Pyramidal tract effects on interneurons in the cat lumbar dorsal horn

Altered regulation of Ia afferent input during voluntary contraction in humans with spinal cord injury

Sensory input converging on the spinal cord contributes to the control of movement. Although sensory pathways reorganize following spinal cord injury (SCI), the extent to which sensory input from Ia afferents is regulated during voluntary contraction after the injury remains largely unknown. To address this question, the soleus H-reflex and conditioning of the H-reflex by stimulating homonymous [depression of the soleus H-reflex evoked by common peroneal nerve (CPN) stimulation, D1 inhibition] and heteronymous (d), [monosynaptic Ia facilitation of the soleus H-reflex evoked by femoral nerve stimulation (FN facilitation)] nerves were tested at rest, and during tonic voluntary contraction in humans with and without chronic incomplete SCI. The soleus H-reflex size increased in both groups during voluntary contraction compared with rest, but to a lesser extent in SCI participants. Compared with rest, the D1 inhibition decreased during voluntary contraction in controls but it was still present in SCI participants. Further, the FN facilitation increased in controls but remained unchanged in SCI participants during voluntary contraction compared with rest. Changes in the D1 inhibition and FN facilitation were correlated with changes in the H-reflex during voluntary contraction, suggesting an association between outcomes. These findings provide the first demonstration that the regulation of Ia afferent input from homonymous and heteronymous nerves is altered during voluntary contraction in humans with SCI, resulting in lesser facilitatory effect on motor neurons.

Pathways mediating functional recovery

Following damage to the motor system (e.g., after stroke or spinal cord injury), recovery of upper limb function exploits the multiple pathways which allow motor commands to be sent to the spinal cord. Corticospinal fibers originate from premotor as well as primary motor cortex. While some corticospinal fibers make direct monosynaptic connections to motoneurons, there are also many connections to interneurons which allow control of motoneurons indirectly. Such interneurons may be placed within the cervical enlargement, or more rostrally (propriospinal interneurons). In addition, connections from cortex to the reticular formation in the brainstem allow motor commands to be sent over the reticulospinal tract to these spinal centers. In this review, we consider the relative roles of these different routes for the control of hand function, both in healthy primates and after recovery from lesion.

Motor cortex electrical stimulation applied to patients with complex regional pain syndrome

Motor cortex stimulation (MCS) is useful to treat patients with neuropathic pain syndromes, unresponsive to medical treatment. Complex regional pain syndrome (CRPS) is a segmentary disease treated successfully by spinal cord stimulation (SCS). However, CRPS often affects large body segments difficult to cover by SCS. This study analyzed the MCS efficacy in patients with CRPS affecting them. Five patients with CRPS of different etiologies underwent a small craniotomy for unilateral 20-grid-contact implantation on MC, guided by craniometric landmarks. Neurophysiological and clinical tests were performed to identify the contacts position and the best analgesic responses to MCS. The grid was replaced by a definitive 4-contacts-electrode connected to an internalized system. Pain was evaluated by international scales. Changes in sympathetic symptoms, including temperature, perspiration, color and swelling were evaluated. Pre-operative and post-operative monthly evaluations were performed during one year. A double-blind maneuver was introduced assigning two groups. One had stimulators turned OFF from day 30-60 and the other from day 60-90. Four patients showed important decrease in pain, sensory and sympathetic changes during the therapeutic trial, while one patient did not have any improvement and was rejected for implantation. VAS and McGill pain scales diminished significantly (p<0.01) throughout the follow-up, accompanied by disappearance of the sensory (allodynea and hyperalgesia) and sympathetic signs. MCS is effective not only to treat pain, but also improve the sympathetic changes in CPRS. Mechanism of action is actually unclear, but seems to involve sensory input at the level of the spinal cord.

Corticospinal neurons respond differentially to neurotrophins and myelin-associated glycoprotein in vitro

Elucidating the mechanisms that regulate the survival and outgrowth of corticospinal tract (CST) neurons and other CNS tracts will be a key component in developing novel approaches for the treatment of central nervous system (CNS) disorders, including stroke, spinal cord injury (SCI), and motor neuron disease (MND). However, the in vivo complexities of these diseases make a systematic evaluation of potential therapeutics that directly affect corticospinal regeneration or survival very challenging. Here, we use Thy1.2 transgenic mice expressing yellow fluorescent protein (YFP) in postnatal day 8 (P8) corticospinal neurons, as a source of CST neurons that have already established synapses in the spinal cord, to assess factors that influence neurite outgrowth and survival of axotomized CST neurons. After culture, YFP-positive corticospinal neurons represent an enriched neuronal population over other glia and interneurons, survive, and extend processes over time. YFP-positive CST neurons also continue to express the corticospinal markers CTIP2 and Otx1. CST neurons display different degrees of axon extension, dendritic branch length and elaboration, and neurite elongation in response to neurotrophin-3 and ciliary neurotrophic factor, and an inhibitory outgrowth response when cultured on myelin-associated glycoprotein. Some CST neurons are lost with extended culture, which provides a baseline from which we can also assess factors that enhance CST neuron survival. This assay thus allows us to assess independent aspects of CST axonal and dendritic outgrowth kinetics, which allows for the rapid and sensitive investigation of new therapies to address corticospinal neuron outgrowth in the context of CNS injury and neurodegenerative disorders.

Electrical stimulation of the primary somatosensory cortex inhibits spinal dorsal horn neuron activity

Cortical stimulation has been demonstrated to alleviate certain pain conditions. The aim of this study was to determine the responses of the spinal cord dorsal horn neurons to stimulation of the primary somatosensory cortex (SSC). We hypothesized that direct stimulation of the SSC will inhibit the activity of spinal dorsal horn neurons by activating the descending inhibitory system. Thirty-four wide dynamic range spinal dorsal horn neurons were recorded in response to graded mechanical stimulation (brush, pressure, and pinch) at their respective receptive fields while a stepwise electrical stimulation (300 Hz, 0.1 ms, at 10, 20, and 30 V) was applied in the SSC through a bipolar tungsten electrode. The responses to brush at control, 10 V, 20 V, 30 V, and recovery were 16.0 +/- 2.3, 15.8 +/- 2.2, 14.6 +/- 1.8, 14.8 +/- 2.0, and 17.0 +/- 2.2 spikes/s, respectively. The responses to pressure at control, 10 V, 20 V, 30 V, and recovery were 44.7 +/- 5.5, 37.0 +/- 5.6, 29.5 +/- 4.8, 31.6 +/- 5.2, and 43.2 +/- 5.7 spikes/s, respectively. The responses to pinch at control, 10 V, 20 V, 30 V, and recovery were 58.1 +/- 7.0, 42.9 +/- 5.5, 34.8 +/- 3.9, 34.6 +/- 4.4, and 52.6 +/- 6.0 spikes/s, respectively. Significant decreases of the dorsal horn neuronal responses to pressure and pinch were observed during SSC stimulation. It is concluded that electrical stimulation of the SSC produces transient inhibition of the responses of spinal cord dorsal horn neurons to higher intensity mechanical stimuli without affecting innocuous stimuli.

Motor cortical modulation of cutaneous reflex responses in the hindlimb of the intact cat

We have used the technique of spatial facilitation to examine the interactions between the signals conveyed by the corticospinal tract and those of cutaneous afferents in the hindlimb of the intact, walking cat. Microstimulation was applied to 20 cortical sites in the hindlimb representation of the motor cortex and to three different cutaneous nerves innervating the hindpaw in four cats. Conditioning stimuli to the motor cortex induced both facilitation and depression of cutaneous reflexes evoked by stimulation of nerves in the hindlimb contralateral to the cortical stimulation site. Facilitation was most frequently evoked by conditioning stimuli in the range of 10-30 ms before the cutaneous stimulation; depression was normally evoked by shorter and longer conditioning delays. Similar changes were observed after conditioning stimuli to the pyramidal tract, suggesting that the changes were independent of any changes in cortical excitability. Modulation of reflex activity varied according to the muscle under study, the cutaneous nerve used to evoke the reflex and the cortical site used to condition the reflex. Together, these results suggest that there is spatial convergence of corticospinal and cutaneous afferent activity and that this convergence is mediated by distinct subpopulations of spinal interneurons.

Spinal dorsal horn neuron response to mechanical stimuli is decreased by electrical stimulation of the primary motor cortex

Motor cortex stimulation (MCS) has been used clinically as a tool for the control for central post-stroke pain and neuropathic facial pain. The underlying mechanisms involved in the antinociceptive effect of MCS are not clearly understood. We hypothesize that the antinociceptive effect is through the modulation of the spinal dorsal horn neuron activity. Thirty-two wide dynamic range spinal dorsal horn neurons were recorded, in response to graded mechanical stimulation (brush, pressure, and pinch) at their respective receptive fields, while a stepwise electrical stimulation was applied simultaneously in the motor cortex. The responses to brush at control, 10 V, 20 V, and 30 V, and recovery were 11.5+/-1.6, 12.1+/-2.6, 11.1+/-2.2, 10.5+/-2.1, and 13.2+/-2.5 spikes/s, respectively. The responses to pressure at control, 10 V, 20 V, and 30 V, and recovery were 33.2+/-6.1, 22.9+/-5.3, 20.5+/-5.0, 17.3+/-3.8, and 27.0+/-4.0 spikes/s, respectively. The responses to pinch at control, 10 V, 20 V, and 30 V, and recovery were 37.2+/-6.4, 26.3+/-4.7, 25.9+/-4.7, 22.5+/-4.3, and 35.0+/-6.2 spikes/s, respectively. It is concluded that, in the rat, electrical stimulation of the motor cortex produces significant transient inhibition of the responses of spinal cord dorsal horn neurons to higher intensity mechanical stimuli without affecting their response to an innocuous stimulus.

Sensory input to primate spinal cord is presynaptically inhibited during voluntary movement

During normal voluntary movements, re-afferent sensory input continuously converges on the spinal circuits that are activated by descending motor commands. This time-varying input must either be synergistically combined with the motor commands or be appropriately suppressed to minimize interference. The earliest suppression could be produced by presynaptic inhibition, which effectively reduces synaptic transmission at the initial synapse. Here we report evidence from awake, behaving monkeys that presynaptic inhibition decreases the ability of afferent impulses to affect postsynaptic neurons in a behaviorally dependent manner. Evidence indicates that cutaneous afferent input to spinal cord interneurons is inhibited presynaptically during active wrist movement, and this inhibition is effectively produced by descending commands. Our results further suggest that this presynaptic inhibition has appropriate functional consequences for movement generation and may underlie increases in perceptual thresholds during active movement.

Magnetic stimulation of motor and somatosensory cortices suppresses perception of ulnar nerve stimuli

Magnetic stimulation of sensorimotor cortex interferes with the detection of electro-cutaneous stimulation. However, it is uncertain whether this interference is due to activation of the somatosensory or the motor cortex. Here, transcranial magnetic stimuli (TMS) were delivered separately over somatosensory and motor cortex contralateral to the right ulnar nerve in 12 subjects. In separate trials, TMS were given 100 ms before and 20 ms after 60 ms trains of electro-cutaneous ulnar nerve stimuli, and their effect on the subjective perception of peripheral stimuli was assessed. TMS of both motor and somatosensory cortex interfered with the perception of afferent stimuli when given before or after stimulation of the ulnar nerve. Perception was more strongly suppressed by motor cortex stimulation than by somatosensory cortex stimulation, when given before or after the peripheral stimulus. A similar proportion of errors was induced by sensory cortex stimulation between the two stimulus timing intervals. This study suggests that the inhibition of the afferent volley is unlikely to be the result of antidromic activation of thalamocortical connections or corticospinal gating. A phenomenon akin to sensory masking is the most plausible explanation for much of the suppression of sensory perception by stimulation of the motor or somatosensory cortex. The more powerful suppressive effect of motor cortex stimulation may be due to multiple mechanisms.

Long-lasting sprouting and gene expression changes induced by the monoclonal antibody IN-1 in the adult spinal cord

Lesion-induced plasticity of the rat corticospinal tract (CST) decreases postnatally, simultaneously with myelin appearance. In adult rats, compensatory sprouting can be induced by the monoclonal antibody (mAb) IN-1 raised against the growth inhibitory protein Nogo-A. In this study, we examined separately the fate of sensory and motor corticospinal fibers after mAb IN-1 application. Intact adult rats treated with the IN-1 antibody exhibited an increase of aberrant CST projections, i.e., sensory fibers projecting into the ventral horn and motor fibers projecting dorsally. Unilateral lesion of the CST [pyramidotomy (PTX)] in the presence of mAb IN-1 triggered a progressive reorganization of the sprouting of the remaining CST across the midline, with sensory fibers projecting gradually into the denervated dorsal horn and motor fibers projecting into the denervated ventral horn. In unilaterally denervated spinal cords, aberrant sprouts were only transient and disappeared by 6 weeks, whereas midline crossing fibers ending in the appropriate target region were stabilized and persisted over the entire study period. Within the spinal cord, IN-1 antibody treatment was associated with upregulation of growth factors (BDNF, VEGF), growth-related proteins (actin, myosin, GAP-43), and transcription factors (STATs), whereas pyramidotomy induced an enhanced expression of guidance molecules (semaphorins and slits) as well as neurotrophic factors (BDNF, IGFs, BMPs). These gene expression changes may contribute to attraction, guidance, and stabilization of sprouting CST fibers.

Effects of somatosensory cortical stimulation on expression of c-Fos in rat medullary dorsal horn in response to formalin-induced noxious stimulation

We examined the effects of epidural electrical stimulation of primary (SI) and secondary (SII) somatosensory cortex on expression of c-Fos protein in rat medullary dorsal horn neurons (Vc; trigeminal nucleus caudalis) in response to formalin-induced noxious stimulation. Epidural electrical stimulation (single pulse, 0.2 msec duration at 10 Hz) was applied to the left facial region SI or SII at three different stimulus intensities, 0.1, 0.5, and 1.0 mA for 60 min 0 or 2 hr after bilateral injection of formalin into the lower lip. SII stimulation at 1.0 mA immediately after injection of formalin, significantly decreased the number of Fos-positive cells in the right VcI/II by 32.4%. There was no significant change in the number of Fos-positive cells in the VcIII/IV. SII stimulation at 0.5 and 1.0 mA 2 hr after injection of formalin, significantly decreased the number of Fos-positive cells in the right VcI/II by 47.9% and 40.8%, but significantly increased the number of Fos-positive cells in the right VcIII/IV by 178.8% and 324.3%, respectively. In contrast, SI stimulation had no effect on expression of c-Fos in Vc. Possible direct corticotrigeminal projections were labeled anterogradely by injection of WGA-HRP into the SI and SII. In the Vc, labeled terminals were distributed mostly in the contralateral medial half of VcIII/IV and medullary reticular nucleus dorsalis but rarely in VcI/II. These results suggest that activation of SII-medullary fibers suppress nociceptive information from the oro-facial regions.

Response properties of spinal interneurons in awake, behaving primates

This paper reviews studies on spinal interneurons in awake, behaving monkeys inspired by the work of Prof Patrick D. Wall. Early studies documented the sensory responses of spinal interneurons in unanesthetized monkeys to natural cutaneous and proprioceptive stimulation. More recently, cervical interneurons were documented in monkeys performing an active step-tracking task. During alternating wrist movements, most task-related interneurons showed bidirectional activity, firing during both flexion and extension (in surprising contrast to the unidirectional activity of muscles and corticomotoneuronal cells). Premotor interneurons were identified by post-spike effects in spike-triggered averages of forelimb muscle activity. The cells' post-spike effects were generally congruent with their activity in their preferred direction, although many fired during components of movement when their output effects would seem inappropriate. In an instructed delay period task many interneurons showed preparatory delay period activity, much like cortical neurons. Other studies tested the excitability of corticospinal axons to electrical stimulation and demonstrated both post-spike and task-related modulations in excitability. Together, these studies suggest that many behavioral functions of spinal interneurons remain to be revealed by recording their activity in awake, behaving animals.

Effects on muscle activity from microstimuli applied to somatosensory and motor cortex during voluntary movement in the monkey

It is well known that electrical stimulation of primary somatosensory cortex (SI) evokes movements that resemble those evoked from primary motor cortex. These findings have led to the concept that SI may possess motor capabilities paralleling those of motor cortex and speculation that SI could function as a robust relay mediating motor responses from central and peripheral inputs. The purpose of this study was to rigorously examine the motor output capabilities of SI areas with the use of the techniques of spike- and stimulus-triggered averaging of electromyographic (EMG) activity in awake monkeys. Unit recordings were obtained from primary motor cortex and SI areas 3a, 3b, 1, and 2 in three rhesus monkeys. Spike-triggered averaging was used to assess the output linkage between individual cells and motoneurons of the recorded muscles. Cells in motor cortex producing postspike facilitation (PSpF) in spike-triggered averages of rectified EMG activity were designated corticomotoneuronal (CM) cells. Motor output efficacy was also assessed by applying stimuli through the microelectrode and computing stimulus-triggered averages of rectified EMG activity. One hundred seventy-one sites in motor cortex and 68 sites in SI were characterized functionally and tested for motor output effects on muscle activity. The incidence, character, and magnitude of motor output effects from SI areas were in sharp contrast to effects from CM cell sites in primary motor cortex. Of 68 SI cells tested with spike-triggered averaging, only one area 3a cell produced significant PSpF in spike-triggered averages of EMG activity. In comparison, 20 of 171 (12%) motor cortex cells tested produced significant postspike effects. Single-pulse intracortical microstimulation produced effects at all CM cell sites in motor cortex but at only 14% of SI sites. The large fraction of SI effects that was inhibitory represented yet another marked difference between CM cell sites in motor cortex and SI sites (25% vs 93%). The fact that motor output effects from SI were frequently absent or very weak and predominantly inhibitory emphasizes the differing motor capabilities of SI compared with primary motor cortex.

Primary motor cortex influences on the descending and ascending systems

The motor cortex plays a crucial role in the co-ordination of movement and posture. This is possible because the pyramidal tract fibres have access both directly and through collateral branches to structures governing eye, head, neck trunk and limb musculature. Pyramidal tract axons also directly reach the dorsal laminae of the spinal cord and the dorsal column nuclei, thus aiding in the selection of the sensory ascendant transmission. No other neurones in the brain besides pyramidal tract cells have such a wide access to different structures within the central nervous system. The majority of the pyramidal tract fibres that originate in the motor cortex and that send collateral branches to multiple supraspinal structures do not reach the spinal cord. Also, the great majority of the corticospinal neurones that emit multiple intracraneal collateral branches terminate at the cervical spinal cord level. The pyramidal tract fibres directed to the dorsal column nuclei that send collateral branches to supraspinal structures also show a clear tendency to terminate at supraspinal and cervical cord levels. These facts suggest that a substantial co-ordination between descending and ascending pathways might be produced by the same motor cortex axons at both supraspinal and cervical spinal cord sites. This may imply that the motor cortex co-ordination will be mostly directed to motor responses involving eye-neck-forelimb muscle synergies. The review makes special emphasis in the available evidence pointing to the role of the motor cortex in co-ordinating the activities of both descending and ascending pathways related to somatomotor integration and control. The motor cortex may function to co-operatively select a unique motor command by selectively filter sensory information and by co-ordinating the activities of the descending systems related to the control of distal and proximal muscles.

Pyramidal tract and corticospinal neurons with branching axons to the dorsal column nuclei of the cat

Extracellular single activity was recorded from pericruciate neurons in anaesthetized, paralysed, artificially ventilated cats. A total of 309 neurons were identified antidromically by stimulation of the dorsal column nuclei (229 from the nuneate nucleus and 80 from the gracile nucleus). The study addressed the question whether pericruciate-dorsal column nuclei neurons (corticonuclear cells) sent collaterals to the ipsilateral red nucleus and/or to the contralateral nucleus reticularis gigantocellularis. Also, the ipsilateral pyramidal tract was stimulated at mid-olivary level, as was the crossed corticospinal tract at C2, Th2 and L2 levels in order to know whether the corticonuclear cells sent their axons to the spinal cord and if so to which level. It was found that more than 95% of the corticonuclear fibres coursed through the pyramidal tract. A significant (28.4%; 88/309) proportion of the the corticonuclear neurons sent collaterals to the red nucleus and/or to the nucleus reticularis gigantocellularis. About 68% (209/309) of the corticonuclear cells did not send their axons to the spinal cord and the remainder were corticospinal neurons. Most of the corticospinal fibres terminated at the cervical level (72/100) and the remaining ended at thoracic (18/100) and lumbar (10/100) segments of the cord. While 63.4% (123/194) of the corticonuclear fibres coursing through the pyramidal tract and ending at supraspinal levels were slow conducting, the great majority of the corticospinal neurons were fast conducting (91/100). The non-corticospinal neurons were significantly slower conducting than the corticospinal cells. The corticogracile neurons were slower conducting than the corticocuneate cells. Of the 88 corticonuclear neurons that sent at least a branch to the sites tested, 50% branched into the red nucleus, 35.2% into the nucleus reticularis gigantocellularis and 14.7% into both nuclei, without significant difference between non-corticospinal and corticospinal cells. Most of the main axons of the corticonuclear cells ended at bulbar and cervical levels (281/309 or 90.9%). The data indicate that pericruciate-dorsal column nuclei neurons form a particular substrate within pyramidal tract cells. They can serve precise functions in motor coordination associated with the selection of their own sensory input. The results are discussed from this point of view.

Effect of manipulation and fractionated finger movements on subcortical sensory activity in man

Previous studies have shown that the somatosensory evoked potentials (SEPs) recorded from the scalp are modified or gated during motor activity in man. Animal studies show corticospinal tract terminals in afferent relays, viz. dorsal horn of spinal cord, dorsal column nuclei and thalamus. Is the attenuation of the SEP during movement the result of gating in subcortical nuclei? This study has investigated the effect of manipulation and fractionated finger movements of the hand on the subcortically generated short latency SEPs in 9 healthy subjects. Left median nerve SEPs were recorded with electrodes optimally placed to record subcortical activity with the least degree of contamination. There was no statistically significant change in amplitude or latency of the P9, N11, N13, P14, N18 and N20 potentials during rest or voluntary movement of the fingers of the left hand or manipulation of objects placed in the hand. The shape of the N13 wave form was not modified during these 3 conditions. It is concluded that in man attenuation of cortical waves during manipulation is not due to an effect of gating in the subcortical sensory relay nuclei.

Reversible tetracaine block of rat periaqueductal gray (PAG) decreases baseline tail-flick latency and prevents analgesic effects of met-enkephalin injections in nucleus paragigantocellularis (PGC)

One micrograms of tetracaine in the rat periaqueductal gray (PAG) produced a decline in baseline tail-flick latencies (hyperalgesia) from about 5 to 3.5 s over the course of 9 min, after which the latencies increased to about 4.5 s. One micrograms of Met-enkephalin in PGC caused an expected increase in latencies (analgesia) from about 4.25 to 6.2 s in 9 min, with recovery to 4.7 s after 15 min post-injection. Giving the preceding 2 nanoinjections simultaneously led to an essentially total block of the PGC analgesia. A control injection in PAG simultaneous with a Met-enkephalin injection in PGC did not block the latter's analgesic effect. Single control (artificial cerebrospinal fluid) injections in PAG or PGC were without effect. The hyperalgesic effect of PAG tetracaine supports the involvement of PAG in normally occurring, tonic descending pain inhibition. The block of PGC met-enkephalin analgesia by distant injection of tetracaine in PAG supports the necessity of PAG integrity for PGC analgesic function.

Effects of infant versus adult pyramidal tract lesions on locomotor behavior in hamsters

The role of the pyramidal tract in locomotion was studied in hamsters by analyzing their locomotor behavior after lesions of the medullary pyramidal tract. Animals with lesions either as adults or as infants were compared to determine whether early pyramidotomy results in greater functional recovery. Normal and pyramidotomized animals were filmed during locomotion on a runway consisting of either smooth or rough terrain to assess whether the uneven surface would accentuate locomotor deficits. Frame-by-frame analysis of the filmed behavior during all phases of the step cycle was carried out to determine positions of the joints of the forelimb and hindlimb during locomotion. Accuracy of limb placement on the rough terrain was determined by observations of consecutive step cycles. The results show that lesions of the pyramidal tract in both infant and adult hamsters affect locomotion first by causing a reduction in the yielding phase of the step cycle and second by producing inaccuracies of forelimb placement. Rough terrain accentuates deficits in forelimb placement during locomotion. Animals with lesions as infants and those with lesions as adults show surprisingly similar deficits in locomotion, with the exception that animals with lesions as infants show some behavioral compensation in hindlimb movement by developing a normal degree of yielding at the knee. In contrast, hamsters with lesions as either adults or infants never recover normal forelimb behavior in either yielding at the elbow or accuracy of forelimb placement. These results emphasize the sensorimotor role of the pyramidal tract, even in a relatively stereotyped behavior such as locomotion.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of intracerebroventricular injection of naloxone, phentolamine and methysergide on the transmission of nociceptive signals in rat dorsal horn neurons with convergent cutaneous-deep input

In anaesthetized rats, recordings were made from nociceptive dorsal horn neurons with convergent input from the skin and deep somatic tissues. The results of a previous study have shown that in these neurons the input from deep nociceptors is subjected to a much stronger tonic descending inhibition than is the input from cutaneous nociceptors. The aim of the present study was to find out whether at supraspinal levels opioidergic, adrenergic, or serotoninergic transmitters are involved in this quite specific inhibition of deep nociception. Injections of naloxone, phentolamine, and methysergide into the third ventricle showed that only naloxone is capable of abolishing the tonic inhibition of the deep nociceptive input to spinal neurons. The input from cutaneous nociceptors to the same cells was largely unaffected by naloxone. Thus the effects of intracerebroventricular injection of naloxone resembled those obtained with a spinal cold block in a previous study; with the exception that the increase in background activity--which is prominent during cold block--was missing after the injection of naloxone. The present results demonstrate that the tonic descending inhibition of the deep nociception operates with opioidergic synapses at the supraspinal level. In contrast, supraspinal adrenergic and serotoninergic mechanisms do not appear to contribute to the tonic inhibition. The data confirm and extend previous results which suggested that a particular portion of the descending antinociceptive system may act mainly on the input from deep nociceptors. Pharmacologically, this particular portion seems to be opioidergic in nature.

Branching patterns of corticospinal axon arbors in the rodent

Despite extensive study of corticospinal connections in a variety of species, little is known about the detailed morphology of corticospinal axon arbors. Results in previous studies of primates based on intra-axonal filling with horseradish peroxidase (HRP) staining of a limited sample of fibers suggest that corticospinal arbors branch widely to multiple motoneuronal pools. To determine whether this pattern of corticospinal connectivity is present in nonprimate species as well, we studied the branching patterns of corticospinal axon arbors in a rodent species, the golden hamster. The axons were labeled by iontophoretic injection of Phaseolus vulgaris-leucoagglutinin (PHA-L) into small regions of the forelimb and hindlimb sensorimotor cortex, and immunohistochemistry with the peroxidase-antiperoxidase (PAP) method was used to reveal fine details of terminal arbors within the cervical and lumbar enlargements of the spinal cord. As in higher mammals, corticospinal connections are topographically organized. Moreover, corticospinal axons arising from somatosensory cortex project primarily to the dorsal horn, whereas those from motor cortex terminate most heavily in the ventral horn. This differential projection pattern, not previously demonstrated in rodents, implies functional differences between somatosensory and motor components of the corticospinal pathway. Reconstruction of corticospinal arbors in the ventral horn showed that in both cervical and lumbar spinal cord segments, axons branch widely into interneuronal regions. A surprising number appear to extend into motoneuron cell groups, and some of these axons branch into multiple motoneuronal pools. Widely divergent corticospinal axons that branch to multiple motoneuron pools have been shown to mediate activity in functionally related muscle groups of the primate forearm. The present results suggest that in other species, such as the rodent, a similar divergence of corticospinal arbors may also function to facilitate activity in subsets of muscles.

Modulation of spinal reflexes by pyramidal tract stimulation in an in vitro brainstem-spinal cord preparation from the hamster

Electrophysiological evidence is presented showing that the pyramidal tract (PT) of the hamster modulates spinal reflexes in an in vitro brainstem-spinal cord preparation. Three spinal reflexes were studied. Stimulation of a dorsal root (DR) while recording from a ventral root (VR) of the same spinal segment evoked two reflexes: the monosynaptic reflex, and a long latency polysynaptic reflex. Stimulation of a DR while recording from a DR immediately rostral to it elicited a volley of antidromic discharges characteristic of the dorsal root reflex (DRR). The effect of PT stimulation on reflex transmission was tested by stimulating the PT at varying intervals prior to evoking a reflex. The results show that the amplitude of the monosynaptic reflex is progressively inhibited when preceded at shorter delays by a train of PT stimuli. Similarly, PT stimulation also suppresses the long latency reflex. In contrast, the PT facilities the DRR and repeated stimulation of the PT may evoke antidromic discharges recorded from the DRs. These data from the in vitro brainstem-spinal cord preparation indicate that the PT of the hamster exerts both inhibitory and facilitatory effects on reflex transmission in the spinal cord. The present study shows that it is possible to examine the descending control of spinal circuitry using an in vitro brainstem-spinal cord preparation.

Activity of digital area neurons of the primary somatosensory cortex in relation to sensorially triggered and self-initiated digital movements of monkeys

Single-cell activity was examined in digital areas of the primary somatosensory cortex (SI) of monkeys performing sensorially triggered and self-initiated digital movements with the aim of rigorously determining the relative timing of onset of the neuronal activity with respect to movement onset. The activity of prime mover muscles for execution of a key-press movement was recorded simultaneously with the neuronal activity; movement onset was defined as the onset of muscle activity. Neuronal receptive fields were also identified. The following findings emerged from this study: (1) Few neurons, if any, in the SI(areas 3b, 1, 2), including pyramidal tract neurons, were active prior to movement onset. (2) The movement-related activity of SI neurons was basically similar in cases of signal-triggered and self-initiated movement. (3) No neuron in the SI showed activity associated with ipsilateral digital movement. (4) A majority of movement-related neurons in the precentral motor cortex, in contrast, started their activity before movement onset. These findings suggest that SI neuronal activity participates little in providing information necessary for developing motor responses in the initial phase of simple digital movements.

Inhibition of spontaneous activity of spinal dorsal horn neurons in the intact cat is naloxone-insensitive

Low-threshold neurons in the dorsal horn of the spinal cord of physiologically intact, awake, drug-free cats demonstrate minimal rates of spontaneous activity. Studies in acute animals suggest that the lack of spontaneous activity is due to descending inhibitory control mechanisms. The present study suggests that the inhibition of spontaneous activity is naloxone-insensitive. Intravenously administered naloxone in doses of up to 0.4 mg/kg failed to have any effect on the spontaneous activity of the 34 low-threshold neurons recorded from the dorsal horn of the spinal cord of physiologically intact, awake animals. Stimulus-evoked activity was also not significantly influenced by the doses of naloxone used in this study. These results confirm data previously obtained by others using acute preparations. This confirmation is important in that it demonstrates that, for this system, differences between acute and chronic preparations do not appear to alter neuronal sensitivity to specific drug manipulations. They also verify that, for this system, acute experiments accurately reflect conditions that exist in the intact animal.

Cortical involvement in dorsal horn cell hyperactivity and abnormal behavior in rats with dorsal root section

Experiments were performed on rats using neurophysiological and behavioral techniques, in an attempt to study the role played by the somatosensory cortex in the abnormal spinal neuron activity and abnormal behavior observed after brachial plexus lesions. The onsets of both phenomena occur at the same postoperative period. Cortical controls exerted on spinal dorsal horn (DH) cells were studied using a transient and reversible cortical blockade, cortical spreading depression (CSD), applied contralateral to the spinal cord recording. In 28 intact animals, 55 cells were studied during the propagation of at least two CSDs. Only 4 of these cells presented a sustained decrease in their spontaneous activity during CSD, which may correspond to transient arrest of a descending tonic cortical facilitation. In 29 animals with dorsal root sections, 161 DH cells displayed abnormal burst activity, and 52 were examined with the CSD test. Thirty-five cells presented a long-duration change in their spontaneous activity during CSD; of these, 28 showed decreased activity (suppression of descending tonic facilitation) and 7 presented increased activity (suppression of descending tonic inhibition). More DH cells were influenced by the cortex in deafferented rats (67%) than in intact rats (7%). The cortical influence was also stronger, as the hyperactive cells were frequently rendered silent during CSD. These observations suggest that the abnormal activity is partly due to a descending cortical influence. Results of a behavioral study performed on 22 rats (one control group and two experimental groups with cortical ablations) showed that the self-mutilating behavior, which develops at the same time as the abnormal DH cell activity, was reduced by unilateral cortical ablation, independent of the cortical region removed. The possible pathways involved in this cortical influence are examined in the discussion.

Sensorimotor cortical projections to the primate cuneate nucleus

The organization of the corticocuneate pathway was investigated in monkeys by using the anterograde and retrograde axonal transport of either horseradish peroxidase (HRP) or wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Injection of either tracer into the precentral cortex (centered on area 4) results in heavy anterograde labeling in the tegmental region, which lies immediately ventrolateral to the cuneate nucleus, particularly at levels caudal to the obex. On the other hand, injections of the same tracers involving areas 3b, 1, and 2 cause anterograde labeling mainly within the core (pars rotunda of Ferraro and Barrera, '35, Arch. Neurol. Psychol. 33:262-75) of the cuneate nucleus. Anterograde labeling is also evident in the rostral parts of the cuneate nucleus, especially after injections involving areas 1 and 2. Injections restricted largely to area 3b cause anterograde labeling preferentially in the core of the cuneate nucleus. After injection of HRP or WGA-HRP into the dorsal medulla, retrogradely labeled neurons are present both in the pre- and postcentral gyrus, but their location depends upon the sites and extent of the injection site. When the tracer diffuses into the underlying tegmental area, many retrogradely labeled neurons appear in the precentral motor cortex, principally in area 4 although some of them also occur in area 6. With smaller injections, largely restricted within the cuneate nucleus, most labeled neurons are present in the postcentral gyrus, with the largest population in areas 1 and 2; a smaller number of small neurons in area 3b are best demonstrated with WGA-HRP; and area 3a contains the smallest complement of retrogradely labeled neurons. The data from these studies suggest a segregation of pre- and postcentral afferents in the ventral tegmental region and the cuneate nucleus, respectively. These findings pertaining to the corticocuneate projection in the monkey are discussed in relation to the parallelism between monkeys and cats possible physiological implications of the anatomical organization described, and conflicting evidence in the neurophysiological observations obtained, by earlier investigators, by antidromic and orthodromic activation of this pathway.

Brainstem influences on transmission of somatosensory information in the spinocervicothalamic pathway

The inhibition of somatosensory responses of lateral cervical nucleus neurons resulting from stimulation of the brainstem has been investigated. Single unit extracellular recordings were obtained from neurons in the lateral cervical nucleus of chloralose-anesthetized cats. Electrical stimulation of the periaqueductal gray, nucleus raphe magnus, nucleus cuneiformis, and nuclei reticularis gigantocellularis and magnocellularis was found to be very effective in inhibiting the responses of lateral cervical nucleus neurons evoked by electrical or tactile stimulation of the skin. Additional experiments were performed to determine whether the inhibitory effects were mediated in the spinal cord dorsal horn or in the lateral cervical nucleus. These experiments which examined the effect of brainstem stimulation on the responses induced by stimulation of the dorsolateral funiculus or on the antidromic latency of activation of lateral cervical nucleus neurons from thalamus, revealed that most and possibly all the inhibition could be accounted for by an action on the spinal cord. These results are consistent with other studies showing that spinocervical tract cells in the spinal cord can be inhibited by stimulation of the same brainstem regions.

Descending projections to the cervical spinal cord in the developing kitten

The distribution of neurons filled by retrograde axonal transport of horseradish peroxidase from the cervical enlargement is described in kittens prior to and following the time of appearance of mature alpha-motoneuron responses to motor cortical stimulation (at 107-111 days gestational age; about 41 days postnatally). Cortex and brainstem reconstructions of the distributions of filled neurons demonstrate a well-defined, discrete projection from cortical area 4 to spinal cord segments C3 to C8, both in mature and immature (20 and 24 days postnatal) animals. In addition, appropriate rubrospinal, reticulospinal and vestibulospinal projections were present at all ages studied.

Bilateral influences from the motor cortex on lumbar ventral horn interneurons

Microelectrode recordings were made of discharges of ventral horn interneurons (VHIN) in segments L6-7 of the spinal cord during bilateral stimulation of the motor cortex in cats. The overwhelming majority of VHIN was shown to be activated by influences from the contralateral cortex, and about half of them also by ipsilateral influences. Clear correlation was established between convergence of afferent and cortical influences: Neurons with inputs from ipsilateral afferents only were activated, irrespective of their other characteristics, by descending influences from the contralateral cortex only, whereas bilateral cortical influences converged on cells with bilateral afferent connections. It is suggested that VHIN with bilateral segmental and supraspinal connections are important integrative elements in the mechanism of bilateral coordination of motor responses of different degrees of complexity.

Increased receptive field size of dorsal horn neurons following chronic spinal cord hemisections in cats

The somatotopic organization of the 17 dorsal horn was studied using extra-cellular recordings in normal cats, and in cats with acute or chronic spinal cord hemisection at T13, sparing the dorsal columns. Based on data concerning recovery of function and collateral sprouting of afferents following hemisections, we predicted that the lesion would result in increases in receptive field size and decreases in the specificity of the somatotopic map. In normal animals, the usual mediolateral, rostrocaudal and dorsoventral somatotopic sequences were found. Following acute hemisections (6 h-5 days), there were changes in spontaneous and evoked activity, but receptive field sizes and somatotopic organization remained unchanged. Following chronic hemisections (88-174 days), proximal hindlimb receptive fields in the lateral dorsal horn ipsilateral to the lesion increased dramatically in size and were significantly larger than similar receptive fields on the contralateral side. The largest of these fields extended from the dorsal midline to the middle of the foot. Receptive field sizes elsewhere in the dorsal horn remained unchanged, as did somatotopic organization in general. These findings indicate that hemisections result in a complex series of changes consisting of an early stage of anatomically generalized changes in excitability and a later stage of highly localized changes in receptive field size. Possible mechanisms for these changes, as well as their relationship to recovery of function, are discussed.

Effect of peripheral nerve injury on receptive fields of cells in the cat spinal cord

When the sciatic and saphenous nerves are cut and ligated in adult cats, the immediate effect is the production of a completely anesthetic foot and a region in medial lumbar dorsal horn where almost all cells have lost their natural receptive fields (RFs). Beginning at about 1 week and maturing by 4 weeks, some 40% of cells in the medial dorsal horn gain a novel RF on proximal skin, that is, upper and lower leg, thigh, lower back, or perineum. This new RF is supplied by intact proximal nerves and not by sciatic and saphenous nerve fibers that sprouted in the periphery. During the period of switching of RFs from distal to proximal skin there was no gross atrophy of dorsal horn grey matter and no Fink-Heimer stainable degeneration of central arbors and terminals of peripherally axotomized afferents. In intact animals medial dorsal horn cells showed no sign of response to mechanical stimulation of proximal skin. RFs of some of the cells had spontaneous variations in size and sensitivity, but these were not nearly sufficient to explain the large shifts observed after chronic nerve section. Tetanic electrical stimulation of skin or peripheral nerves often caused RFs to shrink, but never to expand. Although natural stimuli of proximal skin would not excite medial dorsal horn cells in intact or acutely deafferented animals, it was found that electrical stimulation of proximal nerves did excite many of these cells, often at short latencies. In the discussion we justify our working hypothesis that the appearance of novel RFs is due to the strengthening or unmasking of normally present but ineffective afferent terminals, rather than to long-distance sprouting of new afferent arbors within the spinal cord.

Organization of corticospinal neurons in the monkey

The retrograde axonal transport method has been employed to identify the cell bodies of cortical neurons projecting directly to the spinal cord in the monkey. The investigation has focused on aspects of the laminar, columnar, and somatotopic organization of corticospinal neurons within each of the cytoarchitectural and functional subdivisions of the sensorimotor cortex. The principle findings of these experiments are that: i) cortical regions containing cell bodies of corticospinal neurons are the first motor cortex (area 4), the first somatic sensory cortex (areas 3a, 3b, 1, and 2), and part of the immediately adjacent posterior parietal cortex (area 5), the second somatic sensory cortex, the supplementary motor cortex (the medial aspect of area 6), and the medial part of the posterior parietal cortex in a region termed the supplementary sensory area; ii) corticospinal neurons display a somatotopic organization within each of these functional subdivisions of the sensorimotor cortex; iii) all corticospinal neurons arise from layer V of the cortex; and iv) corticospinal neurons within the first motor and first somatic sensory cortex often occur in clusters, perhaps reflecting a columnar organization in the sensorimotor cortex. These findings demonstrate the origins of the corticospinal system to be more extensive than previously recognized and show that a number of common features characterize the organization of corticospinal neurons in all cortical areas. Across cortical subdivisions, however, major differences exist in the extent of spinal segmental representations, in the manner in which corticospinal neurons occur in groups, and in the numerical density and sizes of corticospinal neurons. These aspects of the organization of the corticospinal system presumably reflect specialization of the different cortical areas in spinal cord sensory and motor control.

Descending influences on receptive fields and activity of single units recorded in laminae 1,2 and 3 of cat spinal cord

Units (108) were isolated in laminae 1,2 and 3 in segments L7-Sl of decerebrate cat spinal cord. For each unit, the size and nature of its receptive field (RF) was delineated. Then the dorsolateral funiculus (DLF) was stimulated for 1 sec with 10 or 50 Hz, 0.1 msec square waves and the response characteristics of the unit were again examined. Of the 108 units, 55 were excited or facilitated, 6 were inhibited (all in lamina 3) and 47 were unaffected. While some of the excited units responded only during the stimulus train, the majority showed prolonged excitation or facilitation lasting over one minute. The excited units were predominantly those responding to pressure or to brush, touch and pressure. Of the pressure units, 73% were excited or facilitated in contrast to only 29% of the brush/touch units. Most of the excited units showed expansion of their RFs. While many units of this type show ongoing variations of excitability and RF size, the evoked responses reported were sufficiently time-locked to the stimulus for it to be apparent that they were caused by the DLF stimulation. The unit's responses still occurred when the DLF was stimulated caudal to a complete cord transection so that the effects did not pass through the brain stem. The major effect of descending systems or of DLF stimulation previously reported on the large cells of laminae 1,4 and 5 has been inhibition. Here we report that a major descending influence on many units of laminae 1,2 and 3 is excitatory. Therefore it is suggested that a population of small interneurons in the superficial laminae could contribute to the descending inhibition of large dorsal horn neurons.

Origin of the pyramidal tract determined with horseradish peroxidase

The origin of the axons contained in the pyramidal tract (PT) of the cat was established using retrograde transport of horseradish peroxidase (HRP). A complete section was made through a PT at the level of the medulla oblongata and HRP was applied to the sectioned axons. Cat brains were cut in frontal and sagittal planes and HRP-labeled cells were plotted in outlines of the brain sections. The entire cortical region containing PT cells was divided into 8 subregions and the percent of PT cells was determined in each. Surface cortex, subregions 1, 3 and 8, contained only 30--40% of PT cells; the majority resided in deep sulcal cortex, in subregions 2, 4, 5, 6 and 7. Subregion 1 (containing 6--12% of PT cells) extends rostral to the cruciate sulcus; subregion 3 (15--22%) extends from the cruciate sulcus caudally to the ansate sulcus; subregion 8 (7--8%) covers cortex laterally adjacent to subregion 3. The hidden banks of the cruciate sulcus contained the greatest concentration of PT cells, 28--34% in the dorsal bank (subregion 5) and 15--20% in the ventral bank (subregion 4). The coronal sulcus contained only 2--5% of PT cells in its dorsal bank (subregion 6) and 1--4% in its ventral bank (subregion 7). The presylvian sulcus contained 8--12% of all PT cells in its lateral bank (subregion 2). This new cortical area is not yet considered part of 'PT cortex'. Qualitative limitations of this study are discussed.