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Objectives: The aim of this study was to investigate spinal cord structure in patients with cervical spondylosis where conventional MRI fails to reveal spinal cord damage., Methods: We performed a cross-sectional study of patients with cervical spondylosis without conventional MRI findings of spinal cord damage and healthy controls. Subjects were studied using spinal diffusion tensor imaging (DTI), precision grip and foot force-tracking tasks, and a clinical examination including assessment of neurological signs. A regional analysis of lateral and medial spinal white matter across multiple cervical levels (C1-C5) was performed., Results: DTI revealed reduced fractional anisotropy (FA) and increased radial diffusivity (RD) in the lateral spinal cord at the level of greatest compression (lowest Pavlov ratio) in patients (p < 0.05). Patients with spondylosis had greater error and longer release duration in both grip and foot force-tracking. Similar spinal cord deficits were present in patients without neurological signs. Increased error in grip and foot tracking (low accuracy) correlated with increased RD in the lateral spinal cord at the level of greatest compression (p ≤ 0.01)., Conclusions: Spinal DTI can detect subtle spinal cord damage of functional relevance in cervical spondylosis, even in patients without signs on conventional T2-imaging and without neurological signs., Key Points: DTI reveals spinal cord changes in cervical spondylosis with few symptoms. DTI changes were present despite normal spinal cord on conventional MRI. DTI parameters correlated with force control accuracy in hand and foot. Spinal DTI is a promising technique for patients with cervical spondylosis.

Motor skill acquisition in the lower limb may induce modifications of spinal network excitability. We hypothesized that short-term motor adaptation in precision grip tasks would also induce modifications of cervical spinal network excitability. In a first series of experiments, we studied the impact of two different precision grip force control tasks (a visuomotor force-tracking task and a control force task without visual feedback) on cervical spinal network excitability in healthy subjects. We separately tested the efficacy of two key components of the spinal circuitry: (i) presynaptic inhibition on flexor carpi radialis (FCR) Ia terminals, and (ii) disynaptic inhibition directed from extensor carpi radialis (ECR) to FCR. We found that disynaptic inhibition decreased temporarily after both force control tasks, independently of the presence of visual feedback. In contrast, the amount of presynaptic inhibition on FCR Ia terminals decreased only after the visuomotor force tracking task. This temporary decrease was correlated with improved tracking accuracy during the task (i.e. short-term motor adaptation). A second series of experiments confirmed these results and showed that the visuomotor force-tracking task resulted also in an increase of the Hmax/Mmax ratio and the slope of the ascending part of the H-reflex recruitment curve. In order to address the role of presynaptic inhibition in the motor adaptation process, we conducted a third series of experiments during which presynaptic inhibition was recorded before and after two consecutive sessions of visuomotor force tracking. The results showed that (i) improved tracking accuracy occurred during both sessions, and (ii) presynaptic inhibition decreased only after the first session of visuomotor force tracking. Taken together, these results suggest thus that the nature of the motor task performed has a specific impact on the excitability of these cervical spinal circuits. These findings also suggest that early motor adaptation is associated with a modulation of presynaptic Ia inhibition in the upper limb.

Background and Purpose: wallerian degeneration (WD) in descending motor tracts after stroke is described at the level of the internal capsule and the brainstem. We investigated whether diffusion tensor imaging (DTI) can detect degeneration in the lateral cervical spinal cord after stroke., Methods: DTI at 1.5 T of the cervical spinal cord was performed in 4 chronic hemiparetic patients after ischemic stroke. Stroke lesions included the corticospinal tract. DTI was also performed in 12 healthy controls. Diffusion parameters were obtained for left and right (i) half and (ii) lateral spinal cord extending from C2 to C7., Results: relative fractional anisotropy (FA) in the lateral tracts on the affected side compared with the unaffected side (left/right) was reduced in stroke patients as compared with controls (P= .007). FA was lowest in patients with severe upper limb hemiparesis. Relative apparent diffusion coefficient in the lateral tracts was increased in the patients (P= .03)., Conclusions: this study provides preliminary evidence that DTI at 1.5 T can be used for identification and quantification of WD in the lateral cervical spinal cord in stroke patients. This may prove useful for prognosis of motor outcome after stroke.

Diffusion tensor imaging (DTI) can be used to elucidate relations between CNS structure and function. We hypothesized that the degree of spinal white matter organization relates to the accuracy of control of grip force. Healthy subjects of different age were studied using DTI and visuomotor tracking of precision grip force. The latter is a prime component of manual dexterity. A regional analysis of spinal white matter [fractional anisotropy (FA)] across multiple cervical levels (C2-C3, C4-C5, and C6-C7) and in different regions of interest (left and right lateral or medial spinal cord) was performed. FA was highest at the C2-C3 level, higher on the right than the left side, and higher in the lateral than in the medial spinal cord (p < 0.001). FA of whole cervical spinal cord (C2-C7) was lower in subjects with high tracking error (r = -0.56, p = 0.004) and decreased with age (r = -0.63, p = 0.001). A multiple regression analysis revealed an independent contribution of each predictor (semipartial correlations: age, r = -0.55, p < 0.001; tracking error, r = -0.49, p = 0.003). The closest relation between FA and tracking error was found at the C6-C7 level in the lateral spinal cord, in which the corticospinal tract innervates spinal circuitry controlling hand and digit muscles. FA of the medial spinal cord correlated consistently with age across all cervical levels, whereas FA of the lateral spinal cord did not. The results suggest (1) a functionally relevant specialization of lateral spinal cord white matter and (2) an increased sensitivity to age-related decline in medial spinal cord white matter in healthy subjects.

There are conflicting views on the functional importance of the system of C3-C4 propriospinal neurones in the macaque, although the two sets of observations from the opposing laboratories are actually quite similar, both making the system appear much weaker than its well-known equivalent in the cat. One group asserts, mainly via evidence derived from experiments with strychnine, that this is a consequence simply of inhibition of the propriospinal neurone. However, we ague here that this judgement is premature and that much more needs to be known about the neurones involved and their connectivity before the analogy with the system in the cat is safe. This is particularly important because of a similar analogy which has been made with respect to measurements in human subjects.

We studied the activity of 577 neurons in the C6-T1 spinal cord of three awake macaque monkeys while they generated visually guided, isometric flexion/extension torques about the wrist. Spike-triggered averaging of electromyographic activity (EMG) identified the units' correlational linkages with

The activity of C6-T1 spinal cord neurons was recorded in three macaques while they generated isometric wrist flexion and extension torques in visually guided step-tracking tasks. Electromyographic activity (EMG) was recorded in /=3.5 ms from the trigger. Synchrony interneurons (Sy-INs) were non-PreM-Ins that had spike-related features with latencies <3.5 ms in at least one muscle. Unidentified interneurons (U-INs) showed no features in any of the STAs. A total of 572 task-related spinal neurons were studied; 29 cells were MNs, 97 PreM-INs, 32 Sy-INs, and 414 U-INs. MNs were activated predominantly in a tonic fashion during the ramp-and-hold torques and were active in one direction only. The most common response pattern for interneurons, irrespective of their class, was phasic-tonic activity, followed by purely tonic and purely phasic activity. Most interneurons (77%) were bidirectionally active in both flexion and extension. For all classes of interneurons, units with phasic response components tended to be activated first, before torque onset, followed by tonic units. The onset times of PreM-INs relative to onsets of their target muscles were distributed broadly, with a mean of -25 +/- 128 (SD) ms. For most neurons with tonic response components (all MNs, 71% of PreM-INs, 67% of Sy-INs, and 84% of U-INs), activity during the hold period was correlated significantly with the magnitude of static torque exerted by the monkey. The rate-torque regressions generally had positive slopes with higher mean slopes for extension than for flexion. The phasic response components were correlated significantly with rate of change of torque for a smaller percentage of tested PreM-Ins (50%), Sy-INs (83%), and U-INs (77%). In contrast to other premotor neurons [corticomotoneuronal (CM), rubromotoneuronal (RM), and dorsal root ganglion (DRG) afferents] previously characterized under similar conditions, a larger proportion of the spinal PreM-INs were activated after onset of their target muscles, probably reflecting a larger proportion of PreM-INs driven by peripheral input. The rate-torque slopes of PreM-INs tended to be less steep than those of CM and RM cells. Unlike the CM and DRG PreM afferents, which were activated unidirectionally, most spinal PreM-INs showed bidirectional activity, like RM cells.

1. Synaptic responses to electrical stimulation of the contralateral pyramidal tract were measured in intracellular recordings from 206 upper limb motoneurones in ten chloralose-anaesthetized macaque monkeys. The objective was to search for evidence of a disynaptic excitatory pathway via C3-C4 propriospinal interneurones similar to that in the cat. 2. In monkeys with intact spinal cords, only a small proportion of motoneurones (19%) responded with late EPSPs to repetitive stimulation of the pyramid; only 3% had segmental latencies that were appropriate for a disynaptic pathway. 3. From previous studies in the cat, it was expected that a lesion to the dorsolateral funiculus (DLF) at C5 would interrupt the corticospinal input to the spinal segments supplying upper limb muscles, whilst leaving intact excitation transmitted via a C3-C4 propriospinal system, the descending axons of which travel in the ventral part of the funiculus. In five of the monkeys a lesion was made to the DLF at C5 which spared the ventrolateral columns. It severely reduced the monosynaptic EPSPs and disynaptic IPSPs evoked from the pyramidal tract that were present in the intact monkey spinal cord, and which might have masked the presence of disynaptic EPSPs. However, even after the lesion the proportion of motoneurones with such late EPSPs was still low (18%); 14% of motoneurones had EPSPs within the disynaptic range. 4. In addition, some EPSPs with relatively long segmental latencies (> 1.1 ms) were recorded before and after the C5 lesions, but since these effects could be evoked by single stimuli, had stable latencies and did not facilitate with repetitive shocks, it is likely that they represent monosynaptic EPSPs evoked by slowly conducting corticospinal fibres which survived the lesions. 5. In seven of the monkeys motoneurone responses to stimulation of the ipsilateral lateral reticular nucleus (LRN) were also tested. Most motoneurones showed EPSPs with short latencies (1.2-2.5 ms) and other properties characteristic of monosynaptic activation. This is consistent with the presence of collaterals of C3-C4 propriospinal neurones to the LRN, as demonstrated in the cat. 6. These short-latency EPSPs evoked from the LRN were just as common before (77%) as after (75%) the C5 lesion. They had small amplitudes both before (mean +/- s.d. 1.1 +/- 0.59 mV) and after (1.2 +/- 0.72 mV) the lesion. Unlike the situation in the cat, only a small proportion (16%) of motoneurones activated from the LRN showed late EPSPs after repetitive stimulation of the pyramid. 7. The results provide little evidence for significant corticospinal excitation of motoneurones via a system of C3-C4 propriospinal neurones in the monkey. The general absence of responses mediated by such a system in the macaque, under experimental conditions similar to those in which they are seen in the cat, show that extrapolation of results from the cat to the primate should be made with considerable caution.

Most of our information about spinal neurons has been derived from experiments with anesthetized or surgically. reduced preparations. To investigate these neurons under normal behavioral conditions, we recorded the activity of single afferent units in cervical dorsal root ganglia and of single interneurons in the cervical spinal cord of macaque monkeys, as they generated alternating flexion and extension torques about the wrist. Spike-triggered averages of rectified electromyographic activity were used to identify "premotor" (PreM) units associated with correlated postspike effects in active muscles. In addition to postspike effects, some spike-triggered averages showed early increases in average muscle activity, which were attributed to synchronous discharges in other PreM units. In recordings of peripheral afferents, 49% of the task-related dorsal root ganglia units produced postspike facilitation (PSF) of at least one forearm muscle, with a mean PSF latency of 5.8 +/- 0.3 ms (SE). The PSF amplitude was measured as the mean percent increase (MPI): the average increase of the PSF as a percentage of the prespike baseline mean. PreM afferent units produced PSF with an average MPI of 4.6 +/- 0.3%. In a study of cervical interneurons, about 13% (72/562) of the task-related cells showed postspike effects. These PreM interneurons had a mean PSF latency of 7.2 +/- 0.3 ms and a mean MPI of 4.6 +/- 0.2%. The MPI values for spinal neurons were similar to the MPIs reported for rubromotoneuronal and corticomotoneuronal cells. PreM neurons usually facilitated a subset of the coactivated muscles called the unit's "muscle field." The PreM afferents facilitated an average of 46% of the synergistically coactivated muscles, while PreM interneurons facilitated an average of 37%. These are comparable with the percentage of muscles facilitated by corticomotoneuronal (40%) and rubromotoneuronal (50%) cells. During the step-tracking task the monkeys generated ramp-and-hold torques about the wrist. The PreM afferents typically became active during either flexion or extension of the wrist, although a few were bidirectionally active. The most common response pattern in PreM afferents was a tonic discharge, followed by phasic and phasic-tonic discharge. The most common patterns exhibited by PreM interneurons were tonic and phasic-tonic responses. PreM afferent units began to discharge on average 51 +/- 13 ms before activation of their target muscle. This early onset supports our hypothesis that these PreM afferents arose from muscle spindles, which is also consistent with their short-latency PSF and their responses to perturbations that stretched their target muscles. The results reveal some salient differences between the discharge properties of dorsal root ganglia neurons, spinal interneurons, and supraspinal PreM cells in the motor cortex and red nucleus. All four PreM populations include tonic, phasic-tonic, and phasic cells, but in significantly different proportions. Most PreM afferents resembled corticomotoneuronal cells in being active only with their target muscles, unlike rubromotoneuronal cells and spinal PreM interneurons, which tended to exhibit more bidirectional discharges.