurred: relatively increased MRF activity and decreased muscle tone. Calibrations as in Fig. 1.
I
342 actual movement. Isometric contractions in which tone increased without overt movement were commonly observed in this study to coexist with increased BSRF unit activity and theta rhythm. Many of the movements and their associated theta rhythm are not readily attributable to motivational factors. Particularly during IR, it is unlikely that a rabbit could anticipate the stimulus and develop theta rhythms as part of a neural processing preparation for such an involuntary movement as the flinch response. Although the two rabbits with much theta activity during the IR could have been processing information to prepare for eventual performance of the righting reflex, it seems more probable that their theta activity simply resulted from a higher level of muscle tone. Previous studies have shown that the early post-induction period is the period of maximum muscle tone and it is also during that period when theta activity is most common [4]. Also consistent with this view of the importance of muscle tone is the observation that tranquilizer completely abolished theta activity at the same time that it also reduced muscle tone and M R F multiple-unit activity, both in Awake and I R states. Thus, the most straight-forward interpretation of the significance of theta rhythm may be that it is a non-specific correlate of muscle activity, either isotonic or isometric, which may be voluntary or involuntary [7]. The EEG findings of no specific correlate with the IR (except in the MRF) extends the number of brain areas which have now been demonstrated to exhibit dissociated activity in the behavioral states of I R and alert wakefulness [4, 6]. For the brain areas tested, this present study also reveals that the dissociation exists also at the level of multipleunit activity. In other words, it appears that the essence of the IR is that it is a motor disconnection syndrome in which some neural clutch in the brain stem disengages motor drive from the spinal motoneurons, allowing the rostral brain cells to nonetheless continue relatively normal discharge [4]. Interpretations of multiple-unit activity have to be qualified to an extent. Lack of clear changes in integrator output from rostral brain areas during muscle activity or during IR does not necessarily indicate absence of changes. Integration is an averaging technique, and significant individual events can go undetected; changes could be occurring during movements in these rostral areas but, compared to M R F increases, are not great enough to be manifest. Moreover, the method does not distinguish excitatory from inhibitory neurons which, if they completely reversed their relative activity, might not produce any net change in integrator output. The few exceptions (Table 2) to the usual correlations among muscle activity, hippocampal theta rhythm, and BSRF multiple-unit increases could be attributed to the fact that only a small part of a given brain area was monitored at any one time and that only one of the body muscles was monitored. Increases in multiple-unit activity, such as those noted in the M R F during muscle activity and the IR or the entire BSRF during muscle activity, could reflect a recruitment of new neurons into firing or an increased discharge frequency of already active neurons, in either case, neurons in the area are clearly activated. The fact that movement-related increases occurred throughout the BSRF is not surprising, in view of anatomical evidence for extensive rostral and caudal
KL E MM projections of BSRF neurons [15]. During muscle activity, these units could be more active because of excitation from proprioceptive feedback or because they are sending descending influences upon lower motoneurons. The tonic increase in M R F multiple units during the IR confirms an earlier study [5] in which IR-associated multipleunit increases, as recorded from movable electrodes, occurred over a several millimeter vertical span of the MRF. Multipleunit increases in this present study were observed from the same M R F area during two behavioral states with opposite degrees of muscle activity, consistent with the common view that at least two functionally different pools of neurons co-exist in the same area. One neuronal pool could cause the Awake phasic muscular activity while the other might tonically inhibit motion during the IR. To speculate further, during the IR, both pools could be partially active concurrently, as when phasic increases are superimposed on the tonic increase during transient muscle activity. If the motor activating pool becomes greatly active, it might over-ride the tonic inhibitory pool, ending the IR by initiation of the righting reflex. The demonstration of tonic unit increases during the IR is all the more convincing, if, as already suggested, motor facilitating units in the area are less active during the IR, when muscle activity is reduced. Decreased motor facilitating activity would subtract from the voltages created by discharge of neurons involved in actively inhibiting movements during the 1R, thus tending to obscure detection of the latter's existence by the multiple-unit recording method. The clear preferential tranquilizer suppression of BSRF units, as opposed to units from rostral structures, was not accompanied by an equally clear cancellation of iR's enhancing effect on MRF multiple-unit activity. Although the drug depressed MRF units, it apparently depressed the IR-associated units less than the others, inasmuch as iR could still be accompanied by increased MRF multiple-unit activity, even in the drugged state. The well-known potentiating effect of tranquilizers on the IR [2, 16] may result from a disproportionate suppression of motor excitatory activity, thus indirectly enhancing the effectiveness of co-existing motor inhibitory influences. The well-known tranquilizing effects of tranquilizers may well be secondary to BSRF depression, if, as suggested previously, limbic activation results from BSRF activity and BSRF activity in turn is especially susceptible to tranquilizers. The decrease in tonic level of BSRF activity produced by tranquilizers could result from the diminished feedback that results from reduced muscle activity. On the other hand, the drug causes the reduced muscle activity, and could very well do so by inhibiting BSRF units whose normal function is to excite lower motoneurons during movement. This latter view is supported by previous experiments that demonstrated that the MRF can initiate both excitatory and inhibitory effects, including organized patterns of body movement [11, 17] by producing EPSPs and IPSPs on spinal motoneurons [1, 9]. Taken together, the cited literature and present findings have led to a theory, explicitly advanced previously [7], that sensory stimuli or higher brain influences stimulate the BSRF to not only activate the hippocampal theta rhythm, via limbic-midbrain paths, but also to produce a net activation of skeletal muscle tone.
HIPPOCAMPUS, BRAIN STEM AND MUSCLE ACTIVITY
343
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10. Lopes da Silva, F. H. and A. Kamp. Hippocampal theta frequency shifts and operant behavior. Electroenceph. clin. Neurophysiol. 26: 133-143, 1969. 11. Magoun, H. W. and R. Rhines. An inhibitory system in the bulbar reticular formation. J. Neurophysiol. 9: 165-171, 1946. 12. McBride, R. L. and W. R. Klemm. Stereotaxic atlas of rabbit brain, based on the rapid method of photography of frozen, unstained sections. Communs. behav. Biol. 2: 179-215, 1968. 13. Petschc, H., G. Gogolak and P. A. van Zwieten. Rhythmicity of septal cell discharges at various levels of reticular excitation. Electroenceph. clin. Neurophysiol. 19: 25-33, 1965. 14. Pickenhain, L. and F. Klingberg. Hippocampal slow activity as a correlate of basic behavioral mechanisms in the rat. In: Progress in Brain Research, Vol. 27, edited by W. R. Adcy and T. Tokizani. Amsterdam: Elsevier, 1967, pp. 218-227. 15. Scheibel, M. E. and A. B. Scheibel. Anatomical basis of attention mechanisms in vertebrate brains. In: The Neurosciences, edited by G. C. Quarton, T. Melnechuk and F. O. Schmitt. New York: Rockefeller Univ. Press, 1967, pp. 577-601. 16. Schaeppi, U. and P. H. Rubin. Effects of chlorpromazine upon induced periods of tonic immobility in rabbits. Int. J. Neuropsychol. 1: 71-75, 1965. 17. Sprague, J. M. and W. W. Chambers. Control of posture by reticular formation and cerebellum in the intact anesthetized and unanesthetized and in the decerebrated cat. Am. J. Physiol. 176: 52--64, 1954. 18. Vanderwolf, C. H. Hippocampal electrical activity and voluntary movement in the rat. Electroenceph. clin. Neurophysiol. 26: 407-418, 1969.