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Normal cognitive development in infants follows a well‐known temporal sequence, which is assumed to be correlated with the structural maturation of underlying functional networks. Postmortem studies and, more recently, structural MR imaging studies have described qualitatively the heterogeneous spatiotemporal progression of white matter myelination. However, in vivo quantification of the maturation phases of fiber bundles is still lacking. We used noninvasive diffusion tensor MR imaging and tractography in twenty‐three 1–4‐month‐old healthy infants to quantify the early maturation of the main cerebral fascicles. A specific maturation model, based on the respective roles of different maturational processes on the diffusion phenomena, was designed to highlight asynchronous maturation across bundles by evaluating the time‐course of mean diffusivity and anisotropy changes over the considered developmental period. Using an original approach, a progression of maturation in four relative stages was determined in each tract by estimating the maturation state and speed, from the diffusion indices over the infants group compared with an adults group on one hand, and in each tract compared with the average over bundles on the other hand. Results were coherent with, and extended previous findings in 8 of 11 bundles, showing the anterior limb of the internal capsule and cingulum as the most immature, followed by the optic radiations, arcuate and inferior longitudinal fascicles, then the spinothalamic tract and fornix, and finally the corticospinal tract as the most mature bundle. Thus, this approach provides new quantitative landmarks for further noninvasive research on brain‐behavior relationships during normal and abnormal development. Hum Brain Mapp, 2008. © 2007 Wiley‐Liss, Inc.

International audience; BACKGROUND AND PURPOSE: The early postnatal period is perhaps the most dynamic phase of white matter development. We hypothesized that the early postnatal development of the corpus callosum and corticospinal tracts could be studied in unsedated healthy neonates by using novel approaches to diffusion tensor imaging (DTI) and quantitative tractography. MATERIALS AND METHODS: Isotropic 2 x 2 x 2 mm(3) DTI and structural images were acquired from 47 healthy neonates. DTI and structural images were coregistered and fractional anisotropy (FA), mean diffusivity (MD), and normalized T1-weighted (T1W) and T2-weighted (T2W) signal intensities were determined in central midline and peripheral cortical regions of the white matter tracts of the genu and splenium of the corpus callosum and the central midbrain and peripheral cortical regions of the corticospinal tracts by using quantitative tractography. RESULTS: We observed that central regions exhibited lower MD, higher FA values, higher T1W intensity, and lower T2W intensity than peripheral cortical regions. As expected, MD decreased, FA increased, and T2W signal intensity decreased with increasing age in the genu and corticospinal tract, whereas there was no significant change in T1W signal intensity. The central midline region of the splenium fiber tract has a unique pattern, with no change in MD, FA, or T2W signal intensity with age, suggesting different growth trajectory compared with the other tracts. FA seems to be more dependent on tract organization, whereas MD seems to be more sensitive to myelination. CONCLUSIONS: Our novel approach may detect small regional differences and age-related changes in the corpus callosum and corticospinal white matter tracts in unsedated healthy neonates and may be used for future studies of pediatric brain disorders that affect developing white matter.

In humans a substantial part of corticospinal excitation to upper limb motoneurons is mediated through cervical premotoneurons located rostral to motoneurons, analogous to the feline system of C3-C4 propriospinal neurons. The indirect (disynaptic) component of the corticospinal command passing through the propriospinal relay may be updated by the extensive convergence at this level of afferent inputs (both excitatory and inhibitory) from the moving limb. Propriospinal neurons are potently inhibited by feedback inhibitory interneurons facilitated from the motor cortex, and this explains why artificial volleys delivered to the pyramidal system by itself have failed to demonstrate this indirect corticospinal projection. The strength of this indirect corticospinal projection may be estimated in routine practice by the amount of suppression elicited by a cutaneous volley on the ongoing electromyogram of extensor carpi radialis, and is increased in patients recovering from hemiplegia and in early Parkinson's disease.

We sought to determine why a given muscle appears represented in widespread loci in the motor cortex (MCx). To this end, we microstimulated every 500 microm along medio-lateral rows and recorded the evoked electromyographic (EMG) responses of up to a dozen forelimb muscles of the cat. A consistent finding in all animals studied was that along a given row, distal muscle responses could be elicited from medially situated cortical loci and conversely, proximal muscles responses from laterally situated cortical loci. In many such cases, the evoked EMG responses were such that the largest responses from a distal muscle were obtained by stimulation at a medially situated point and those of a proximal muscle from a laterally situated point. A Spearman correlation analysis showed that there was no correlation between cortical position and where the peak response of a given muscle occurred. These quantitative results strongly support the view that in the forelimb area of the cat MCx there exists widespread 'colonies' of corticospinal neurons with common spinal cord targets.

International audience; This paper will discuss experiments dedicated to the exploration of pathways linking the sensorimotor cortex (SMC) and the main bulbar nuclei involved in cardiovascular control: the nucleus tractus solitarius (NTS), the dorsal nucleus of the vagus (DMV) and the rostral ventrolateral medulla (RVLM). Results obtained through neurofunctional and neuroanatomical methods are presented in order to bring new answers to relevant points concerning somato-cardiovascular integration: firstly to show the ability of the SMC to influence neurons in bulbar cardiovascular nuclei, and secondly to identify pathways that transmit such influences. The neurofunctional approach, based on the identification of Fos-like immunoreactive neurons, indicated that the SMC has functional connections with cardiovascular bulbar nuclei. The neuroanatomical approach, which employed retrograde and anterograde axonal tracing methods, provided evidence of direct projections from the SMC to NTS/DMV and RVLM. Furthermore, experiments showed clearly that corticospinal neurons sent collaterals to bulbar cardiovascular nuclei, especially to the RVLM. Direct cortical projections to the NTS/DMV and the RVLM provide the anatomical basis for cortical influences on the baroreceptor reflex and sympathetic vasomotor mechanisms for blood pressure control, and support the hypothesis of cortical commands coupling somatic and cardiovascular outputs for action.