Cord Dorsum Potentials Research Articles

Influence of Short-Term Ischemia on Segmental Spinal Cord Reflex Functions in Cats

In the lumbar spinal cord in cats, microcirculatory (laser Doppler flowmetry) and segmental neurophysiological parameters [monosynaptic reflexes of a flexor and extensor, polysynaptic reflexes from a cutaneous nerve, and the cord dorsum potential (CDP)] were determined in context with ischemias of 1-10 min. Ischemias were achieved by aortic snare occlusion of the descending aorta and were pooled into deep and moderate ones (0-20% and 20-50% residual spinal cord blood flow). Three phases of reaction to ischemia were defined: a period of decreasing responses, a period of delay until recovery, and a recovery period from beginning to completeness of recovery. Although the period of decreasing responses was relatively constant, the delay until recovery could be correlated with duration and depth of ischemia. The recovery period depended mainly on the duration of ischemia. For 2-6-min ischemias, the correlation between depth and duration of ischemia to the delay until recovery or to the recovery period tended to be linear. The period of decreasing responses, the delay until recovery, and the recovery period of mono- and polysynaptic reflexes behaved in a comparable way and demonstrated similar susceptibility to ischemia. The CDP turned out to be the most stable response, indicating a higher resistance of the first-order interneurons to ischemia than of motor neurons.

Synchronous inherent oscillations of potentials within the rat lumbar spinal cord

Dorsal root potentials (DRPs) and dorsal cord potentials (DCPs) were recorded from the lumbar spinal cord in anaesthetised rats. With the spinal cord intact, low-voltage low-frequency spontaneous DRPs were recorded in synchrony on all lumbar dorsal roots. When the cord was cut at T12, spontaneous large-voltage approximately 10 Hz DRPs appeared immediately in synchrony on all dorsal roots. Section of the dorsolateral funiculus (DLF) was necessary to release these 10 Hz waves. They persisted unchanged for at least 2 h and were not affected by section of all lumbar, sacral and coccygeal dorsal roots. Selective transverse lesions were made to locate the fibres responsible for the synchrony of the oscillations of DRPs recorded on L1 and L6 dorsal roots. Synchrony was maintained with lesions of the entire cord medial to the Lissauer tract but disappeared when the lesion was extended to include the Lissauer tract. We conclude that the isolated cord contains a synchronous oscillatory mechanism inhibited by impulses descending in the DLF and synchronised by way of intrinsic axons in the Lissauer tract.

Plasticity of dorsal horn cell receptive fields after peripheral nerve regeneration.

1. The tibial and sural nerves were transected and repaired in nine adult cats. The receptive field (RF) properties of dorsal horn neurons were examined at three different intervals (5-6, 9, or 12 mo) after axotomy. The properties examined included RF location, area, and modality convergence. In some cases, discrete areas of the cell's RF were stimulated electrically while the evoked cord dorsum potentials (CDPs) and any intracellularly recorded responses were simultaneously recorded. 2. At the shortest interval following reinnervation, the somatotopic organization in the affected areas of the dorsal horn was lost. Dorsal horn cells that received input primarily from regenerated fibers had large, low-threshold excitatory RFs that contained much of the reinnervated skin. Those cells with RFs restricted to a fraction of the reinnervated skin had significant components of their RFs on the foot dorsum supplied by intact fibers (i.e., superficial peroneal nerve). 3. At longer intervals the somatotopic organization remained scrambled. Dorsal horn cell low-threshold RFs were significantly reduced in size. Many cells exhibited large areas of excitatory subliminal fringe and concise inhibitory RFs. In addition, those cells that responded to peripheral stimuli across a wide range of stimulus intensities (wide-dynamic-range cells) also exhibited plasticity in the relative sizes of their low- and high-threshold RFs. 4. At the shortest recovery time, focal electrical stimulation of the skin within the RF of an impaled cell and simultaneous recordings of the evoked CDPs and postsynaptic potentials revealed that at numerous locations within the initial large RFs, single fibers or small groups of fibers could be electrically activated that were not connected to the dorsal horn cell. At the longer recovery times there was a much higher incidence of connectivity. 5. These results suggest that mechanisms affecting both synaptic efficacy of afferent fiber connections and/or the establishment of afferent-driven inhibitory inputs may effect the reshaping of dorsal horn cell RFs after reinnervation. These results are discussed in relation to their potential contribution to previously observed cortical plasticity and functional recovery following similar lesions.

Bilaterally enhanced dorsal horn postsynaptic currents in a rat model of peripheral mononeuropathy

Surface compound potentials were recorded from the surgically exposed lumbar spinal cord in anaesthetized rats which had had one sciatic nerve loosely ligatured 12–15 days previously, resulting in unilateral allodynia and hyperalgesia, as assessed behaviourally. These cord dorsum potentials were recorded in response to electrical stimulation of the ligatured and non-ligatured sciatic nerve, respectively, on both the ipsi- and contralateral side with respect to the stimulated nerve. Compared to potentials produced by stimulation of the non-ligatured sciatic nerve, electrical stimulation of large diameter fibres proximal to the ligatures resulted in a smaller afferent fibre input arriving at the spinal cord. However, larger net postsynaptic currents in the contralateral dorsal horn and a larger net post-synaptic current per unit of afferent fibre input were found in the ipsilateral and contralateral spinal cord. Such changes may result from structural changes or increased synaptic efficacy in the dorsal horn following peripheral nerve injury.

低出力レーザー照射による除痛、創傷治癒促進機序に関する基礎的研究

Pain Low power laser analgesic effect was generally accepted in clinical cases, whereas there was no direct evidence to indicate that low power laser irradiation suppressed an impulse conduction within a peripheral nerve. The effects of low power laser irradiation on electrically evoked responses within the sural nerve was electrophysiologically analyzed in unanesthetized, decerebrate cats and in anesthetized rabbits. Firstly, the effect of a low power (1mW; 632.5nm) helium-neon laser irradiation to the cord dorsum potentials evoked by the sural nerve stimulation was analyzed in unanesthetized, decerebrate cats. The cord dorsum potentials which were induced by the electrical nerve stimulation applied to a distal portion of exposed sural nerve was significantly suppressed during a low power laser irradiation. Secondary, the effects of low power laser irradiation upon multiple units' discharges within a peripheral nerve, which were induced by a noxious stimulation, were analyzed in anesthetized rabbits. Responding to a pinch stimulation of plantar skin, transitory increase of neuronal discharges, which reduced within a minute, and persistent increases, which continued during a period of the stimulation, were induced in the sural nerve. These increases became significantly smaller than the control value during low power laser irradiation that applied to the exposed sural nerve at distal to the recording site. Thirdly, the effects of low power laser irradiation on electrically evoked responses within the sural nerve were analyzed in anesthetized rabbits. Only high threshold evoked responses (unmyelinated A delta) which was induced by an electrical stimulation to the peripheral stamp of the nerve were significantly suppressed during low power laser irradiation which applied to the exposed sural nerve between the stimulus site and the recording site. The suppressive effect was reversible and recovered to the control level after the irradiation, Experimental evidences indicated that low power laser irradiation suppressed the impulse conduction of unmyelinated A delta afferents in peripheral sensory nerve which caused a pain sensation. Our data suggest that low power laser act as a reversible direct suppressor of neuronal activity.Wound healing Although many clinical studies have tested usefulness of low power laser (LPL) irradiation for the wound healing, the mechanism of the effects by LPL irradiation has not been fully elucidated. The present study demonstrated that the enhanced LPL irradiation effect to the procollagen production was consistently observed in the serum-starved medium, not in the FCS-contained medium. These results suggested that some factors in FCS might interfere with LPL irradiation onto procollagen synthesis. This view is supported by the fact that LPL irradiation was much more effective in intractable ulcers and bloodless wounds, which could be lack in FCS. The LPL irradiation effect was not blocked by dexamethasone but methylene blue. It seems probable that the LPL enhanced effects to the procollagen product of the fibroblast might be occurred at the translational level not at the pretranslational level of the mRNA synthesis and cGMP might be involved in the enhanced effect of the procollagen synthesis in fibroblast.In vivo study, the effect of the low power laser on the inflammatory stage of wound healing was investigated using delayed wound healing animal model. Round open wounds were made on the bilateral backs of doxorubicin treated pigs. After making wounds, low power laser was ipsilaterally irradiated on the wounds for 3 minutes everyday. On the 5th day, the wounds were removed and immunohistologocally stained using anti-fibronectin antibody and anti-keratin antibody. Intensity of the fibronectin-like immunoreactivity, the length of new epithelium and the number of the inflammatory cells were measured and compared in the presence and absence of LPL irradiation by using computed analyzing method.

Morphology of functional long-ranging primary afferent projections in the cat spinal cord.

1. A beta-cutaneous primary afferent fibers were impaled in the dorsal columns of alpha-chloralose-anesthetized cats. Penetrations were made with the use of electrodes filled with 2 or 5% N-(2-aminoethyl) biotinamide hydrochloride (Neurobiotin, NB) in 0.1 or 1 M KCl. After determining its adequate stimulus, each fiber was activated by current pulses (18 Hz) injected via the microelectrode. The resulting cord dorsum potential (CDP) was recorded at four locations. NB was then injected into the fiber with the use of positive current pulses (11-22 nA) and a 75% duty cycle. 2. After allowing 2-8 h for diffusion, animals were perfused with saline (37 degrees C) followed by 4% paraformaldehyde (4 degrees C). Frozen 50-microns sections were cut in either the transverse or sagittal plane, processed on slides with the use of standard avidin-biotin protocols, and visualized by the nickel-enhanced diaminobenzidine (DAB) reaction. 3. A total of 15 A beta-cutaneous afferents innervating both rapidly (RA) and slowly adapting (SA) receptors were adequately stained and their central projections recovered. For selected fibers the rostrocaudal and laminar bouton distributions were determined and compared with the distribution of monosynaptic CDP amplitudes recorded at the four surface locations. 4. The rostrocaudal extent of a single A beta-afferent fiber bouton distribution visualized with NB ranged from 8 to 17.5 mm (14.4 +/- 2.4 mm, mean +/- SD), or two to three times greater than that previously shown with the use of horseradish peroxidase (HRP). 5. The strong correlation between the rostrocaudal distribution of boutons and monosynaptic CDP amplitudes, and the improved agreement between modeled and observed CDP amplitudes over that seen previously with the use of HRP (mean percent error, HRP = 23 +/- 2.9%; NB = 9 +/- 2.3%), suggest that boutons along the entire length of the visualized distribution contribute to the recorded potentials. 6. Taken together, these findings suggest that inputs from a given point on the skin can directly influence sensory information processing over a much greater rostrocaudal extent than predicted by dorsal horn somatotopic maps. These findings are discussed in terms of their implications for spinal cord plasticity.

Properties of regenerated primary afferents and their functional connections.

1. The tibial and sural nerves in cats were cut, anastomosed to their distal stumps, and allowed to regenerate for 3-17.5 mo. In the terminal acute experiment, individual afferents were impaled in the dorsal root ganglion to study their receptive field properties, somatic spike parameters, and spinal projections using cord dorsum potential (CDP) measurements. Properties of the CDPs provided evidence on whether the afferent fibers were originally proprioceptive or cutaneous (rapidly or slowly adapting). 2. Fibers with the largest conduction velocity were selectively slowly adapting, suggesting that large muscle afferents maintained their adaptation properties regardless of the peripheral structure innervated. Similarly, the relationship of somatic spike configuration to mechanical threshold was largely normal. Cells with narrow spikes innervated low-threshold mechanoreceptors, whereas cells with broad spikes and an inflection on the descending limb innervated high-threshold mechanoreceptors. 3. Spikes with intermediate properties were observed in some cells that innervated low-threshold mechanoreceptors in the periphery. These were classified as "hybrid" spikes. 4. The largest CDPs were evoked by afferents classified as having originally been cutaneous fibers, regardless of whether they had reinnervated cutaneous or subcutaneous receptors. Fibers classified as has having originally been proprioceptive afferents produced much smaller CDPs; however, these afferents never produce CDPs in intact preparations. Afferents nonresponsive to peripheral stimulation, classified putatively as having been cutaneous, also evoked small CDPs. 5. Fibers classified as putatively cutaneous or proprioceptive could reinnervate foreign target tissue (subcutaneous tissue or skin, respectively), but a propensity to reinnervate the original target tissue was observed. 6. Among putative cutaneous afferents, those with rostrocaudal CDP distributions somatotopically correct for the location of their receptive fields evoked the largest CDPs regardless of the peripheral tissue innervated. 7. We conclude that receptive field properties (adaptation, mechanical threshold) of regenerated afferents are well matched with the electrophysiological properties of the soma and axon. The properties of the central projections of these afferents are not as well matched with their peripheral receptor properties. This is discussed in terms of the plasticity of the central projections of axotomized afferents.

Rescue of motoneuron and muscle afferent function in cats by regeneration into skin. I. Properties of afferents

1. In this study we investigate the peripheral receptive field properties and spinal cord connections of low-threshold muscle afferent fibers cross-regenerated into the skin to determine whether a cutaneous target can rescue physiological functions lost after chronic axotomy. 2. In adult cats the medial gastrocnemius (MG) muscle nerve was coated with the distal cut end of either the caudal or lateral cutaneous sural nerves and allowed to regenerate into the hairy skin (postoperative period 6-30 mo). During terminal acute experiments we made recordings of single MG afferent fibers in dorsal root filaments and peripheral nerve. Conduction velocity and receptive field characteristics were determined for each fiber. In addition, the MG nerve was stimulated to elicit cord dorsum potentials and monosynaptic excitatory postsynaptic potentials (EPSPs) in heteronymous motoneurons. As controls, studies were carried out after MG nerve axotomy (postoperative period 2.5-12 mo). 3. After innervation of the skin, MG muscle afferent fibers exhibited firing characteristics and proximal segment conduction velocities like those of normal MG afferents. Responses to skin and hair stimulation consisted primarily of slowly adapting, stretch-sensitive, and steady discharge patterns, all common in normal muscle afferents but not in cutaneous afferents. These properties were observed despite the innervation of touch domes and single hairs, suggesting that the peripheral physiology of muscle afferents is a function of the axonal membrane and is not respecified by a cutaneous target and/or receptors. 4. Cord dorsum potentials were characteristic of those elicited by intact muscle afferents rather than skin afferents and showed recovery of configurations lost after chronic axotomy. 5. The monosynaptic EPSPs elicited in lateral gastrocnemius-soleus motoneurons also recovered from the reduction in amplitude observed after chronic axotomy. The configurations of these EPSPs were characteristic of muscle afferents rather than skin afferents. 6. These experiments demonstrate that the peripheral and central physiological properties of muscle afferents are rescued from the axotomy state if the afferents are allowed to reinnervate skin. We found no evidence that respecification had occurred to bring the function of muscle afferents into accord with the new cutaneous target.

Central sprouting and functional plasticity of regenerated primary afferents

A combination of neuroanatomical and electrophysiological techniques was used to study the effects of peripheral axotomy and regeneration of primary afferents on their central projections in the spinal cord. Individual regenerated afferent fibers were impaled with HRP-filled electrodes in the dorsal columns of alpha-chloralose-anesthetized cats and activated by current pulses delivered via the intracellular electrode. The resulting cord dorsum potentials (CDPs) were recorded at four rostrocaudal locations and HRP was iontophoretically injected into the fiber. Central distributions of boutons and CDPs were compared with peripheral receptor type to determine the accuracy of peripheral regeneration and the effects of central-peripheral mismatches. Reconstruction of the central projections of 13 individual afferents for which the adequate stimulus and CDPs had been recorded revealed many abnormalities. For example, unlike controls, four group I and II afferents with central projections typical of proprioceptors (concentrated in laminae V, VI, and VII) innervating either cutaneous or noncutaneous targets evoked measurable CDPs. Three other group II or A beta afferents innervating low-threshold mechanoreceptors with central terminations confined to the dorsal horn exhibited extensive collateralization in laminae I and II in addition to large numbers of terminals in laminae III-IV. These fibers activated central networks whose adaptation behavior was identical to those evoked by high-threshold mechanoreceptive afferents in controls. These results suggest that primary afferents and their central connections are capable of significant modifications following axotomy and regeneration. In addition, the anatomical studies indicate some reorganization in the laminar distribution of boutons as well as in bouton size.

Rescue of neuronal function by cross-regeneration of cutaneous afferents into muscle in cats.

1. This study investigates the relation between the peripheral innervation of low-threshold cutaneous afferents and the postsynaptic potentials elicited by electrical stimulation of those afferents. 2. In cats deeply anesthetized with pentobarbital sodium, cord dorsum potentials (CDPs) and postsynaptic potentials (PSPs) in spinal motoneurons were elicited by stimulation of the caudal cutaneous sural nerve (CCS), the lateral cutaneous sural nerve (LCS), and the medial gastrocnemius (MG) muscle nerve. We tested 1) unoperated cats, and cats in which CCS has been 2) chronically axotomized and ligated, 3) cut and self-reunited, 4) cut and cross-united with LCS, or 5) cut and cross-united with the MG. Terminal experiments were performed 3-36 mo after initial surgery. 3. In cats in which the CCS had been self-reunited or cross-united distally with LCS, tactile stimulation of the hairy skin normally innervated by the distal nerve activated afferents in the CCS central to the coaptation, indicating that former CCS afferents had regenerated into native or foreign skin, respectively. 4. In cats in which the CCS had been cross-united distally with the MG, both stretch and contraction of the MG muscle activated the former CCS afferents. 5. In unoperated cats, CDPs elicited by stimulation of CCS and of LCS exhibited a low-threshold N1 wave and a higher-threshold N2 wave. These waves were greatly delayed and appeared to merge after chronic axotomy of CCS. Regeneration of CCS into itself, into LCS, or into MG restored the normal latencies and configurations of these potentials. 6. In unoperated cats, stimulation of CCS, of LCS, and of MG each produced PSPs of characteristic configurations in the various subpopulations of motoneurons of the triceps surae. CDPs and PSPs elicited by the CCS cross-regenerated into LCS or MG were typical of those generated by the normal CCS, i.e., there was no evidence of respecification of central synaptic connections to bring accord between center and periphery after cross-regeneration. 7. Chronic axotomy of CCS increased the latencies of PSPs from CCS; regeneration of CCS into the CCS, LCS, or MG restored normal latencies. 8. In summary, CDPs and PSPs in motoneurons from the cutaneous sensory nerve CCS are altered by chronic axotomy of CCS, thus indicating their target dependency. These alterations are restored by regeneration of CCS into not only a native or foreign cutaneous target, but also into skeletal muscle. We conclude that muscle as well as skin is capable of providing trophic support for cutaneous afferents.

A relay for input from group II muscle afferents in sacral segments of the cat spinal cord.

1. A neuronal relay for input from group II afferents of hindlimb muscle nerves has been found in the previously little explored sacral segments of the cat spinal cord. 2. Electrical stimulation of group II muscle afferents of a number of nerves evoked negative potentials on the surface (cord dorsum potentials) and population postsynaptic potentials (field potentials) within the sacral segments. The largest potentials were evoked by stimulation of the posterior biceps-semitendinosus and triceps surae nerves which evoke much smaller potentials in other segments. Group II afferents of other nerves, notably those which have their main relay within the middle lumbar segments, were much less effective. 3. The sites at which cord dorsum and field potentials evoked by group II muscle afferents were recorded varied in relation to the external topography of the L7-S2 spinal segments but were consistent in their location relative to the pudendal motor nucleus (Onuf's nucleus). Potentials evoked by group II afferents of the posterior biceps and semitendinosus nerves peaked at a level corresponding to the rostral half of Onuf's nucleus and potentials evoked by afferents of the gastrocnemius nerves peaked just rostral to this nucleus. The largest field potentials (of 0.5-1.0 mV) were recorded within the dorsal horn. Field potentials in the intermediate zone were much smaller (< 0.3 mV) and were seen less frequently. 4. Evidence was obtained that the dorsal horn field potentials are to a great extent evoked monosynaptically by the fast conducting fraction of group II muscle afferents: (i) they were evoked at short latencies (2.4-2.7 ms from the stimulus; 1.3-1.7 ms from group I components of afferent volleys and 0.5-0.7 ms from group II components of these volleys), (ii) the conduction times of impulses in the fastest conducting fraction of group II afferents, between the sacral segments (where these impulses were induced by intraspinal stimuli) and the peripheral nerves, were only about 0.5 ms shorter than the latencies of field potentials recorded at the site of intraspinal stimulation and evoked by stimulation of the same peripheral nerves and, (iii) the field potentials followed repetitive stimuli without temporal facilitation. 5. Negative cord dorsum and field potentials were also evoked by small stretches of the semitendinosus and triceps surae muscles. Although they were smaller than potentials evoked by electrical stimulation of sensory fibres and appeared at longer latencies, their presence is consistent with a contribution of muscle spindle afferents to the actions of group II muscle afferents within the sacral segments.(ABSTRACT TRUNCATED AT 400 WORDS)

Cord dorsum potentials suppressed by low power laser irradiation on a peripheral nerve in the cat.

The effects of low-power helium-neon laser irradiation on the cord dorsum potentials (CDP) evoked by electrical nerve stimulation of a distal portion of exposed sural nerve were observed in unanesthetized decerebrate cats. These evoked CDP were significantly suppressed (25.6 +/- 2.5%, p less than 0.01) during low-power laser irradiation. It is suggested that the analgesic effects of low power laser irradiation is based on the decrease of ascending signals from the spinal cord to the higher central nervous system. The suppressive effect of low power laser irradiation upon the impulse transmission of nerve fibers is discussed.

Dorsal root potentials and afferent input to the spinal cord in rats with an experimental peripheral neuropathy

Compound action potentials (CAPs), dorsal root potentials (DRPs) and cord dorsum potentials evoked by stimulating the sciatic nerves have been measured in 4 control rats and in 19 rats with a constriction injury of one sciatic nerve produced by loose ligation of the nerve at mid-thigh level 5 days ( n = 8) or 10 days ( n = 11) before the acute experiments. The contralateral nerve was exposed but not ligated in a sham procedure. In all cases, the nerve was stimulated proximal to the lesion. At 5 post-operative (PO) the maximal A-fibre CAPs on the nerve-injured side were not significantly different from those on the sham-operated side. At 10 days PO all animals showed a decrease in the CAP on the nerve-injured side. The mean CAP area on the nerve-injured side was74.0% ± 4.2 of the sham-operated side, which was significantly different ( P < 0.005). The sciatic nerves and L 5 dorsal roots from 4 of the 10 day PO animals were examined histologically and showed no signs of demyelination or degeneration. The amplitude and area of the maximal DRPs were significantly smaller on the nerve-injured side than on the sham-operated side in all of the nerve-injured animals ( P < 0.01at 5 days PO; P < 0.05at 10 days PO). The mean area of DRPs from the nerve-injured side was61.7% ± 7.5 and 46.8% ± 7.5 of the DRPs from the sham-operated side in the 5 and 10 day PO animals, respectively. The DRPs evoked by sub-maximal afferent volleys were also measured. In all of the nerve-injured animals the CAP-DRP curve on the nerve-injured side was shifted to the right compared to that of the sham-operated side, such that a given size of CAP evoked a smaller DRP on the nerve-injured side than on the sham-operated side. We conclude that the constriction injury produces a decrease in the DRP generated by a volley in the injured nerve and that this change is independent of the decrease in the CAP seen in the injured nerve. We propose that the constriction injury affects the central mechanism responsible for generating primary afferent depolarization (PAD), and thus the pre-synaptic inhibitory control of the afferent input from the injured nerve is impaired.

Effects of pentobarbital on heterosegmentally activated dorsal root depolarization in the rat. Investigation by sucrose-gap technique in vivo.

Slow positive cord dorsum (P-) potentials activated by segmental stimulation are believed to reflect primary afferent depolarizations and have been shown to be augmented by barbiturates. However, there have been no data to confirm whether heterosegmentally activated P-potentials also represent primary afferent depolarizations and are similarly affected by barbiturates. We therefore tested whether heterosegmental P-potentials reflect primary afferent depolarizations and how these heterosegmental potentials are affected by barbiturates. Heterosegmentally activated dorsal root (DR) depolarizations (depolarizations evoked in DRs of lumbar segments in response to afferent volleys to cervical segments produced by electrical stimulation of the forepaw) and P-potentials were simultaneously recorded, adapting the sucrose-gap technique for recording DR depolarization in vivo in the rat. Forepaw (heterosegmental) stimulations produced a large depolarization in the DRs of L5-S1 as well as a slow P-potential in the lumbosacral enlargement. Transection of the spinal cord at the level of C1-C2 abolished both the P-potential and DR depolarization activated by heterosegmental stimulation as well as the second component of segmentally (hind-paw) activated P-potential. Bicuculline (100 micrograms/kg, intravenous) augmented the P-potential and DR depolarization produced by heterosegmental stimulation, but larger doses, 400-600 micrograms/kg, eventually suppressed these. However, the drug, in a dose-dependent manner, suppressed both the P-potential and DR depolarization produced by the segmental stimulation. Pentobarbital (10-40 mg/kg, intravenous) suppressed in a dose-dependent manner both the heterosegmental P-potential and heterosegmental DR depolarization and prolonged their peak latencies. By contrast, pentobarbital augmented and prolonged the segmental P-potential and segmental DR depolarization.(ABSTRACT TRUNCATED AT 250 WORDS)

Slow positive dorsal cord potentials activated by heterosegmental stimuli

Heterosegmental slow positive waves (HSPs) and segmental spinal cord potentials were recorded from the cord dorsum in ketamine-anesthetized rats. Forepaw stimulation produced HSPs in the lumbo-sacral enlargement (lumbar HSPs), whereas hind paw stimulation evoked HSPs in the cervical cord (cervical HSP). Both the HSP and the secondary component of the slow positive wave (P 2s) in the segmental spinal cord potential were highly vulnerable to anesthetics and completely disappeared after spinal cord transection at the C 1 2 level, indicating that both the HSP and P 2s are produced by a long feedback loop via supraspinal structures. The lumbar HSP evoked by forepaw stimulation was maximal in amplitude at the L 5 level and more dominant in the ipsilateral cord dorsum than in the contralateral one, but widely distributed in the lumbo-sacral cord. A variability of onset (7–18 msec for cervical and 5–17 msec for lumbar HSPs) and peak (22–35 msec for cervical and 12–50 msec for lumbar HSPs) suggests the existence of several nuclei to form the feedback loops for descending impulses to produce the HSPs. There were no peak latency differences between the HSP and P 2s. Since there were several similar characteristics between the P 2s and HSP such as high vulnerability to anesthetic, a complete disappearance after high spinal transection and similar response curves to graded intensities of stimulation, there may be a close relationship between their feedback nuclei and the pathways mediating them. All wide dynamic range (WDR) neurons ( 12 12 ) in lamina V of Rexed responded to heterosegmental stimulation with inhibition of firing. The time-course of inhibitio of WDR neurons was similar to that of the HSP from the same segment. These results suggest that the HSP reflects an inhibitory process, such as primary afferent depolarization, activated by a feedback loop via supraspinal structures.

Contribution of antidromic efferent-fiber excitation to mass spinal potentials in the cat

The contribution of antidromic excitation of motoneurons to cord dorsum potentials (CDP) was studied in the spinal cord of anesthetized cats. It was shown that stimulation of ventral roots (VR) or peripheral nerves following deafferentiation of a number of segments by crosscutting of dorsal roots on the dorsal surface evokes appreciable positive-negative CDP (VR-CDP). Under intact conditions, VR effects of antidromic stimulation of efferent fibers brings appreciable input to the initial "fast" CDP component (the "afferent" peak); input values for the main mixed nerves of the hindlimb are presented. After conditioning stimulation of a mixed nerve, VR-CDP undergo inhibition with two maximums, associated with blocking of the effects of antidromic excitation of efferents by orthodromic mono- and polysynaptic reflex discharges of motoneurons. The hypothesis that intactness of efferents in nerves under stimulation can be determined from an analysis of initial CDP components is stated.

Tuning of spinal networks to frequency components of spike trains in individual afferents

Cord dorsum potentials (CDPs) evoked by primary afferent fiber stimulation reflect the response of postsynaptic dorsal horn neurons. The properties of these CDPs have been shown to vary in accordance with the type of primary afferent fiber stimulated. The purpose of the present study was to determine the relationships between frequency modulation of the afferent input trains, the amplitude modulation of the evoked CDPs, and the type of primary afferent stimulated. The somata of individual primary afferent fibers were impaled in the L7 dorsal root ganglion of alpha-chloralose-anesthetized cats. Action potentials (APs) were evoked in single identified afferents via the intracellular microelectrode while simultaneously recording the response of dorsal horn neurons as CDPs, or activity of individual target interneurons recorded extracellularly or intracellularly. APs were evoked in afferents using temporal patterns identical to the responses of selected afferents to natural stimulation of their receptive fields. Two such physiologically realistic trains, one recorded from a hair follicle and the other from a slowly adapting type 1 receptor, were chosen as standard test trains. Modulation of CDP amplitude in response to this frequency-modulated afferent activity varied according to the type of peripheral mechanoreceptor innervated. Dorsal horn networks driven by A beta afferents innervating hair follicles, rapidly adapting pad (Krause end bulb), and field receptors seemed "tuned" to amplify the onset of activity in single afferents. Networks driven by afferents innervating down hair follicles and pacinian corpuscles required more high-frequency activity to elicit their peak response. Dorsal horn networks driven by afferents innervating slowly adapting receptors including high-threshold mechanoreceptors exhibited some sensitivity to the instantaneous frequency, but in general they reproduced the activity in the afferent fiber much more faithfully. Responses of synaptically coupled dorsal horn neurons belonging to either hair follicle or SA1 fiber-driven networks to frequency-modulated input were in agreement with the CDP results, confirming that CDP amplitude modulation is a true reflection of EPSP amplitude modulation in at least a subset of dorsal horn neurons comprising the network.

Two forms of inhibition of spinothalamic tract neurons produced by stimulation of the periaqueductal gray and the cerebral cortex

1. Recordings were made from the lumbosacral spinal cord in anesthetized macaque monkeys. The inhibitory effects of electrical stimulation of the periaqueductal gray (PAG) and the cerebral cortex or cerebral peduncle (CP) were tested and compared by recording 1) cord dorsum potentials evoked by stimulation of the sural nerve, 2) discharges recorded extracellularly, and 3) membrane potentials recorded intracellularly from spinothalamic tract (STT) neurons at rest (background activity) or in response to stimulation of the sural nerve. 2. Stimulation of the cortex or in the CP preferentially reduced the amplitude of the N1 and N2 waves of the cord dorsum potential evoked by stimulation of the sural nerve, without affecting the N3 wave. Stimulation of the PAG, on the other hand, reduced the amplitude of the N3 wave with little effect on the N1 and N2 waves. 3. The activity of 62 STT neurons was recorded extracellularly. Stimulation of the PAG or the cortex/CP inhibited nonpreferentially the responses of the neurons in the superficial laminae to all afferent inputs. On the other hand, stimulation of the PAG or the cortex/CP inhibited preferentially the responses of most STT neurons in deep layers of the dorsal horn to the small or large afferent input, respectively. 4. Thirty-five neurons were recorded intracellularly. The membrane potential of the neurons averaged -45.5 +/- 10.1 (SD) mV. All neurons were recorded in laminae III-VI; the neurons were of the wide-dynamic-range (WDR) type and had background activity. 5. The inhibitory effects of stimulation of the PAG were tested on all 35 neurons. In 32 of the neurons, stimulation of the PAG evoked a hyperpolarization. The background activity of the neurons was reduced (generally it completely ceased) by the hyperpolarization. In three neurons stimulation of the PAG did not evoke a hyperpolarization and the background activity of the neurons did not change. Nevertheless, the responses of these three neurons to afferent input were inhibited by stimulation in the PAG. 6. The inhibitory effects of stimulating the cortex and/or the CP were tested in 26 of the 35 neurons. Stimulation of the cortex and/or the CP evoked a hyperpolarization in all the neurons, although, in 10 of the 26 neurons, stimulation of the CP also evoked a depolarization. The hyperpolarization generally blocked the background activity of the neurons. 7. The effective stimuli in the PAG and the cortex/CP to evoke a hyperpolarization in STT neurons were short, high-frequency trains of pulses.(ABSTRACT TRUNCATED AT 400 WORDS)

Skin and epidural recording of spinal somatosensory evoked potentials following median nerve stimulation: correlation between the absence of spinal N13 and impaired pain sense

A clinical lesion study and intraoperative epidural recordings were made to test the origin and clinical significance of the spinal N13 and P13 of somatosensory evoked potentials (SEP) that follow median nerve stimulation. Intraoperatively, the respective peak latencies of spinal P13 and N13 coincided with those of the N1 component of the dorsal cord potential and its phase reversed positivity. On both the ventral and dorsal sides of the cervical epidural space, maximal amplitude was at the C5 vertebral level to which nerve input from the C6 dermatome is the main contributor. The modality of sensory impairment in the hand dermatome was examined in selected patients with cervical lesions, who showed such normal conventional SEP components as Erb N9, far-field P9, P11, P14, N18 and cortical N20, with or without loss of spinal N13. Statistically, the loss of spinal N13 was associated with decrease of pain sensation in the C6 dermatome. This was interpreted as being due to damage to the central grey matter of the cord, including the dorsal horn. Our results suggest the spinal N13 and P13 originate from the same source in the C6 spinal cord segment and that they are good indicators for the detection of centromedullary cervical cord damage.

Correlation of monosynaptic field potentials evoked by single action potentials in single primary afferent axons and their bouton distributions in the dorsal horn

The relationship between structure and function of the projections of single identified primary cutaneous axons was investigated by recording cord dorsum potentials at 4 sites in response to electrical stimulation of the single axon and visualizing the boutons of the axon stained by intracellular injection of horseradish peroxidase. The rostrocaudal extent of boutons differed from fiber to fiber ranging from 4.14-11.50 mm; their location in the dorsal horn also varied in agreement with the known somatotopy of the presynaptic neuropil and dorsal horn neurons. Rostrocaudal distributions of cord dorsum potentials and boutons of individual fibers revealed good agreement. Cord dorsum potential amplitude and length of the spinal projection were positively correlated with number of boutons, but no correlation with bouton density was found. The spinal projection of afferents innervating slowly adapting type 1 mechanoreceptors exhibited a greater rostrocaudal extent (mean: 8.48 mm) than those innervating rapidly adapting mechanoreceptors (i.e., hair follicle and field receptors: mean: 5.87 mm). Although the mean total number of boutons was greater for axons with slowly adapting receptors (7,250/fiber) than for axons of rapidly adapting receptors (4,677/fiber), no differences in the longitudinal density of boutons (boutons/mm) were observed. Likewise, summed amplitudes of cord dorsum potentials at the 4 recording electrodes were larger for SA1 afferents than for those of field and hair follicle afferents. A major role for the number of boutons in determining these differences is supported by the finding that the calculated average contribution per bouton to cord dorsum potentials (expressed as an amplitude coefficient a) was similar for slowly and rapidly adapting afferents. No evidence was found for regions in which boutons did not contribute to the cord dorsum potential.