Functional corticospinal projections are established prenatally in the human foetus permitting involvement in the development of spinal motor centres

High-energy and -protein diet increases brain and corticospinal tract growth in term and preterm infants after perinatal brain injury

OBJECTIVE

Our hypothesis was that infants with perinatal brain injury fail to thrive in the first postnatal year because of increased energy and protein requirements from deficits that accumulated during neonatal intensive care. Our aim was to assess whether dietary energy and protein input was a rate-limiting factor in brain and body growth in the first year after birth.

METHODS

We conducted a prospective, double-blind and randomized, 2-stage group sequential study and controlled for gestation, gender, and brain lesion. Neonates with perinatal brain damage were randomly allocated to receive either a high- (120% recommended average intake) or average (100% recommended average intake) energy and protein diet. The study began at term and continued for 12 months. Three-day dietary diaries estimated energy and protein intake. The primary outcome measure was growth of occipitofrontal circumference. Other measures were growth of axonal diameters in the corticospinal tract, which were estimated by using transcranial magnetic stimulation, weight gain, and length.

RESULTS

The study was terminated at the first analysis when the 16 subjects had completed the protocol, because the predetermined stopping criterion of >1 SD difference in occipitofrontal circumference at 12 months' corrected age in those receiving the higher-energy and -protein diet had been demonstrated. Axonal diameters in the corticospinal tract, length, and weight were also significantly increased.

CONCLUSIONS

These data support our hypothesis that infants with significant perinatal brain damage have increased nutritional requirements in the first postnatal year and suggest that decreased postnatal brain growth may exacerbate their impairment. There are no measures of cognitive ability at 12 months of age, and whether there will be any improvement in the status of these children, therefore, remains to be shown.

Is hemiplegic cerebral palsy equivalent to amblyopia of the corticospinal system?

OBJECTIVE

Subjects with severe hemiplegic cerebral palsy have increased ipsilateral corticospinal projections from their noninfarcted cortex. We investigated whether their severe impairment might, in part, be caused by activity-dependent, competitive displacement of surviving contralateral corticospinal projections from the affected cortex by more active ipsilateral corticospinal projections from the nonaffected cortex, thereby compounding the impairment.

METHODS

Transcranial magnetic stimulation (TMS) characterized corticospinal tract development from each hemisphere over the first 2 years in 32 healthy children, 14 children with unilateral stroke, and 25 with bilateral lesions. Magnetic resonance imaging and anatomic studies compared corticospinal tract growth in 13 patients with perinatal stroke with 46 healthy subjects.

RESULTS

Infants with unilateral lesions initially had responses after TMS of the affected cortex, which became progressively more abnormal, and seven were eventually lost. There was associated hypertrophy of the ipsilateral corticospinal axons projecting from the noninfarcted cortex. Magnetic resonance imaging and anatomic studies demonstrated hypertrophy of the corticospinal tract from the noninfarcted hemisphere. TMS findings soon after the stroke did not predict impairment; subsequent loss of responses and hypertrophy of ipsilateral corticospinal axons from the noninfarcted cortex predicted severe impairment at 2 years. Infants with bilateral lesions maintained responses to TMS from both hemispheres with a normal pattern of development.

INTERPRETATION

Rather than representing "reparative plasticity," increased ipsilateral projections from the noninfarcted cortex compound disability by competitively displacing surviving contralateral corticospinal projections from the infarcted cortex. This may provide a pathophysiological explanation for why signs of hemiplegic cerebral palsy appear late and progress over the first 2 years of life.

Corticospinal dysgenesis and upper-limb deficits in congenital hemiplegia: a diffusion tensor imaging study

OBJECTIVES

Precision grasping critically relies on the integrity of the corticospinal tract as evidenced in congenital hemiplegia by the correlation found between corticospinal dysgenesis and hand-movement deficits. Therefore, corticospinal dysgenesis could be used to anticipate upper-limb deficits in young infants with congenital hemiplegia. However, most studies have quantified corticospinal dysgenesis by measuring the cross-sectional area of cerebral peduncles on T1 MRI, a measure biased by other structures present in the peduncles. The purpose of this study was to evaluate the extent to which this may have hampered the conclusions of previous studies. We also aimed to investigate the relationship between upper-limb deficits and a more accurate measure of corticospinal dysgenesis to provide a tool for anticipating upper-limb deficits in infants with congenital hemiplegia.

METHODS

To address this issue, we measured corticospinal tract areas in 12 patients with congenital hemiplegia and 12 matched control subjects by using the diffusion tensor imaging technique. Corticospinal dysgenesis was quantified by computing a symmetry index between the area of the contralateral and ipsilateral corticospinal tracts. This value was then compared with that resulting from the conventional MRI method.

RESULTS

The symmetry indexes gathered with these 2 methods were highly correlated, although the diffusion tensor imaging symmetry indexes were significantly smaller. This indicates that, in patients with congenital hemiplegia, the conventional MRI measurement has led to a systematic underestimate of corticospinal dysgenesis. These 2 estimates of corticospinal dysgenesis were also correlated with upper-limb impairments and disabilities. Although the symmetry index computed from peduncle measurements was correlated solely with deficits in stereognosis, the diffusion tensor imaging index correlated with stereognosis, digital and manual dexterities, and ABILHAND-Kids, a measure of manual ability in daily life activities.

CONCLUSIONS

The diffusion tensor imaging symmetry index provides a useful prognostic tool for anticipating upper-limb deficits and their consequences in daily life activities.

Functional asymmetries in the stepping response of the human newborn: a kinematic approach

In order to investigate subtle expressions of functional asymmetries in newborn leg movements, kinematic registrations were made on a sample of 40 healthy fullterm newborn infants during performance of the stepping response. Time-position data were collected from markers attached to the hip, knee and ankle joints of the left and right leg, and movements of both legs recorded simultaneously. Findings included evident side differences in terms of smoother trajectories of the right leg as a consequence of less movement segmentation compared to the left leg. Additionally, analyses of intralimb coordination revealed side differences with regard to stronger ankle-knee couplings and smaller phase shifts in the right leg. The findings suggest that asymmetries in newborn stepping responses are present in terms of spatio-temporal parameters and intralimb coordination. No evidence of a lateral preference in terms of frequency of the first foot moved was found. The present study adds new understanding to the lateralized attributes of the stepping response in the human newborn and as such points to new directions of research on the nature of laterality in the future.

A molecular neuroanatomical study of the developing human neocortex from 8 to 17 postconceptional weeks revealing the early differentiation of the subplate and subventricular zone

We have employed immunohistochemistry for multiple markers to investigate the structure and possible function of the different compartments of human cerebral wall from the formation of cortical plate at 8 postconceptional weeks (PCW) to the arrival of thalamocortical afferents at 17 PCW. New observations include the subplate emerging as a discrete differentiated layer by 10 PCW, characterized by synaptophysin and vesicular gamma-aminobutyric acid transporter expression also seen in the marginal zone, suggesting that these compartments may maintain a spontaneously active synaptic network even before the arrival of thalamocortical afferents. The subplate expanded from 13 to 17 PCW, becoming the largest compartment and differentiated further, with NPY neurons located in the outer subplate and KCC2 neurons in the inner subplate. Glutamate decarboxylase and calretinin-positive inhibitory neurons migrated tangentially and radially from 11.5 PCW, appearing in larger numbers toward the rostral pole. The proliferative zones, marked by Ki67 expression, developed a complicated structure by 12.5 PCW reflected in transcription factor expression patterns, including TBR2 confined to the inner subventricular and outer ventricular zones and TBR1 weakly expressed in the subventricular zone (SVZ). PAX6 was extensively expressed in the proliferative zones such that the human outer SVZ contained a large reservoir of PAX6-positive potential progenitor cells.

Modulation of soleus H-reflexes during gait in children with cerebral palsy

In healthy adults, soleus H-reflexes are rhythmically modulated and generally depressed during gait compared with rest. From ages 6 to 13 yr, there is a progressive increase in the tonic inhibition of H-reflexes during walking, especially during the stance phase of the step cycle. In adults, rhythmic modulation and tonic depression are severely disturbed after bilateral spinal lesions but remain partly preserved after unilateral cerebral lesions. Children with diplegic cerebral palsy (CP) suffer from a bilateral supraspinal lesion of the corticospinal tract that occurs before the maturation of the CNS is complete. If supraspinal structures are involved in the tonic, but not rhythmic, age-dependent reflex depression, it could be hypothesized that the tonic reflex depression with age is disturbed in CP, whereas the rhythmic part of the modulation remains unaffected. To test this hypothesis, soleus H-reflexes were assessed during gait in 16 CP children aged 5-11 and 15-16 and compared with 25 age-matched healthy children walking at similar velocities. Although the rhythmic part of the modulation pattern was present in CP, there was no significant tonic reflex depression with age, thus reflecting a lack of maturation of the corticospinal tract. It is argued the rhythmic part of the modulation may be generated on a spinal or brain stem level and is therefore not affected by the bilateral supraspinal lesion, whereas the tonic depression that occurs with maturation of the CNS is under supraspinal control. In conclusion, the supraspinal structures affected in CP are therefore likely involved in this age-dependent tonic depression.

Cerebro-muscular and cerebro-cerebral coherence in patients with pre- and perinatally acquired unilateral brain lesions

The cerebral networks involved in motor control were analyzed in four young hemi-paretic patients (21-25 years) with pre- and perinatally acquired brain lesions (3 with left periventricular brain lesions, 1 with left schizencephaly) by means of MEG source coherence analysis. Previous TMS and fMRI studies on the same patients had investigated their residual ability to move the paretic hand by means of a reorganized primary motor cortex (M1) representation in the contralesional hemisphere. The purpose of this study is to identify the effects of such a cerebral reorganization and the related dynamic aspects which allow the patients to move the paretic arm. Patients underwent a pinch grip task (1-N isometric contraction) using their paretic and non-paretic hands in alternation. MEG signals were recorded using a whole-head 151-channel magnetoencephalograph. EMG was simultaneously recorded as a reference for coherence calculations. 3D coherence mapping was performed in the beta frequency range (14-30 Hz). This approach confirmed the relocation of motor functions from the lesioned (left) to the contralesional (right) hemisphere. In case of left, non-paretic pinch grip, coherent activity originated from contralateral (right) M1 exclusively. In the case of right (paretic) grip, coherent activity in ipsilateral M1 as well as significant coherence of ipsilateral cerebellum with both muscle activity and M1 itself was detected in 3 out of 4 subjects. As expected, the patient with no cerebellar involvement during paretic hand contraction showed the worst motor performance in the grip task. Coupling direction analysis demonstrated that throughout pinch grip the coupling direction goes from M1 to cerebellum. The present study verified the assumption that the intact hemisphere takes over motor control from the paretic (ipsilateral) hand in the presence of early unilateral brain lesion. Moreover, the role of cerebellum in motor deficit compensation and its close interaction with ipsilateral primary motor cortex was studied in detail.

Activity- and use-dependent plasticity of the developing corticospinal system

The corticospinal (CS) system, critical for controlling skilled movements, develops during the late prenatal and early postnatal periods in all species examined. In the cat, there is a sequence of development of the mature pattern of terminations of CS tract axons in the spinal gray matter, followed by motor map development of the primary motor cortex. Skilled limb movements begin to be expressed as the map develops. Development of the proper connections between CS axons and spinal neurons in cats depends on CS neural activity and motor behavioral experience during a critical postnatal period. Reversible CS inactivation or preventing limb use produces an aberrant distribution of CS axon terminations and impairs visually guided movements. This altered pattern of CS connections after inactivation in cats resembles the aberrant pattern of motor responses evoked by transcranial magnetic stimulation in hemiplegic cerebral palsy patients. Left untreated in the cat, these impairments do not resolve. We have found that activity-dependent processes can be harnessed in cats to reestablish normal CS connections and function. This finding suggests that aspects of normal CS connectivity and function might some day be restored in hemiplegic cerebral palsy.

The dependence of spinal cord development on corticospinal input and its significance in understanding and treating spastic cerebral palsy

The final phase of spinal cord development follows the arrival of descending pathways which brings about a reorganisation that allows mature motor behaviours to emerge under the control of higher brain centres. Observations made during typical human development have shown that low threshold stretch reflexes, including excitatory reflexes between agonist and antagonist muscle pairs are a feature of the newborn. However, perinatal lesions of the corticospinal tract can lead to abnormal development of spinal reflexes that includes retention and reinforcement of developmental features that do not emerge in adult stroke victims, even though they also suffer from spasticity. This review describes investigations in animal models into how corticospinal input may drive segmental maturation. It compares their findings with observations made in humans and discusses how therapeutic interventions in cerebral palsy might aim to correct imbalances between descending and segmental inputs, bearing in mind that descending activity may play the crucial role in development.

Locomotor training remodels fMRI sensorimotor cortical activations in children after cerebral hemispherectomy

PURPOSE

This study examined whether locomotor training, which included body weight-supported treadmill therapy, improved walking and induced cortical representational adaptations using functional magnetic resonance imaging in the remaining sensorimotor network after cerebral hemispherectomy.

METHODS

Hemispherectomy patients (n = 12) underwent 2 weeks of gait training for at least 30 hours each. They were tested pre- and posttraining with the Fugl-Meyer Motor Assessment, unassisted single-limb stance time, and usual and fastest walking speeds. Three patients performed voluntary ankle movements as the functional magnetic resonance imaging activation task pre- and posttraining. Control subjects included 5 healthy children tested 2 weeks apart, 2 of whom trained on the treadmill, and 2 hemispherectomy patients who received upper extremity rehabilitation and no gait therapy.

RESULTS

Although patients reported improvements with gait training, behavioral outcomes did not significantly change. Training was associated with increased volume and intensity of cortical activation in the primary sensorimotor (S1M1), supplementary motor, motor cingulate, and secondary somatosensory cortex for the paretic foot, along with greater overlap in the representation for each moving foot in S1M1 and the supplementary motor area of the remaining hemisphere. Control subjects showed a decrease in activation in these cortical regions after training.

CONCLUSIONS

Locomotor training of hemispherectomy patients improved mobility subjectively in association with functional magnetic resonance imaging evidence of cortical remodeling with ankle dorsiflexion. These findings support the notion that hemispherectomy patients may respond to rehabilitation interventions through mechanisms of activity-dependent cortical plasticity. The authors hypothesize that developmentally persistent descending ipsilateral and contralateral corticospinal tracts may allow the remaining hemisphere to maintain bilateral lower extremity motor control after surgery.

Spinal cord plasticity in acquisition and maintenance of motor skills

Throughout normal life, activity-dependent plasticity occurs in the spinal cord as well as in brain. Like other central nervous system (CNS) plasticity, spinal cord plasticity can occur at numerous neuronal and synaptic sites and through a variety of mechanisms. Spinal cord plasticity is prominent early in life and contributes to mastery of standard behaviours like locomotion and rapid withdrawal from pain. Later in life, spinal cord plasticity has a role in acquisition and maintenance of new motor skills, and in compensation for peripheral and central changes accompanying ageing, disease and trauma. Mastery of the simplest behaviours is accompanied by complex spinal and supraspinal plasticity. This complexity is necessary, in order to preserve the complete behavioural repertoire, and is also inevitable, due to the ubiquity of activity-dependent CNS plasticity. Explorations of spinal cord plasticity are necessary for understanding motor skills. Furthermore, the spinal cord's comparative simplicity and accessibility makes it a logical starting point for studying skill acquisition. Induction and guidance of activity-dependent spinal cord plasticity will probably play an important role in realization of effective new rehabilitation methods for spinal cord injuries, cerebral palsy and other motor disorders.

Changes in EMG coherence between long and short thumb abductor muscles during human development

In adults, motoneurone pools of synergistic muscles that act around a common joint share a common presynaptic drive. Common drive can be revealed by both time domain and frequency domain analysis of EMG signals. Analysis in the frequency domain reveals significant coherence in the range 1-45 Hz, with maximal coherence in low (1-12 Hz) and high (16-32 Hz) ranges. The high-frequency range depends on cortical drive to motoneurones and is coherent with cortical oscillations at approximately 20 Hz frequencies. It is of interest to know whether oscillatory drive to human motoneurone pools changes with development. In the present study we examined age-related changes in coherence between rectified surface EMG signals recorded from the short and long thumb abductor muscles during steady isometric contraction obtained while subjects abducted the thumb against a manipulandum. We analysed EMG data from 36 subjects aged between 4 and 14 years, and 11 adult subjects aged between 22 and 59 years. Using the techniques of pooled coherence analysis and the chi(2) difference of coherence test we demonstrate that between the ages of 7 and 9 years, and 12 and 14 years, there are marked increases in the prevalence and magnitude of coherence at frequencies between 11 and 45 Hz. The data from subjects aged 12-14 years were similar to those obtained from adult controls. The most significant differences between younger children and the older age groups were detected at frequencies close to 20 Hz. We believe that these are the first reported results demonstrating significant late maturational changes in the approximately 20 Hz common oscillatory drive to human motoneurone pools.

The development of cerebral connections during the first 20-45 weeks' gestation

We have correlated data on neuroanatomical organization and magnetic resonance imaging of transient fetal zones shown to contain connectivity elements (growing axons, synapses, dendrites). In the fetal phase, afferent fibres 'wait' within the subplate zone which is the most prominent lamina on histological and magnetic resonance images and is a substrate of endogenous neuronal activity. In early preterm the thalamocortical afferents accumulate within the superficial subplate and grow into cortical plate developing synapses. In late preterm, the resolution of the subplate and growth of cortico-cortical fibres into the cortical plate occur simultaneously with gyration. Both preterm phases characterize the coexistence of endogenous and sensory-driven circuitries and occurrence of the transient electrical phenomena. In neonates, the long cortico-cortical pathways stop growth, and the main histogenetic events are an elaboration of intracortical circuitry and synaptogenesis. In conclusion, the growth of the axonal pathways preterm explains their vulnerability and plasticity. In neonates the vulnerability is related to the intracortical circuitry.

Toy-oriented changes in early arm movements II--joint kinematics

Our recent work suggests that infants begin to change their hand and joint kinematics in the presence of a toy months before the onset of purposeful reaching. Moreover, these 'toy-oriented' changes in hand kinematics cluster into Early, Mid and Late phases. The purpose of the present study was to test hypotheses regarding toy-oriented changes in joint kinematics in the same infants.

METHODS

Thirteen infants were observed every other week from 8 weeks up to the first week of reaching around 20 weeks. At each session, a high-speed motion analysis system recorded infants' arm movements with and without a toy present.

RESULTS

During the Early phase, infants scaled down their movements. In contrast, during the Mid phase infants scaled up their movements and did not change the relationship between the shoulder and elbow for speed and smoothness-related variables. In addition, infants showed toy-oriented changes such as increase in shoulder flexion and adduction. In the Late phase, infants continued to produce toy-oriented changes in shoulder orientation, and increased shoulder excursion and speed relative to the elbow. Thus, the toy-oriented changes in hand kinematics in the Mid and Late phases [Bhat, A. N., & Galloway, J. C. (2006). Toy-oriented changes in early arm movements of young infants: Hand kinematics. Infant Behavior and Development, 29(3), 358-372] more closely followed changes in shoulder kinematics. Lastly, results are discussed in terms of shoulder-elbow dissociations, speed-amplitude relationships, and the key role of spontaneous movements in the development of reaching.

Spinal and brain control of human walking: implications for retraining of walking

In this update, the authors will discuss evidence for both spinal and brain regulation of walking in humans. They will consider the sensory control of walking in young babies and spinal cord-injured adults, two models with weak descending input from the brain, to suggest that subcortical structures are important in shaping walking behavior. Based on evidence from development, the authors suggest that the primitive pattern of walking seen in babies forms the base upon which additional features are added by supraspinal input as independent walking develops. Increasing evidence suggests the motor cortex is important in the control of level-ground walking in adults, in contrast to quadrupeds. This brain input seems particularly important for distal flexors in the leg. Finally, the authors will consider evidence that the recovery of walking after incomplete spinal cord injuries is dependent on the presence of descending input from the motor cortex and our ability to strengthen that input. These findings imply that training methods for improving walking after injury to the nervous system must promote the involvement of both spinal and brain circuits.

Detection of impaired growth of the corpus callosum in premature infants

OBJECTIVE

There is an urgent need for a bedside method to assess the effectiveness of neonatal therapies designed to improve cerebral development in very low birth-weight infants. The aim of this study was to assess the impact of preterm birth on the serial growth of the corpus callosum and how soon it could be detected after birth with cranial ultrasound.

METHODS

We recruited 61 very low birth-weight infants admitted to a single regional level III NICU from 1998 to 2000. Study infants had 2 cranial sonograms > or = 7 days apart in the first 2 weeks of life and further sonograms at 6 weeks and at term equivalent. At each time point, the length of the corpus callosum and cerebellar vermis was measured on midline sagittal images, with growth rates calculated in millimeters per day. We compared growth of corpus callosum and cerebellar vermis in individuals, between birth age groups, and with corrected gestational age. We used antenatal growth rate of the corpus callosum of 0.2 to 0.27 mm/day as a reference. Relationships between corpus callosum growth rates and neurodevelopmental outcome at 2 years of age (corrected) were also examined.

RESULTS

Growth of the corpus callosum was normal in most infants during the first 2 weeks of life but slowed after this (0.21 mm/day from 0-2 weeks vs 0.11 mm/day for weeks 2-6). Slowing of corpus callosum growth below expected reference range was consistently detectable by age 6 weeks for 96% of infants born between 23 and 33 weeks' gestation. Although some improvement in growth rate was observed for 15% of infants after 6 weeks, this was confined to infants born after 28 weeks. Vermis length correlated strongly with corpus callosum length. By 2 years of age, serious motor delay and cerebral palsy were associated with poorer growth of the length of the corpus callosum between 2 and 6 weeks after birth.

CONCLUSIONS

The effect of preterm birth on growth of the corpus callosum is detectable by 6 weeks after delivery in preterm infants born at gestations of 23 to 33 weeks. Reduced growth of the corpus callosum in weeks 2 to 6, places these infants at elevated risks of later psychomotor delay and cerebral palsy.

Ontogeny of the human central nervous system: what is happening when?

The present paper reviews current data on the structural development of the human nervous system. Focus is on the timing of ontogenetic events in the telencephalon. Neuronal proliferation and migration especially occur during the first half of gestation; the second half of gestation is the period of the existence of the functionally important transient structure 'subplate' and the major period of glial cell proliferation and programmed cell death. Axon and dendrite sprouting and synapse formation bloom during the last trimester of gestation and the first postnatal year. Major part of telencephalic myelination occurs during the first year after birth. Many developmental processes, such as myelination, synapse formation and synapse elimination continue throughout childhood and adolescence. Evidence is emerging that the peak of synapse elimination occurs between puberty and the onset of adulthood. Neurotransmitter systems are present from early foetal life onwards and their pre- and perinatal development is characterized by periods of transient overexpression. The latter is for instance true for the acetylcholinergic, catecholaminergic and glutamate systems. Thus, the development of the human brain is characterized by a protracted, neatly orchestrated chain of specific ontogenetic events. The continuous changes of the nervous system have consequences for vulnerability to adverse conditions, for diagnostics and for physiotherapeutical intervention.

Movement analysis in neonates with spina bifida aperta

INTRODUCTION

In neonates with spina bifida aperta (SBA), leg movements by myotomes caudal to the meningomyelocele (MMC) are transiently observed. It is unclear whether these leg movements relate to functional neural conduction through the MMC. For optimal therapeutical intervention, pathophysiological insight in these transient leg movements seems relevant. If leg movements by myotomes caudal to the MMC concur with the execution of general movements (GMs), functional neural conduction through the MMC is implicated.

OBJECTIVE

In neonates with SBA, we aimed to determine whether the transiently present leg movements caudal to the MMC indicate functional neural conduction through the MMC.

METHODS

During the perinatal period, fetuses and neonates with SBA (n = 7 and n = 13, respectively) were longitudinally analysed for concurrency between leg movements caudal to the MMC and GMs. To address the integrity of the reflex arc in spinal segments (at, or) caudal to the MMC, tendon leg reflexes were assessed during the first postnatal week.

RESULTS

At postnatal day 1, leg movements caudal to the MMC concurred with GMs in 12 of 13 infants. Isolated leg movements were observed in only 3 of these 12 infants (isolated vs. concurrent; p < 0.005). Leg movements concurring with GMs lasted longer than isolated leg movements (median duration = 11 s vs. 2 s; p < 0.05). Between days 1 and 7, tendon leg reflexes (at, or) caudal to the MMC had disappeared in all but 1 neonate. However, leg movements caudal to the MMC remained concurrently present with GMs in all five neonates available for follow-up after day 7. Comparing these leg movements between days 1 and 7 indicated a decreased duration (-44%, p < 0.05).

CONCLUSIONS

In neonates with SBA, leg movements caudal to the MMC concur with GMs, indicative of functional neural conduction through the MMC. The disappearance of these leg movements is caused by lower motor neuron dysfunction at the reflex arc, whereas neural conduction through the MMC is still functional.

Toy-oriented changes during early arm movements: hand kinematics

In a recent cross-sectional study, we found that young infants changed their spontaneous arm movements in the presence of a toy, termed 'toy-oriented changes', in systematic ways beginning many weeks before their first consistent reaches [Bhat, A. N., Heathcock, J. H., & Galloway, J. C. (2005). Toy-oriented changes in hand and joint kinematics during the emergence of purposeful reaching. Infant Behavior and Development, 28(4), 445-465]. The purpose of the present study was to test specific hypotheses regarding toy-oriented changes in a longitudinal design.

METHODS

Thirteen infants were observed every other week from 8 weeks of age up to the onset of reaching. At each session, hand and joint motions were observed with and without a toy present using a high-speed motion capture system. This paper focuses on the toy-oriented changes in hand variables.

RESULTS

As predicted, infants displayed a meaningful pattern of toy-oriented changes, which systematically changed as infants approached the first week of reaching. During the Early phase (8-10 weeks before reaching), infants scaled down their movement length and speed in the presence of a toy. During the Mid phase (4-6 weeks before reaching), infants scaled up movement number and speed, increased movement smoothness, and decreased their hand-toy distance in the presence of a toy. During the Late phase (within 2 weeks of reaching), infants continued to change their hand's position to get closer to the toy and began contacting it. Interestingly, movement number and smoothness displayed similar developmental patterns, where movement length and speed displayed similar patterns.

CONCLUSION

Toy-oriented adaptation of arm movements emerges in the first months of life and forms a complex, yet tractable continuum with purposeful reaching. These results provide a foundation to test more specific hypotheses of hand and joint coordination in both typically developing infants and those infants born at risk for coordination impairments.

MRI of normal fetal brain development

Normal fetal brain maturation can be studied by in vivo magnetic resonance imaging (MRI) from the 18th gestational week (GW) to term, and relies primarily on T2-weighted and diffusion-weighted (DW) sequences. These maturational changes must be interpreted with a knowledge of the histological background and the temporal course of the respective developmental steps. In addition, MR presentation of developing and transient structures must be considered. Signal changes associated with maturational processes can mainly be ascribed to the following changes in tissue composition and organization, which occur at the histological level: (1) a decrease in water content and increasing cell-density can be recognized as a shortening of T1- and T2-relaxation times, leading to increased T1-weighted and decreased T2-weighted intensity, respectively; (2) the arrangement of microanatomical structures to create a symmetrical or asymmetrical environment, leading to structural differences that may be demonstrated by DW-anisotropy; (3) changes in non-structural qualities, such as the onset of a membrane potential in premyelinating axons. The latter process also influences the appearance of a structure on DW sequences. Thus, we will review the in vivo MR appearance of different maturational states of the fetal brain and relate these maturational states to anatomical, histological, and in vitro MRI data. Then, the development of the cerebral cortex, white matter, temporal lobe, and cerebellum will be reviewed, and the MR appearance of transient structures of the fetal brain will be shown. Emphasis will be placed on the appearance of the different structures with the various sequences. In addition, the possible utility of dynamic fetal sequences in assessing spontaneous fetal movements is discussed.

The education and re-education of the spinal cord

In normal life, activity-dependent plasticity occurs in the spinal cord as well as in the brain. Like CNS plasticity elsewhere, this spinal cord plasticity can occur at many neuronal and synaptic sites and by a variety of mechanisms. Spinal cord plasticity is prominent in postnatal development and contributes to acquisition of standard behaviors such as locomotion and rapid withdrawal from pain. Later on in life, spinal cord plasticity contributes to acquisition and maintenance of specialized motor skills, and to compensation for the peripheral and central changes associated with aging, disease, and trauma. Mastery of even the simplest behaviors is accompanied by complex spinal and supraspinal plasticity. This complexity is necessary, to preserve the full roster of behaviors, and is also inevitable, due to the ubiquity of activity-dependent plasticity in the CNS. Careful investigation of spinal cord plasticity is essential for understanding motor skills; and, because of the relative simplicity and accessibility of the spinal cord, is a logical and convenient starting point for exploring skill acquisition. Appropriate induction and guidance of activity-dependent plasticity in the spinal cord is likely to be a key part of the realization of effective new rehabilitation methods for spinal cord injuries, cerebral palsy, and other chronic motor disorders.

The importance of ‘awareness’ for understanding fetal pain

Our understanding of when the fetus can experience pain has been largely shaped by neuroanatomy. However, completion of the cortical nociceptive connections just after mid-gestation is only one part of the story. In addition to critically reviewing evidence for whether the fetus is ever awake or aware, and thus able to truly experience pain, we examine the role of endogenous neuro-inhibitors, such as adenosine and pregnanolone, produced within the feto-placental unit that contribute to fetal sleep states, and thus mediate suppression of fetal awareness. The uncritical view that the nature of presumed fetal pain perception can be assessed by reference to the prematurely born infant is challenged. Rigorously controlled studies of invasive procedures and analgesia in the fetus are required to clarify the impact of fetal nociception on postnatal pain sensitivity and neural development, and the potential benefits or harm of using analgesia in this unique setting.

Index finger movement imitation by human neonates: motivation, learning, and left-hand preference

Imitation of a fine motor movement, index finger protrusion, was examined in 39 neonates using an ethologically based objective coding system. Results confirmed that imitation of finger movements exists, and infants demonstrated "learning" as imitation developed through an incomplete imitation stage. Neonatal imitation was more frequently left-handed, an early sign of laterality in motivation to be investigated further. The existence of index finger imitation in human neonates indicates that volitional control of individuated finger movements develops much earlier than previously thought. The differential increase of index finger protrusion movements during the imitation periods suggests that this behavior is not an automatic response triggered by general arousal but instead is a true indicator of purposeful neonatal imitation.

Living a normal life with the nondominant hemisphere: magnetic resonance imaging findings and clinical outcome for a patient with left-hemispheric hydranencephaly

In hemihydranencephaly, the human brain lacks 1 complete hemisphere. An occlusion of the carotid artery, affecting all supplied territories, is thought to be the underlying mechanism. This extremely rare disorder, of which only 7 cases have been reported to date, is thought to occur before the last trimester of gestation (20th to 27th week), after neural migration but before synaptogenesis. We report on a 36-year-old man born at term, with no complications, from nonconsanguineous healthy parents. Cranial computed tomography had been performed because of left-sided headaches. Because of the imaging findings, the patient presented at our institution for additional MRI and clinical testing (including the Motor Activity Log, Wolf motor function test, 2-point discrimination test, Purdue pegboard test, gross motor function test, Physician Rating Scale, and Aachener aphasia test, including patterns for spontaneous speech, repetition, naming, comprehension, written language, and the token test). The patient's disabilities were related to deficits in fine motor control and reduced precision. Therefore, the patient was unable to perform the Purdue pegboard test with his affected hand. According to the Aachener aphasia test, no aphasia could be demonstrated for this strongly left-handed patient. Strong mirror movements were found. Cortical reorganization is possible if damage occurs in very early childhood. Motor function and speech were controlled by the remaining, nonaffected hemisphere, with a remarkable outcome. Because the damage is thought to occur before synaptogenesis, existing or prepared cortical areas and pathways have the potential to execute the lacking functions of the destroyed hemisphere.

Corticospinal regeneration into lumbar grey matter correlates with locomotor recovery after complete spinal cord transection and repair with peripheral nerve grafts, fibroblast growth factor 1, fibrin glue, and spinal fusion

Knowledge of which tracts are essential for the recovery of locomotor function in rats after repair is unknown. To assess the mechanism of recovery, we examined the correlation between functional recovery and axonal regeneration. All rats underwent complete cord transection and repair with peripheral nerves, fibroblast growth factor 1, fibrin glue, and spinal fixation. Repaired rats recovered both motor-evoked potentials recorded at the lumbar level and locomotor function. Cord retransection rostral to the repair abolished the recovery, indicating improvement was due to long tract regeneration. To determine which long tracts correlated with recovery, a novel technique of simultaneous bidirectional axonal tracing and immunohistochemical examination of axonal type was used to quantitate the regeneration of corticospinal, rubrospinal, reticulospinal, vestibulospinal, raphespinal, propriospinal, serotonergic, and calcitonin gene-related peptide containing axons. Multiple linear regression analysis revealed recovery of function correlated only with regeneration of corticospinal axons into the gray matter of the lumbar spinal cord (R = 0.977, p < 0.02). For the first time, we show that regeneration of the corticospinal tract into the lumbar gray matter is a mechanism of functional locomotor recovery after complete cord transection and repair.

Infant stepping: a window to the behaviour of the human pattern generator for walking

The aim of this paper is to provide evidence, both published and new, to support the notion that human infants are particularly good subjects for the study of the pattern generator for walking. We and others have shown that stepping can be initiated by sensory input from the legs or by general heightened excitability of the infant. New results are presented here to suggest that weight support through the feet and rapid extension of the legs are important proprioceptive inputs to initiate stepping. Our previous work has shown that infants can step at many different speeds when supported on a treadmill. The step cycle duration shortens as the speed increases, with the changes coming largely from the stance phase, just as in most other terrestrial animals. Moreover, we have shown that infants will step in all directions. Regardless of the direction of stepping, the step cycle changes in the same way with walking speed, suggesting the circuitry that controls different directions of walking share common elements. We have also shown that infant stepping is highly organized. Sensory inputs, whether proprioceptive or touch, are gated in a functional way so that only important sensory inputs generate a response. For example, touch to the lateral surface of the foot elicits a response only in sideways walking, and only in the leading limb. New data is presented here to show that the pattern generators from each limb can operate somewhat independently. On a split-belt treadmill with the 2 belts running at different speeds or in different directions, the legs showed considerable independence in behaviour. Yet, the pattern generators on each side interact to ensure that swing phase does not occur at the same time. These studies have provided insight into the organization of the pattern generator for walking in humans. It will be interesting in the future to study how maturation of the descending tracts changes walking behaviour to allow independent bipedal walking.

Rescuing transient corticospinal terminations and promoting growth with corticospinal stimulation in kittens

Development of corticospinal (CS) terminations is activity dependent. In the cat, activity-dependent refinement of termination topography occurs between weeks 3 and 6. Initially, sparse terminals are present in the gray matter bilaterally, including the motor nuclei. By week 6, virtually all motor nuclear terminations are eliminated, as are most ipsilateral terminations. In this study, we determined whether electrical stimulation of CS axons could be used to rescue transient terminations and promote their growth. We implanted microwires in the pyramid or spinal white matter to stimulate CS axons (2 hr/d, 330 Hz, 45 msec burst, 2 sec intervals) for 2-3 weeks during the refinement period. CS terminations were traced using wheat germ agglutinin conjugated to horseradish peroxidase. Animals were killed after week 6. Stimulation produced dense terminations bilaterally, including within the motor nuclei. Termination density was least in lamina 1 and ventral lamina 9. Reticular formation stimulation produced a control (i.e., nonstimulated) termination pattern. To determine whether CS stimulation affected development of the nonstimulated CS system, we traced terminations from the contralateral cortex using biotinylated dextran amine. Compared with controls or after reticular formation stimulation, there was a shift in the distribution of terminations of the nonstimulated side to more dorsal laminas, which is where the stimulated CS system had fewer terminals. This distribution shift is consistent with competition for termination space between the CS systems on both sides. Our findings indicate that activity can be harnessed to bias CS axon terminal development. This has important implications for using activity to modify motor system organization after perinatal CNS trauma.

Comparing the function of the corticospinal system in different species: Organizational differences for motor specialization?

An appreciation of the comparative functions of the corticospinal tract is of direct relevance to the understanding of how results from animal models can advance knowledge of the human motor system and its disorders. Two critical functions of the corticospinal tract are discussed: first, the role of descending projections to the dorsal horn in the control of sensory afferent input, and second, the capacity of direct cortico-motoneuronal projections to support voluntary execution of skilled hand and finger movements. We stress that there are some important differences in corticospinal projections from different cortical regions within a particular species and that these projections support different functions. Therefore, any differences in the organization of corticospinal projections across species may well reflect differences in their functional roles. Such differences most likely reflect features of the sensorimotor behavior that are characteristic of that species. Insights into corticospinal function in different animal models are of direct relevance to understanding the human motor system, providing they are interpreted in relation to the functions they underpin in a given model. Studies in non-human primates will continue to be needed for understanding special features of the human motor system, including feed-forward control of skilled hand movements. These movements are often particularly vulnerable to neurological disease, including stroke, cerebral palsy, movement disorders, spinal injury, and motor neuron disease.

Development and malformations of the human pyramidal tract

The corticospinal tract develops over a rather long period of time, during which malformations involving this main central motor pathway may occur. In rodents, the spinal outgrowth of the corticospinal tract occurs entirely postnatally, but in primates largely prenatally. In mice, an increasing number of genes have been found to play a role during the development of the pyramidal tract. In experimentally studied mammals, initially a much larger part of the cerebral cortex sends axons to the spinal cord, and the site of termination of corticospinal fibers in the spinal grey matter is much more extensive than in adult animals. Selective elimination of the transient corticospinal projections yields the mature projections functionally appropriate for the pyramidal tract. Direct corticomotoneuronal projections arise as the latest components of the corticospinal system. The subsequent myelination of the pyramidal tract is a slow process, taking place over a considerable period of time. Available data suggest that in man the pyramidal tract develops in a similar way. Several variations in the funicular trajectory of the human pyramidal tract have been described in otherwise normally developed cases, the most obvious being those with uncrossed pyramidal tracts. A survey of the neuropathological and clinical literature, illustrated with autopsy cases, reveals that the pyramidal tract may be involved in a large number of developmental disorders. Most of these malformations form part of a broad spectrum, ranging from disorders of patterning, neurogenesis and neuronal migration of the cerebral cortex to hypoxic-ischemic injury of the white matter. In some cases, pyramidal tract malformations may be due to abnormal axon guidance mechanisms. The molecular nature of such disorders is only beginning to be revealed.

Topological aspects of axonal regeneration

The fibers making up any sensory system in the spinal cord come from the same cells as do the fibers in peripheral nerves yet severed nerve fibers in the adult spinal cord do not regenerate but damaged peripheral nerves - those in the extremities - do heal themselves. Why should spinal cord regeneration even be an issue, why should an inhibiting protein have evolved to prevent it and what causes this protein to be expressed? From a holistic perspective, an answer to this conundrum shows that these questions are intertwined, and suggests that: (1) The model of the neurons as 'wires' is too simplistic. (2) In humans, the 'map' of individual connections is (topologically, at least) locally variable, though the overall global topology and 'functionality' of each normal spinal cord is constant. Both of these issues have to be addressed if functional restoration is to be achieved.

Use of botulinum toxin in pediatric spasticity (cerebral palsy)

Local injection of botulinum toxin (BT) is a well-established treatment option for spastic movement disorders in children. BT blocks the release of acetylcholine from the axon terminal into the synaptic cleft of the motor endplate resulting in paresis of the injected musculature. Such localised, temporary chemodenervation of affected muscles can lead to functional gains and may improve the child's daily routine and rehabilitative care. We summarise state-of-the-art treatment of spasticity in children with BT type A, addressing critical issues and introducing recent advances, such as sonography-guided injection of BT and the distal injection of the psoas muscle without the need for general anaesthesia. First-hand experience with BT type B in children is presented.

Corticospinal system development depends on motor experience

Early motor experiences have been shown to be important for the development of motor skills in humans and animals. However, little is known about the role of motor experience in motor system development. In this study, we address the question of whether early motor experience is important in shaping the development of the corticospinal (CS) tract. We prevented limb use by the intramuscular injection of botulinum toxin A into selected forelimb muscles to produce muscle paralysis during the period of development of CS connection specificity, which is between postnatal weeks 3 and 7. CS terminations were examined using an anterograde tracer. Preventing normal forelimb use during CS axon development produced defective development of CS terminations at week 8 and in maturity. There were reductions in the topographic distribution of axon terminals, in terminal and preterminal branching, and in varicosity density. This suggests that limb use is needed to refine CS terminals into topographically specific clusters of dense terminal branches and varicosities. To determine correlated effects on motor behavior, cats were tested in a prehension task, to reach and grasp a piece of food from a narrow food well, when the neuromuscular blockade dissipated (by week 10) and in maturity (week 16). Preventing normal limb use also produced a prehension deficit later in development and in maturity, in which there was a loss of the supination component of grasping. This component of prehension in the cat depends on CS projections from the paw representation of rostral motor cortex to the cervical enlargement. Our findings show that motor experiences are necessary for normal development of CS terminations and function.

Kicking coordination captures differences between full-term and premature infants with white matter disorder

This study explores the relation of white matter disorder (WMD) to intralimb coordination patterns in premature infants with very low birth weight (VLBW). We specifically measured the temporal-spatial characteristics of intralimb coordination patterns of the legs. Three groups of infants were compared at one month corrected age (CA): 10 premature infants born VLBW and WMD (PTWMD), 10 premature infants born VLBW without WMD (PT) and 10 full term infants (FT). Using kinematic variables, we discriminate among VLBW infants with WMD from the two comparison groups. Infants born with WMD maintain patterns of tight coupling among leg joints (all flexion or all extension) while PT and FT term infants have begun to decouple leg joints by this age (combinations of flexion with extension). The coupling pattern is captured through joint correlations, discrete relative phase, and phase plane portraits. The PTWMD infants also demonstrate aberrant patterns of coordination evident through both temporal and spatial characteristics of the kicks. This is the first evidence that movement disorder associated with brain lesions can be identified and quantified with kinematic variables as early as one month of age.

Functional recovery after lesions of the primary motor cortex

After a lesion in the motor cortex of an adult primate, are cortical motor maps reorganized? This important question has attracted much interest throughout the past decade. In human subjects, substantial progress has resulted from the use of noninvasive imaging and stimulation techniques. For example, there is recent, well-accepted, albeit indirect evidence that following such a lesion on one side of the human brain, a dramatic reorganization of the hand representation occurs within either the ipsilateral primary motor cortex, nonprimary motor areas or both. The contribution of contralateral motor areas to functional recovery of the paretic hand remains uncertain, however, because of the lack of direct confirmatory evidence obtained from experiments undertaken on nonhuman primates. A better understanding of how the brain selects the optimal strategy for functional recovery following cortical lesions, and the neuronal mechanisms underlying cortical plasticity, will be important challenges for the next decade. To this end, the purpose of the present chapter is to provide an update on what is truly known about the functional recovery that takes place after a lesion in the primary motor cortex of both the nonhuman primate and the human. It bears emphasis that work on these fundamental issues is an essential prerequisite to the development of improved therapeutic and rehabilitation procedures for the brain-injured human.

Postnatal development of corticospinal postsynaptic action

The corticospinal system has a delayed and prolonged postnatal development. In the cat, lesion, inactivation, or stimulation of the system influence motor output minimally when corticospinal (CS) terminals have an immature topographic pattern but produce robust effects immediately after developing the mature pattern by weeks 6-7. In this study, we directly tested if the delay in expression of cortical motor functions is due to the inability of the corticospinal synapse to activate spinal neurons. We stimulated corticospinal axons in the pyramid and recorded evoked field potentials from the surface of the cervical spinal cord and locally from within the gray matter in anesthetized cats during development and in adults. Pyramidal stimulation in animals between week 4 and maturity evoked an initial corticospinal surface volley followed by a postsynaptic field response. Depth recordings from the superficial dorsal horn to the ventral white matter showed that local pre- and postsynaptic field potentials could be recorded over the full extent of the gray matter in 4- to 5-wk animals but were restricted to the intermediate zone in older animals and adults. Dorsoventral refinement of CS field potentials parallels anatomical refinement of individual CS axon terminals shown in our earlier studies. Our present findings indicate that the developing corticospinal system could influence the excitability of virtually the entire contralateral gray matter before cortical motor functions are expressed. Given the importance of activity-dependent axon terminal refinement, this capacity for activating spinal neurons during early postnatal life could play an important role in development of CS circuit connectivity.

The effect on motor cortical neuronal development of focal lesions to the sub-cortical white matter in the neonatal rat: a model for periventricular leukomalacia

Periventricular leukomalacia (PVL) is either a diffuse or cystic lesion of the periventricular white matter that leaves the overlying cortical grey matter largely intact. It is believed to result from hypoxia occurring pre- or perinatally and is a major cause of cerebral palsy. We have modelled PVL in rats comparing the effects of discrete injections of 3-nitropropionic acid (3-NP), a mitochondrial toxin, ibotenic acid (IBA), a glutamate analogue, or saline into the sub-cortical white matter on postnatal day 7 (P7). Following recovery times ranging from 3 days to 4 weeks, forebrain sections were Nissl stained or immunostained for Bax, cJun, calbindin (CB), parvalbumin (PV) or non-phosphorylated neurofilaments (NPNF). Compared to saline injections, ibotenic acid caused large lesions of both grey and white matter not characteristic of periventricular leukomalacia. 3-Nitropropionic acid injections caused small focal lesions restricted to the sub-cortical white matter. 3-Nitropropionic acid treatment initially increased expression of the apoptosis promoting proteins Bax and cJun, as well as non-phosphorylated neurofilaments in cortical layer V overlying the injection site. Non-phosphorylated neurofilament expression distal to the lesion was decreased representing a loss of cortical axons, but persisted and even increased with time within the cortex, demonstrating persistence of the parent cell bodies and local sprouting of neurites. There were significantly fewer calbindin and parvalbumin positive neurones in the motor cortex (MC) side ipsilateral to the 3-nitropropionic acid injection compared to the contralateral side. These persistent differences in expression of activity sensitive calcium binding proteins suggest alterations in local cortical circuitry without substantial loss of grey matter as is characteristic of periventricular leukomalacia. Changes in expression of Bax, cJun and non-phosphorylated neurofilaments during normal development are also described.

Premotoneuronal and direct corticomotoneuronal control in the cat and macaque monkey

The literature on premotoneuronal and direct corticomotoneuronal (CM) control in the cat and macaque monkey is reviewed. The available experimental findings are not in accordance with a recently proposed hypothesis that direct CM connections have "replaced" the premotoneuronal pathways. Instead, we propose that premotoneuronal CM control plays an important role in motor control also in primates and that the direct CM connection has been added during phylogeny.

Independent digit movements and precision grip patterns in 1–5-month-old human infants: hand-babbling, including vacuous then self-directed hand and digit movements, precedes targeted reaching

Previous work has described human reflexive grasp patterns in early infancy and visually guided reaching and grasping in late infancy. There has been no examination of hand movements in the intervening period. This was the purpose of the present study. We video recorded the spontaneous hand and digit movements made by alert infants over their first 5 months of age. Over this period, spontaneous hand and digit movements developed from fists to almost continuous, vacuous movements and then to self-directed grasping movements. Amongst the many hand and digit movements observed, four grasping patterns emerged during this period: fists, pre-precision grips associated with numerous digit postures, precision grips including the pincer grasp, and self-directed grasps. The finding that a wide range of independent digit movements and grasp patterns are displayed spontaneously by infants within their first 5 months of age is discussed in relation to the development of the motor system, including the suggestion that direct connections of the pyramidal tract are functional relatively early in infancy. It is also suggested that hand babbling, consisting of first vacuous and then self-directed movements, is preparatory to targeted reaching.

Postnatal development of connectional specificity of corticospinal terminals in the cat

The purpose of this study was to examine postnatal development of connectional specificity of corticospinal terminals. We labeled a small population of primary motor cortex neurons with the anterograde tracer biotinylated dextran amine. We reconstructed individual corticospinal segmental axon terminals in the spinal gray matter in cats of varying postnatal ages and adults. We found that at days 25 and 35 the segmental termination field of reconstructed axons was large, estimated to cover more than half of the contralateral gray matter. Branches and varicosities were sparse and had a relatively uniform distribution. When we examined the terminal fields of multiple axons, reconstructed over the same set of spinal sections (120-200 microm), we found that there was extensive overlap. By day 55, the morphology and termination fields had changed remarkably. There were many short branches, organized into discrete clusters, and varicosities were preferentially located within these clusters. The termination field of individual axons was substantially reduced compared with that of younger animals, and there was minimal overlap between the terminals of neighboring corticospinal neurons. In adults, a further reduction was seen in the spatial extent of terminals, branching, and varicosity density. Termination overlap was not substantially different from that in PD 55 animals. Development of spatially restricted clusters of short terminal branches and dense axonal varicosities occurred just prior to development of the motor map in primary motor cortex and may be necessary for ensuring that the corticospinal system can exert a dominant influence on skilled limb movement control in maturity.

Postnatal development of corticospinal axon terminal morphology in the cat

The corticospinal system undergoes important postnatal development, leading to the mature topography and specificity of connections. The purpose of this study was to determine the time-course of development of corticospinal axonal branching and varicosity density within the cervical gray matter. Corticospinal neurons were labeled after small injections of the anterograde tracer biotinylated dextran amine into the primary motor cortex of cats. Tracer injection and transport times were adjusted to examine labeling at 25, 35, 55, and 75 days and in adults. We measured the numbers and lengths of nonreconstructed terminal and preterminal branches and the numbers and locations of axon varicosities. We found significant age-dependent increases in all morphologic measures. At 25 days, corticospinal axon branching was sparse, with only a few scattered varicosities. By day 35, the mean number of branches, varicosities per branch, and varicosity density increased. Several morphologic measures did not increase between day 35 and 55, but further changes occurred between 55 days and maturity. Beginning around day 55, there was extensive development of small terminal axon branches with high densities of varicosities. We also found, by using spatial point analysis, that there was an age-dependent increase in varicosity clustering. Our results show for the first time that terminal and preterminal corticospinal axon branches increase in complexity during a protracted early postnatal period. This developmental period extended beyond the early postnatal period of activity-dependent refinement of the topography of terminations. Comparison with the time-course of maturation of the cortical motor representation revealed development of substantial, albeit incomplete, branching and varicosity density of CS axons before cortical motor circuits effectively drive their spinal targets.

Reciprocal and Renshaw (recurrent) inhibition are functional in man at birth

The aims were (1) to determine when in human postnatal development Group Ia reciprocal and Renshaw inhibition can be demonstrated; (2) to explore the relationship between the expression reciprocal inhibition and the disappearance of Group Ia excitatory reflexes between agonist and antagonist muscles. Studies were performed on 99 subjects, aged 1 day to 31 years, of whom 53 were neonates. A longitudinal study was also performed on 29 subjects recruited at birth and studied 3 monthly until 12 months of age. Reciprocal inhibitory and excitatory reflexes were recorded in the surface EMG of contracting biceps brachii (Bi), evoked by taps applied to the tendon of triceps brachii (Tri). Reciprocal excitatory reflexes were recorded in all but one neonate. Reciprocal inhibition was observed in 25% of neonates; evidence is provided that it was likely to have been masked by low threshold reciprocal excitation in the remaining neonates. Reciprocal inhibition was demonstrated in all subjects after 9 months of age. In four neonates there was depression of inhibition of Bi during co-contraction of Bi and Tri implying that Group Ia interneurones may be under segmental and suprasegmental control at birth. Renshaw cells, identified in human postmortem cervical spinal cord by their morphology, location and calbindin D28K immunoreactivity, were present at 11 weeks post-conceptional age (PCA) and by 35 weeks PCA had mature morphological characteristics. In four neonates reciprocal inhibitory responses in Bi disappeared when the tap to Tri evoked its own homonymous phasic stretch reflex, providing neurophysiological evidence for Renshaw inhibition of Group Ia inhibitory interneurones.

Short latency heteronymous excitatory and inhibitory reflexes between antagonist and heteronymous muscles of the human shoulder and upper limb

The aims were (i) to investigate heteronymous excitatory and inhibitory Group Ia reflexes linking agonist/antagonist muscle pairs acting at the shoulder and elbow; clavicular pectoralis major (Pmajor) and posterior deltoid (Pdeltoid); biceps brachii (Bi) and Tri brachii (Tri), and linking muscles acting at the elbow (Bi and Tri) with muscles acting at the shoulder (Pmajor and Pdeltoid). (ii) To test the hypothesis that the excitability of the reflexes would vary between different tasks in a functionally relevant manner. The study was performed on 45 adults. Reflexes were recorded in the surface EMG when the target muscle was contracting at 10% maximum voluntary contraction. Reflexes were recorded in Bi and Tri with the elbow joint in one of three positions: 105 degrees, 80 degrees, or 55 degrees from full extension. Group Ia reflexes were evoked using a small, brief tap to the tendon of the muscle being stimulated. Reflexes were recorded by cross-correlation of the surface EMG and pseudo-random series of taps. All subjects demonstrated short latency inhibition and excitation between agonist/antagonist muscle pairs; inhibition was significantly more frequent than excitation. Excitation and inhibition occurred with equal frequency between muscle pairs acting between elbow and shoulder. Minimum central delays for excitatory reflexes were 1 ms, consistent with monosynaptic projections and for inhibitory responses were 2 ms consistent with disynaptic linkage. Later excitatory and inhibitory reflexes with central delays of up to 15 ms also occurred. The probability of evoking excitation or inhibition in Tri or Bi changed with the different elbow positions.

Postnatal development of the motor representation in primary motor cortex

The purpose of this study was to examine when the muscles and joints of the forelimb become represented in primary motor cortex (M1) during postnatal life and how local representation patterns change. We examined these questions in cats that were anesthetized (45-90 days, n = 14; adults, n = 3) and awake (n = 4; 52-86 days). We used intracortical microstimulation (45 ms duration train, 330 Hz, 0.2-ms balanced biphasic pulses, with a leading cathodic pulse; up to 100 microA). In young animals (less than day 70), we also used stimulus trains and pulses that could produce greater temporal summation (up to 200-ms train duration, down to 143-Hz stimulus frequency, up to 0.8-ms pulse width). Anesthetized animals were areflexic, and muscle tone was similar to that of the awake cats (i.e., relaxed, not weight or load bearing, with minimal resistance to passive stretch). We monitored the kinematic effects of microstimulation and changes in electromyographic (EMG) activity in forelimb muscles. There was an age-dependent reduction in the number of sites where microstimulation did not produce a motor effect (i.e., ineffective sites), from 95% in animals younger than 60 days to 33% between 81 and 90 days. In adults, 24% of sites were ineffective. Median current thresholds for evoking movements dropped from 79 microA in animals younger than day 60 to 38 and 28 microA in day 81-90 animals and adults, respectively. There was a proximal-to-distal development of the somatotopic organization of the motor map. Stimulation at the majority of sites in animals younger than day 71 produced shoulder and elbow movement. Wrist sites were first present by day 71, and digit sites by day 81. Sites at which multiple responses were evoked, between 1.0 and 1.5 times threshold, were present after day 71, and increased with age. A higher percentage of distal joints were co-represented with other joints, rather than being represented alone. We found that effective sites initially were scattered and new sites representing proximal and distal joints filled in the gaps between effective sites. During most of the period examined, development of the caudal M1 subregion lagged that of the rostral subregion (percent of effective sites; threshold currents), although these differences were minimal or absent in adults. Our results show that the M1 motor representation is absent at day 45 and, during the subsequent month, the motor map is constructed by progressively representing more distal forelimb joints.