Neuronal Subtype-Specific Genes that Control Corticospinal Motor Neuron Development In Vivo

Within the vertebrate nervous system, the presence of many different lineages of neurons and glia complicates the molecular characterization of single neuronal populations. In order to elucidate molecular mechanisms underlying the specification and development of corticospinal motor neurons (CSMN), we purified CSMN at distinct stages of development in vivo and compared their gene expression to two other pure populations of cortical projection neurons: callosal projection neurons and corticotectal projection neurons. We found genes that are potentially instructive for CSMN development, as well as genes that are excluded from CSMN and are restricted to other populations of neurons, even within the same cortical layer. Loss-of-function experiments in null mutant mice for Ctip2 (also known as Bcl11b), one of the newly characterized genes, demonstrate that it plays a critical role in the development of CSMN axonal projections to the spinal cord in vivo, confirming that we identified central genetic determinants of the CSMN population.

The injured spinal cord spontaneously forms a new intraspinal circuit in adult rats

In contrast to peripheral nerves, central axons do not regenerate. Partial injuries to the spinal cord, however, are followed by functional recovery. We investigated the anatomical basis of this recovery and found that after incomplete spinal cord injury in rats, transected hindlimb corticospinal tract (CST) axons sprouted into the cervical gray matter to contact short and long propriospinal neurons (PSNs). Over 12 weeks, contacts with long PSNs that bridged the lesion were maintained, whereas contacts with short PSNs that did not bridge the lesion were lost. In turn, long PSNs arborize on lumbar motor neurons, creating a new intraspinal circuit relaying cortical input to its original spinal targets. We confirmed the functionality of this circuit by electrophysiological and behavioral testing before and after CST re-lesion. Retrograde transynaptic tracing confirmed its integrity, and revealed changes of cortical representation. Hence, after incomplete spinal cord injury, spontaneous extensive remodeling occurs, based on axonal sprout formation and removal. Such remodeling may be crucial for rehabilitation in humans.

Concurrent overproduction of synapses in diverse regions of the primate cerebral cortex

Synapses develop concurrently and at identical rates in different layers of the visual, somatosensory, motor, and prefrontal areas of the primate cerebral cortex. This isochronic course of synaptogenesis in anatomically and functionally diverse regions indicates that the entire cerebral cortex develops as a whole and that the establishment of cell-to-cell communication in this structure may be orchestrated by a single genetic or humoral signal. This is in contrast to the traditional view of hierarchical development of the cortical regions and provides new insight into the maturation of cortical functions.

What songbirds teach us about learning

Bird fanciers have known for centuries that songbirds learn their songs. This learning has striking parallels to speech acquisition: like humans, birds must hear the sounds of adults during a sensitive period, and must hear their own voice while learning to vocalize. With the discovery and investigation of discrete brain structures required for singing, songbirds are now providing insights into neural mechanisms of learning. Aided by a wealth of behavioural observations and species diversity, studies in songbirds are addressing such basic issues in neuroscience as perceptual and sensorimotor learning, developmental regulation of plasticity, and the control and function of adult neurogenesis.

Columnar distribution of cortico-cortical fibers in the frontal association, limbic, and motor cortex of the developing rhesus monkey

The terminal distribution of cortico-cortical connections was examined by autoradiography 7-8 days following injections of tritium labeled amino acids into the dorsal bank of the principal sulcus, the posterior part of the medial orbital gyrus, or the hand and arm area of the primary motor cortex in monkeys ranging in age from 4 days to 5.5 months. Labeled axons originating in these various regions of the frontal lobe have topographically diverse ipsilateral and contralateral destinations but virtually all of these projections share a common mode of distribution: they terminate in distinct vertically oriented columns, 200-500 mum wide, that extend across all layers of cortex and alternate in regular sequence with columns of comparable width in which grains do not exceed background. Spatial periodicity in the pattern of transported label in such regions as the prefrontal association cortex, the retrosplenial limbic cortex and the motor cortex indicates that columination in the intracortical distribution of afferent fibers is not unique to sensory specific cortex but is instead a general feature of neocortical organization. A columnar mode of distribution of cortico-cortical projections is present in monkeys at all ages investigated but is especially well delineated in the youngest of them. Thus, grain concentrations within columns are very high in monkeys injected at 4 days of age, somewhat lower in monkeys injected at 39-45 days of age, and least dense in those injected at 5.5 months. The distinctness of the spatially segregated pattern of innervation in the cortex of neonates indicates that the columnar organization of association-fiber systems in the frontal and limbic cortex is achieved before or shortly after birth.

Differences in genetic and environmental influences on the human cerebral cortex associated with development during childhood and adolescence

In this report, we present the first regional quantitative analysis of age-related differences in the heritability of cortical thickness using anatomic MRI with a large pediatric sample of twins, twin siblings, and singletons (n = 600, mean age 11.1 years, range 5-19). Regions of primary sensory and motor cortex, which develop earlier, both phylogenetically and ontologically, show relatively greater genetic effects earlier in childhood. Later developing regions within the dorsal prefrontal cortex and temporal lobes conversely show increasingly prominent genetic effects with maturation. The observation that regions associated with complex cognitive processes such as language, tool use, and executive function are more heritable in adolescents than children is consistent with previous studies showing that IQ becomes increasingly heritable with maturity(Plomin et al. 1997: Psychol Sci 8:442-447). These results suggest that both the specific cortical region and the age of the population should be taken into account when using cortical thickness as an intermediate phenotype to link genes, environment, and behavior.

Cortical axons branch to multiple subcortical targets by interstitial axon budding: implications for target recognition and "waiting periods"

We are studying how axons branch in vivo. Individual cortical neurons send axons to both the spinal cord and the basilar pons. Here we show that the corticopontine projection develops by an interstitial budding of collaterals from parent axons rather than a reported mechanism of axon branching, growth cone bifurcation. This mechanism is used regardless of whether the parent axon's postpontine segment, which forms the corticospinal projection, is permanent (motor cortex) or transient (visual cortex). Budding occurs days after the parent axons grow spinally past the pons, accounting for the "waiting period" reported in this system in contrast to an alternative explanation that the growth cones pause outside of their target. Timing and location of pontine collateral budding vary with cortical origin of the parent axon and are correlated with the temporal ordering of axon arrival.

The Corticospinal System: From Development to Motor Control

The corticospinal system is the principal motor system for controlling movements that require the greatest skill and flexibility. It is the last motor system to develop. The pattern of termination of corticospinal axons, as they grow into the spinal gray matter, bears little resemblance to the pattern later in development and in maturity. Refinement of corticospinal terminations occurs during a protracted postnatal period and includes both elimination of transient terminations and growth to new targets. This refinement is driven by neural activity in the motor cortical areas and by limb motor experience. Developing corticospinal terminals compete with each other for synaptic space on spinal neurons. More active terminals are more competitive and are able to secure more synaptic space than their less active counterparts. Corticospinal terminals can activate spinal neurons from very early in development. The importance of this early synaptic activity appears to be more for refining corticospinal connections than for transmitting signals to spinal motor circuits for movement control. The motor control functions of the corticospinal system are not expressed until development of connectional specificity with spinal cord neurons, a strong capacity for corticospinal synapses to facilitate spinal motor circuits, and the formation of the cortical motor map.

Postnatal maturation of the dopaminergic innervation of monkey prefrontal and motor cortices: a tyrosine hydroxylase immunohistochemical analysis

The mature functional architecture of the primate prefrontal cortex arises during a protracted period of postnatal development. Although catecholaminergic afferents arrive in the primate cortex quite early during fetal development, several lines of evidence suggest that substantial changes in the dopaminergic innervation of prefrontal cortex may occur during postnatal development. In this study, we used immunocytochemical techniques and antibodies against tyrosine hydroxylase, the rate-limiting enzyme in catecholamine biosynthesis, to examine the precise time course from birth to adulthood of the maturational changes of tyrosine hydroxylase-labeled axons in prefrontal cortical areas 9 and 46 and primary motor cortex (area 4) of rhesus monkeys. In area 9, the densities of tyrosine hydroxylase-labeled axons and varicosities in the superficial and deep cortical layers remained relatively constant during postnatal development. In contrast, marked developmental changes in innervation density occurred in the middle cortical layers. For example, in deep layer III, the density of tyrosine hydroxylase-positive varicosities was relatively low and uniform in animals under 1 month of age but then increased by a factor of three in animals 2-3 months of age. The density of labeled varicosities continued to increase, reaching a peak (sixfold greater than in the youngest animals) in animals 2-3 years of age before declining to stable adult levels. Similar laminar-specific patterns of change also occurred in areas 46 and 4, although regional differences were present in the magnitude and precise time course of these developmental changes. These findings demonstrate that the innervation of monkey frontal cortex by tyrosine hydroxylase-immunoreactive axons undergoes a protracted, laminar-specific pattern of change during postnatal development that continues through adolescence and into early adulthood. These developmental refinements may interact with other modifications of cortical circuitry that underlie the functional maturation of these regions.

The Role of Task-Specific Training in Rehabilitation Therapies

Task-oriented therapy is important. It makes intuitive sense that the best way to relearn a given task is to train specifically for that task. In animals, functional reorganization is greater for tasks that are meaningful to the animal. Repetition alone, without usefulness or meaning in terms of function, is not enough to produce increased motor cortical representations. In humans, less intense but task-specific training regimens with the more affected limb can produce cortical reorganization and associated, meaningful functional improvements.

Single word reading in developmental stutterers and fluent speakers

Ten fluent speakers and nine developmental stutterers read isolated nouns aloud in a delayed reading paradigm. Cortical activation sequences were mapped with a whole-head magnetoencephalography system. The stutterers were mostly fluent in this task. Although the overt performance was essentially identical in the two groups, the cortical activation patterns showed clear differences, both in the evoked responses, time-locked to word presentation and mouth movement onset, and in task-related suppression of 20-Hz oscillations. Within the first 400 ms after seeing the word, processing in fluent speakers advanced from the left inferior frontal cortex (articulatory programming) to the left lateral central sulcus and dorsal premotor cortex (motor preparation). This sequence was reversed in the stutterers, who showed an early left motor cortex activation followed by a delayed left inferior frontal signal. Stutterers thus appeared to initiate motor programmes before preparation of the articulatory code. During speech production, the right motor/premotor cortex generated consistent evoked activation in fluent speakers but was silent in stutterers. On the other hand, suppression of motor cortical 20-Hz rhythm, reflecting task-related neuronal processing, occurred bilaterally in both groups. Moreover, the suppression was right-hemisphere dominant in stutterers, as opposed to left-hemisphere dominant in fluent speakers. Accordingly, the right frontal cortex of stutterers was highly active during speech production but did not generate synchronous time-locked responses. The speech-related 20-Hz suppression concentrated in the mouth area in fluent speakers, but was evident in both the hand and mouth areas in stutterers. These findings may reflect imprecise functional connectivity within the right frontal cortex and incomplete segregation between the adjacent hand and mouth motor representations in stutterers during speech production. A network including the left inferior frontal cortex and the right motor/premotor cortex, likely to be relevant in merging linguistic and affective prosody with articulation during fluent speech, thus appears to be partly dysfunctional in developmental stutterers.

Growth factor-stimulated generation of new cortical tissue and functional recovery after stroke damage to the motor cortex of rats

Recent studies suggest that proliferation in the adult forebrain subventricular zone increases in response to a forebrain stroke and intraventricular infusions of growth factors enhance this response. The potential for growth factor infusions to regenerate the damaged motor cortex and promote recovery of motor function after stroke has not been examined. Here, we report that intraventricular infusions of epidermal growth factor and erythropoietin together, but not individually, promote substantial regeneration of the damaged cerebral cortex and reverse impairments in spontaneous and skilled motor tasks, in a rat model of stroke. Cortical regeneration and functional recovery occurred even when growth factor administration was delayed for up to 7 days after the stroke-induced lesion. Cell tracking demonstrated the contribution of neural precursors originating in the forebrain subventricular zone to the regenerated cortex. Strikingly, removal of the regenerated cortical tissue reversed the growth factor-induced functional recovery. These findings reveal that specific combinations of growth factors can mobilize endogenous adult neural stem cells to promote cortical tissue re-growth and functional recovery after stroke.

Motor-sensory cortex-corticospinal system and developing locomotion and placing in rats

Normal and abnormal development of movement in the rat were studied by investigating the growth and organization of the motor-sensory cortexcorticospinal tract system (MSC-CST) and the functional and morphologic effects of ablating the MSC or quadrants of it at different ages. Major growth of the MSC outflow, the CST, in the brain stem and rostral cord occurred in the second and third weeks postnatally, coinciding approximately with the normal mid-week transition from infantile to mature locomotion. Ablation of the MSC at birth revealed that while the MSC-CST was not essential for ordinary locomotion on flat terrain, its presence hastened normal development of this kind of movement, and that it was absolutely essential for locomotion on difficult terrain. The MSC quadrants showed quite different, and in some domains mutually exclusive, CST projection patterns to forebrain, diencephalon, brain stem, and spinal destinations (determined by Fink-Heimer-Nauta fiber degeneration studies). Ablation of some quadrants produced distinctive syndromes of disordered movement: the posterolateral quadrant related to active grasping in positioning limbs, while the posteromedial quadrant related to tactile motor-sensory positioning of limbs. Thus in addition to the classic somatotopic organization of the MSC, there was another kind of organization into regions concerned with components of integrated movement of a number of parts of the body. Several forms of aberrant circuitry developed after MSC ablations in infants, but their possible roles in functional adaptation remain to be determined.

Changes in synaptic density in motor cortex of rhesus monkey during fetal and postnatal life

The density and proportion of synaptic contacts in the primate motor cortex (Brodmann area 4) were determined in 21 rhesus monkeys ranging in age from embryonic day 41 (E41) to 20 years. Two to 4 vertical electron microscopic probes, each consisting of 150-250 overlapping micrographs traversing the thickness of the cortex, were prepared for each specimen. Synapses were categorized according to their morphology (symmetrical or asymmetrical), cellular location (on spines, shafts or soma), number, and ratio of laminar distribution. The density of synapses was expressed per unit area and volume of neuropil (excluding neuronal and glia cell bodies, myelin sheath, blood vessels and extracellular space). The first synapse in the area of the emerging motor cortex were observed at E53 in the marginal zone (prospective layer I) and in the transient subplate zone situated beneath the developing cortical plate. Around midgestation (E89) synapses were observed over the entire width of the cortical plate, and their density was about 5/100 microns 3 of neuropil. During the last two months of gestation synaptic density increased 8-fold across all layers to reach about 40/100 microns 3 at the time of birth (E165). Synaptic production continued postnatally and by the end of the second postnatal month attained a level of 60/100 microns 3 neuropil which is two times higher than in the adults. This level decreased at a slow rate until sexual maturity (3 years of age) and then more rapidly to the adult level which is characterized by relative stability of about 30/100 microns 3. The decline in synaptic density after the peak in infancy occurs predominantly at the expense of asymmetric synapses situated on dendritic spines; the population of symmetric synapses on dendritic shafts remains relatively constant. The development of synaptic connections in the motor cortex of non-human primates involves initial overproduction followed by selective elimination and structural alterations.

Synchronized overproduction of neurotransmitter receptors in diverse regions of the primate cerebral cortex

A remarkable diversity of neurotransmitter receptors develops concurrently in disparate areas of the primate cerebral cortex. The density of dopaminergic, adrenergic, serotonergic, cholinergic, and GABAergic receptors (where GABA is gamma-aminobutyric acid) in rhesus monkey reaches a maximum level between 2 and 4 months of age and then declines gradually to adult levels in all layers of sensory, motor, and association regions. The synchronized development of neurotransmitter receptors in diverse layers and regions of the neocortex occurs pari passu with synaptogenesis, demonstrating unusual coordination of biochemical and structural maturation and supporting the hypothesis that the entire cerebral cortex matures as an integrated network, rather than as a system-by-system cascade.

Older adults exhibit more intracortical inhibition and less intracortical facilitation than young adults

BACKGROUND

Aging results in decreased neuromuscular function, which is likely associated with neurologic alterations. At present little is known regarding age-related changes in intracortical properties.

METHODS

In this study we used transcranial magnetic stimulation (TMS) to measure intracortical facilitation (ICF), short- and long-interval intracortical inhibition (SICI and LICI), motor evoked potential amplitude, and silent period duration in young and older adults (21.4+/-0.8years and 70.9+/-1.8years). These variables were assessed from the flexor carpi radialis muscle of the non-dominant arm under resting conditions, and during a submaximal contraction (intensity 15% maximum strength).

RESULTS

Older adults exhibited increased SICI and LICI in comparison to young adults (SICI: 29.0+/-9.2% vs. 46.2+/-4.8% of unconditioned pulse; LICI: 6.5+/-1.7% vs. 15.8+/-3.3% of unconditioned pulse; P=0.04), and less ICF under resting conditions (74.6+/-8.7% vs. 104.9+/-6.9% of unconditioned pulse; P=0.02). These age-related differences disappeared during contraction, although the older adults did exhibit a longer silent period during contraction (112.5+/-6.5 vs. 84.0+/-3.9ms; P<0.01).

CONCLUSIONS

Collectively, these findings suggest increased GABA mediated intracortical inhibition with age.

Neural correlates of age-related changes in cortical neurophysiology

Functional imaging studies of cortical motor systems in humans have demonstrated age-related reorganisation often attributed to anatomical and physiological changes. In this study we investigated whether aspects of brain activity during a motor task were influenced not only by age, but also by neurophysiological parameters of the motor cortex contralateral to the moving hand. Twenty seven right-handed volunteers underwent functional magnetic resonance imaging whilst performing repetitive isometric right hand grips in which the target force was parametrically varied between 15 and 55% of each subject's own maximum grip force. For each subject we characterised two orthogonal parameters, B(G) (average task-related activity for all hand grips) and B(F) (the degree to which task-related activity co-varied with peak grip force). We used transcranial magnetic stimulation (TMS) to assess task-related changes in interhemispheric inhibition from left to right motor cortex (IHIc) and to perform measures relating to left motor cortex excitability during activation of the right hand. Firstly, we found that B(G) in right (ipsilateral) motor cortex was greater with increasing values of age(2) and IHIc. Secondly, B(F) in left ventral premotor cortex was greater in older subjects and in those in whom contralateral M1 was less responsive to TMS stimulation. In both cases, neurophysiological parameters accounted for variability in brain responses over and above that explained by ageing. These results indicate that neurophysiological markers may be better indicators of biological ageing than chronological age and point towards the mechanisms by which reconfiguration of distributed brain networks occurs in the face of degenerative changes.

Cortical maturation and myelination in healthy toddlers and young children

The maturation of cortical structures, and the establishment of their connectivity, are critical neurodevelopmental processes that support and enable cognitive and behavioral functioning. Measures of cortical development, including thickness, curvature, and gyrification have been extensively studied in older children, adolescents, and adults, revealing regional associations with cognitive performance, and alterations with disease or pathology. In addition to these gross morphometric measures, increased attention has recently focused on quantifying more specific indices of cortical structure, in particular intracortical myelination, and their relationship to cognitive skills, including IQ, executive functioning, and language performance. Here we analyze the progression of cortical myelination across early childhood, from 1 to 6 years of age, in vivo for the first time. Using two quantitative imaging techniques, namely T1 relaxation time and myelin water fraction (MWF) imaging, we characterize myelination throughout the cortex, examine developmental trends, and investigate hemispheric and gender-based differences. We present a pattern of cortical myelination that broadly mirrors established histological timelines, with somatosensory, motor and visual cortices myelinating by 1 year of age; and frontal and temporal cortices exhibiting more protracted myelination. Developmental trajectories, defined by logarithmic functions (increasing for MWF, decreasing for T1), were characterized for each of 68 cortical regions. Comparisons of trajectories between hemispheres and gender revealed no significant differences. Results illustrate the ability to quantitatively map cortical myelination throughout early neurodevelopment, and may provide an important new tool for investigating typical and atypical development.

The pattern of dendritic development in the cerebral cortex of the rat

The pattern of dendritic development of layer V pyramidal cells in the neocortex of the rat was studied using a variety of quantitative techniques in an attempt to determine what rules govern dendritic differentiation. Animals were sacrificed on postnatal days (P) 1, 3, 5, 7, 10, 15, 20, 25, 30 and 60, their brains impregnated with the rapid Golgi technique, and cells from the sensorimotor cortex examined for maximal apical and basilar dendritic field, number of dendritic branches at 20 micron intervals from the cell body, number of apical and basilar branch types (branching order), length of dendritic branch segments, and dendritic spine density. Primary dendrites are formed early in development, with no new ones formed after P7-10. Once a dendritic segment has bifurcated, all further development appears to occur at the tip, i.e. the trunk does not seem to undergo additional elongation, and new branches do not appear to form from the trunk. There is a plateau in dendritic differentiation close to the cell body after approximately P20; however, there is a continued increase in the length of terminal dendritic branches in the distal portions of the dendritic field into adulthood. During early development, dendrites bifurcate on reaching approximately 20-30 microns; however, during adulthood additional length is added to terminal dendrites without branching. Dendritic spines increase dramatically early in development, and then decline on proximal dendrites but continue to increase on terminal branches into adulthood. These results suggest that the terminal portion of the dendritic field remains plastic into adulthood, and that during development several general rules govern the pattern of dendritic differentiation.

Broca's region: cytoarchitectonic asymmetry and developmental changes

Functional imaging and clinical studies in children and adults have provided evidence of developmental changes in the hemispheric specialization for language. Whereas cytoarchitectonic asymmetry has been demonstrated in Broca's region of adults, the anatomical correlates of developmental changes in language dominance are largely unknown. In the present postmortem study of 34 human brains (ages ranging from 3.5 months to 85 years), the cytoarchitecture of areas 44 and 45 as the putative anatomical correlates of Broca's region, their developmental changes, and interhemispheric asymmetry were analyzed. Asymmetry as estimated by Euclidean distances between feature vectors of cytoarchitectonic profiles of left and right areas 44 and 45 was already found in 1-year-old infants. Asymmetry tended to increase with age, which was significant in area 45, but not in area 44. An adult-like, left-larger-than-right asymmetry in the volume fraction of cell bodies [gray level index (GLI)] was reached at approximately 5 years in area 45 and 11 years in area 44. These time points indicate a delayed development of the cytoarchitectonic asymmetry in Broca's region in comparison with that of the primary motor cortex. It may be hypothesized that the delayed maturation is the microstructural basis of the development of language abilities and the influence of language practice on cytoarchitecture during childhood. Interhemispheric asymmetry in the cytoarchitecture of areas 44 and 45 continues to change throughout life. We conclude that the cytoarchitectonic asymmetry of areas 44 and 45 is a result of microstructural plasticity that endures throughout almost the whole lifespan.

New dopaminergic terminal fields in the motor, visual (area 18b) and retrosplenial cortex in the young and adult rat. Immunocytochemical and catecholamine histochemical analyses

New dopaminergic terminal fields have been visualized in the rat cerebral neocortex, using two morphological methods based on distinct properties of the dopaminergic system: presence of the first synthetic enzyme, tyrosine hydroxylase, and high-affinity uptake of amines. Tyrosine hydroxylase was used as an immunocytochemical marker after destruction of the cortical noradrenergic system, induced either neonatally by 6-hydroxydopamine or later on by DSP4, and controlled by the absence of dopamine beta-hydroxylase immunoreactivity. The uptake and storage of exogenous amines in tissue sections, in the presence of selective high-affinity transport inhibitors, enabled the specific visualization of the dopaminergic system with fluorescence histochemistry. A dopaminergic innervation of low density was observed along a dorsal sagittal strip which extended from the genu of corpus callosum until about 2 mm behind the splenium and encompassed several distinct cytoarchitectonic areas in the sensorimotor and visual cortex (medial and lateral agranular field, area 18b), as well as in discrete zones of the retrosplenial granular 29c,b, and agranular 29d areas. The distribution of these dopaminergic fields suggested a columnar organization. Several characteristics of the dopaminergic innervation were similar to that of the superficial anterior cingulate cortex (area 24): the laminar distribution to the superficial I-III layers, the secondarily developed varicose aspect in catecholamine fluorescence histochemistry and the delayed postnatal ingrowth in contrast with the early prenatal dopaminergic input to the prefrontal cortex. These similarities suggested that the subpopulation of dopaminergic neurons which provides projections to the anterior cingulate cortex could also contribute to the motor and visual cortex and thus play a role in sensorimotor integration. The predictive value of these results in the ascent of the phylogenetic scale are further considered.