EXPERIMENTAL
NECROLOGY
A Quantitative in Single
50, 180-193 (1976)
Investigation
Coding
Cells of the Cat Mesencephalic Reticular Formation KAREN
Divisiovk
of Somatosensory
L. BARNES 1
of Ncuroskrvgrvy, Drpartwkmt Urkiversity School of Medicitk&, Rrccivrd
of Szwgcvy, Case Clcvclaud,
Ohio
Wcstcrrk 44106
Rcscrve
Jwke 21,19i.5
Single cell response patterns in cat mesencephalic reticular formation to electronically controlled touch-pressure and noxious radiant heat stimulation were compared. Cells were recorded from 32 acutely decorticated cats in response to transient and continuous s’omatic stimulation. Analysis of 136 cells revealed that 65% responded to both stimuli, 23% only to noxious heat, and 12% only to touch-pressure. Response adaptation was observed to be slow in 93% of the heat responders, in contrast to rapid adaptation in 67% of the touch units. Although the majority of cells was excited by stimulation, both excitatory and inhibitory responses were found. Considerable differences. in statistical characteristics of the response to touch versus heat were seen in all but two of the 89 bimodal cells. The differential adaptation to touch versus heat and the finding of both excitatory and inhibitory responses support predictions of Wall and Melzack (10). The predominance of cells responding to both stimuli reinforces Magoun’s (16, 17) view of the mesencephalic reticular formation as part of the reticular activating system.
INTRODUCTION This investigation focused on a quantitative analysis of the single cell response patterns in the cat mesencephalic reticular formation to electronically controlled touch-pressure and noxious radiant heat stimulation. Comparison of the responsesto noxious versus nonnoxious skin stimulation was undertaken to determine whether this region of the central nervous 1 This research was supported by NIH Grant NS-00779-16 and NIMH Predoctoral Z-FLMH-32, 300-02 (PS). Part of this study was described in a doctoral dissertation submitted in partial fulfillment of the requirements for the Ph.D. degree, Department of Psychology, Case Western Reserve University. The auth’or appreciates the advice and criticism of Drs. H. Gluck, D. P. Becker, J. S. Brodkey, and J, P. Conomy.
ResearchFellowship
180 Copyright 6 1976 by AcademicPress, A11rights of reproduction in any form
MESENCEPHALIC
RETICULAR
FORMATION
181
system (CNS) differentially processes information from painful, as opposed to innocuous, stimuli. The choice of the mesencephalic reticular formation as the recording site was based first upon considerable evidence for somatic responses in this region (l-3), and second upon its probable function as a region for integration of information from visual, auditory, and somatosensory inputs, including pain (3, 9, 12, 13). Early theories of cutaneous sensation tended to espouse one of two diametrically opposed positions, specificity theory or pattern theory. However, recent findings have indicated the inadequacy of either of these theories to explain all the phenomena of somesthesis (4, 7, 14, 19, 20). Hence this study takes its theoretical basis from the attempts of Wall and Melzack (10, 11) to integrate the best aspects of the older approaches to cutaneous sensation. The suggestion originally made by Magoun (16, 17) that the mesencephalic reticular formation has an arousing, alerting function as part of the reticular activating system is also considered. METHODS Recordings were made from 32 adult cats in the unanesthetized acutely decorticated preparation because of the well documented reduction of CNS responsiveness to sensory stimulation under anesthesia. Several animals were allowed to recover from the halothane and nitrous oxide anesthesia under which initial preparation was done in order to assess whether decortication provided an adequately pain-free preparation. These cats were blind and relatively unresponsive to noxious stimuli sucl~ as pinprick, intense radiant heat, and pinch. All wounds were infiltrated with Xylocaine. During recording, the pupils remained constricted, another indication of the preparation adequacy. The cats were paralyzed with gallamine triethiodide, artificially ventilated, and placed in a stereotaxic apparatus with head holder. An infrared lamp and Telethermometer kept rectal temperature between 36 and 39 C. Body fluids were maintained by slow intravenous drip of lactated Ringer’s solution. Systemic arterial blood pressure was monitored continuously during recording. Whenever the systolic pressure dropped below 100 mm Hg or the pupils became dilated and fixed, the experiment was terminated. One or two electrode passes through the mesencephalic reticular formation were made on each side of the brain. Boundaries of the region studied were the stereotaxic coordinates A0 to A3, Ll to L4, and D+l to D-4, according to the Snider and Niemer atlas of the cat brain (15). To make sure that the cells recorded actually lay within the mesencephalic reticular formation anatomical bounds, lesions were made in selected brains and sections prepared alon,m an electrode track, after perfusion with formalin. The brain stem was then cut at 20 pm, stained with cresyl violet, and
KAREN
L.
BARNES
FIG. 1. Cross-section of cat brain stem at anterior 3.0, 20 pm frozen section with cresyl violet Nissl cell stain. Black outline denotes the limits within which cells were recorded for this study.
examined microscopically to confirm the electrode location. In all cases the tracks were seen to pass through the region outlined in Fig. 1, which shows a cross section of one of the brain stems at approximately anterior 3.0. Recordings were taken from glass capillary microelectrodes filled with 3 M KCI, having impedances between 1 and 3 megohms. Unit potentials were led into a unity gain Bak preamplifier. An amplitude discriminator was set to window spikes within a chosen voltage range, allowing unit isolation from noise and background cell activity. The constant voltage pulse output of the window was fed into an audio monitor and displayed on an oscilloscope along with the original recording of brain activity. The original data, amplitude window output, and various stimulus markers were recorded on a Precision Instrument FM tape recorder. Data analysis was done on a Line-8 computer system. During the course of the study, an on-line data acquisition program was developed for the Lint-8, so that data for the last ten cats were recorded directly on Lint-8 tape. During exploration of the midbrain reticular regions, touch stimulation and brief electrical shock to the footpads were applied so that the cell sample was not limited to those units having spontaneous activity. Receptive fields for touch and heat were mapped with the audio monitor. Responses of isolated mesencephalic reticular formation cells to transient
and continuous
somatic
stimulation
were determined
as follows:
Brief
ME~ENcEPHALI~
RETICULAR
FORMATION
183
touch and noxious heat were applied for 1 or 2 set, while continuous stimulation was given for 1 min. Touch-pressure was applied by an electronically driven probe, patterned after one described by Werner and Mountcastle (21) . A 4- mm Lucite probe tip was applied perpendicular to the skin surface. The transient stimulus was a 400 pm static indentation of the skin for 1 set, while during the minute of continuous stimulation a 200 Hz tip vibration of 50 pm was superimposed on the static displacement. The vibration was added because almost all units ceased responding after the first few seconds of static indentation. Heat was provided by a miniature projection lamp with built-in reflector (GE EJM) behind a tachistoscope shutter limiting the area and duration of stimulation. Transient heat was applied for 2 set, because of the time lag for heating the skin. A thermistor on the surface of the skin registered 54 C during stimulation. Since this temperature is well above the 45 C threshold for pain in humans (5), and since it produced pain when applied to the experimenter, it clearly qualifies as a noxious somatic stimulus. In order to eliminate bias from any changes in cell activity over the recording period, both the order of stimulus modalities and the application of transient and continuous stimulation were counterbalanced over successive cells. Transient stimulus records were 24 set long: 5 set control for spontaneous activity, 1 or 2 set of stimulation, and 17 or 18 set poststimulus firing. Running averages of the unit firings per second (O.l-set increments) were outputted graphically on a Calcomp plotter to reveal spontaneous, stimulation, and poststimulus activity patterns. Both interspike interval histograms and running averages of unit firing were plotted for the continuous stimuli and for minute control records taken before and after each stimulus record. While the interval histograms provide a good index of overall changes in firing pattern with different stimuli, they give no information about the time course of changes in firing. Hence the running average analysis was included to preserve information about response variation during the minute record. RESULTS This series of experiments yielded 157 cells responsive to somatic stimulation and observed within the limits of the mesencephalic reticular formation. Of these cells, 136 were recorded in sufficient detail for analysis. The great majority of these somatic responders were found in the area bounded by A0 to A3, Ll to L3, and DO to D-2.5 on the stereotaxic axes. Nearly all (93%) of these units were spontaneously active, most having between 200 and 1500 discharges per minute. In addition, there was a definite decrease in the variability of discharge as the overall rate increased.
184
KAREN
L.
BARNES
TABLE RESIWNSE
RESPONDING
ONLY
TO OYE
Type Firing
1
OF 47 MESENCEPHALIC
CHARACTEKISTICS
MODE
RETICULAR
OF SOMATOSENSORY
of Change
in Firing
change Touch -..-__
Excitatory Inhibitor) Rate
of Adaptation
mode Heat
__12 ‘4
Totals
CELLS
Rate
Response
__-
FORMATION STIMULATION
16 of Response
19 12 31
.__ Rapid Slow Totals
11 5
2 29
16
31
Receptive field size and location varied extensively for these units. Responses were evoked bilaterally in 57% of the cells, while unilateral fields were mainly (51%) contralateral. Only 38% of the fields were restricted to the region of a single limb or less, with many covering all four limbs or two limbs plus part of the trunk. Responses to touch and to noxious heat generally had congruent skin fields. All these characteristics are in accord with the findings of previous investigators (l-3, S, 9). Looking now at the response patterns of the 136 cells which were analysed, 47 (35%) were unimodal, responding only to heat or to touch, while the other 89 (65%) were bimodal, responding to both stimuli. Of the unimodal responders 31 (66%) modified their activity for noxious heat, and 16 (34%) were touch responders (Table 1). Since unit activity is generally a nonstationary process, tests of statistical significance for mean differences have limited meaning. Thus a change of 20% in discharges per minute was arbitrarily set to define a response. In most cases the firing changes were well above the minimum. Figure 2 displays the analysis of a typical touch responder: the top row shows the interspike interval histograms for the three recording conditions, while the bottom plots display the corresponding running averages of each minute record. The left column indicates that the spontaneous activity of this cell, unit 2303, was quite rapid (1678 discharges per minute, mean interval 34.7 msec, standard deviation 13.7 msec) . In the middle set, continuous touch stimulation has produced an increase in total firing, particularly during the first 15 set of stimulation (1983 discharges, mean interval 29.2 msec), and some
MESENCEPHALIC
RETICULAR
FORMATION
FIG. 2. Unimodal touch responder 2303: excitatory response. Top row displays interspike interval histogram, bottom row contains running average plots. Control records on left, touch stimulation in middle, heat stimulation at right. Histogram abscissa gives interval length in msec, ordinate displays interval frequencies. M = mean interval length in msec, SD = interval standard deviation in msec, N = total interspike intervals during minute record. Note differences in histogram ordinate scale values. Running average abscissa gives elapsed recording time in seconds, ordinate shows average unit firing rate per second. N = total unit discharges during minute record. decrease in interspike interval variability (SD 10.2 msec). The right two plots show no clear changes in firing to heat. The response pattern of unimodal heat responder 2110 is shown in Fig. 3. This cell fired at a moderate spontaneous rate (564 discharges, mean 93 msec, SD 110 msec). Touch stimulation had virtually no effect
FIG. 3. Unimodal heat responder 2110: excitatory response. General same as Fig. 2. Note differences in histogram ordinate scale values.
description
186
KAREN
L.
TABLE TYPE
BARNES
2
OF FIKING CHANCE OF 89 MESENCEPHALIC RETICULAR FORMATION RESPONDING TO Two MODES OF SOMATOSENSORY STIMULATION
Number of cells having same direction of both changes Both excitatory
40
Both inhibitory
19
Q Twelve additional cells had a normal lus mode, but responded with a widely text.
CELLS
Number of cells having different direction of both changes” Touch excitatory, heat inhibitory 11
excitatory or inhibitory varying discharge rate
Touch inhibitory, heat excitatory 7
response to one stimuto the other mode. See
upon any of the firing pattern parameters. In contrast, heat stimulation produced a marked acceleration, with discharge rate rising to 971, mean interval dropping to 61 msec, and SD falling to 63 msec. For both varieties of unimodal responder the majority of responses were excitatory, raising the activity above the spontaneous level. Of 16 touch units, 12 (7570) increased their firing during stimulation; 19 of the 31 heat responders (60%) accelerated their discharge rate when stimulated (Table 1). Heat responder 2110 (Fig. 3) exemplifies the excitatory response, indicated by the increase in discharges and decrease in mean interval during heat stimulation compared to the control record. Examination of the average firing rate plots (shown on the bottom in each figure) for these unimodal responders revealed their characteristics of response adaptation to continuous stimulation. Adaptation was classified as rapid if the firing rate returned to the spontaneous level within the first 15 set of stimulation, and slow if the response was maintained for more than 15 sec. On this basis 11 of the 16 touch responders (67%) adapted rapidly; of the 31 noxious heat responders, only two adapted rapidly, while the other 29 (9370) exhibited a long duration response, many lasting well beyond the minute of stimulation recorded (Table 1). Touch responder 2303 (Fig. 2) adapted rapidly, its firing dropping off into the spontaneous range after the first 15 set of stimulation. In contrast, heat responder 2110 displays a firing acceleration which lasted throughout the entire minute record (Fig. 3). Description of the responses for the bimodal units was more complex, since it must account for two response patterns (Table 2). Again excitation was the predominant feature: forty of the 89 bimodal responders (45%) increased their firing to both stimuli. Unit 1907 (Fig. 4) exempli-
MESENCEPHALIC
3lGo
RETICULAR
FORMATION
CoNTRtil 19077-‘1 0
187
,;fio SEC 60
FIG. 4. Bimodal responder 1907: touch excitatory, heat excitatory. General description same as Fig. 2. Note differences in ordinate scale values for both histograms and running averages.
fies this pattern. Touch stimulation nearly doubled the cell’s firing rate and halved the mean interval, while heat produced an even greater increment of activity. From the running average plots, it appears that the touch response magnitude increased gradually over the minute record, while the heat response varied irregularly. It seems that the average plot is a more sensitive index of firing variability than is the standard deviation, since the greater oscillations of the heat average response compared to the touch average plot are not substantiated by the interval standard deviations. The second most frequent bimodal response pattern consisted of inhibition by both stimuli, seen in 19 cells (21%). In addition, the nature of these two most frequent response patterns reveals that 66% of this sampleof mesencephalicreticular formation bimodal responders show the same basic response direction of excitation or of inhibition to both touch-pressure and noxious heat. Another group of 18 cells (20%) exhibited opposing response patterns to the stimuli: Eleven were excited by touch and inhibited by heat, while the other seven displayed the inverse patterns. The remaining 12 cells (14%) were difficult to classify because they had a widely varying discharge rate to one of the stimulus modes, so that the response could be labeled neither excitation nor inhibition, Unit 1305 (Fig. 5) exhibits this pattern in that there is little change from the control in the mean or total discharge values for the touch interval histogram, yet the running average plot reveals a widely varying firing rate compared to the control. These units demonstrate the particular value of both the standard deviation and the running average plots in assessingresponsepatterns, becauseunit 1305 would have been classified as an excitatory unimodal heat responder if
188
KAREN
L.
BARiiES
FIG. 5. Bimodal responder 1305: touch oscillatory, heat excitatory. General description same as Fig. 2. Note differences in ordinate scale values for both histograms and running averages.
analysis had been limited to the interspike interval mean and total firings per minute. Very few bimodal responders having transient responsesto both stimulus modeswere found. Although it was possibleto record from only seven such mesencephalic reticular formation cells, all of them showed the same clifferential response adaptation to touch compared with heat. In Fig. 6 the firing increase of unit 3002 to touch dropped back to the spontaneous level within a few seconds after stimulus offset, while the response to heat tapered off very slowly. One factor to be considered is the time lapse after heat stimulus offset necessary for the skin temperature to fall below 45 C, the threshold for thermal pain in man. However, the skin cools below 45 C within 4 to 5 set, while the cell response appeared to last 8 to 10 set beyond stimulus offset. 0:
12
6
ii !I : ', i \
UNIT 3002 TOUCH RFP
02
. :' "> --.I+ 0
'wL
,
FIG. 6. Bimodal responder 3002: touch excitatory, plays firing histogram for brief touch stimulation shows firing histogram for brief heat stimulation of indicates stimulus duration, vertical mark corresponds sec. Ordinate gives discharge rate per second.
L
heat excitatory. Left graph disof right forepaw, right graph right forepaw. Horizontal mark to firing rate of 3 discharges/
MESENCEPHALIC
RETICULAR
FORMATION
189
A final analysis of tire bimodal responders involved classifying them with respect to which stimulus mode produced the greatest response, determined by the absolute deviation in total discharges from the spontaneous level. Of the 89 cells, 53 (60%) responded more strongly to heat, while 34 (38%) responded more intensely to touch. Although the remaining two units showed no difference in total discharges, examination of the running average plots revealed differences in the time course of the response, DISCUSSION The work of Melzack and Wall (10, 11) attempting to integrate the best aspects of the older theories of cutaneous sensation has suggested several hypotheses about somatosensory systems whose predictions may be tested by the results of this study. The first hypothesis proposes that CNS cells may detect or discriminate between somatosensory stimuli of different types by means of the adaptation characteristics of the responses. Based on evidence for the differential firing patterns of primary cutaneous fibers to touch-pressure versus noxious stimulation (6, 21)) Melzack and Wall (10) propose that CNS unimodal touch responders will adapt rapidly to continuous stimulation, while noxious heat units will adapt their response only very slowly, if at all. As a test of this prediction the adaptation characteristics of the 47 unimodal responders were determined from the running average plots of their response to continuous stimulation: Sixtyseven percent of the 16 touch cells adapted rapidly, while 93% of the 31 heat responders showed little if any decrement in response during the minute of stimulation (Table 1). Unit 2303 (Fig. 2) displays a typical brief excitatory response to touch, while unit 2110 (Fig. 3) exhibits a prolonged acceleration to heat. Hence these results affirm the existence of differential adaptation patterns in the two varieties of unimodal mesencephalic reticular formation units. The above findings were extended with a comparison of the poststimulus histograms for brief touch and noxious heat stimulation. Both of the unimodal transient heat responses observed were prolonged excitation ; of the ten touch units analyzed in this manner, eight displayed a brief firing increase during the stimulus, one showed brief inhibition, and the remaining cell had a prolonged excitation. When the analysis was expanded to include 19 bimodal mesencephalic reticular formation cells from which at least one transient response was recorded, 15 (79%) of these touch responses were brief excitation, and two were brief inhibition. All heat responses were clearly prolonged excitation or inhibition. Unit 3002 (Fig. 6) exemplifies the response patterns of these cells. In summary, the analysis of both continuous and transient stimulus records confirmed a predominance
190
KAREN
I,.
BARNES
of rapid Xkl~‘t~lti~Jll to toucll alIt Jaw a(laptation tu heat stiniulatiou by mesencephalic reticular formation cells. The existence of these differential adaptation patterns does not necessarily imply that mesencephalic reticular formation cells have functional significance in the organism’s ability to respond to stimulus differences. Because the mesencephalic reticular formation is not part of a “straight line” sensory path, but instead receives collaterals from those paths (13), its function is more probably one of alerting or arousing the organism so that it is ready to respond appropriately to specific stimulus information arriving on the straight line inputs. This view of the mesencephalic reticular formation as part of the reticular activating system has been substantiated by many studies since its proposal by Magoun and his associates (16, 17). A second suggestion of Wall and Melzack (10) is that CNS cells differentiate stimulus modes by means of the predominance of excitatory versus inhibitory inputs to the cell. This study has demonstrated both excitatory and inhibitory responses, with the great majority being excitatory: Two-thirds of the unimodal responders exhibited firing increases when stimulated (Table l), while the largest bimodal response category comprised 40 cells excited by both touch-pressure and noxious heat (Table 2). Hence more than half (52%) of the 136 cells observed showed only excitatory responses to somatosensory stimuli, while another 2670 were exclusively inhibited by stimulation. The predominance of excitatory responses also supports Magoun’s theory of the arousing, activating function of the mesenceplialic reticular formation. The multimodal responsiveness of these cells, with 89 (65%) responding to both touch-pressure and noxious heat, may be explained by the anatomical characteristics of the mesencephalic reticular formation described by the Scheibels (13). First, afferent input to mesencephalic reticular formation cells arises from a great variety of sources: spinothalamic, spinoreticular, and ventrolateral systems, brachium conjuctivum, vestibular complexes, and pyramidal tracts all make contributions. Second, mesencephalic reticular formation cells are profusely interconnected, with richly branching collaterals which could influence adjacent cells. Finally, most reticular axons have been traced beyond the mesencephalic reticular formation boundaries to such structures as cranial nerve nuclei, most of the thalamic nuclei, subthalamus, hypothalamus, colliculi, periaqueductal gray, geniculates, and cerebellum. These anatomical features of diffuse input, interconnections, and output suggest extensive possibilities for integration of somatic and other sensory information by mesencephalic reticular formation cells. In addition, predominance of multimodal responders in our sample is in accord with the views of Magoun and colleagues (16, 17) that the
ME~ENCEPHAL~C RETICULAR FORMATION
191
mesenceplialic reticular formation is part ot the reticular activating system. According to Magoun’s electrophysiological studies ( 17)) and confirmed by the Scheibels (13), collaterals from the lemniscal sensory fibers enter the mesencephalic reticular formation, so that somatic and auditory stimuli activate these cells. Reticular output is then transmitted by both thalamic and extrathalamic routes to the cortex, resulting in both electrophysiological and behavioral signs of arousal. The predominance of cells responding to both touch and heat stimulation, and the fact that over half of our sample of 136 mesencephalic reticular formation units (71, 52%) had only excitatory responses to the stimuli presented further reinforce Magoun’s idea of the activating, arousing nature of this system. Comparison of the results of this study with those of Becker et al. (2) reveals some units with response patterns similar to Becker’s six “nonnoxious responders,” characterized by short latency and short duration excitatory responses to both brief touch and electrical shock to the footpad. Nonnoxious cells had either no response to a minute of noxious heat, or a response of similar direction but smaller magnitude than that to touch. In this study the 16 unimodal touch responders and the 34 bimodal units which had their greatest response to touch confirm the existence of a group of somatic nonnoxious responders. However, the majority of mesencephalic reticular formation cells observed in this sample were either exclusively or maximally excited by noxious heat stimulation. Becker et al. (2) described their eight “noxious responders” as bimodal units having a long latency response with considerable afterdischarge to brief footpad shock. Continuous stimulation produced opposite directions of response : Heat was excitatory, while touch sharply inhibited the spontaneous activity. The 53 bimodal units having their greatest response to heat have similar transient response characteristics, but give little support to the importance of contrasting continuous response directions. Only five of these 53 cells exhibited excitation by heat and inhibition by touch. It is not surprising that this study found much more diversity of response pattern than did Becker ; his sample was so small (16 units) that he surely missed observing many of the mesencephalic reticular formation response varieties. Failure to find opposing response patterns of heat excitation and touch inhibition in more than seven of the 89 bimodal mesencephalic reticular formation responders provides an interesting parallel to the findings of Zimmerman (22) and Vyklicky et al. (18). Both of these studies report that noxious heat stimuli produce negative dorsal root potentials at the spinal cord just as does touch stimulation. Hence, large and small fiber inputs seem to have similar effects at the cord rather than the opposite sign dorsal root potentials implied by Melzack and Wall’s gate control
192
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BARNES
model (1 1). \\‘hcther there is a couriection ~J~t\wxll the findings of similar response directions of the mesencephalic reticular formation units observed in this study and similar dorsal root potentials at the spinal cord for touchpressure and noxious heat requires further studies. In conclusion, the results of this study have uncovered several patterns of somatic response, previously unreported, that extend our understanding of this region. The quantitative analysis was invaluable in the classification of response patterns and the precise description of response characteristics. The great variety of response patterns observed with the same stimulus is consistent with the fact that the mesencephalic reticular formation is not part of the straight line sensory input routes, but serves to alert the organism about events in its environment to which it must respond. REFERENCES 1. A~~ASSIAN, V. E., and H. J. WALLER. 1958. Spatiotemporal activity patterns in individual reticular units, pp. 69-108. 112 “Reticular Formation of the Brain.” H. H. Jasper [Ed.]. Little, Brown, Boston. 2. BECKER, D. P., H. GLUCK, F. E. NULSEN, and J. A. JANE. 1969. An inquiry into the neurophysiological basis for pain. J. Nezlroszlrg. 30: 1-13. 3. BELL, C., G. SIERRA, N. BLJENDIA, and J. P. SEGUNDO. 1964. Sensory properties of neurons in mesencephalic reticular formation. J. Nezlvophysiol. 27 : 961-987. 4. DOUGLAS, W. W., and J. M. RITCHIE. 1957. Non-medulated fibers in the cat saphenous nerve which signal touch. J. Physiol. 139: 385-399. 5. HARDY, J. D., H. GOODELL, and H. G. WOLFF. 1951. Influence of skin temperature on pain threshold to thermal radiation. Sc&?lce 114: 149-150. 6. HUNT, C. C., and A. K. MCINTYRE. 1960. Cutaneous touch receptors in the cat. J. Physiol. 153 : 83-98. 7. LELE, P. P., and G. WEDDELL, 1956. The relationship between neurohistology and cornea1 sensitivity. Bra&z 129 : 119-154. 8. MACHNE, X., J. CALMA, and H. W. MAGOUN. 1955. Unit activity of central cephalic brain stem in EEG arousal. J. Nrzwophysiol. 18: 547-558. 9. MANCIA, M., K. MECHELESE, and A. MOLLICA. 1957. Microelectrode recording from the mesencephalic reticular formation in decerebrate cat. Arck. Ital. Biol. 9s : 110-119. 10. MELZACK, R., and P. D. WALL. 1962. On the nature of cutaneous sensory mechanisms. Brain 85 : 331-356. 11. MELZACK, R., and P. D. WALL. 1965. Pain mechanisms: A new theory. Science 150 : 971-979. 12. SCHEIBEL, M. E., A. B. SCHEIREL, A. MOLLICA, and G. MORUZZI. 1955. Convergence and interaction of afferent impulses on single reticular formation units. J. Nrwophysiol. 18 : 309-331. 13. SCHEIBEL, M. E., and A. B. SCHEIBEL. 1958. Structural substrates for integrative patterns in brain stem reticular formation, pp. 31-55. I+L “Reticular Formation of the Brain.” H. H. Jasper [Ed.]. Little, Brown, Boston. 14. SINCLAIR, D. C. 1955. Cutaneous sensation and the doctrine of specific energy. Braitt 78: 584614. 1.5. SNIDER, R. S., and W. T. NIEMER. 1961. “A Stereotaxic Atlas of the Cat Brain.” Univ. of Chicago Press, Chicago.
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16. STAKZL, T. E., C. W. TAYLOR, and H. W. MAGOUN. 1951. Ascending conduction in the reticular activating system: The diencephalon. J. Ncrrro/Jz~ls&~I. 14: 461478. 17. STARZL, T. E., C. W. TAYLOR, and H. W. MAGOWN. 1951. Collateral afferent excitation of reticular formation of brain stem. J. Neurophysiol. 4 : 479-496. 18. VYKLICKY, L., P. RUDOMIN, F. E. ZAJAC, and R. E. BURKE. 1969. Primary afferent depolarization evoked by a painful stimulus. Science 165: 184-186. 19. WALL, P. D. 1960. Cord cells responding to touch, damage, and temperature of the skin. J. NeurophysioL 23 : 197-210. 20. WEDDELL, G. 1955. Somesthesis and the chemical senses. Amt. REV. Psychol. 6: 119-136. 21. WERNER, G., and V. B. MOUNTCASTLE. 1965. Mechanoreceptive cutaneous afferents : S-R relations, Weber functions, and information. 1. Neurophysiol. 28: 359-397. 22. ZIMMERMAN, M. 1968. Dorsal root potentials after C fiber stimulation. Science 160 : 896-898.
NECROLOGY
A Quantitative in Single
50, 180-193 (1976)
Investigation
Coding
Cells of the Cat Mesencephalic Reticular Formation KAREN
Divisiovk
of Somatosensory
L. BARNES 1
of Ncuroskrvgrvy, Drpartwkmt Urkiversity School of Medicitk&, Rrccivrd
of Szwgcvy, Case Clcvclaud,
Ohio
Wcstcrrk 44106
Rcscrve
Jwke 21,19i.5
Single cell response patterns in cat mesencephalic reticular formation to electronically controlled touch-pressure and noxious radiant heat stimulation were compared. Cells were recorded from 32 acutely decorticated cats in response to transient and continuous s’omatic stimulation. Analysis of 136 cells revealed that 65% responded to both stimuli, 23% only to noxious heat, and 12% only to touch-pressure. Response adaptation was observed to be slow in 93% of the heat responders, in contrast to rapid adaptation in 67% of the touch units. Although the majority of cells was excited by stimulation, both excitatory and inhibitory responses were found. Considerable differences. in statistical characteristics of the response to touch versus heat were seen in all but two of the 89 bimodal cells. The differential adaptation to touch versus heat and the finding of both excitatory and inhibitory responses support predictions of Wall and Melzack (10). The predominance of cells responding to both stimuli reinforces Magoun’s (16, 17) view of the mesencephalic reticular formation as part of the reticular activating system.
INTRODUCTION This investigation focused on a quantitative analysis of the single cell response patterns in the cat mesencephalic reticular formation to electronically controlled touch-pressure and noxious radiant heat stimulation. Comparison of the responsesto noxious versus nonnoxious skin stimulation was undertaken to determine whether this region of the central nervous 1 This research was supported by NIH Grant NS-00779-16 and NIMH Predoctoral Z-FLMH-32, 300-02 (PS). Part of this study was described in a doctoral dissertation submitted in partial fulfillment of the requirements for the Ph.D. degree, Department of Psychology, Case Western Reserve University. The auth’or appreciates the advice and criticism of Drs. H. Gluck, D. P. Becker, J. S. Brodkey, and J, P. Conomy.
ResearchFellowship
180 Copyright 6 1976 by AcademicPress, A11rights of reproduction in any form
MESENCEPHALIC
RETICULAR
FORMATION
181
system (CNS) differentially processes information from painful, as opposed to innocuous, stimuli. The choice of the mesencephalic reticular formation as the recording site was based first upon considerable evidence for somatic responses in this region (l-3), and second upon its probable function as a region for integration of information from visual, auditory, and somatosensory inputs, including pain (3, 9, 12, 13). Early theories of cutaneous sensation tended to espouse one of two diametrically opposed positions, specificity theory or pattern theory. However, recent findings have indicated the inadequacy of either of these theories to explain all the phenomena of somesthesis (4, 7, 14, 19, 20). Hence this study takes its theoretical basis from the attempts of Wall and Melzack (10, 11) to integrate the best aspects of the older approaches to cutaneous sensation. The suggestion originally made by Magoun (16, 17) that the mesencephalic reticular formation has an arousing, alerting function as part of the reticular activating system is also considered. METHODS Recordings were made from 32 adult cats in the unanesthetized acutely decorticated preparation because of the well documented reduction of CNS responsiveness to sensory stimulation under anesthesia. Several animals were allowed to recover from the halothane and nitrous oxide anesthesia under which initial preparation was done in order to assess whether decortication provided an adequately pain-free preparation. These cats were blind and relatively unresponsive to noxious stimuli sucl~ as pinprick, intense radiant heat, and pinch. All wounds were infiltrated with Xylocaine. During recording, the pupils remained constricted, another indication of the preparation adequacy. The cats were paralyzed with gallamine triethiodide, artificially ventilated, and placed in a stereotaxic apparatus with head holder. An infrared lamp and Telethermometer kept rectal temperature between 36 and 39 C. Body fluids were maintained by slow intravenous drip of lactated Ringer’s solution. Systemic arterial blood pressure was monitored continuously during recording. Whenever the systolic pressure dropped below 100 mm Hg or the pupils became dilated and fixed, the experiment was terminated. One or two electrode passes through the mesencephalic reticular formation were made on each side of the brain. Boundaries of the region studied were the stereotaxic coordinates A0 to A3, Ll to L4, and D+l to D-4, according to the Snider and Niemer atlas of the cat brain (15). To make sure that the cells recorded actually lay within the mesencephalic reticular formation anatomical bounds, lesions were made in selected brains and sections prepared alon,m an electrode track, after perfusion with formalin. The brain stem was then cut at 20 pm, stained with cresyl violet, and
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FIG. 1. Cross-section of cat brain stem at anterior 3.0, 20 pm frozen section with cresyl violet Nissl cell stain. Black outline denotes the limits within which cells were recorded for this study.
examined microscopically to confirm the electrode location. In all cases the tracks were seen to pass through the region outlined in Fig. 1, which shows a cross section of one of the brain stems at approximately anterior 3.0. Recordings were taken from glass capillary microelectrodes filled with 3 M KCI, having impedances between 1 and 3 megohms. Unit potentials were led into a unity gain Bak preamplifier. An amplitude discriminator was set to window spikes within a chosen voltage range, allowing unit isolation from noise and background cell activity. The constant voltage pulse output of the window was fed into an audio monitor and displayed on an oscilloscope along with the original recording of brain activity. The original data, amplitude window output, and various stimulus markers were recorded on a Precision Instrument FM tape recorder. Data analysis was done on a Line-8 computer system. During the course of the study, an on-line data acquisition program was developed for the Lint-8, so that data for the last ten cats were recorded directly on Lint-8 tape. During exploration of the midbrain reticular regions, touch stimulation and brief electrical shock to the footpads were applied so that the cell sample was not limited to those units having spontaneous activity. Receptive fields for touch and heat were mapped with the audio monitor. Responses of isolated mesencephalic reticular formation cells to transient
and continuous
somatic
stimulation
were determined
as follows:
Brief
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RETICULAR
FORMATION
183
touch and noxious heat were applied for 1 or 2 set, while continuous stimulation was given for 1 min. Touch-pressure was applied by an electronically driven probe, patterned after one described by Werner and Mountcastle (21) . A 4- mm Lucite probe tip was applied perpendicular to the skin surface. The transient stimulus was a 400 pm static indentation of the skin for 1 set, while during the minute of continuous stimulation a 200 Hz tip vibration of 50 pm was superimposed on the static displacement. The vibration was added because almost all units ceased responding after the first few seconds of static indentation. Heat was provided by a miniature projection lamp with built-in reflector (GE EJM) behind a tachistoscope shutter limiting the area and duration of stimulation. Transient heat was applied for 2 set, because of the time lag for heating the skin. A thermistor on the surface of the skin registered 54 C during stimulation. Since this temperature is well above the 45 C threshold for pain in humans (5), and since it produced pain when applied to the experimenter, it clearly qualifies as a noxious somatic stimulus. In order to eliminate bias from any changes in cell activity over the recording period, both the order of stimulus modalities and the application of transient and continuous stimulation were counterbalanced over successive cells. Transient stimulus records were 24 set long: 5 set control for spontaneous activity, 1 or 2 set of stimulation, and 17 or 18 set poststimulus firing. Running averages of the unit firings per second (O.l-set increments) were outputted graphically on a Calcomp plotter to reveal spontaneous, stimulation, and poststimulus activity patterns. Both interspike interval histograms and running averages of unit firing were plotted for the continuous stimuli and for minute control records taken before and after each stimulus record. While the interval histograms provide a good index of overall changes in firing pattern with different stimuli, they give no information about the time course of changes in firing. Hence the running average analysis was included to preserve information about response variation during the minute record. RESULTS This series of experiments yielded 157 cells responsive to somatic stimulation and observed within the limits of the mesencephalic reticular formation. Of these cells, 136 were recorded in sufficient detail for analysis. The great majority of these somatic responders were found in the area bounded by A0 to A3, Ll to L3, and DO to D-2.5 on the stereotaxic axes. Nearly all (93%) of these units were spontaneously active, most having between 200 and 1500 discharges per minute. In addition, there was a definite decrease in the variability of discharge as the overall rate increased.
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TABLE RESIWNSE
RESPONDING
ONLY
TO OYE
Type Firing
1
OF 47 MESENCEPHALIC
CHARACTEKISTICS
MODE
RETICULAR
OF SOMATOSENSORY
of Change
in Firing
change Touch -..-__
Excitatory Inhibitor) Rate
of Adaptation
mode Heat
__12 ‘4
Totals
CELLS
Rate
Response
__-
FORMATION STIMULATION
16 of Response
19 12 31
.__ Rapid Slow Totals
11 5
2 29
16
31
Receptive field size and location varied extensively for these units. Responses were evoked bilaterally in 57% of the cells, while unilateral fields were mainly (51%) contralateral. Only 38% of the fields were restricted to the region of a single limb or less, with many covering all four limbs or two limbs plus part of the trunk. Responses to touch and to noxious heat generally had congruent skin fields. All these characteristics are in accord with the findings of previous investigators (l-3, S, 9). Looking now at the response patterns of the 136 cells which were analysed, 47 (35%) were unimodal, responding only to heat or to touch, while the other 89 (65%) were bimodal, responding to both stimuli. Of the unimodal responders 31 (66%) modified their activity for noxious heat, and 16 (34%) were touch responders (Table 1). Since unit activity is generally a nonstationary process, tests of statistical significance for mean differences have limited meaning. Thus a change of 20% in discharges per minute was arbitrarily set to define a response. In most cases the firing changes were well above the minimum. Figure 2 displays the analysis of a typical touch responder: the top row shows the interspike interval histograms for the three recording conditions, while the bottom plots display the corresponding running averages of each minute record. The left column indicates that the spontaneous activity of this cell, unit 2303, was quite rapid (1678 discharges per minute, mean interval 34.7 msec, standard deviation 13.7 msec) . In the middle set, continuous touch stimulation has produced an increase in total firing, particularly during the first 15 set of stimulation (1983 discharges, mean interval 29.2 msec), and some
MESENCEPHALIC
RETICULAR
FORMATION
FIG. 2. Unimodal touch responder 2303: excitatory response. Top row displays interspike interval histogram, bottom row contains running average plots. Control records on left, touch stimulation in middle, heat stimulation at right. Histogram abscissa gives interval length in msec, ordinate displays interval frequencies. M = mean interval length in msec, SD = interval standard deviation in msec, N = total interspike intervals during minute record. Note differences in histogram ordinate scale values. Running average abscissa gives elapsed recording time in seconds, ordinate shows average unit firing rate per second. N = total unit discharges during minute record. decrease in interspike interval variability (SD 10.2 msec). The right two plots show no clear changes in firing to heat. The response pattern of unimodal heat responder 2110 is shown in Fig. 3. This cell fired at a moderate spontaneous rate (564 discharges, mean 93 msec, SD 110 msec). Touch stimulation had virtually no effect
FIG. 3. Unimodal heat responder 2110: excitatory response. General same as Fig. 2. Note differences in histogram ordinate scale values.
description
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TABLE TYPE
BARNES
2
OF FIKING CHANCE OF 89 MESENCEPHALIC RETICULAR FORMATION RESPONDING TO Two MODES OF SOMATOSENSORY STIMULATION
Number of cells having same direction of both changes Both excitatory
40
Both inhibitory
19
Q Twelve additional cells had a normal lus mode, but responded with a widely text.
CELLS
Number of cells having different direction of both changes” Touch excitatory, heat inhibitory 11
excitatory or inhibitory varying discharge rate
Touch inhibitory, heat excitatory 7
response to one stimuto the other mode. See
upon any of the firing pattern parameters. In contrast, heat stimulation produced a marked acceleration, with discharge rate rising to 971, mean interval dropping to 61 msec, and SD falling to 63 msec. For both varieties of unimodal responder the majority of responses were excitatory, raising the activity above the spontaneous level. Of 16 touch units, 12 (7570) increased their firing during stimulation; 19 of the 31 heat responders (60%) accelerated their discharge rate when stimulated (Table 1). Heat responder 2110 (Fig. 3) exemplifies the excitatory response, indicated by the increase in discharges and decrease in mean interval during heat stimulation compared to the control record. Examination of the average firing rate plots (shown on the bottom in each figure) for these unimodal responders revealed their characteristics of response adaptation to continuous stimulation. Adaptation was classified as rapid if the firing rate returned to the spontaneous level within the first 15 set of stimulation, and slow if the response was maintained for more than 15 sec. On this basis 11 of the 16 touch responders (67%) adapted rapidly; of the 31 noxious heat responders, only two adapted rapidly, while the other 29 (9370) exhibited a long duration response, many lasting well beyond the minute of stimulation recorded (Table 1). Touch responder 2303 (Fig. 2) adapted rapidly, its firing dropping off into the spontaneous range after the first 15 set of stimulation. In contrast, heat responder 2110 displays a firing acceleration which lasted throughout the entire minute record (Fig. 3). Description of the responses for the bimodal units was more complex, since it must account for two response patterns (Table 2). Again excitation was the predominant feature: forty of the 89 bimodal responders (45%) increased their firing to both stimuli. Unit 1907 (Fig. 4) exempli-
MESENCEPHALIC
3lGo
RETICULAR
FORMATION
CoNTRtil 19077-‘1 0
187
,;fio SEC 60
FIG. 4. Bimodal responder 1907: touch excitatory, heat excitatory. General description same as Fig. 2. Note differences in ordinate scale values for both histograms and running averages.
fies this pattern. Touch stimulation nearly doubled the cell’s firing rate and halved the mean interval, while heat produced an even greater increment of activity. From the running average plots, it appears that the touch response magnitude increased gradually over the minute record, while the heat response varied irregularly. It seems that the average plot is a more sensitive index of firing variability than is the standard deviation, since the greater oscillations of the heat average response compared to the touch average plot are not substantiated by the interval standard deviations. The second most frequent bimodal response pattern consisted of inhibition by both stimuli, seen in 19 cells (21%). In addition, the nature of these two most frequent response patterns reveals that 66% of this sampleof mesencephalicreticular formation bimodal responders show the same basic response direction of excitation or of inhibition to both touch-pressure and noxious heat. Another group of 18 cells (20%) exhibited opposing response patterns to the stimuli: Eleven were excited by touch and inhibited by heat, while the other seven displayed the inverse patterns. The remaining 12 cells (14%) were difficult to classify because they had a widely varying discharge rate to one of the stimulus modes, so that the response could be labeled neither excitation nor inhibition, Unit 1305 (Fig. 5) exhibits this pattern in that there is little change from the control in the mean or total discharge values for the touch interval histogram, yet the running average plot reveals a widely varying firing rate compared to the control. These units demonstrate the particular value of both the standard deviation and the running average plots in assessingresponsepatterns, becauseunit 1305 would have been classified as an excitatory unimodal heat responder if
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FIG. 5. Bimodal responder 1305: touch oscillatory, heat excitatory. General description same as Fig. 2. Note differences in ordinate scale values for both histograms and running averages.
analysis had been limited to the interspike interval mean and total firings per minute. Very few bimodal responders having transient responsesto both stimulus modeswere found. Although it was possibleto record from only seven such mesencephalic reticular formation cells, all of them showed the same clifferential response adaptation to touch compared with heat. In Fig. 6 the firing increase of unit 3002 to touch dropped back to the spontaneous level within a few seconds after stimulus offset, while the response to heat tapered off very slowly. One factor to be considered is the time lapse after heat stimulus offset necessary for the skin temperature to fall below 45 C, the threshold for thermal pain in man. However, the skin cools below 45 C within 4 to 5 set, while the cell response appeared to last 8 to 10 set beyond stimulus offset. 0:
12
6
ii !I : ', i \
UNIT 3002 TOUCH RFP
02
. :' "> --.I+ 0
'wL
,
FIG. 6. Bimodal responder 3002: touch excitatory, plays firing histogram for brief touch stimulation shows firing histogram for brief heat stimulation of indicates stimulus duration, vertical mark corresponds sec. Ordinate gives discharge rate per second.
L
heat excitatory. Left graph disof right forepaw, right graph right forepaw. Horizontal mark to firing rate of 3 discharges/
MESENCEPHALIC
RETICULAR
FORMATION
189
A final analysis of tire bimodal responders involved classifying them with respect to which stimulus mode produced the greatest response, determined by the absolute deviation in total discharges from the spontaneous level. Of the 89 cells, 53 (60%) responded more strongly to heat, while 34 (38%) responded more intensely to touch. Although the remaining two units showed no difference in total discharges, examination of the running average plots revealed differences in the time course of the response, DISCUSSION The work of Melzack and Wall (10, 11) attempting to integrate the best aspects of the older theories of cutaneous sensation has suggested several hypotheses about somatosensory systems whose predictions may be tested by the results of this study. The first hypothesis proposes that CNS cells may detect or discriminate between somatosensory stimuli of different types by means of the adaptation characteristics of the responses. Based on evidence for the differential firing patterns of primary cutaneous fibers to touch-pressure versus noxious stimulation (6, 21)) Melzack and Wall (10) propose that CNS unimodal touch responders will adapt rapidly to continuous stimulation, while noxious heat units will adapt their response only very slowly, if at all. As a test of this prediction the adaptation characteristics of the 47 unimodal responders were determined from the running average plots of their response to continuous stimulation: Sixtyseven percent of the 16 touch cells adapted rapidly, while 93% of the 31 heat responders showed little if any decrement in response during the minute of stimulation (Table 1). Unit 2303 (Fig. 2) displays a typical brief excitatory response to touch, while unit 2110 (Fig. 3) exhibits a prolonged acceleration to heat. Hence these results affirm the existence of differential adaptation patterns in the two varieties of unimodal mesencephalic reticular formation units. The above findings were extended with a comparison of the poststimulus histograms for brief touch and noxious heat stimulation. Both of the unimodal transient heat responses observed were prolonged excitation ; of the ten touch units analyzed in this manner, eight displayed a brief firing increase during the stimulus, one showed brief inhibition, and the remaining cell had a prolonged excitation. When the analysis was expanded to include 19 bimodal mesencephalic reticular formation cells from which at least one transient response was recorded, 15 (79%) of these touch responses were brief excitation, and two were brief inhibition. All heat responses were clearly prolonged excitation or inhibition. Unit 3002 (Fig. 6) exemplifies the response patterns of these cells. In summary, the analysis of both continuous and transient stimulus records confirmed a predominance
190
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of rapid Xkl~‘t~lti~Jll to toucll alIt Jaw a(laptation tu heat stiniulatiou by mesencephalic reticular formation cells. The existence of these differential adaptation patterns does not necessarily imply that mesencephalic reticular formation cells have functional significance in the organism’s ability to respond to stimulus differences. Because the mesencephalic reticular formation is not part of a “straight line” sensory path, but instead receives collaterals from those paths (13), its function is more probably one of alerting or arousing the organism so that it is ready to respond appropriately to specific stimulus information arriving on the straight line inputs. This view of the mesencephalic reticular formation as part of the reticular activating system has been substantiated by many studies since its proposal by Magoun and his associates (16, 17). A second suggestion of Wall and Melzack (10) is that CNS cells differentiate stimulus modes by means of the predominance of excitatory versus inhibitory inputs to the cell. This study has demonstrated both excitatory and inhibitory responses, with the great majority being excitatory: Two-thirds of the unimodal responders exhibited firing increases when stimulated (Table l), while the largest bimodal response category comprised 40 cells excited by both touch-pressure and noxious heat (Table 2). Hence more than half (52%) of the 136 cells observed showed only excitatory responses to somatosensory stimuli, while another 2670 were exclusively inhibited by stimulation. The predominance of excitatory responses also supports Magoun’s theory of the arousing, activating function of the mesenceplialic reticular formation. The multimodal responsiveness of these cells, with 89 (65%) responding to both touch-pressure and noxious heat, may be explained by the anatomical characteristics of the mesencephalic reticular formation described by the Scheibels (13). First, afferent input to mesencephalic reticular formation cells arises from a great variety of sources: spinothalamic, spinoreticular, and ventrolateral systems, brachium conjuctivum, vestibular complexes, and pyramidal tracts all make contributions. Second, mesencephalic reticular formation cells are profusely interconnected, with richly branching collaterals which could influence adjacent cells. Finally, most reticular axons have been traced beyond the mesencephalic reticular formation boundaries to such structures as cranial nerve nuclei, most of the thalamic nuclei, subthalamus, hypothalamus, colliculi, periaqueductal gray, geniculates, and cerebellum. These anatomical features of diffuse input, interconnections, and output suggest extensive possibilities for integration of somatic and other sensory information by mesencephalic reticular formation cells. In addition, predominance of multimodal responders in our sample is in accord with the views of Magoun and colleagues (16, 17) that the
ME~ENCEPHAL~C RETICULAR FORMATION
191
mesenceplialic reticular formation is part ot the reticular activating system. According to Magoun’s electrophysiological studies ( 17)) and confirmed by the Scheibels (13), collaterals from the lemniscal sensory fibers enter the mesencephalic reticular formation, so that somatic and auditory stimuli activate these cells. Reticular output is then transmitted by both thalamic and extrathalamic routes to the cortex, resulting in both electrophysiological and behavioral signs of arousal. The predominance of cells responding to both touch and heat stimulation, and the fact that over half of our sample of 136 mesencephalic reticular formation units (71, 52%) had only excitatory responses to the stimuli presented further reinforce Magoun’s idea of the activating, arousing nature of this system. Comparison of the results of this study with those of Becker et al. (2) reveals some units with response patterns similar to Becker’s six “nonnoxious responders,” characterized by short latency and short duration excitatory responses to both brief touch and electrical shock to the footpad. Nonnoxious cells had either no response to a minute of noxious heat, or a response of similar direction but smaller magnitude than that to touch. In this study the 16 unimodal touch responders and the 34 bimodal units which had their greatest response to touch confirm the existence of a group of somatic nonnoxious responders. However, the majority of mesencephalic reticular formation cells observed in this sample were either exclusively or maximally excited by noxious heat stimulation. Becker et al. (2) described their eight “noxious responders” as bimodal units having a long latency response with considerable afterdischarge to brief footpad shock. Continuous stimulation produced opposite directions of response : Heat was excitatory, while touch sharply inhibited the spontaneous activity. The 53 bimodal units having their greatest response to heat have similar transient response characteristics, but give little support to the importance of contrasting continuous response directions. Only five of these 53 cells exhibited excitation by heat and inhibition by touch. It is not surprising that this study found much more diversity of response pattern than did Becker ; his sample was so small (16 units) that he surely missed observing many of the mesencephalic reticular formation response varieties. Failure to find opposing response patterns of heat excitation and touch inhibition in more than seven of the 89 bimodal mesencephalic reticular formation responders provides an interesting parallel to the findings of Zimmerman (22) and Vyklicky et al. (18). Both of these studies report that noxious heat stimuli produce negative dorsal root potentials at the spinal cord just as does touch stimulation. Hence, large and small fiber inputs seem to have similar effects at the cord rather than the opposite sign dorsal root potentials implied by Melzack and Wall’s gate control
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model (1 1). \\‘hcther there is a couriection ~J~t\wxll the findings of similar response directions of the mesencephalic reticular formation units observed in this study and similar dorsal root potentials at the spinal cord for touchpressure and noxious heat requires further studies. In conclusion, the results of this study have uncovered several patterns of somatic response, previously unreported, that extend our understanding of this region. The quantitative analysis was invaluable in the classification of response patterns and the precise description of response characteristics. The great variety of response patterns observed with the same stimulus is consistent with the fact that the mesencephalic reticular formation is not part of the straight line sensory input routes, but serves to alert the organism about events in its environment to which it must respond. REFERENCES 1. A~~ASSIAN, V. E., and H. J. WALLER. 1958. Spatiotemporal activity patterns in individual reticular units, pp. 69-108. 112 “Reticular Formation of the Brain.” H. H. Jasper [Ed.]. Little, Brown, Boston. 2. BECKER, D. P., H. GLUCK, F. E. NULSEN, and J. A. JANE. 1969. An inquiry into the neurophysiological basis for pain. J. Nezlroszlrg. 30: 1-13. 3. BELL, C., G. SIERRA, N. BLJENDIA, and J. P. SEGUNDO. 1964. Sensory properties of neurons in mesencephalic reticular formation. J. Nezlvophysiol. 27 : 961-987. 4. DOUGLAS, W. W., and J. M. RITCHIE. 1957. Non-medulated fibers in the cat saphenous nerve which signal touch. J. Physiol. 139: 385-399. 5. HARDY, J. D., H. GOODELL, and H. G. WOLFF. 1951. Influence of skin temperature on pain threshold to thermal radiation. Sc&?lce 114: 149-150. 6. HUNT, C. C., and A. K. MCINTYRE. 1960. Cutaneous touch receptors in the cat. J. Physiol. 153 : 83-98. 7. LELE, P. P., and G. WEDDELL, 1956. The relationship between neurohistology and cornea1 sensitivity. Bra&z 129 : 119-154. 8. MACHNE, X., J. CALMA, and H. W. MAGOUN. 1955. Unit activity of central cephalic brain stem in EEG arousal. J. Nrzwophysiol. 18: 547-558. 9. MANCIA, M., K. MECHELESE, and A. MOLLICA. 1957. Microelectrode recording from the mesencephalic reticular formation in decerebrate cat. Arck. Ital. Biol. 9s : 110-119. 10. MELZACK, R., and P. D. WALL. 1962. On the nature of cutaneous sensory mechanisms. Brain 85 : 331-356. 11. MELZACK, R., and P. D. WALL. 1965. Pain mechanisms: A new theory. Science 150 : 971-979. 12. SCHEIBEL, M. E., A. B. SCHEIREL, A. MOLLICA, and G. MORUZZI. 1955. Convergence and interaction of afferent impulses on single reticular formation units. J. Nrwophysiol. 18 : 309-331. 13. SCHEIBEL, M. E., and A. B. SCHEIBEL. 1958. Structural substrates for integrative patterns in brain stem reticular formation, pp. 31-55. I+L “Reticular Formation of the Brain.” H. H. Jasper [Ed.]. Little, Brown, Boston. 14. SINCLAIR, D. C. 1955. Cutaneous sensation and the doctrine of specific energy. Braitt 78: 584614. 1.5. SNIDER, R. S., and W. T. NIEMER. 1961. “A Stereotaxic Atlas of the Cat Brain.” Univ. of Chicago Press, Chicago.
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16. STAKZL, T. E., C. W. TAYLOR, and H. W. MAGOUN. 1951. Ascending conduction in the reticular activating system: The diencephalon. J. Ncrrro/Jz~ls&~I. 14: 461478. 17. STARZL, T. E., C. W. TAYLOR, and H. W. MAGOWN. 1951. Collateral afferent excitation of reticular formation of brain stem. J. Neurophysiol. 4 : 479-496. 18. VYKLICKY, L., P. RUDOMIN, F. E. ZAJAC, and R. E. BURKE. 1969. Primary afferent depolarization evoked by a painful stimulus. Science 165: 184-186. 19. WALL, P. D. 1960. Cord cells responding to touch, damage, and temperature of the skin. J. NeurophysioL 23 : 197-210. 20. WEDDELL, G. 1955. Somesthesis and the chemical senses. Amt. REV. Psychol. 6: 119-136. 21. WERNER, G., and V. B. MOUNTCASTLE. 1965. Mechanoreceptive cutaneous afferents : S-R relations, Weber functions, and information. 1. Neurophysiol. 28: 359-397. 22. ZIMMERMAN, M. 1968. Dorsal root potentials after C fiber stimulation. Science 160 : 896-898.
