Significance of peripheral feedback in the generation of stepping movements during epidural stimulation of the spinal cord

Acute experiments on decerebrate and spinal cats were performed to study the role of the peripheral afferent input from hindlimb receptors in forming the locomotor pattern during epidural stimulation of the spinal cord. Evoked electromyographic activity in the muscles of the hindlimbs was analyzed, along with the kinematic parameters of stepping movements. Epidural stimulation (20–100 μA, 5 Hz) of segments L4–5 of the spinal cord was found to elicit well coordinated walking in the hindlimbs on a moving treadmill band. When the support conditions were changed (non-moving treadmill, unsupported position), epidural stimulation initiated walking with an unstable rhythm. This was associated with a change in the overall nature of the locomotor pattern and the internal structure of the stepping cycle. Alteration of the direction of movement of the treadmill band led to the appearance of backward walking. An increase in the speed of movement of the treadmill band increased the stepping frequency, mainly due to decreases in the extensor phase. Epidural stimulation applied 2–4 h after complete transection of the spinal cord at the T8–T9 level could elicit stepping movements, but only when the treadmill was moving. The role of peripheral feedback in generating the locomotor pattern in conditions of complete disconnection from supraspinal control increased significantly. These data show that peripheral feedback during epidural stimulation of the spinal cord can define the properties of the motor output. __________ Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 91, No. 12, pp. 1407–1420, December, 2005.     

Initiation of Locomotor Activity in Spinal Cats by Epidural Stimulation of the Spinal Cord

Acute and chronic experiments on lower spinal (T10–T12) cats were performed to investigate the effects of epidural stimulation of the dorsal surface of the spinal cord on the initiation of locomotor activity. A zone located at the border between segments L4 and L5 was identified, stimulation of which induces locomotor activity. The parameters of epidural stimulation of the spinal cord effective in activating the stepping movement generator were identified. Epidural stimulation leading to the initiation of movement activity was shown to depend on intracentral and peripheral mechanisms activating the segmental, intersegmental and propriospinal reflex systems of the spinal cord. A leading role was demonstrated for the propriospinal system of the dorsolateral funiculi in activating the generators of stepping movements in epidural stimulation of the spinal cord.     

The role of cutaneous afferents in controlling locomotion evoked by epidural stimulation of the spinal cord in decerebrate cats

The effects of the cutaneous input on the formation of the locomotor pattern in conditions of epidural stimulation of the spinal cord in decerebrate cats were studied. Locomotor activity was induced by rhythmic stimulation of the dorsal surface of spinal cord segments L4-L5 at a frequency of 3-5 Hz. Electromyograms (EMG) recorded from the antagonist muscles quadriceps, semitendinosus, tibialis anterior, and gastrocnemius lateralis were recorded, along with the kinematics of stepping movements during locomotion on a moving treadmill and reflex responses to single stimuli. Changes in the pattern of reactions observed before and after exclusion of cutaneous receptors (infiltration of lidocaine solution at the base of the paw or irrigation of the paw pads with chlorothane solution) were assessed. This treatment led to impairment of the locomotor cycle: the paw was placed with the rear surface downward and was dragged along in the swing phase, and the duration of the stance phase decreased. Exclusion of cutaneous afferents suppressed the polysynaptic activity of the extensor muscles and the distal flexor muscle of the ipsilateral hindlimb during locomotion evoked by epidural stimulation of the spinal cord. The effects of exclusion of cutaneous afferents on the monosynaptic component of the EMG response were insignificant.     

Dose dependence of the 5-HT agonist quipazine in facilitating spinal stepping in the rat with epidural stimulation

Epidural electrical stimulation (ES) at spinal cord segment L2 can produce coordinated step-like movements in completely spinalized adult rats [R.M. Ichiyama, Y.P. Gerasimenko, H. Zhong, R.R. Roy, V.R. Edgerton, Hindlimb stepping movements in complete spinal rats induced by epidural spinal cord stimulation, Neurosci. Lett. 383 (2005) 339-344]. Plantar placement of the paws, however, was rarely observed. Here, we sought to determine the dose dependence of a 5-HT agonist (quipazine) on stepping kinematics when administered in combination with ES. Six adult female Sprague-Dawley rats received a complete mid-thoracic spinal cord transection and were implanted with epidural electrodes at the L2 spinal cord level. Quipazine (i.p.) was tested at doses of 0.1, 0.2, 0.3, 0.4, and 0.5 mg/kg. Rats were placed in a body weight support system, allowing them to walk bipedally on a moving treadmill belt (7 cm/s). 3D step kinematics analysis revealed that coordinated alternating bilateral stepping was induced by L2 stimulation (50 Hz) alone and by quipazine alone. Furthermore, the combination treatment produced significantly greater numbers of plantar steps and improved quality of stepping compared to either intervention alone. Both number and quality of stepping peaked at the intermediate dose of 0.3-0.4 mg/kg. The results indicate that quipazine and ES can have complementary effects on spinal circuits and that quipazine dosage is an important factor in differentially modulating these circuitries to improve the quality of the bipedal stepping on a treadmill belt.     

Effects of Spinal Cord Electrical Stimulation in Patients with Vertebrospinal Pathology

Until now, no scientific neurophysiologic methods of diagnostics and treatment of vertebrospinal pathologies were developed. Previous study showed that electrical stimulation of lumbar segments of the spinal cord in animals with complete spinal cord transection induced a well-coordinated weight-bearing locomotion. The present comparative study of motor activity triggered by electrical epidural stimulation of one or two segments of the spinal cord in spinal patients showed that stimulation of lumbar (L2-L4) or sacral (S2) segments facilitated generation of motor patterns of muscle activity. Combination of electrical stimulation with locomotor training resulted in the appearance of stepping patterns characteristic of normal walking and tonic activity of the muscles needed for body balance maintenance.     

Formation of locomotor patterns in decerebrate cats in conditions of epidural stimulation of the spinal cord

Acute experiments on decerebrate cats were performed to study the mechanism of formation of the locomotor pattern in conditions of epidural stimulation of the spinal cord. These studies showed that only segments L3–L5 contributed to generating the stepping pattern in the hindlimbs. At the optimum frequency (5–10 Hz) of stimulation of these segments, formation of electromyographic burst activity in the flexor muscles was mainly due to polysynaptic reflex responses with latencies of 80–110 msec. In the extensor muscles, this process involved the interaction of a monosynaptic reflex and polysynaptic activity. In epidural stimulation, the stepping pattern was specified by spinal structures, while peripheral feedback had modulatory influences.Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 89, No. 9, pp. 1046–1057, September, 2003.     

Plasticity of spinal cord reflexes after a complete transection in adult rats: relationship to stepping ability

Changes in epidurally induced (S1) spinal cord reflexes were studied as a function of the level of restoration of stepping ability after spinal cord transection (ST). Three types of responses were observed. The early response (ER) had a latency of 2.5 to 3 ms and resulted from direct stimulation of motor fibers or motoneurons. The middle response (MR) had a latency of 5 to 7 ms and was monosynaptic. The late response (LR) had a latency of 9 to 11 ms and was polysynaptic. After a complete midthoracic ST, the LR was abolished, whereas the MR was facilitated and progressively increased. The LR reappeared about 3 wk after ST and increased during the following weeks. Restoration of stepping induced by epidural stimulation at 40 Hz coincided with changes in the LR. During the first 2 wk post-ST, rats were unable to step and electrophysiological assessment failed to show any LR. Three weeks post-ST, epidural stimulation resulted in a few steps and these coincided with reappearance of the LR. The ability of rats to step progressively improved from wk 3 to wk 6 post-ST. There was a continuously improved modulation of rhythmic EMG bursts that was correlated with restoration of the LR. These results suggest that restoration of polysynaptic spinal cord reflexes after complete ST coincides with restoration of stepping function when facilitated by epidural stimulation. Combined, these findings support the view that restoration of polysynaptic spinal cord reflexes induced epidurally may provide a measure of functional restoration of spinal cord locomotor networks after ST.     

Initiation of Locomotion in Decerebrate Cats by Pulsed Magnetic Fields Projected onto Spinal Cord Segments

The motor effects induced by pulsed magnetic fields (PMF) projected onto the lumbar and cervical spinal cord were studied in decerebrate cats. A magnetic coil (inductor) of diameter 8 cm was positioned 1–2 cm above the surface of the spinal cord. Stimulation of the spinal cord with PMF was performed in two regimes: with single impulses with an intensity of 0.5–1 T and with continuous rhythmic stimulation at a frequency of 1 Hz and an intensity of 0.5 T. Application of single stimuli to the lumbar enlargement evoked reflex responses in the proximal and distal hindlimb muscles. Rhythmic stimulation initiated locomotor activity of the limb on a running treadmill, i.e., activated the neural locomotor network of the spinal cord (stepping movement generator). Magnetic stimulation of the lumbar enlargement evoked coordinated stepping movements of the hindlimbs only. Application of PMF to the cervical enlargement induced coordinated stepping movements of all four limbs, hindlimb movements starting before forelimb movements. After cessation of magnetic stimulation, the limbs completed several further coordinated movement cycles. This is the first report of the triggering of limb stepping movement generators with PMF in decerebrate cats. The results obtained here demonstrate that the neural locomotor networks of the spinal cord can be activated noninvasively and open new perspectives for the clinical use of PMF.     

Epidural spinal cord stimulation plus quipazine administration enable stepping in complete spinal adult rats

We hypothesized that epidural spinal cord stimulation (ES) and quipazine (a serotonergic agonist) modulates the excitability of flexor and extensor related intraspinal neural networks in qualitatively unique, but complementary, ways to facilitate locomotion in spinal cord-injured rats. To test this hypothesis, we stimulated (40 Hz) the S(1) spinal segment before and after quipazine administration (0.3 mg/kg, ip) in bipedally step-trained and nontrained, adult, complete spinal (mid-thoracic) rats. The stepping pattern of these rats was compared with control rats. At the stimulation levels used, stepping was elicited only when the hindlimbs were placed on a moving treadmill. In nontrained rats, the stepping induced by ES and quipazine administration was non-weight bearing, and the cycle period was shorter than in controls. In contrast, the stepping induced by ES and quipazine in step-trained rats was highly coordinated with clear plantar foot placement and partial weight bearing. The effect of ES and quipazine on EMG burst amplitude and duration was greater in flexor than extensor motor pools. Using fast Fourier transformation analysis of EMG bursts during ES, we observed one dominant peak at 40 Hz in the medial gastrocnemius (ankle extensor), whereas there was less of dominant spectral peak in the tibialis anterior (ankle flexor). We suggest that these frequency distributions reflect amplitude modulation of predominantly monosynaptic potentials in the extensor and predominantly polysynaptic pathways in the flexor muscle. Quipazine potentiated the amplitude of these responses. The data suggest that there are fundamental differences in the circuitry that generates flexion and extension during locomotion.     

Control of Locomotor Activity in Humans and Animals in the Absence of Supraspinal Influences

Electrical epidural stimulation of the dorsal surface of the spinal cord at the level of the second lumbar segment induced step-like movements accompanied by the corresponding electromyographic activity in the leg muscles in patients lacking supraspinal influences as a result of vertebral trauma. Triggering of stepping movements was shown to occur with particular stimulation parameters. The results provide evidence that in humans, as in other mammals, the spinal cord contains a network of interneurons acting as generators of stepping movements and producing coordinated patterns of movement activity. Experiments on chronic spinal cats demonstrated the leading role of the propriospinal system of the spinal cord in activating the spinal generators of stepping in response to epidural stimuli.     

Axonal projection of descending pathways responsible for eliciting forelimb stepping into the cat cervical spinal cord

Summary The descending pathways responsible for eliciting forelimb stepping are located in the lateral funiculus (Yamaguchi 1986). In order to determine into which spinal segments the descending pathways project and to know the projections and functions of the other descending system, the ventral funicular pathways, we placed various lesions in the cervical spinal cord of decerebrate cats with the lower thoracic cord transected and studied their effects on forelimb stepping evoked by stimulation of the midbrain locomotor region. (1) The lateral funiculus was transected on one side. The operation removes descending input to all the segments caudal to the lesion. Experiments with serial transections from the caudal to rostral segment revealed that stepping activity of the limb on the lesioned side is reduced when the lesion is placed at the level between the C6 and C7 segment and then between C5 and C6. A slight reduction of activity was also observed after a lesion placed between C7 and C8. (2) Consistently, bilateral transection of the lateral funiculus at the level between C5 and C6 abolished stepping movements of both forelimbs. (3) The cervical cord was split in the parasagittal plane through the dorsal root entry. The operation removes the descending input to the segment in which the lesion is placed. The parasagittal lesions from the C1 to C6 did not abolish stepping activity, although a lesion placed between C5 and C6 could slightly affect stepping. The results, (1)–(3) suggest that the lateral funicular pathways project into the spinal segments mainly at the C6–C7 level with some rostrocaudal extension into C5 and C8. (4) Complete transections of the medial part of the spinal cord cut extensor bursts short and raised stepping frequency. Nevertheless, if the lesion at C1–C5 spared the ventromedial part of the ventral funiculus, it did not result in such high-frequency stepping or in weakened extensor activity. In the case of segments caudal to C6, medial transections which spared the corresponding region could result in such stepping. It is suggested that the pathways descending through the ventromedial part of the ventral funiculus in the rostral segments provide extensor activity during stepping. They may change their course in the more dorsal part of the ventral funiculus below the C6 and presumably project into the grey matter of more caudal segments.     

Characteristics and mechanisms of locomotion induced by intraspinal microstimulation and dorsal root stimulation in spinal cats

Intraspinal microstimulation (ISMS) through a single microelectrode can induce locomotion in cats spinalized at T(13) 1 wk before (untrained) or after 3-5 wk of treadmill training. Here we study the optimal parameters of ISMS and the characteristics of locomotion evoked. ISMS was applied in the dorsal region of segments L(3)-S(1) at different lateralities (midline to 2.5 mm) and after an intravenous injection of clonidine (noradrenergic agonist). Kinematics and electromyographic recordings were used to characterize locomotion. ISMS could induce a bilateral locomotor pattern similar to that obtained with perineal stimulation, and the characteristics of locomotion varied according to the spinal segment stimulated. Mechanisms by which ISMS could evoke locomotion were then investigated by stimulating, inactivating, or lesioning different spinal structures. Dorsal root stimulation (DRS), just like ISMS, could evoke a variety of ipsi- and bilateral nonlocomotor movements as well as locomotor responses. This suggests that sensory afferent pathways are involved in the production of locomotion by ISMS. Microinjections of yohimbine (noradrenergic antagonist) in L(3) and L(4) segments or a complete second spinal lesion at L(3)-L(4) abolished all locomotor activity evoked by ISMS applied at more caudal segments. Progressive dorsoventral spinal lesions at L(3) or L(4) and restricted ventral lesions at L(4) further suggest that the integrity of the ventral or ventrolateral funiculi as well as the L(3)-L(4) segments are critical for the induction of locomotion by ISMS at L(5) to S(1) or by DRS at these caudal segments.     

Monoaminergic establishment of rostrocaudal gradients of rhythmicity in the neonatal mouse spinal cord

Bath application of monoamines is a potent method for evoking locomotor activity in neonatal rats and mice. Monoamines also promote functional recovery in adult animals with spinal cord injuries by activating spinal cord networks. However, the mechanisms of their actions on spinal networks are largely unknown. In this study, we tested the hypothesis that monoamines establish rostrocaudal gradients of rhythmicity in the thoracolumbar spinal cord. Isolated neonatal mouse spinal cord preparations (P0-P2) were used. To assay excitability of networks by monoamines, we evoked a disinhibited rhythm by bath application of picrotoxin and strychnine and recorded neurograms from several thoracolumbar ventral roots. We first established that rostral and caudal segments of the thoracolumbar spinal cord had equal excitability by completely transecting preparations at the L3 segmental level and recording the frequency of the disinhibited rhythm from both segments. Next we established that a majority of ventral interneurons retrogradely labeled by calcium green dextran were active during network activity. We then bath applied combinations of monoaminergic agonists [5-HT and dopamine (DA)] known to elicit locomotor activity. Our results show that monoamines establish rostrocaudal gradients of rhythmicity in the thoracolumbar spinal cord. This may be one mechanism by which combinations of monoaminergic compounds normally stably activate locomotor networks.     

Somatosensory control of balance during locomotion in decerebrated cat

Postmammillary decerebrated cats can generate stepping on a moving treadmill belt when the brain stem or spinal cord is stimulated tonically and the hindquarters are supported both vertically and laterally. While adequate propulsion seems to be generated by the hindlimbs under these conditions, the ability to sustain equilibrium during locomotion has not been examined extensively. We found that tonic epidural spinal cord stimulation (5 Hz at L5) of decerebrated cats initiated and sustained unrestrained weight-bearing hindlimb stepping for extended periods. Detailed analyses of the relationships among hindlimb muscle EMG activity and trunk and limb kinematics and kinetics indicated that the motor circuitries in decerebrated cats actively maintain equilibrium during walking, similar to that observed in intact animals. Because of the suppression of vestibular, visual, and head-neck-trunk sensory input, balance-related adjustments relied entirely on the integration of somatosensory information arising from the moving hindquarters. In addition to dynamic balance control during unperturbed locomotion, sustained stepping could be reestablished rapidly after a collapse or stumble when the hindquarters switched from a restrained to an unrestrained condition. Deflecting the body by pulling the tail laterally induced adaptive modulations in the EMG activity, step cycle features, and left-right ground reaction forces that were sufficient to maintain lateral stability. Thus the brain stem-spinal cord circuitry of decerebrated cats in response to tonic spinal cord stimulation can control dynamic balance during locomotion using only somatosensory input.     

Effects of electrical stimulation of the thoracic spinal cord on bladder and external urethral sphincter activity in the decerebrate cat

Summary Electrical stimulation of the spinal cord above the sacral segments was used to produce coordinated micturition in the paralysed decerebrate cat. Stimulation of the superficial aspect of the dorsolateral funiculus (DLF) within the lower thoracic (T9-T13) segments produced a bladder contraction coordinated with decreased activity in the external urethral sphincter (EUS) branch of the pudendal nerve during which time fluid was expelled. In addition, a similar response was observed with DLF stimulation at the boundary of the L5/L6 segments. At the second cervical spinal segment, however, stimulation of a more lateral and ventral portion of the superficial spinal white matter was the only effective site for producing micturition. The spinal cord-evoked response was comparable to the micturition evoked by electrical stimulation of the pontine micturition centre (PMC) within the brainstem. A bilateral lesion of the dorsal columns (DC) and the dorsolateral funiculi (DLF) at the lower thoracic levels abolished reflex micturition evoked by bladder distension. However stimulation rostral to the lesion, within the PMC or thoracic DLF, continued to produce coordinated bladder and sphincter response during voiding. Stimulation caudal to the lesion produced a decrease in pudendal nerve activity but did not produce a void or bladder pressure change. This reduction in pudendal nerve activity could be abolished with a second lesion of the superficial DLF caudal to the stimulation site. It was concluded that stimulation of the thoracic dorsolateral funiculus activates both ascending and descending fibres which can influence the bladder and/or sphincter muscles. The spinal cordevoked voiding was hypothesized to be due to activation of some portion of the ascending limb of the spinobulbospinal micturition reflex loop. The decreased activity produced by stimulation of the thoracic DLF caudal to a bilateral DC/DLF subtotal cord lesion may be mediated by fibres descending in the dorsolateral funiculus. The possibility that the spinal cord stimulation antidromically activated axons of neurons having segmental collaterals capable of influencing pudendal neural activity cannot be exclused at this time.     

Propriospinal neurons are sufficient for bulbospinal transmission of the locomotor command signal in the neonatal rat spinal cord

We recently showed that propriospinal neurons contribute to bulbospinal activation of locomotor networks in the in vitro neonatal rat brainstem–spinal cord preparation. In the present study, we examined whether propriospinal neurons alone, in the absence of long direct bulbospinal transmission to the lumbar cord, can successfully mediate brainstem activation of the locomotor network. In the presence of staggered bilateral spinal cord hemisections, the brainstem was stimulated electrically while recording from lumbar ventral roots. The rostral hemisection was located between C1 and T3 and the contralateral caudal hemisection was located between T5 and mid-L1. Locomotor-like activity was evoked in 27% of the preparations, which included experiments with staggered hemisections placed only two segments apart. There was no relation between the likelihood of developing locomotor-like activity and the distance separating the two hemisections or specific level of the hemisections. In some experiments, where brainstem stimulation alone was ineffective, neurochemical excitation of propriospinal neurons (using 5-HT and NMDA) at concentrations subthreshold for producing locomotor-like activity, promoted locomotor-like activity in conjunction with brainstem stimulation. In other experiments, involving neither brainstem stimulation nor cord hemisections, the excitability of propriospinal neurons in the cervical and/or thoracic region was selectively enhanced by bath application of 5-HT and NMDA or elevation of bath K⁺ concentration. These manipulations produced locomotor-like activity in the lumbar region. In total, the results suggest that propriospinal neurons are sufficient for transmission of descending locomotor command signals. This observation has implications for regeneration strategies aimed at restoration of locomotor function after spinal cord injury.     

Role of Neurotrophin 3 in spinal neuroplasticity in rats subjected to cord transection

That neuroplasticity occurs in mammalian spinal cord is well known, though the underlying mechanism still awaits elucidation. This study evaluated the role of endogenous Neurotrophin-3 (NT-3) in the spinal neuroplasticity. Following cord transection at the junction between T9 and T10, the hindlimb locomotor functions of rats showed gradual but significant improvement from 7 to 28 days post-operation. Corresponding to this was a significant increase in the level of NT-3 in the cord segments caudal to injury site. Significantly, after NT-3-antibody administration, the spinal transected rats displayed poor hindlimb locomotor functions and a decrease in the number of neurons in spinal laminae VIII–IX. Whether NT-3-antibody was administered, corticospinal tract regeneration and somatosensory evoked potentials could not be detected. Our findings suggested that endogenous NT-3 could play an important role in spinal plasticity in adult spinal cords subjected to transection, possibly through a regulation of neuronal activity in the local circuitry.     

Level of spinal cord lesion determines locomotor activity in spinal man

Recent studies have demonstrated that coordinated stepping movements can be induced in patients with complete para-/tetraplegia, when they were standing on a moving treadmill with their body weight partially unloaded and external assistance. The aim of this study was to determine which part of the spinal cord generated the locomotor pattern. In patients with complete paraplegia due to lesions at different levels of the spinal cord, the locomotor pattern was compared with that of healthy subjects. Any similarities in electromyographic (EMG) activity of gastrocnemius and tibialis anterior muscles between the patients and healthy subjects were reflected by the analysis of the variation ratio and amplitudes of the EMG activity. It was found that the higher the level of spinal cord lesion the more ”normal” was the locomotor pattern. This suggests that neuronal circuits underlying locomotor ”pattern generation” in man are not restricted to any specific level(s) of the spinal cord, but that an intricate neuronal network contributing to bipedal locomotion extends from thoracolumbal to cervical levels. Received: 16 October 1998 / Accepted: 21 April 1999     

Neurochemical excitation of propriospinal neurons facilitates locomotor command signal transmission in the lesioned spinal cord

Previous studies of the in vitro neonatal rat brain stem-spinal cord showed that propriospinal relays contribute to descending transmission of a supraspinal command signal that is capable of activating locomotion. Using the same preparation, the present series examines whether enhanced excitation of thoracic propriospinal neurons facilitates propagation of the locomotor command signal in the lesioned spinal cord. First, we identified neurotransmitters contributing to normal endogenous propriospinal transmission of the locomotor command signal by testing the effect of receptor antagonists applied to cervicothoracic segments during brain stem-induced locomotor-like activity. Spinal cords were either intact or contained staggered bilateral hemisections located at right T1/T2 and left T10/T11 junctions designed to abolish direct long-projecting bulbospinal axons. Serotonergic, noradrenergic, dopaminergic, and glutamatergic, but not cholinergic, receptor antagonists blocked locomotor-like activity. Approximately 73% of preparations with staggered bilateral hemisections failed to generate locomotor-like activity in response to electrical stimulation of the brain stem alone; such preparations were used to test the effect of neuroactive substances applied to thoracic segments (bath barriers placed at T3 and T9) during brain stem stimulation. The percentage of preparations developing locomotor-like activity was as follows: 5-HT (43%), 5-HT/N-methyl-D-aspartate (NMDA; 33%), quipazine (42%), 8-hydroxy-2-(di-n-propylamino)tetralin (20%), methoxamine (45%), and elevated bath K(+) concentration (29%). Combined norepinephrine and dopamine increased the success rate (67%) compared with the use of either agent alone (4 and 7%, respectively). NMDA, Mg(2+) ion removal, clonidine, and acetylcholine were ineffective. The results provide proof of principle that artificial excitation of thoracic propriospinal neurons can improve supraspinal control over hindlimb locomotor networks in the lesioned spinal cord.     

Anatomical studies on the ovine spinal cord

The root attachment lengths were consistently greater in the cranial cervical (C3), midthoracic (T7), caudal lumbar (L5) and cranial sacral (S1) cord segment levels than the corresponding caudal cervical, caudal thoracic, cranial lumbar and caudal sacral levels respectively. As to the root emergence length the greatest values were obtained bilaterally at C3, T1, L4 and S1 cord segment levels respectively. The interroot intervals were maximum at C3, T13, L1 and S1 cord levels in the respective regions. The longest cord segments were located at C2, T13, L3 and S1 levels; the shortest were at C8, T1, L6, and S4 cord levels. The greatest diameter and cross-sectional area were confined to the last cervical, first 2 thoracic, last lumbar and first sacral cord segment levels. The spinal cord segments C2, T13, L4 and S1 were most voluminous in the respective regions. The topography of cord segments and the level of termination of the spinal cord have been studied and recorded.