diameter, i.e. the largest afferent fibres from PC. The motoneurones active stimuli for the reflex were evaluated. It was concluded that the receptors

Transcription

1 J. Physiol. (1971), 216, pp With 7 text- gure Printed in Great Britain THE PLANTAR CUSHION REFLEX CIRCUIT: AN OLIGOSYNAPTIC CUTANEOUS REFLEX BY M. DAVID EGGER* AND PATRICK D. WALL From the M.R.C. Cerebral Functions Research Group, Department of Anatomy, University College London, London WCIE 6 BT (Received 15 February 1971) SUMMARY 1. Reflex toe extension elicited by pressure on the plantar cushion (PC) was studied in cats anaesthetized with Dial. Receptive fields and adequate stimuli for the reflex were evaluated. It was concluded that the receptors for the reflex were chiefly cutaneous pressure receptors in PC. 2. The fastest impulses from the PC receptors for this reflex are conducted to the spinal cord at about 64 m/sec via fibres about am in diameter, i.e. the largest afferent fibres from PC. The motoneurones active in the reflex mainly supplied the intrinsic plantar muscles. Most active axons ran in the SI ventral root. 3. Extracellular recordings of interneurones in the dorsal horn of L7 spinal segment revealed that many units at the medial edge of the dorsal horn, chiefly in Rexed's laminae IV and V, were activated by stimuli similar to those eliciting the PC-toe extension reflex. These were termed intermediate threshold PC units. Some of these medially located units were activated monosynaptically by PC stimulation. Intermediate threshold PC units activated disynaptically or polysynaptically were also found in this medial region of the dorsal horn, as well as ventrolaterally and caudally in lamina V. 4. No intermediate threshold PC units sent axons into dorsolateral ipsilateral thoracic white matter, in contrast to lower threshold PC units, 42% of which were driven by lateral column stimulation. 5. Extracellular and intracellular recordings were made from motoneurones activated by adequate stimuli for the reflex. Minimum latencies of EPSPs revealed that, for the fastest component of the reflex, at most two interneurones could be interposed between a primary sensory neurone and a motoneurone. * Present address: Department of Anatomy, Yale University School of Medicine, New Haven, Conn , U.S.A. i6-3

2 484 M. DAVID EGGER AND PATRICK D. WALL 6. Although convergence of low threshold PC units on to intermediate threshold PC units or on to motoneurones may play a part in the PC-toe extension reflex, it appears probable that the two populations of intermediate threshold PC interneurones described above, that is, the monosynaptic and the disynaptic (with higher order interneurones), mediate the reflex. INTRODUCTION A tap on the plantar cushion (PC) of the foot of an acutely spinalized cat elicits a brisk extension of the toes of that foot (Engberg, 1964). This PC reflex is also easily obtained in cats anaesthetized with Dial, at a threshold markedly below that at which a flexion reflex is elicited. Although there has been no study of the entire circuit of this reflex, the anatomy (Malinovsky, 1966) and physiology (Armett & Hunsperger, 1961; Jhnig, Schmidt & Zimmerman, 1968; Lynn, 1969) of the sensory innervation of PC, the properties of some of the dorsal horn cells responding to PC stimulation (Armett, Gray & Palmer, 1961; Armett, Gray, Hunsperger & Lal, 1962; Fuller & Gray, 1969), the motoneurones and the gross morphology of the reflex itself (Engberg, 1964) have all been studied. In addition, aspects of this reflex have been studied in freely moving cats (Engberg, 1964; Engberg & Lundberg, 1969; Lund & Pompeiano, 1970). Our work is an attempt to add to and to synthesize the relevant earlier findings, leading to a proposed minimal circuit diagram for the PC reflex, including the anatomical locations within the spinal cord of the interneurones and motoneurones mediating the reflex. METHODS The PC reflex was studied in forty-six adult cats of either sex, weighing kg, anaesthetized with Dial, 60 mg/kg i.p. Atropine sulphate was routinely administered, 0-15 mg I.M. Both carotid arteries were tied. For each cat, the receptive field of the PC reflex and the mechanical and electrical thresholds were determined. The cats were then immobilized with Flaxedil and artificially respirated. Conventional neurophysiological methods of cord exposure, stabilization, electrical stimulation and extracellular recording were used. One hind limb was suspended by a loop of rope passed under the Achilles tendon. Following the initial reflex testing, electrical stimuli were applied to PC through 26-gauge hypodermic needles inserted into the septa which separate medial and lateral lobes from the central lobe. In some experiments, the medial plantar nerve and the tibial nerve which contain afferents from PC were exposed and placed on pairs of silver electrodes. The spinal cord and the bared peripheral nerves were flooded with mineral oil, and maintained near C with a radiant heater and heat lamps. Extracellular recordings were made with glass micropipettes containing nearly saturated KCl or NaCl, with impedances at 1 khz of 1-5 Mn. Intracellular recordings were made with KCl-filled micropipettes of impedances of 5-25 Mn. No reversals or marked changes of PSPs in the motoneurones were observed during prolonged recording with these electrodes. The preamplifiers for both extra and intracellular

3 CUTANEOUS REFLEX 485 recording were designed and built by Dr G. Merrill. Motoneurones were identified by recording antidromic action potentials following ventral root stimulation. Localization of recording sites in the spinal cord was based on a method previously described (Wall, 1967). The distance of the electrode tip from the cord dorsum at each recording site was noted. When the electrode had reached a sufficient depth in the cord, it was left in place; the shaft was cut off a few mm above the surface of the cord. At the end of an experiment, the entire lumbosacral cord was soaked in 10 % formal saline, removed and immersed in 10 % formal saline for at least 48 hr. The cord was then cut transversely into 1 mm slices. These slices were dehydrated and cleared in oil of wintergreen. The cleared slices with the electrodes in 8itU were projected microscopically on to drawing paper, and cross-sectional drawings were made of each cord section with its electrodes. After correction for tissue shrinkage, usually %, the position of each recording site was plotted on the drawings. RESULTS In the cats under Dial, the PC reflex was similar to the reflex described by Engberg (1964) in acutely spinalized cats. Contraction occurred chiefly in muscles that plantar flex the toes, particularly the intrinsic plantar muscles. When PC was stimulated above threshold, either mechanically or electrically, all four toes moved in a plantar direction, usually with apparent abduction of the most medial toe. The PC reflex is distinct from the general flexion reflex that involves flexors of the ankle and knee. When the flexion reflex could be obtained, it was invariably elicited at a much higher threshold, electrical and mechanical, than the PC reflex. Although the hind limbs were studied primarily, similar responses could be elicited from stimulation of the palmar cushions. The reflex involves the stimulated limb only. Often quite light pressure can elicit a response. Engberg (1964) observed thresholds as low as 1 g pressure on the central PC lobe. Usually the adequate stimulus was a firm tap with the tip of an anatomical probe. The response could also be obtained with the blunt end of the probe, but greater force was required. When the probe tip was applied to the experimenter's finger tip with approximately the force required to elicit the PC reflex, it was perceived as firm pressure, well below threshold for pain or discomfort. Light brushing of PC, or the application of a vibrating tuning fork (chosen at a frequency near 250 Hz, known to activate Pacinian corpuscles) never elicited the reflex. The lowest mechanical thresholds for the response tended to be found on the central lobe of the trilobed PC. In some cats, at threshold, the most medial toes responded when the medial lobe was probed, while probing the central lobe often activated only the middle two toes. When the probe was applied with a single tap, the response occurred promptly and phasically. If, instead of a tap, the probe was applied with maintained force, the reflex occurred several times in rapid succession, but

4 486 M. DAVID EGGER AND PATRICK D. WALL with diminishing amplitude until the toes returned to, or near to, their original relaxed position. The PC reflex could also be elicited by running a probe tip gently over the central lobe so as to pull the skin, or by pressing gently on stimulating electrodes that had been inserted just beneath the skin. The PC reflex could be elicited by probing anywhere on PC, or the skin around PC, but not from toes, toe pads, or the proximal skin of the foot some distance from PC. In several cats, squeezing the skin between the toes during PC stimulation inhibited or reduced the response. To test that the receptors for the PC reflex were in or near the skin of PC, and not in the deep tissue below it, the surface of PC was cooled by applying a piece of ice. Cooling PC greatly increased both mechanical and electrical thresholds for the PC reflex, in some cases abolishing the reflex altogether; the thresholds returned to normal as the skin warmed following removal of the ice. The tissue within PC was explored with a monopolar electrode, insulated to the tip. The threshold for the reflex was lowest about 5 mm below the surface and was lower in the proximal than in the distal part of PC. During intraspinal explorations, the electrical stimulation of PC was typically a monophasic rectangular wave, 0 5 msec in duration, delivered at 2 Hz. With such stimuli, the medial toe generally exhibited the lowest threshold, with the other three toes beginning to plantar flex at a slightly higher threshold, one response per stimulus. At electrical stimulation frequencies below 10 Hz, each stimulus produced a brief twitch. When the frequency of electrical stimulation was increased toward Hz, the twitches fused to a moderate maintained contraction. In summary, our evidence, as well as that of Engberg (1964), indicates that the PC reflex is elicited by cutaneous stimulation of PC. No evidence that proprioceptive stimuli are involved was obtained by us or by Engberg (1964). Furthermore, neither the exquisitely sensitive Pacinian corpuscles (Janig et al. 1968; Lynn, 1969) nor very high threshold cutaneous receptors (responding to great pressure and to tissue damage) appear to be primarily involved. Rather, the receptors for the PC reflex seem to be in an intermediate range of sensitivity, quite likely including the encapsulated interdermal receptors described by Malinovsky (1966). Conduction of impulses to the spinal cord Most of the afferent fibres activated by stimulating PC enter the spinal cord in the middle, i.e. neither most rostral nor most caudal, rootlets of L7 dorsal root. At the end of experiments, these rootlets were cut as close as possible to the entry zone and were mounted on bipolar recording elec-

5 CUTANEOUS REFLEX4 487 trodes. PC was stimulated at 4-5 times threshold for the reflex. Conduction velocities of the earliest recorded impulses ranged from m/sec, with a median of 64 m/sec. By stimulating the medial plantar nerve innervating PC, and recording the volley from the intact tibial nerve in the popliteal fossa, we found that the medial plantar fibres with the lowest electrical thresholds elicited the PC reflex. Electrical stimulation of PC at threshold for the reflex also activated fibres with the same conduction velocities as those with the lowest electrical thresholds, in the m/sec range. Thus, it seems likely that the fastest conducting afferents from PC mediate the reflex. Extrapolating from the data of Boyd & Davey (1968), the measured conduction velocities indicate that the largest afferent fibres mediating the PC reflex are um in diameter. Our conduction velocity measurements agree well with those of Janig et al. (1968), who found additionally that afferent fibres from the most sensitive receptors, presumably Pacinian corpuscles, averaged 8-5 m/sec slower than those of large myelinated afferents conducting impulses from other PC mechanoreceptors. This supports our observation that Pacinian corpuscles are not essential for the PC reflex. Dorsal horn units In exploring the spinal grey matter, we found units in the dorsal horn with receptive field properties very similar to those of the PC reflex itself. These neurones responded to a probe tap centred on, or confined to PC, but were not activated by brushing or vibration. Units responding to this class of stimuli shall be termed 'intermediate threshold', in contrast to 'low threshold' units responding to light brushing or vibration, and to 'high threshold' units responding only to stimuli of much greater force. Fig. lb shows a latency histogram for sixty-eight such intermediate threshold PC units in the dorsal horn. The latencies are the intervals between the arrival of the earliest afferent impulses at the entry zone of L7 dorsal root following electrical stimulation of PC at 4-5 times threshold for reflex elicitation, and the occurrence of an action potential in the interneurone. Note that the latency distribution is markedly bimodal, in agreement with Armett et al. (1961), who, however, included low threshold units in their sample. Almost all of our PC dorsal horn units which were not intermediate threshold PC units had lower thresholds to brushing or hair movement than the intermediate PC units. When the latencies of these PC dorsal horn units are plotted, omitting the intermediate threshold units, the bimodality (or trimodality) of the distribution, while still apparent, is not nearly as marked (Fig. 2).

6 488 M. DAVID EGGER AND PATRICK D. WALL There were no marked differences in receptive fields or thresholds for electrical stimulation between the monosynaptic (latency: msec) intermediate threshold PC units on the one hand, and the di- (latency: 24-4'0 msec) and polysynaptic (latency > 441 msec) intermediate threshold PC units on the other. However, in addition to the differences in firing latency to electrical stimulation, the monosynaptic, and the di- and poly Time (msec) Fig. 1. Latency histograms showing the intervals between the arrival of afferent impulses at L 7 dorsal root entry zone following electrical stimulation of the plantar cushion (PC) and (a) slow waves recorded extracellularly near intermediate threshold PC units, (b) action potentials of intermediate threshold PC units, (c) slow waves recorded extracellularly or intracellularly in or near motoneurones activated by PC stimulation, (d) extracellularly and intracellularly recorded action potentials of motoneurones. Ordinates indicate the number of units at each interval.

7 CUTANEOUS REFLEX 489 synaptic populations differed slightly in the maximum frequency at which they followed PC stimulation, with the median for the monosynaptic group about 20 Hz, and the median for the di- and polysynaptic group about 12-5 Hz. They also differed in location within the dorsal horn. All the monosynaptically driven intermediate threshold PC units were found to be very medial in the dorsal horn, at a depth corresponding quite precisely to the location of laminae IV and V described by Rexed (1954, p. 326, Fig. 24) _! C z Time (msec) Fig. 2. Histogram of latencies of action potentials of dorsal horn units activated by stimulation of PC, omitting units with latencies greater than 7-2 msec and intermediate threshold PC units whose latencies are shown in Fig. l b. Ordinate indicates the number of units at each interval. for this portion of the cat's spinal cord. Fig. 3a shows the location of twenty-five such units. The di- and polysynaptic intermediate threshold PC units were also found medial in the dorsal horn, but either slightly caudal, or caudal and ventrolateral to the monosynaptic units (Fig. 3b shows the location of twenty such units).

8 490 M. DAVID EGGER AND PATRICK D. WALL The entire population ofintermediate threshold PC units differed from the other PC units encountered in the dorsal horn in that they lacked almost totally the high threshold excitatory surround fields found for many of the other PC interneurones; furthermore, none of the intermediate threshold (d) \~~~~~~~~~- mm (b) 10mm 1-0mm Fig. 3. Diagram of (a) monosynaptically activated intermediate threshold PC units shown on a transverse section of L 7 (shaded portion summarizing location of twenty-five units); (b) di- and polysynaptically activated intermediate threshold PC units (shaded portion summarizing the location of twenty units); (c) motoneurones activated by PC stimulation sending axons in S I ventral root (shaded portion summarizes locations of ten units). Scale mark in (b) refers also to (a) and (c). The same unit locations summarized in (a), (b) and (c) are also presented on a horizontal section of spinal cord (d). The rostral-caudal location of the transverse sections are indicated. The shaded portion medially shows the approximate location of the intermediate threshold PC units. The shaded portion laterally refers to the motoneurones. Scale in d refers only to d.

9 CUTANEOUS REFLEX 491 PC units could be driven, either antidromically or orthodromically, by stimulation of the ipsilateral thoracic lateral column, compared with the 42 % of low threshold PC units which were driven by such lateral column stimulation. In systematic exploration of the spinal gray matter including the ventral horn, no other interneurones were found that appeared to be involved in mediating the PC reflex. Moreover, the locations ofunits found in the dorsal horn form an orderly somatotopic mapping of the plantar surface of the foot on to the dorsal horn, with all PC units occupying a consistent position within that mapping (Fig. 4). Inhibitory fields for the PC interneurones were not observed, though interneurones with excitatory fields similar to those of the inhibitory PC reflex fields, i.e. skin areas between the toes, were found in proximity to the PC units. Motoneurone8 When the PC reflex was monitored by recording from the proximal portions of intradurally sectioned ventral roots, the largest compound action potential following electrical stimulation of PC was usually found in SI ventral root. The compound action potential recorded from L7 ventral root, while somewhat smaller than that recorded from SI ventral root, occurred typically a few tenths of a millisecond earlier. The cell bodies of some of the Si motoneurones taking part in the PC reflex were located by intra- and extracellular recording in the dorsolateral portion of the ventral horn (Fig. 3c, summarizing the location of 10 units). The location of these recording sites corresponded to the motor cell column of the intrinsic plantar foot muscles, plantaris, flexor hallucis longus and flexor digitorum longus (Romanes, 1964). The receptive fields of the motoneurones were similar to those of the intermediate threshold PC interneurones, comprising a part of one or more of the PC lobes. The motoneurone following frequencies for stimulation of PC ranged from 4 to 100 Hz, with a median of 12 Hz. Inhibition of motoneurone, firing during squeezing of skin between the toes was observed for several motoneurones. As noted above, such stimulation between the toes also inhibited the reflex itself. Fig. 1 shows the latencies of extracellular slow waves recorded near the intermediate threshold PC interneurones (1 a),the latencies of interneuronal action potentials (1 b), slow waves recorded in or near motoneurones (1 c) and the latencies of motoneuronal action potentials recorded extra - or intracellularly. All latencies were measured relative to the arrival of the afferent volley at the dorsal root entry zone following 4-5 times threshold stimulations of PC.

10 492 M. DAVID EGGER AND PATRICK D. WALL Because the motoneurones studied were selected for short latencies, the motoneuronal latency distribution is biased in that direction. The interval between slow wave and action potential for twenty-nine motoneurones is shown in Fig. 5. (a) TRANSVERSE ROSTRAL LATERAL MEDIAL CAUDAL Fig. 4a. For legend see facing page. The usual response to a single tap applied to PC for both intermediate threshold interneurones and motoneurones was a single action potential, but some units fired 2-3 times per tap. A few interneurones and motoneurones fired continuously during sustained pressure. High frequency bursts following a single stimulation, rather common for the low threshold

11 CUTANEOUS REFLEX 493 PC interneurones, were only rarely seen for either intermediate threshold PC interneurones or for the motoneurones. Reflex circuit The second peak of interneuronal firing occurred at 2-5-2*8 msec after the arrival of the afferent volley at the spinal cord (Fig. 1 b). The peak for the occurrence of motoneuronal slow waves was at msec (Fig. 1 c). The time interval between these two peaks, 0-8 msec, is consistent with the possibility that the later firing interneurones excited the motoneurones monosynaptically. HORIZONTAL MEDIAL LATERAL SAGITTAL VENTRAL Fig. 4. Summary of somatotopic projection of plantar skin surface on to L7 dorsal horn. Except for transverse sections in (a), drawings are not to scale. (a) A series of transverse sections of spinal cord labelled to indicate the locations of units responding to stimulation of toes, PC, and proximal foot. The first cross-hatched region, labelled PC, summarizes the location of fifty-five units, including the intermediate threshold units, responding monosynaptically to PC stimulation. The second, larger, cross-hatched PC portion summarizes the location of fifty units, including the intermediate threshold units, responding di- or polysynaptically to PC stimulation. The shaded portions of the drawings of the plantar surface of the foot correspond roughlyto those parts of the plantarsurface that project onto the corresponding transverse spinal cord section. Schematic (b) horizontal and (c) sagittal sections of middle portion (approximately laminae IV and V) of L 7 dorsal horn.

12 494 M. DAVID EGGER AND PATRICK D. WALL Fig. 6, illustrating data from a single cat, shows the arrival at the entry zone of impulses in L7 dorsal root (6a), an intraspinal slow wave with action potentials of interneurones (6 b), and intercellular recordings of a motoneurone fired by PC stimulation (6c). Fig. 6c also shows the extracellular field potential recorded following withdrawal of the electrode from the motoneurone C 0 6 z I Time (msec) Fig. 5. Interval histogram of the times between the occurrence of slow waves recorded extra- or intracellularly from a motoneurone, and the action potential of that motoneurone, for twenty-nine motoneurones fired by PC stimulation. Ordinate indicates the number of units at each interval. Fig. 7, also summarizing the data from a single cat, shows the receptive fields, intraspinal vocalizations and latencies of interneurones and motoneurones activated by PC stimulation (7a), intraspinal slow waves with action potentials of a PC interneurone (7 b), extracellular recordings of an activated motoneurone (7c), and compound action potentials recorded from SI ventral root in response to PC stimulation (7d). In summary, our evidence is consistent with a trisynaptic, four neurone basic circuit, as depicted in Fig. 7a, though some very fast responses observed at the motoneurones suggest that a disynaptic pathway, while probably not the most common connexion, cannot be ruled out. And, of course, higher order connexions among the interneurones may be common

13 CUTANEOUS REFLEX 495 and important in the functional control of the reflex; our attention was focused on the fastest component of the PC reflex. The paths by which the interneurones project to the motoneurones are unknown, but in recording from motoneurones, it was observed that a large afferent barrage could be recorded in the lateral column just above the cell columns of the motoneurones activated by PC stimulation, suggesting that some interneurones may project along the ventral portion of the lateral column before sending fibres down to make synaptic contact with the motoneurones. (a) (b) jia _ s - i i i* w iw STA i. i i i i I111 I 4 + w F * TO.IT' in ~ * * ~ + Mw 4 Fig. 6. Recordings from a single cat. Arrows indicate beginning of stimulus artifacts. Horizontal calibration for all traces: 2-0 msec. (a) Five consecutive traces from cut ends of L 7 dorsal rootlets, recorded distal to cut at dorsal root entry zone, following stimulation of PC at 1 Hz. Vertical calibration: 17 gv. (b) A single trace recorded intraspinally near intermediate threshold PC interneurones. Recorded with negativity downward. Vertical calibration: 400 tv. (c) Lower trace: five consecutive recordings at 1 Hz from within a motoneurone activated by PC stimulation. Upper trace is identical, except that electrode has been removed from within cell. Vertical calibration: 25 mv.

14 496 M. DAVID EGGER AND PATRICK D. WALL DISCUSSION Prior to Engberg's study (1964), the effects of cutaneous stimulation (i.e. stimulation of the skin and not just electrical stimulation of a cutaneous nerve) on motoneurones had been studied by Hagbarth (1952) and Megirian (1962), who were able to suggest general rules about how skin stimulation affected some populations of motoneurones of the cat's hind limb. But Engberg was the first to study motoneurones concerned with a (a) Action Slow wave potential Receptive (msec) (ms6c) field S 4( ( VRS 1 Fig. 7a. For legend see facing page. specific reflex activated from a restricted cutaneous region, in this case, PC. Our observations in anaesthetized cats agree with Engberg's concerning the PC reflex in acutely spinalized cats with only minor exceptions. In contrast to Engberg, we found that slight differences in the distribution of contraction occurred, depending upon which PC lobe was stimulated. In our preparations, the PC reflex was more phasic than Engberg observed. With continued application of mechanical stimulation, we observed several responses occurring with decreasing amplitude until the response disappeared altogether, whereas Engberg observed reflex extension to continue virtually undiminished for sec.

15 CUTANEOUS REFLEX 497 (c) _m- ~J Fig. 7. (a) Schematic drawing of possible circuit of the PC reflex as exemplified by the data from a single cat. Stimulating electrodes are shown inserted into the septa dividing the lobes of PC. Afferent impulses arrive at the dorsal root entry zone 50 msec following PC stimulation. An intermediate threshold PC unit fires at 7 0 msee (slow wave at 6-0 msec), with receptive field as indicated, and location in spinal cord as indicated. Two other intermediate threshold PC interneurones are depicted in a section slightly caudal to that of the first intermediate threshold PC interneurones. These have receptive fields as indicated, and fire at 8-1 and 8-6 msec following PC stimulation. A motoneurone is shown, with its receptive field, that has a slow wave at 7-4 msec, and fires at 9-1 msec. The latency of the compound action potential recorded from S 1 ventral root a few cm from the spinal cord is 9-8 msec. Note that the neurones depicted here are only representative of their populations; they are not to be thought of as the actual or only possible combinations of neurones forming the PC reflex. (b) Five consecutive traces at 1 Hz recorded in the neighbourhood of an intermediate threshold PC interneurone. Recorded with negativity downward. Vertical calibration: 33 #V. Horizontal calibration for (b), (c) and (d): 2-0 msec. (c) Five consecutive traces at 1 Hz recorded near a motoneurone activated by PC stimulation. Arrow indicates beginning of stimulus artifact. Vertical calibration: 1*7 mv. (d) Five consecutive traces at 1 Hz recorded from S1 ventral root. Vertical calibration: 400 jav.

16 498 M. DAVID EGGER AND PATRICK D. WALL Engberg speculated that a 'possible function of this activation of all the plantar musculature would be to stabilize the foot during standing or during the stance phase of the step.' However, in freely moving cats, Engberg found that applying a local anaesthetic to PC did not seem to affect the patterns of activation of plantar muscles during walking or running. The effect on more demanding climbing or jumping was not determined. Lund & Pompeiano (1970) studied what appears to have been the PC reflex in freely moving cats. They found that stimulation of the tibial nerve elicited a polysynaptic response at a lower threshold than the monosynaptic response. Engberg (1964) studied intracellularly about 100 motoneurones with axons in the plantar nerves. All received excitation from PC afferents. Neither Engberg nor we found inhibitory components in synaptic action evoked by moderate stimulation of PC. The only point of disagreement between Engberg's and our results from intracellular motoneuronal recording was that Engberg found a minimal value of 3-5 msec between arrival of PC afferent impulses at the dorsal root entry zone and the occurrence of an EPSP. Our minimum central latency for an EPSP was 1*7 msec (Fig. 1 c). This discrepancy may have been due to differences in interneuronal transmission in spinal and anaesthetized cats. Spinal cord units in the dorsal horn responding to PC stimulation were studied by Armett et al. 1961, who studied units driven by electrical stimulation ofthe medial plantar nerve that could also be fired by mechanical stimulation of PC. It seems likely that the 45 % of units Armett et al. (1961) described as driven by 'light' mechanical stimuli would probably have included our intermediate threshold units. Their latency histogram of firing to medial plantar stimulation agrees well with our latency histograms of firing to PC stimulation, though the latencies were calculated in slightly different ways. For both of our populations, the short and longer latency components of the histograms separate at a latency of 2-2 msec. However, the peak of our short latency distributions occur 0-8 (Fig. lb) and 0-6 msec (Fig. 2) later than theirs. We calculated the arrival time at the dorsal root entry zone from the fastest fibres activated by PC stimulation, whereas Armett et al. (1961) calculated it from the arrival of the fastest fibres in the volley that elicited the firing of the unit under study; this may account for some or all of the latency difference. Also, Armett et al. (1961) found an interval between the first two peaks of the latency distribution of 2-0 msec, while for our two interneuronal populations it was 0-8 (Fig. 1 b) and 1 0 msec (Fig. 2). This may have been due to more intense, or less mixed, synaptic drive on the units in our study, or perhaps to anaesthetic

17 CUTANEOUS REFLEX 499 differences (Armett et al. (1961) studied acutely spinalized cats under chloralose-urethane anaesthesia). Both in the Armett et al. (1961) study and in our study (Figs. 1 b and 2), the number of units found in the short and in the longer latency groups are about equal, not exceeding a ratio of 1:2. While Armett et al. (1961) observed that PC units tended to fire multiply to a single PC shock, this was true of our low threshold PC units only, but not of the intermediate PC threshold units. This difference in adaptation rate to firing might have been due to anaesthetic differences. Also, we did not find a relationship between latency and threshold for our intermediate threshold units. The intraspinal vocalizations of PC units agrees with Armett et al. (1961), with the exception that our units were in L7 rather than L6. Also, their short latency units were found over a much wider area in the dorsal horn than our monosynaptic intermediate threshold units. This wider distribution is due to the inclusion by Armett et al. ( 1961) of low threshold units, which meant that units with receptive fields partly on PC and partly on other parts of the foot were included in their sample. A map including all our short latency PC units (Fig. 4 a), not just the intermediate threshold units (Fig. 3 a), agrees very closely with theirs. In both studies, the longer latency units tend to lie ventral and lateral to the earlier firing units (Figs. 3b and 4a). Armett et al. (1962) re-examined the dorsal horn PC units using a precisely controllable mechanical stimulator. Their plot of the anatomical locations of monosynaptically excited PC units (their Fig. 4) agrees well with ours (Fig. 4a), except for units located centrally and medially, near the central canal, where we did not observe any monosynaptic units. Most of the units studied by Gray and colleagues were true interneurones as defined by Eccles, Eccles & Lundberg (1960), that is, neurones whose axons are not relayed up the cord beyond lumbosacral levels. Fuller & Gray (1969), citing unpublished evidence from R. M. Eccles, estimate that of the neurones they studied, about 75 % were true interneurones. In our study, we did not limit ourselves to units that were true interneurones by this definition, but found that none of the intermediate threshold PC units in our sample could be fired antidromically by lateral column stimulation of the thoracic cord. Wall (1967) examined dorsal horn units responding to stimulation of the skin of the hind limb of cats, in which he observed that monosynaptic units were found only in Rexed's lamina IV, with lamina V cells typically having latencies msec longer than the cells of lamina IV. We did not observe this for the monosynaptic intermediate threshold PC units. On the contrary, we found no differences in latency, threshold, or following

18 500 M. DAVID EGGER AND PATRICK D. WALL frequency between the monosynaptic units of lamina IV and lamina V. Other investigators have reported monosynaptic cutaneous units in lamina V as well as IV, including Hongo, Jankowska & Lundberg (1966, 1968), Armett et at. (1961), Armett et al. (1962), and Willis (1969). However, there was a tendency for the di- and polysynaptically activated units to be found ventral to the monosynaptically activated units (Figs. 3a, b and 4a). Many investigators have observed that the intermediate nucleus of the spinal cord receives a concentration of afferent signals from the periphery (Coombs, Curtis & Landgren, 1956; Eccles et al. 1960; Hongo et al. 1966). The ventrolateral locations of some of our longer latency intermediate threshold PC units included the region dorsal to and including part of the intermediate nucleus (Fig. 3b), suggesting that this part of nucleus may be involved in relaying cutaneous information directly to motoneurones. In summary, the evidence we have collected, as well as most of the relevant evidence from the experimental literature, supports our contention that the PC reflex is minimally a four-neurone reflex mediated by two interneurones, the first in the medial portion of the dorsal horn of L7, the second caudal and ventrolateral to the first (Fig. 7a). We thank A. Ainsworth, A. Campbell, and Miss E. Clark for technical assistance, B. Lynn for good suggestions and Mrs Virginia Simons for the illustrations. The work was supported by funds from the Medical Research Council, as well as by NIH grantsnb andnb M.D.E. holds aresearch Scientist Development grant, 5-K02-MH-11, 952 from NIMH. REFERENCES ARmErr, C. J., GRAY, J. A. B., HuwNSPERGER, R. W. & LAL, S. (1962). The transmission of information in primary receptor neurones and second-order neurones of a phasic system. J. Physiol. 164, ArmETT, C. J., GRAY, J. A. B. & PAiMER, J. F. (1961). A group of neurones in the dorsal horn associated with cutaneous mechanoreceptors. J. Phystol. 156, AREtMTT, C. J. & HumNSPERGER, R. W. (1961). Excitation of receptors in the pad of the cat by single and double mechanical pulses. J. Physiol. 158, BoYD, I. A. & DAVEY, M. R. (1968). Composition of Peripheral Nerves. Edinburgh and London: E. and S. Livingstone Ltd. CooMBs, J. S., CITms, D. R. & LANDGREN, S. (1956). Spinal cord potentials generated by impulses in muscle and cutaneous afferent fibres. J. Neurophysiol. 19, EccuEs, J. C., Ecciss, R. M. & LUNDBERG, A. (1960). Types ofneurone in and around the intermediate nucleus of the lumbosacral cord. J. Physiol. 154, ENGBERG, I. (1964). Reflexes to foot muscles in the cat. Acta physiol. scand. 62, suppl. 235, ENGBERG, I. & LUNDBERG, A. (1969). An electromyographic analysis of muscular activity in the hindlimb of the cat during unrestrained locomotion. Acta physiol. scand. 75, FULLER, D. R. G. & GRAY, J. A. B. (1969). A quantitative analysis of the responses of certain dorsal horn neurones to mechanical stimulation of the large foot pad in cats. J. Physiol. 200,

19 CUTANEOUS REFLEX 501 HAGBARTH, K. E. (1952). Excitatory and inhibitory skin areas for flexor and extensor motoneurones. Acta physiol. 8cand. 26, suppi. 94, HONGO, T., JANKOWSxA, E. & LUNDBERG, A. (1966). Convergence of excitatory and inhibitory action on interneurones in the lumbosacral cord. Expl Brain Res. 1, HONGO, T., JANxowsKA, E. & LUNDBERG, A. (1968). Post-synaptic excitation and inhibition from primary afferents in neurones of the spinocervical tract. J. Physiol. 199, JANIG, W., SCHMIDT, R. F. & ZIMMERMANN, M. (1968). Single unit responses and the total afferent outflow from the cat's foot pad upon mechanical stimulation. Expl Brain Re8. 6, LUND, S. & POMrEIANO, 0. (1970). Electrically induced monosynaptic and polysynaptic reflexes involving the same motoneuronal pool in the unrestrained cat. Arch8 ital. Biol. 108, LYNN, B. (1969). The nature and location of certain phasic mechanoreceptors in the -cat's foot. J. Phy8iol. 201, MALINOVSKY, L. (1966). Variability of sensory nerve endings in foot pads of a domestic cat. Acta anat. 64, MEGIRIAN, D. (1962). Bilateral facilitatory and inhibitory skin areas of spinal motoneurones of cat. J. Neurophy8iol. 23, REXED, B. (1954). A cytoarchitectonic atlas of the spinal cord in the cat. J. comp. Neurol. 100, RoMANEs, G. J. (1964). The motor pools of the spinal cord. In Progr"s8 in Brain Research, vol. ii, pp , ed. ECCLES, J. C. & SCHADE, J. P. Amsterdam: Elsevier. WALL, P. D. (1967). The laminar organization of dorsal horn and effects of descending impulses. J. Physiol. 188, WILLIS, W. D., Jr. (1969). The localization of functional groups of interneurons. In The Interneuron, pp , ed. BRAZIER, M.A.B. Berkeley and Los Angeles: University of California Press.