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Spinal cord potentials produced by ventral cord volleys in the cat
EXPERIMENTAL
NEUROLOGY
27, 305-317
Spinal
Cord
Ventral R. D. Departrnertt
of
Anatomy,
Cord SKINNER
(1970)
Potentials
Produced
Volleys AND
in the
W. D.
WILLIS
by Cat 1
The University of Texas Sor~thzwcstcm at Dallas, Dallas, Texas 75235
Received January
dledirnl
School
7, 1970
Electrical field potentials produced by volleys in the ventral white matter were recorded along the dorsal surface and in the depth of the cat lumbosacral spinal cord. The cord dorsum potential was positive in sign and composed of three phases. Potentials in the depth of the cord were located with reference to the histology of the cord using a contour mapping technique. A current sink was found in the medial ventral horn at the time of phase III. It was attributed to the excitation of neurons in laminae VII and VIII. Sources were found in the dorsal horn. They were probably distributed along the dorsally projecting axons of many ventral horn interneurons. Introduction
The activity of neurons in the lumbosacral enlargement of the spinal cord is influenced by pathways descending from the brain and from more rostra1 parts of the spinal cord. Several of these tracts travel in the ventral white matter of the cord, either in the ventral part of the lateral funiculus or in the ventral funiculus (1, 10, 12-14, 21). Most of the ventral cord pathways terminate in the medial part of the ventral horn( 11) in the regions designated by Rexed as laminae VIII and VII (15, 16). When ventral cord pathways are stimulated electrically, a characteristic potential sequencecan be recorded from the spinal cord. Several aspects of the potential sequence were analyzed by Lloyd (8). An electrode in the ventrolateral white matter will record a successionof two spike potentials in response to a single stimulus applied at any level from the brain stem downward. The first spike is the activity of the fibers directly escited by the stimulus, while the second is due to activity relayed after a single synaptic delay. Lloyd recorded a slow negative potential and single 1 This Institutes for their fellow in chusetts.
work was supported by a research grant, NB 04779, from the of Health. The authors thank Miss Anita Bird and Miss Patsy technical assistance. Dr. R. D. Skinner is presently a postdoctoral the Department of Physiology, Harvard Medical School, Boston, 305
Kational Hatldley
research Massa-
306
SKINNER
AND
WILLIS
unit activity in the ventral horn in response to a similar stimulus. He suggested that the relayed spike was due to monosynaptic excitation of ventral horn interneurons whose axons projected into the ventral white matter as part of the propriospinal system. When records are made from the dorsal surface of the spinal cord following excitation of fibers in the ventral white matter, the potential observed is composed of a series of positive waves. Illustrations of this have been published by several laboratories (3, 7, 24). However, there has been no attempt to interpret this potential sequence in detail, nor has there been any explanation of the positive sign of the slow component. The present investigation was undertaken to answer these questions and to relate the potentials recorded to the anatomical studies of the regions of termination of the tracts descending in the ventral cord. A correlation of the field potential recordings reported in this study with single unit recordings will be made in the following paper (19). A preliminary report of some of this work has been given ( 18). Method In cats anesthetized with chloralose (80 mg/kg, iv (after induction with halothane and nitrous oxide), the dorsal half of the cord was cut at L2, so that electrical stimulation of the ventral surface of the cord at Ll produced a volley which was conducted down just the ventral white matter. The ventral half of the cord will be called the ventral cord (VC) . In some experiments only the one ventral quadrant was left intact. The general operative, stimulating, and recording techniques in this laboratory have been described previously (23 ) . Potentials produced by ventral cord volleys were recorded on the dorsal surface of the cord from L4 to Sl with a ball-tipped platinum electrode (Fig. 1). A photograph was made of superimposed oscilloscope traces. These potentials could also be averaged using a small computer (NuclearChicago). The output of the averager was recorded in analog form on a pen writer (Brush) and in digital form on magnetic tape (Kennedy). The magnetic tape records were processed further by digital computer. Field potentials in the depth of the cord produced by ventral cord volleys were recorded by a microelectrode at 250-p intervals in a series of tracks placed transversely across the cord at about L6 (Fig. 1). The microelectrodes, filled with 4 M sodium chloride, had tips broken to a diameter of l-2 p under microscopic control. These potentials were recorded in the same manner as the cord dorsum potentials. From measurements of averaged records, isopotential maps were made and related to the cord histology using techniques described previously (22).
SPINAL
CORD
POTENTIALS
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Sm
SC Y FIG. 1. Diagram of the experimental arrangement. The dorsal half of the spinal cord was cut at L2, and the ventral cord was stimulated with an electrode, aVc Muscular and cutaneous nerves in the hindlimb were stimulated electrically with electrodes S, and C,. Potentials from the microelectrode placed in the gray matter of the ventral horn and from the ball electrode on the cord dorsum nearby were amplified and displayed on a cathode ray oscilloscope from which they were photographed. These potentials could be averaged. The averaged potential was read out in analog form onto a pen writer and in digital form onto magnetic tape. The tape was interpreted by a digital computer, which produced a print out. Selected data from the print out were used to make isopotential field maps by using a plotter under computer control. Results
Cord Dorsum Pofentials. Stimulation of ventral cord (VC) axons at the upper lumbar level produced a complex positive potential when records were made from the dorsal surface of the lumbosacral spinal cord. An example of a typical potential sequenceis shown by the record in Fig. 2D taken at the junction of the L6 and L7 segments. This and the other records in Figs. 2 and 3 were each the averaged responseto 200 ventral cord volleys as displayed on a pen writer. The potential was divisible into three phases. Some of the phases were more discernible in sonle preparations than in others. The general form of this potential, however, was the same in each preparation.
308
SKINNER
E
AND
WILLIS
“C CORD ,
DORSUM
POTENTIALS
FIG. 2. Relation of cord dorsum potentials to ,stimulus strength and an example of facilitation of the P-wave. All records are averaged responses to 200 stimuli at lO/sec, and measurements were made from averaged records. A-D are records of the cord dorsum potential with different strength stimuli made at the junction of the L6 and L7 segments. E is a plot of the amplitude of the axon volley (triangles) and the P-wave (circles), both expressed as percentage of maximum versus the stimulus strength in terms of multiples of threshold of the most excitable ventral cord axons. F is a plot of the amplitude of the facilitated test P-wave, after electronic subtraction of the conditioning cord dorsum potential versus the interval between the two stimuli. G-I are samples of the records from which F was made. They are, respectively, the potentials produced by the conditioning stimulus (same as the test response), the conditioning stimulus plus the test stimulus delayed 4.0 msec, and the test stimulus after subtraction of the conditioning stimulus. The calibration pulse of 0.2 mv and 1 msec in A-D applies to all records.
Phase I, indicated by an arrow in Fig. 2D, was a short latency, spikeshaped potential. It coincided in time with spike potentials in axons in the ventral cord and, therefore, will be referred to as the axon volley. Phase II (Fig. 2D) was first distinguishable when the stimulus strength was 1.5 2.0 times the threshold of the most excitable axons in the ventral cord (Fig. 2C). It reached a peak 0.8 msec after the positive peak of the axon volley (average of 10 experiments). Phase III (Fig. 2D) reached a peak 2.2 msec after the axon volley (average of eight experiments) and had a duration of about 9 msec (average of 12 experiments). This phase will be referred to as the P-wave. Figure 2E is a plot of the amplitudes of the axon volley (triangles) and the P-wave (circles) recorded at the L6-L7 junction as a function of the stimulus strength. The ason volley and P-wave are expressed as a percentage of the maximum response, while the stimulus strength is ex-
SPINAL
B
VENTRAL
CORD
P-WAVE
CORD
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POTENTIALS
C
VENTRAL
CORD AXON
VOLLEY
o!ZmZqti, SPINAL
CORD SEGMENT
SPINAL
CORD
SEGMENT
FIG. 3. Distribution of the axon volley and P-wave along the lumbosacral spinal cord. Sample records at strength 3.OT are shown in .L\. They are the averaged responses to 200 stimuli at lO/sec. B shows the distribution of the amplitude of the P-wave expressed as a percentage of the maximum along the lumbasacral cord. Each point is the average of the number of experiments indicated, and the vertical bars represent one standard deviation. C shows the distribution of the amplitude of the axon volley in one experiment. The calibration pulse is 0.2 mv and 1 msec in all records.
pressed as a multiple of the threshold of the most excitable fibers in the ventral cord. The amplitude of the P-wave increased with that of the axon volley. Sample records are shown in Fig. 2 ri-11. Facilitation of the P-wave was demonstrated by using two ventral cord volleys, one as a conditioning stimulus and another at the same strength as a test stimulus. The interval between the volleys was varied. An example of facilitation is illustrated in Fig. 2G-I. The response to the conditioning volley, G, was identical to the unconditioned test response; H was the response to the conditioning stimulus plus the response to the test stimulus at an interval of 4 msec; and I was the test response after electronic subtraction of the conditioning stimulus response. This was accomplished by use of the subtraction mode of the signal-averaging computer. The subtraction was not perfect, since a portion of the first stimulus artifact can still be seen in Fig. 21. The result of measurements taken at several intervals is shown in F. The amplitude of the test P-wave is espressed as a percentage of the control response at various intervals between the two responses.The facilitation had a time course similar to that of the P-wave itself. The distribution of the P-wave along the surface of the lumbosacral spinal cord is plotted in Fig. 3B, and sample averaged records are shown
310
SKlNNER
AND
WILLIS
in A. Each point on the graph was the average response at that level from the number of experiments indicated. The vertical bars represent one standard deviation. The amplitude was essentially constant along the lumbar enlargement except for two relative maxima. One occurred at L4 and the other at the L6-L7 junction. The amplitude decreased rapidly caudal to L7. The axon volley decreased in amplitude with distance along the cord as shown for one experiment in Fig. 3C. The amplitude of the axon volley is expressed as a percentage of the maximum. Potentials Recorded Within the Cord. In order to study the action of ventral cord pathways on populations of spinal cord neurons, the extracellular field potentials elicted by ventral cord stimulation were recorded from a grid of points within the cord. The location of an array of microelectrode tracks in a projected drawing of a cross section of the spinal cord is shown for one experiment in Fig. 4. The recording points were at 250-p intervals, as indicated by the circles along one of the tracks. Sample records from the points shown by
FIG.
sample across intervals. on the dorsal tion of interval 0.2 mv
4. Spinal cord section showing the position of a set of electrode tracks and records. Six parallel electrode tracks 250 p apart were made transversely the cord. The circles on one of these mark the recording points at 250 B Sample averaged field potentials in response to VC stimulation are shown right for the solid circles. The phase III potential was positive in the horn and negative in the ventral horn. Reversal occurred at about the junclaminae VI and VII. The vertical line on the records illustrates a typical at which values were taken to make a field map. The calibration pulse was and 1 msec.
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CORD
POTENTIALS
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the filled circles are at the right margin of the drawing. The potential sequence recorded in the dorsal horn was similar to that recorded from the dorsal surface of the cord. The P-wave reversed, however, in the intermediate gray matter and became negative in the ventral horn. The vertical line in Fig. 4 indicates an interval after the stimulus which corresponded to the peak of the negative wave in lamina VIII. Measurements of all of the records in this experiment were made at this interval. An isopotential map was constructed with the aid of a digital computer from these measurements, and the map was superimposed on a drawing of a histological section of the spinal cord, as shown in Fig. 5. The map shows a large area of negativity in the ventral part of the gray matter with a focus in lamina \‘III. The negativity covered all of lamina VIII and most of the medial and ventral portions of lamina VII. Even though lamina IX and the lateral part of lamina VII were not recorded from in this animal, the negativity appeared to decrease rapidly in the direction of these areas. The entire dorsal horn was positive with a focus in the lateral part
FIG. 5. Isopotential map of responses to bilateral ventral cord stimulation. The spinal cord lesion is shown in the inset. Most of the fibers in the ventral part of the lateral funicuhrs were interrupted on the ipsilateral side. Negativity is indicated by solid line, positivity by the thin broken line, and zero by the heavy broken line. The maximum negativity is denoted by a minus sign and the maximum positivity by a plus sign. The arrows indicate the electrode tracks.
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of lamina IV. The zero line followed the border of laminae VI and VII. The lesion in the dorsal part of the spinal cord at L2 in this experiment was reconstructed from serial sections and is indicated by the shaded area in the inset in Fig. 5. The lateral funiculi were interrupted to a level well below the denticulate ligament. In other experiments, more of the lateral funiculus was spared. For instance, in Fig. 6 the inset shows that some of the dorsolateral part of the lateral funiculus was intact on the left side, while the lesion reached a level approximating that of the denticulate ligament on the right. The isopotential map extended across most of the gray matter on both sides of the cord. The distribution of sources and sinks on the right side was nearly identical to that shown in Fig. 5. However, on the left side there were additional foci of negativity in the central and medial parts of lamina VII, and the zero line was shifted dorsally to the middle of lamina VI. The results shown in Figs. 5 and 6 were confirmed in two other experiments. Two animals had lesions designed for the investigation of crossed path-
’
mm
’
6. Isopotential map of responses fibers of the dorsal part of the lateral at the right. Negativity is indicated by positivity by the broken lines. The effects noticeable on the left which has additional more dorsal zero line. The arrows indicate FIG.
to bilateral ventral cord stimulation with funiculus. The spinal cord lesion is shown solid lines, zero by the dash-dot line, and of the lateral funicular tracts are especially foci of negativity in lamina VII and a the electrode tracks.
SPINAL
CORD
POTENTIALS
313
ways. One lesion is shown in Fig. 7C with the corresponding map in A. This lesion spared one ventral quadrant, including some of the dorsolateral part of the lateral funiculus on that side. The map shows a small amount of negativity in lamina VIII and the ventral and medial parts of lamina VII on the side contralateral to the intact pathways. The other lesion spared the lateral and ventral funiculi on one side. The lesion and the field potential map are shown in Fig. 7D and B, respectively. Again, there was an area of negativity in the ventromedial gray matter on the side of the cord contralateral to the intact axons. On the ipsilateral side, there was intense negativity in the ventromedial gray matter. The negativity extended into the central and lateral parts of lamina VII and the lateral part of VI. The zero line in this esperiment was shifted to the border between lamina V and 19 in the lateral part of the dorsal horn.
FIG. 7. Isopotential maps of responses to stimulation of the contralateral cord. Maximum negativity is shown by minus signs and maximum positivity by plus signs. In both A and B negativity is denoted by a solid line, positivity by a broken line, and zero by a dash-dot line. For the map in A the spinal cord lesion in C left only the contralateral ventral quadrant intact. For the map in B the spinal cord lesion shown in D left the entire ventral and lateral funiculi on one side. The negativity present on the contralateral side was relatively much less than that on the ipsilateral side. The arrows indicate the positions of the electrode tracks.
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WILLIS
Discussion
Activation of fibers in the ventral white matter of the spinal cord results in a sequence of potentials which can be recorded in the lumbosacral enlargement. The initial event is a spikelike potential. This is undoubtedly due to the conduction of activity in axons directly excited by the stimulus. This spike is positive in records from the dorsal surface of the cord, but it may be diphasic (positive-negative) (Fig. 4) or triphasic (Fig. 1 in ref. 8) in records made in the ventral horn or ventral white matter. The amplitude of this potential is progressively less with distance from the point of stimulation (Fig. 3C ; Fig. 1 in ref. 8). This may reflect the termination of some of the fibers at more rostra1 levels, temporal dispersion of the volley, and possibly changes in recording conditions. The second potential (phase II in our terminology for the cord dorsum records) is probably due to a complicated mixture of events. It was positive in the dorsal part of the cord (Fig. ZD), and it could be negative and spikelike in the ventral horn (Fig. 4). In records made near the ventral white matter, a spike could sometimes be seen at the time of phase II (not illustrated). Lloyd’s second spike potential occurred at this time. It therefore seems likely that the potential designated phase11 in the cord dorsum record is associated wrth action potentials in ventral horn neurons and ventral white matter axons following monosynaptic excitation by the volley represented by phase I. This is supported by the observation that phase II has a higher threshold than phase III (Fig. 2). In addition, at least two other events would contribute. Monosynaptic excitatory postsynaptic potentials are generated by many ventral horn interneurons and motoneurons in response to ventral cord volleys (3-6, 9, 19, 23-25). These synaptic potentials would begin during phase II, although their peaks would be represented by phase III. Stimuli below threshold for phase II produce a smooth waveform which consists entirely of phase III (Fig. 2B). In addition, single unit records from the ventral horn showed that some neurons having an ascending axon which projected rostrally to the level of the stimulating electrode could be antidromically activated by the stimulus. A portion of these cells had axons of sufficiently slow conduction velocity that their antidromic action potentials would contribute to phase II. The emphasis of the present study was on the potential we have termed phase III or the P-wave. This potential was positive in sign in the dorsal part of the cord. It reversed at the base of the dorsal horn, and it was negative in the ventral horn (Fig. 4). In general, it had a smooth waveform, although there were often superimposed spikes from single units in records from the ventral horn. This potential corresponds to the negative slow wave that Lloyd observed in the ventral horn. We interpret this potential to be due largely to extracellular current flow associated with monosynaptic
SI’IliAL
CORD
l’OTENTI.\LS
3 15
and disynaptic excitatory postsynaptic potentials which are produced in ventral horn neurons by volleys in the ventral white matter (cf., 19). This potential was produced by relatively large-sized axons, as judged by the effects of grading the strength of the stimulus applied to the cord (Fig. 2AE). The fact that it showed a marked facilitation (Fig. 2F-I) is in agreement with Lloyd’s findings and his interpretation that this is a postsynaptic event. The facilitation curve of Fig. -3F has a time course similar to that of the excitatory potentials recorded from ventral horn neurons (19), and it is reasonable to assume that the facilitation is due to the EPSPs. The amplitude of the P-wave is nearly constant throughout the lumboscacral enlargement (Fig. 3A-B). It declines below L7, perhaps because of the decrease in numbers of neurons in the conus medullaris. The isopotential maps (Fig. 5-7) provided a means of determining the distribution of current sinks and sources across the spinal cord gray matter at a time corresponding to the peak of phase III. The maximum region of negativity was generally in lamina VIII and in the adjacent regions of lamina VII. However, this varied with the extent of the lesion placed in the dorsal part of the spinal cord at L2 to limit the number of pathways activated by the stimulus. When the volley was limited to fibers in the ventral funiculus and the ventral part of the lateral funiculus, the negativity was distributed as mentioned above. However, when some of the dorsal part of the lateral funiculus was spared, there was a greater amount of negativity in lamina VII, and the reversal line was situated more dorsally. It is our interpretation that the sinks in lamina VIII and adjacent regions of lamina VII are due to the synaptic potentials evo!;ed in neurons in these regions by fibers in the ventral funiculus which are known to terminate in these areas, the major ones being the vestibulospinal, pontine reticulospinal, and long propriospinal tracts (1, 10-13). The additional sinks produced in lamina VII when some of the dorsal part of the lateral funiculus was left intact could be due to activity in the medullary reticulospinal tract, which is found in this region of the white matter and which terminates largely in the appropriate part of lamina VII ( 10). Our interpretation of the positivity recorded in the dorsal horn during phase III of the potential sequence is less secure. One possible explanation would be that the ventral cord volleys evoked inhibitory postsynaptic potentials in dorsal horn interneurons, thus generating positive extracellular field potentials. However, intracellular recordings from dorsal horn interneurons did not substantiate this idea (19). The most likely explanation is based on the observation that many ventral horn interneurons give rise to axons which project dorsally, with collaterals on both the ipsilateral and the contralateral side (17). The positive field potentials could then represent current sources along dorsally projecting axons. The ventral horn inter-
316
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neurons, especially those in lamina VIII, would thus form dipoles oriented in the opposite direction of those produced by dorsal horn interneurons which are responsible for the cord dorsum negative potentials evoked by stimulation of peripheral cutaneous aff erents (2). In two experiments, it was found that negativity and hence presumably activity could be evoked in the contralateral spinal cord by volleys in a ventral quadrant (Fig. 7). This could have been mediated by fibers crossing in the ventral commissure (10). However, it was not certain whether the field potentials were due to monosynaptic or to disynaptic events. A portion of the activity studied in these experiments could have been caused by antidromic activation of the ascending axons of neurons projecting to more rostra1 parts of the nervous system. It is known that the axons of many such tract cells have collaterals (20) which can be presumed to exert synaptic effects at the same or adjacent segmental levels. The degree of this kind of effect will need to be studied in chronically spinalized animals. However, it is anticipated that the bulk of the activity observed was due to activation of descending tracts, since others have shown that stimulation of specific brain stem nuclei, such as the lateral vestibular nucleus, or descending brain stem tracts produce comparable field potentials, at least as judged by records from the dorsal surface of the cord (4-6, 9). References 1. BARILARI,
M.
G., and
H.
G. J. M.
KUYPERS. 1969. Propriospinal fibers interconthe cat. Brain Res. 14 : 321-330. J. S., D. R. CURTIS. and S. LANDGREN. 1956. Spinal cord potentials genby impulses in muscle and cutaneous afferent fibres. J. Ncu~ophysiol. 19:
necting the spinal enlargements in 2.
COOMBS,
erated
452-467. 3. EIDE, E., A. LUNDBERG, and P. VOORHOEVE. 1961. Monosynaptically evoked inhibitory post-synaptic potentials in motoneurones. &4&a Physiol, Scand. 53 : 185-195. 4. GRILLNER, S., T. HONGO, and S. LUND. 1968. The origin of descending fibres monosynaptically activating spinoreticular neurones. Brain. Res. 10 : 259-262. 5. GRILLNER, S., T. HONGO, and S. LUND. 1969. Descending monosynaptic and reflex control of y-motoneurones. Acta Physiol. Scaled. 75 : 592-613. 6. GRILLNER, S., and S. Luxn. 1968. The origin of a descending pathway with monosynaptic action on flexor motoneurones. Acta Physiol. Scared. 74 : 274-284. 7. LINDBLOM, U. F.. and J. 0. OTTOSSON. 1956. Bulbar influence on spinal cord dorsum potentials and ventral root reflexes. .4cta PhqGoZ. Scmd. 35 : 203-214. 8. LLOYD. D. P. C. 1941. Activity in neurons of the bulbospinal correlation system. J. Neurophysiol. 4 : 115-134. 9. LUND, S., and 0. POMPEIANO. 1968. Monosynaptic excitation of a alpha motoneurones from supraspinal structures in the cat. Acta PhysioE. Scarrd. 73 : I-21. 10. NYBFXG-HANSEN. R. 1965. Sites and mode of termination of reticulospinal fibers in the cat. J. Camp. Newel. 124 : 71-100. 11. NYBERG-HANSEN, R. 1966. Functional organization of descending supraspinal fibre systems to the spinal cord. Ergeh. i-lust. E1ztz~ic./zlll,igsgeScR. 39 : l-48.
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12. NYBERG-HANSEN, R., and T. A. MASCITTI. 1964. Sites and mode of termination of fibers of the vestibulospinal tract in the cat. J. Camp. Nezcrol. 132: 369-387. 13. PETRAS, J. M. 1967. Cortical, tectal and tegmental fiber connections in the spinal cord of the cat. Brain Res. 6 : 275-321. 14. POMPEIANO, O., and A. BRODAL. 1957. The origin of vestibulospinal fibres in the cat. Arch. Ital. Biol. 95 : 166-195. 15. REXED, B. 1952. The cytoarchitectonic organization of the spinal cord in the cat. .I. Cow@. Neural. 96 : 415-496. 16. REXED, B. 1954. A cytoarchitectonic atlas of the spinal cord in the cat. J. Camp. Neural. lo0 : 297-380. 17. SCHEIBEL, M. E., and A. M. SCHEIBEL. 1966. Spinal motoneurons, interneurons and Renshaw cells. A Golgi study. &-1rch. Ital. Biol. 194: 328-353. 18. SKINNER, R. D., and W. D. WILLIS. 1969. Population and unit responses of interneurons in the cat spinal cord to stimulation of the ventral columns. Amt. Rec. 163 : 266. 19. SKINNER, R. D., W. D. WILLIS. and M. B. HANCOCK. 1970. Action of ventral cord pathways on spinal neurons. Exp. Newel. 27 : 318-333. 20. SZENTAG~THAI, J. 1967. Synaptic architecture of the spinal motoneuron pool. Electroencephalogr. Clin. Ncwophysiol. Suppl. 25 : 419. 21. TORVIK, A., and A. BRODAL. 1957. The origin of reticulospinal fibers in the cat. Anat. Rec. 126 : 113-135. 22. WILLIS, W. D., R. D. SKINNER, and M. A. WEIR. 1969. Responses of alpha and gamma motoneurons and Renshaw cells to activation of motor axons. Exp. Neurol.
23.
25 : 57-69.
W. D. and J. C. WILLIS. 1966. Properties of interneurons in the ventral spinal cord. Arch. Jtal. Biol. 104 : 354-386. 24. WILLIS, W. D., J. C. WILLIS, and W. M. THOMPSON. 1967. Synaptic actions of fibers in the ventral spinal cord upon lumbosacral motoneurons. J. Neurophysiol. 30 : 382-397. 25. WILSON, V. J. and M. YOSHIDA. 1968. Vestibulospinal and reticulospinal effects on hindlimb, forelimb, and neck alpha motoneurons of the cat. Proc. Nat. WILLIS,
Acad.
Sci. U.S.
60 : 836-840.
NEUROLOGY
27, 305-317
Spinal
Cord
Ventral R. D. Departrnertt
of
Anatomy,
Cord SKINNER
(1970)
Potentials
Produced
Volleys AND
in the
W. D.
WILLIS
by Cat 1
The University of Texas Sor~thzwcstcm at Dallas, Dallas, Texas 75235
Received January
dledirnl
School
7, 1970
Electrical field potentials produced by volleys in the ventral white matter were recorded along the dorsal surface and in the depth of the cat lumbosacral spinal cord. The cord dorsum potential was positive in sign and composed of three phases. Potentials in the depth of the cord were located with reference to the histology of the cord using a contour mapping technique. A current sink was found in the medial ventral horn at the time of phase III. It was attributed to the excitation of neurons in laminae VII and VIII. Sources were found in the dorsal horn. They were probably distributed along the dorsally projecting axons of many ventral horn interneurons. Introduction
The activity of neurons in the lumbosacral enlargement of the spinal cord is influenced by pathways descending from the brain and from more rostra1 parts of the spinal cord. Several of these tracts travel in the ventral white matter of the cord, either in the ventral part of the lateral funiculus or in the ventral funiculus (1, 10, 12-14, 21). Most of the ventral cord pathways terminate in the medial part of the ventral horn( 11) in the regions designated by Rexed as laminae VIII and VII (15, 16). When ventral cord pathways are stimulated electrically, a characteristic potential sequencecan be recorded from the spinal cord. Several aspects of the potential sequence were analyzed by Lloyd (8). An electrode in the ventrolateral white matter will record a successionof two spike potentials in response to a single stimulus applied at any level from the brain stem downward. The first spike is the activity of the fibers directly escited by the stimulus, while the second is due to activity relayed after a single synaptic delay. Lloyd recorded a slow negative potential and single 1 This Institutes for their fellow in chusetts.
work was supported by a research grant, NB 04779, from the of Health. The authors thank Miss Anita Bird and Miss Patsy technical assistance. Dr. R. D. Skinner is presently a postdoctoral the Department of Physiology, Harvard Medical School, Boston, 305
Kational Hatldley
research Massa-
306
SKINNER
AND
WILLIS
unit activity in the ventral horn in response to a similar stimulus. He suggested that the relayed spike was due to monosynaptic excitation of ventral horn interneurons whose axons projected into the ventral white matter as part of the propriospinal system. When records are made from the dorsal surface of the spinal cord following excitation of fibers in the ventral white matter, the potential observed is composed of a series of positive waves. Illustrations of this have been published by several laboratories (3, 7, 24). However, there has been no attempt to interpret this potential sequence in detail, nor has there been any explanation of the positive sign of the slow component. The present investigation was undertaken to answer these questions and to relate the potentials recorded to the anatomical studies of the regions of termination of the tracts descending in the ventral cord. A correlation of the field potential recordings reported in this study with single unit recordings will be made in the following paper (19). A preliminary report of some of this work has been given ( 18). Method In cats anesthetized with chloralose (80 mg/kg, iv (after induction with halothane and nitrous oxide), the dorsal half of the cord was cut at L2, so that electrical stimulation of the ventral surface of the cord at Ll produced a volley which was conducted down just the ventral white matter. The ventral half of the cord will be called the ventral cord (VC) . In some experiments only the one ventral quadrant was left intact. The general operative, stimulating, and recording techniques in this laboratory have been described previously (23 ) . Potentials produced by ventral cord volleys were recorded on the dorsal surface of the cord from L4 to Sl with a ball-tipped platinum electrode (Fig. 1). A photograph was made of superimposed oscilloscope traces. These potentials could also be averaged using a small computer (NuclearChicago). The output of the averager was recorded in analog form on a pen writer (Brush) and in digital form on magnetic tape (Kennedy). The magnetic tape records were processed further by digital computer. Field potentials in the depth of the cord produced by ventral cord volleys were recorded by a microelectrode at 250-p intervals in a series of tracks placed transversely across the cord at about L6 (Fig. 1). The microelectrodes, filled with 4 M sodium chloride, had tips broken to a diameter of l-2 p under microscopic control. These potentials were recorded in the same manner as the cord dorsum potentials. From measurements of averaged records, isopotential maps were made and related to the cord histology using techniques described previously (22).
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Sm
SC Y FIG. 1. Diagram of the experimental arrangement. The dorsal half of the spinal cord was cut at L2, and the ventral cord was stimulated with an electrode, aVc Muscular and cutaneous nerves in the hindlimb were stimulated electrically with electrodes S, and C,. Potentials from the microelectrode placed in the gray matter of the ventral horn and from the ball electrode on the cord dorsum nearby were amplified and displayed on a cathode ray oscilloscope from which they were photographed. These potentials could be averaged. The averaged potential was read out in analog form onto a pen writer and in digital form onto magnetic tape. The tape was interpreted by a digital computer, which produced a print out. Selected data from the print out were used to make isopotential field maps by using a plotter under computer control. Results
Cord Dorsum Pofentials. Stimulation of ventral cord (VC) axons at the upper lumbar level produced a complex positive potential when records were made from the dorsal surface of the lumbosacral spinal cord. An example of a typical potential sequenceis shown by the record in Fig. 2D taken at the junction of the L6 and L7 segments. This and the other records in Figs. 2 and 3 were each the averaged responseto 200 ventral cord volleys as displayed on a pen writer. The potential was divisible into three phases. Some of the phases were more discernible in sonle preparations than in others. The general form of this potential, however, was the same in each preparation.
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“C CORD ,
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FIG. 2. Relation of cord dorsum potentials to ,stimulus strength and an example of facilitation of the P-wave. All records are averaged responses to 200 stimuli at lO/sec, and measurements were made from averaged records. A-D are records of the cord dorsum potential with different strength stimuli made at the junction of the L6 and L7 segments. E is a plot of the amplitude of the axon volley (triangles) and the P-wave (circles), both expressed as percentage of maximum versus the stimulus strength in terms of multiples of threshold of the most excitable ventral cord axons. F is a plot of the amplitude of the facilitated test P-wave, after electronic subtraction of the conditioning cord dorsum potential versus the interval between the two stimuli. G-I are samples of the records from which F was made. They are, respectively, the potentials produced by the conditioning stimulus (same as the test response), the conditioning stimulus plus the test stimulus delayed 4.0 msec, and the test stimulus after subtraction of the conditioning stimulus. The calibration pulse of 0.2 mv and 1 msec in A-D applies to all records.
Phase I, indicated by an arrow in Fig. 2D, was a short latency, spikeshaped potential. It coincided in time with spike potentials in axons in the ventral cord and, therefore, will be referred to as the axon volley. Phase II (Fig. 2D) was first distinguishable when the stimulus strength was 1.5 2.0 times the threshold of the most excitable axons in the ventral cord (Fig. 2C). It reached a peak 0.8 msec after the positive peak of the axon volley (average of 10 experiments). Phase III (Fig. 2D) reached a peak 2.2 msec after the axon volley (average of eight experiments) and had a duration of about 9 msec (average of 12 experiments). This phase will be referred to as the P-wave. Figure 2E is a plot of the amplitudes of the axon volley (triangles) and the P-wave (circles) recorded at the L6-L7 junction as a function of the stimulus strength. The ason volley and P-wave are expressed as a percentage of the maximum response, while the stimulus strength is ex-
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FIG. 3. Distribution of the axon volley and P-wave along the lumbosacral spinal cord. Sample records at strength 3.OT are shown in .L\. They are the averaged responses to 200 stimuli at lO/sec. B shows the distribution of the amplitude of the P-wave expressed as a percentage of the maximum along the lumbasacral cord. Each point is the average of the number of experiments indicated, and the vertical bars represent one standard deviation. C shows the distribution of the amplitude of the axon volley in one experiment. The calibration pulse is 0.2 mv and 1 msec in all records.
pressed as a multiple of the threshold of the most excitable fibers in the ventral cord. The amplitude of the P-wave increased with that of the axon volley. Sample records are shown in Fig. 2 ri-11. Facilitation of the P-wave was demonstrated by using two ventral cord volleys, one as a conditioning stimulus and another at the same strength as a test stimulus. The interval between the volleys was varied. An example of facilitation is illustrated in Fig. 2G-I. The response to the conditioning volley, G, was identical to the unconditioned test response; H was the response to the conditioning stimulus plus the response to the test stimulus at an interval of 4 msec; and I was the test response after electronic subtraction of the conditioning stimulus response. This was accomplished by use of the subtraction mode of the signal-averaging computer. The subtraction was not perfect, since a portion of the first stimulus artifact can still be seen in Fig. 21. The result of measurements taken at several intervals is shown in F. The amplitude of the test P-wave is espressed as a percentage of the control response at various intervals between the two responses.The facilitation had a time course similar to that of the P-wave itself. The distribution of the P-wave along the surface of the lumbosacral spinal cord is plotted in Fig. 3B, and sample averaged records are shown
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in A. Each point on the graph was the average response at that level from the number of experiments indicated. The vertical bars represent one standard deviation. The amplitude was essentially constant along the lumbar enlargement except for two relative maxima. One occurred at L4 and the other at the L6-L7 junction. The amplitude decreased rapidly caudal to L7. The axon volley decreased in amplitude with distance along the cord as shown for one experiment in Fig. 3C. The amplitude of the axon volley is expressed as a percentage of the maximum. Potentials Recorded Within the Cord. In order to study the action of ventral cord pathways on populations of spinal cord neurons, the extracellular field potentials elicted by ventral cord stimulation were recorded from a grid of points within the cord. The location of an array of microelectrode tracks in a projected drawing of a cross section of the spinal cord is shown for one experiment in Fig. 4. The recording points were at 250-p intervals, as indicated by the circles along one of the tracks. Sample records from the points shown by
FIG.
sample across intervals. on the dorsal tion of interval 0.2 mv
4. Spinal cord section showing the position of a set of electrode tracks and records. Six parallel electrode tracks 250 p apart were made transversely the cord. The circles on one of these mark the recording points at 250 B Sample averaged field potentials in response to VC stimulation are shown right for the solid circles. The phase III potential was positive in the horn and negative in the ventral horn. Reversal occurred at about the junclaminae VI and VII. The vertical line on the records illustrates a typical at which values were taken to make a field map. The calibration pulse was and 1 msec.
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the filled circles are at the right margin of the drawing. The potential sequence recorded in the dorsal horn was similar to that recorded from the dorsal surface of the cord. The P-wave reversed, however, in the intermediate gray matter and became negative in the ventral horn. The vertical line in Fig. 4 indicates an interval after the stimulus which corresponded to the peak of the negative wave in lamina VIII. Measurements of all of the records in this experiment were made at this interval. An isopotential map was constructed with the aid of a digital computer from these measurements, and the map was superimposed on a drawing of a histological section of the spinal cord, as shown in Fig. 5. The map shows a large area of negativity in the ventral part of the gray matter with a focus in lamina \‘III. The negativity covered all of lamina VIII and most of the medial and ventral portions of lamina VII. Even though lamina IX and the lateral part of lamina VII were not recorded from in this animal, the negativity appeared to decrease rapidly in the direction of these areas. The entire dorsal horn was positive with a focus in the lateral part
FIG. 5. Isopotential map of responses to bilateral ventral cord stimulation. The spinal cord lesion is shown in the inset. Most of the fibers in the ventral part of the lateral funicuhrs were interrupted on the ipsilateral side. Negativity is indicated by solid line, positivity by the thin broken line, and zero by the heavy broken line. The maximum negativity is denoted by a minus sign and the maximum positivity by a plus sign. The arrows indicate the electrode tracks.
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of lamina IV. The zero line followed the border of laminae VI and VII. The lesion in the dorsal part of the spinal cord at L2 in this experiment was reconstructed from serial sections and is indicated by the shaded area in the inset in Fig. 5. The lateral funiculi were interrupted to a level well below the denticulate ligament. In other experiments, more of the lateral funiculus was spared. For instance, in Fig. 6 the inset shows that some of the dorsolateral part of the lateral funiculus was intact on the left side, while the lesion reached a level approximating that of the denticulate ligament on the right. The isopotential map extended across most of the gray matter on both sides of the cord. The distribution of sources and sinks on the right side was nearly identical to that shown in Fig. 5. However, on the left side there were additional foci of negativity in the central and medial parts of lamina VII, and the zero line was shifted dorsally to the middle of lamina VI. The results shown in Figs. 5 and 6 were confirmed in two other experiments. Two animals had lesions designed for the investigation of crossed path-
’
mm
’
6. Isopotential map of responses fibers of the dorsal part of the lateral at the right. Negativity is indicated by positivity by the broken lines. The effects noticeable on the left which has additional more dorsal zero line. The arrows indicate FIG.
to bilateral ventral cord stimulation with funiculus. The spinal cord lesion is shown solid lines, zero by the dash-dot line, and of the lateral funicular tracts are especially foci of negativity in lamina VII and a the electrode tracks.
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ways. One lesion is shown in Fig. 7C with the corresponding map in A. This lesion spared one ventral quadrant, including some of the dorsolateral part of the lateral funiculus on that side. The map shows a small amount of negativity in lamina VIII and the ventral and medial parts of lamina VII on the side contralateral to the intact pathways. The other lesion spared the lateral and ventral funiculi on one side. The lesion and the field potential map are shown in Fig. 7D and B, respectively. Again, there was an area of negativity in the ventromedial gray matter on the side of the cord contralateral to the intact axons. On the ipsilateral side, there was intense negativity in the ventromedial gray matter. The negativity extended into the central and lateral parts of lamina VII and the lateral part of VI. The zero line in this esperiment was shifted to the border between lamina V and 19 in the lateral part of the dorsal horn.
FIG. 7. Isopotential maps of responses to stimulation of the contralateral cord. Maximum negativity is shown by minus signs and maximum positivity by plus signs. In both A and B negativity is denoted by a solid line, positivity by a broken line, and zero by a dash-dot line. For the map in A the spinal cord lesion in C left only the contralateral ventral quadrant intact. For the map in B the spinal cord lesion shown in D left the entire ventral and lateral funiculi on one side. The negativity present on the contralateral side was relatively much less than that on the ipsilateral side. The arrows indicate the positions of the electrode tracks.
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Discussion
Activation of fibers in the ventral white matter of the spinal cord results in a sequence of potentials which can be recorded in the lumbosacral enlargement. The initial event is a spikelike potential. This is undoubtedly due to the conduction of activity in axons directly excited by the stimulus. This spike is positive in records from the dorsal surface of the cord, but it may be diphasic (positive-negative) (Fig. 4) or triphasic (Fig. 1 in ref. 8) in records made in the ventral horn or ventral white matter. The amplitude of this potential is progressively less with distance from the point of stimulation (Fig. 3C ; Fig. 1 in ref. 8). This may reflect the termination of some of the fibers at more rostra1 levels, temporal dispersion of the volley, and possibly changes in recording conditions. The second potential (phase II in our terminology for the cord dorsum records) is probably due to a complicated mixture of events. It was positive in the dorsal part of the cord (Fig. ZD), and it could be negative and spikelike in the ventral horn (Fig. 4). In records made near the ventral white matter, a spike could sometimes be seen at the time of phase II (not illustrated). Lloyd’s second spike potential occurred at this time. It therefore seems likely that the potential designated phase11 in the cord dorsum record is associated wrth action potentials in ventral horn neurons and ventral white matter axons following monosynaptic excitation by the volley represented by phase I. This is supported by the observation that phase II has a higher threshold than phase III (Fig. 2). In addition, at least two other events would contribute. Monosynaptic excitatory postsynaptic potentials are generated by many ventral horn interneurons and motoneurons in response to ventral cord volleys (3-6, 9, 19, 23-25). These synaptic potentials would begin during phase II, although their peaks would be represented by phase III. Stimuli below threshold for phase II produce a smooth waveform which consists entirely of phase III (Fig. 2B). In addition, single unit records from the ventral horn showed that some neurons having an ascending axon which projected rostrally to the level of the stimulating electrode could be antidromically activated by the stimulus. A portion of these cells had axons of sufficiently slow conduction velocity that their antidromic action potentials would contribute to phase II. The emphasis of the present study was on the potential we have termed phase III or the P-wave. This potential was positive in sign in the dorsal part of the cord. It reversed at the base of the dorsal horn, and it was negative in the ventral horn (Fig. 4). In general, it had a smooth waveform, although there were often superimposed spikes from single units in records from the ventral horn. This potential corresponds to the negative slow wave that Lloyd observed in the ventral horn. We interpret this potential to be due largely to extracellular current flow associated with monosynaptic
SI’IliAL
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3 15
and disynaptic excitatory postsynaptic potentials which are produced in ventral horn neurons by volleys in the ventral white matter (cf., 19). This potential was produced by relatively large-sized axons, as judged by the effects of grading the strength of the stimulus applied to the cord (Fig. 2AE). The fact that it showed a marked facilitation (Fig. 2F-I) is in agreement with Lloyd’s findings and his interpretation that this is a postsynaptic event. The facilitation curve of Fig. -3F has a time course similar to that of the excitatory potentials recorded from ventral horn neurons (19), and it is reasonable to assume that the facilitation is due to the EPSPs. The amplitude of the P-wave is nearly constant throughout the lumboscacral enlargement (Fig. 3A-B). It declines below L7, perhaps because of the decrease in numbers of neurons in the conus medullaris. The isopotential maps (Fig. 5-7) provided a means of determining the distribution of current sinks and sources across the spinal cord gray matter at a time corresponding to the peak of phase III. The maximum region of negativity was generally in lamina VIII and in the adjacent regions of lamina VII. However, this varied with the extent of the lesion placed in the dorsal part of the spinal cord at L2 to limit the number of pathways activated by the stimulus. When the volley was limited to fibers in the ventral funiculus and the ventral part of the lateral funiculus, the negativity was distributed as mentioned above. However, when some of the dorsal part of the lateral funiculus was spared, there was a greater amount of negativity in lamina VII, and the reversal line was situated more dorsally. It is our interpretation that the sinks in lamina VIII and adjacent regions of lamina VII are due to the synaptic potentials evo!;ed in neurons in these regions by fibers in the ventral funiculus which are known to terminate in these areas, the major ones being the vestibulospinal, pontine reticulospinal, and long propriospinal tracts (1, 10-13). The additional sinks produced in lamina VII when some of the dorsal part of the lateral funiculus was left intact could be due to activity in the medullary reticulospinal tract, which is found in this region of the white matter and which terminates largely in the appropriate part of lamina VII ( 10). Our interpretation of the positivity recorded in the dorsal horn during phase III of the potential sequence is less secure. One possible explanation would be that the ventral cord volleys evoked inhibitory postsynaptic potentials in dorsal horn interneurons, thus generating positive extracellular field potentials. However, intracellular recordings from dorsal horn interneurons did not substantiate this idea (19). The most likely explanation is based on the observation that many ventral horn interneurons give rise to axons which project dorsally, with collaterals on both the ipsilateral and the contralateral side (17). The positive field potentials could then represent current sources along dorsally projecting axons. The ventral horn inter-
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neurons, especially those in lamina VIII, would thus form dipoles oriented in the opposite direction of those produced by dorsal horn interneurons which are responsible for the cord dorsum negative potentials evoked by stimulation of peripheral cutaneous aff erents (2). In two experiments, it was found that negativity and hence presumably activity could be evoked in the contralateral spinal cord by volleys in a ventral quadrant (Fig. 7). This could have been mediated by fibers crossing in the ventral commissure (10). However, it was not certain whether the field potentials were due to monosynaptic or to disynaptic events. A portion of the activity studied in these experiments could have been caused by antidromic activation of the ascending axons of neurons projecting to more rostra1 parts of the nervous system. It is known that the axons of many such tract cells have collaterals (20) which can be presumed to exert synaptic effects at the same or adjacent segmental levels. The degree of this kind of effect will need to be studied in chronically spinalized animals. However, it is anticipated that the bulk of the activity observed was due to activation of descending tracts, since others have shown that stimulation of specific brain stem nuclei, such as the lateral vestibular nucleus, or descending brain stem tracts produce comparable field potentials, at least as judged by records from the dorsal surface of the cord (4-6, 9). References 1. BARILARI,
M.
G., and
H.
G. J. M.
KUYPERS. 1969. Propriospinal fibers interconthe cat. Brain Res. 14 : 321-330. J. S., D. R. CURTIS. and S. LANDGREN. 1956. Spinal cord potentials genby impulses in muscle and cutaneous afferent fibres. J. Ncu~ophysiol. 19:
necting the spinal enlargements in 2.
COOMBS,
erated
452-467. 3. EIDE, E., A. LUNDBERG, and P. VOORHOEVE. 1961. Monosynaptically evoked inhibitory post-synaptic potentials in motoneurones. &4&a Physiol, Scand. 53 : 185-195. 4. GRILLNER, S., T. HONGO, and S. LUND. 1968. The origin of descending fibres monosynaptically activating spinoreticular neurones. Brain. Res. 10 : 259-262. 5. GRILLNER, S., T. HONGO, and S. LUND. 1969. Descending monosynaptic and reflex control of y-motoneurones. Acta Physiol. Scaled. 75 : 592-613. 6. GRILLNER, S., and S. Luxn. 1968. The origin of a descending pathway with monosynaptic action on flexor motoneurones. Acta Physiol. Scared. 74 : 274-284. 7. LINDBLOM, U. F.. and J. 0. OTTOSSON. 1956. Bulbar influence on spinal cord dorsum potentials and ventral root reflexes. .4cta PhqGoZ. Scmd. 35 : 203-214. 8. LLOYD. D. P. C. 1941. Activity in neurons of the bulbospinal correlation system. J. Neurophysiol. 4 : 115-134. 9. LUND, S., and 0. POMPEIANO. 1968. Monosynaptic excitation of a alpha motoneurones from supraspinal structures in the cat. Acta PhysioE. Scarrd. 73 : I-21. 10. NYBFXG-HANSEN. R. 1965. Sites and mode of termination of reticulospinal fibers in the cat. J. Camp. Newel. 124 : 71-100. 11. NYBERG-HANSEN, R. 1966. Functional organization of descending supraspinal fibre systems to the spinal cord. Ergeh. i-lust. E1ztz~ic./zlll,igsgeScR. 39 : l-48.
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12. NYBERG-HANSEN, R., and T. A. MASCITTI. 1964. Sites and mode of termination of fibers of the vestibulospinal tract in the cat. J. Camp. Nezcrol. 132: 369-387. 13. PETRAS, J. M. 1967. Cortical, tectal and tegmental fiber connections in the spinal cord of the cat. Brain Res. 6 : 275-321. 14. POMPEIANO, O., and A. BRODAL. 1957. The origin of vestibulospinal fibres in the cat. Arch. Ital. Biol. 95 : 166-195. 15. REXED, B. 1952. The cytoarchitectonic organization of the spinal cord in the cat. .I. Cow@. Neural. 96 : 415-496. 16. REXED, B. 1954. A cytoarchitectonic atlas of the spinal cord in the cat. J. Camp. Neural. lo0 : 297-380. 17. SCHEIBEL, M. E., and A. M. SCHEIBEL. 1966. Spinal motoneurons, interneurons and Renshaw cells. A Golgi study. &-1rch. Ital. Biol. 194: 328-353. 18. SKINNER, R. D., and W. D. WILLIS. 1969. Population and unit responses of interneurons in the cat spinal cord to stimulation of the ventral columns. Amt. Rec. 163 : 266. 19. SKINNER, R. D., W. D. WILLIS. and M. B. HANCOCK. 1970. Action of ventral cord pathways on spinal neurons. Exp. Newel. 27 : 318-333. 20. SZENTAG~THAI, J. 1967. Synaptic architecture of the spinal motoneuron pool. Electroencephalogr. Clin. Ncwophysiol. Suppl. 25 : 419. 21. TORVIK, A., and A. BRODAL. 1957. The origin of reticulospinal fibers in the cat. Anat. Rec. 126 : 113-135. 22. WILLIS, W. D., R. D. SKINNER, and M. A. WEIR. 1969. Responses of alpha and gamma motoneurons and Renshaw cells to activation of motor axons. Exp. Neurol.
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W. D. and J. C. WILLIS. 1966. Properties of interneurons in the ventral spinal cord. Arch. Jtal. Biol. 104 : 354-386. 24. WILLIS, W. D., J. C. WILLIS, and W. M. THOMPSON. 1967. Synaptic actions of fibers in the ventral spinal cord upon lumbosacral motoneurons. J. Neurophysiol. 30 : 382-397. 25. WILSON, V. J. and M. YOSHIDA. 1968. Vestibulospinal and reticulospinal effects on hindlimb, forelimb, and neck alpha motoneurons of the cat. Proc. Nat. WILLIS,
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