Axon and myelin morphology in animal and human spinal cord

Characterizing precisely the microstructure of axons, their density, size and myelination is of interest for the neuroscientific community, for example to help maximize the outcome of studies on white matter (WM) pathologies of the spinal cord (SC). The existence of a comprehensive and structured database of axonal measurements in healthy and disease models could help the validation of results obtained by different researchers. The purpose of this article is to provide such a database of healthy SC WM, to discuss the potential sources of variability and to suggest avenues for robust and accurate quantification of axon morphometry based on novel acquisition and processing techniques. The article is organized in three sections. The first section reviews morphometric results across species according to range of densities and counts of myelinated axons, axon diameter and myelin thickness, and characteristics of unmyelinated axons in different regions. The second section discusses the sources of variability across studies, such as age, sex, spinal pathways, spinal levels, statistical power and terminology in regard to tracts and protocols. The third section presents new techniques and perspectives that could benefit histology studies. For example, coherent anti-stokes Raman spectroscopy (CARS) imaging can provide sub-micrometric resolution without the need for fixation and staining, while slide scanners and stitching algorithms can provide full cross-sectional area of SC. In combination with these acquisition techniques, automatic segmentation algorithms for delineating axons and myelin sheath can help provide large-scale statistics on axon morphometry

Evaluation of q-ball metrics for assessing the integrity of the injured spinal cord

International audienceAssessment of spinal cord integrity following injury is crucial for evaluating the potential for functional rehabilitation [1]. Previous studies showed the benefits of diffusion tensor imaging (DTI) for the non-invasive characterization of the healthy and injured spinal cord [2]. However, biases related to the incapability of DTI to represent complex diffusion profiles suggested the use of less constraining techniques. Recently, we demonstrated that q-ball imaging (QBI) is capable of partly solving fiber crossing information in the intact spinal cord [3]. In this study, we extended the application of QBI in a model of cat partial spinal cord injury and we compared various QBI quantitative metrics to the ones used in DTI. We also proposed an original QBI-based metric to quantify the homogeneity of diffusion directions

Auditory influence on motor systems.

This study on the influence of auditory stimuli on motor systems was divided into 4 experimental series. The first one consisted in studying in Man the changes in excitability of spinal motoneurones in response to sounds using monosynaptic testing (H-reflex). The results showed an audio-spinal potentiation occurring 80 msec after the onset of the tone, attaining a peak at 100-120 msec and lasting on average 200 msec. The second study aimed at establishing how this audiospinal influence could be integrated to repetitive hopping at the preferred frequency (2.05 Hz) in synchrony with dance music. The muscular activity responsible for landing and takeoff in hopping was synchronized to music so as to take advantage of the potential audio-spinal facilitation of the musical sound. The following experiments were devoted to the study of startle reactions to sound since the observed audio-spinal facilitation was believed to have evolved from such primitive reactions. Patterns of excitation and inhibition in raw electromyograms of startle responses, suggestive of rudimentary locomotor movements, were observed. In final experiments, the pattern of responses of identified reticulo-spinal cells to sound was studied. These, cells were shown to be audio-responsive and were postulated to constitute the motor system mediating audio-spinal influence and integrating auditory and motor events in sound-guided behavior.Cette étude portant sur l'influence de stimuli auditifs sur la motricité fut divisée en quatre phases expérimenta1es. La première a consisté à étudier, chez l'Homme, les changements d'excitabilité de motoneurones spinaux en réponse aux sons en utilisant un réflexe monosynaptique (le réflexe H). Les résultats ont montré une potentiation audio-spinale survenant 80 msec après le début du son, atteignant son maximum à 100-120 msec et se prolongeant en moyenne 200 msec. La seconde étude chercha a établir comment une telle influence audio-spinale pouvait s'intégrer lors de sauts répétitifs à la fréquence préférentielle (2.05 Hz) synchronisés à une musique de danse. L'activité musculaire reliée à l'atterrissage et au décollage pendant le saut se synchronise à la musique de façon à tirer le meilleur bénéfice de la facilitation audio-spinale potentielle originant de la musique. Les expériences suivantes furent consacrées à l'étude chez l'Homme des réactions de sursaut étant donné que la facilitation audio-spinale observée prend peut-être origine de réactions primitives semblables. Les enregistrements électromyographiques de sursaut suggèrent que certaines réponses excitatrices et inhibitrices, ressemblent à des mouvements locomoteurs rudimentaires. Dans les expériences finales, la réponse au son de cellules réticulo-spinales identifiées fut étudiée chez le chat. Ces cellules, sensibles à la stimulation auditive, apparaissent constituer le centre moteur responsable des influences, audio-spinales et de l'intégration d'informations auditives dans le contrôle moteur

Sharp asymptotics for the multidimensional KPP equation

In this article sharp asymptotics for the solution of nonhomogeneous Kolmogorov, Petrovskii and Pisciunov equation depending on a small parameter are considered when the initial condition is the characteristic function of a set . We show how to extend the Ben Arous and Rouault's result that dealt with d=1 and the initial condition as the characteristic function of A={x[less-than-or-equals, slant]0}. The dependance of the asymptotics on the geometry of the boundary of A is precisely described for the problem with constraints.Reaction diffusion equation Large deviations