The Corticospinal System: From Development to Motor Control

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.

Premotor interneurones contributing to actions of feline pyramidal tract neurones on ipsilateral hindlimb motoneurones

The aim of the study was to analyse the potential contribution of excitatory and inhibitory premotor interneurones in reflex pathways from muscle afferents to actions of pyramidal tract (PT) neurones on ipsilateral hindlimb motoneurones. Disynaptic EPSPs and IPSPs evoked in motoneurones in deeply anaesthetized cats by group Ia, Ib and II muscle afferents were found to be facilitated by stimulation of the ipsilateral, as well as of contralateral, PT. The ipsilateral actions were evoked by either uncrossed or double-crossed pathways. The results show that interneurones mediating reflex actions of muscle afferents may be activated strongly enough by PT stimulation to contribute to movements initiated by ipsilateral PT neurones and that PT actions relayed by them might be enhanced by muscle stretches and/or contractions. However, in some motoneurones disynaptic IPSPs and EPSPs evoked from group Ib or II afferents were depressed by PT stimulation. In order to analyse the basis of this depression, the transmitter content in terminals of 11 intracellularly labelled interneurones excited by PT stimulation was defined immunohistochemically and their axonal projections were reconstructed. The interneurones included 9 glycinergic and 2 glutamatergic neurones. All but one of these neurones were mono- or disynaptically excited by group I and/or II afferents. Several projected to motor nuclei and formed contacts with motoneurones. However, all had terminal projections to areas outside the motor nuclei. Therefore both inhibitory and excitatory interneurones could modulate responses of other premotor interneurones in parallel with direct actions on motoneurones.

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.

Bilateral activity-dependent interactions in the developing corticospinal system

Activity-dependent competition between the corticospinal (CS) systems in each hemisphere drives postnatal development of motor skills and stable CS tract connections with contralateral spinal motor circuits. Unilateral restriction of motor cortex (M1) activity during an early postnatal critical period impairs contralateral visually guided movements later in development and in maturity. Silenced M1 develops aberrant connections with the contralateral spinal cord whereas the initially active M1, in the other hemisphere, develops bilateral connections. In this study, we determined whether the aberrant pattern of CS tract terminations and motor impairments produced by early postnatal M1 activity restriction could be abrogated by reducing activity-dependent synaptic competition from the initially active M1 later in development. We first inactivated M1 unilaterally between postnatal weeks 5-7. We next inactivated M1 on the other side from weeks 7-11 (alternate inactivation), to reduce the competitive advantage that this side may have over the initially inactivated side. Alternate inactivation redirected aberrant contralateral CS tract terminations from the initially silenced M1 to their normal spinal territories and reduced the density of aberrant ipsilateral terminations from the initially active side. Normal movement endpoint control during visually guided locomotion was fully restored. This reorganization of CS terminals reveals an unsuspected late plasticity after the critical period for establishing the pattern of CS terminations in the spinal cord. Our findings show that robust bilateral interactions between the developing CS systems on each side are important for achieving balance between contralateral and ipsilateral CS tract connections and visuomotor control.

Locomotor training within an inpatient rehabilitation program after pediatric incomplete spinal cord injury

BACKGROUND AND PURPOSE

The outcomes of intense locomotor training after incomplete spinal cord injury (SCI) have been described in adults with acute and chronic injuries and with various levels of ambulatory function. This case report describes a comprehensive inpatient rehabilitation program with a locomotor training component in a child with a severe incomplete SCI.

CASE DESCRIPTION

A 5-year-old girl injured at C4 participated in locomotor training for 5 months during inpatient rehabilitation.

OUTCOMES

The patient's Functional Independence Measure for Children II (WeeFIM II) mobility score increased from 5/35 to 21/35. Her Walking Index for Spinal Cord Injury II (WISCI II) score improved from 0 to 12. The patient returned to walking in the community with assistive devices.

DISCUSSION

It is feasible to include an intense locomotor training program in the clinical rehabilitation setting for a child with a severe SCI, and the outcomes were consistent with results in adults. Further investigation with experimental designs and more participants will determine the extent to which this intervention benefits the pediatric population with SCI.

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.

Differential activity-dependent development of corticospinal control of movement and final limb position during visually guided locomotion

Although we understand that activity- and use-dependent processes are important in determining corticospinal axon terminal development in the spinal cord, little is known about the role of these processes in development of skilled control of limb movements. In the present study we determined the effects of unilateral motor cortex activity blockade produced by muscimol infusion during the corticospinal axon terminal refinement period, between postnatal weeks 5-7, on visually guided locomotion. We examined stepping and forepaw placement on the rungs of a horizontal ladder and gait modifications as animals stepped over obstacles during treadmill walking. When cats traversed the horizontal ladder, the limb contralateral to inactivation was placed significantly farther forward on the rungs than the ipsilateral limb, indicating defective endpoint control. Similarly, when animals stepped over obstacles on a treadmill, the contralateral limb was placed farther in front of the obstacle, but only when it was the first (i.e., leading) limb to step over the obstacle, not when it was the second (i.e., trailing) limb. This is also indicative of an endpoint control deficit. In contrast, neither during ladder walking, nor when stepping over obstacles on the treadmill, was there any consistent evidence for a major impairment in limb trajectory. These results point to distinct and possibility independent corticospinal mechanisms for movement trajectory control and endpoint control. Although corticospinal activity during early postnatal development is needed to refine circuits for accurate endpoint control, this activity-dependent refinement is not needed for movement trajectory control.

Plasticity of the corticospinal tract in early blindness revealed by quantitative analysis of fractional anisotropy based on diffusion tensor tractography

Early visual deprivation may induce plastic changes, not only in the visual system, but also in the remaining sensory systems, secondary to altered experience in these spared modalities. Most of previous studies were focused on the plasticity of cortical areas of sensory modalities, but little attention was paid to the plasticity of motor system and white matter fiber tracts. Our purpose is to investigate the plasticity of the corticospinal tract (CST) in early blindness by tract-based quantitative analysis of fractional anisotropy (FA). Diffusion tensor imaging was performed in 17 early blind and 17 gender- and age-matched sighted subjects. The entire CST of each subject was reconstructed and the average FA of the tract was analyzed. To validate the results derived from the entire CST, we further analyzed a segment of the CST between the lowest slice of the cerebral peduncle and the uppermost slice of the lateral ventricle, in which the fibers are coherently arranged and the anatomical correspondence of the CST across subjects is established. On comparison with matched sighted participants, the average FA of the CST was significantly increased in the early blind men, but not in the early blind women. In conclusion, the plasticity of the CST is present in the early blind men, which might be related to the changes of motor experience during critical developmental period of the CST. This study also supports the perspective that experience-dependent plasticity occurs not only in the cortical areas but also in the white matter fiber tracts.

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.

Uncrossed actions of feline corticospinal tract neurones on lumbar interneurones evoked via ipsilaterally descending pathways

Effects of stimulation of ipsilateral pyramidal tract (PT) fibres were analysed in interneurones in midlumbar segments of the cat spinal cord in search of interneurones mediating disynaptic actions of uncrossed PT fibres on hindlimb motoneurones. The sample included 44 intermediate zone and ventral horn interneurones, most with monosynaptic input from group I and/or group II muscle afferents and likely to be premotor interneurones. Monosynaptic EPSPs evoked by stimulation of the ipsilateral PT were found in 12 of the 44 (27%) interneurones, while disynaptic or trisynaptic EPSPs were evoked in more than 75%. Both appeared at latencies that were either longer or within the same range as those of disynaptic EPSPs and IPSPs evoked by PT stimuli in motoneurones, making it unlikely that premotor interneurones in pathways from group I and/or II afferents relay the earliest actions of uncrossed PT fibres on motoneurones. These interneurones might nevertheless contribute to PT actions at longer latencies. Uncrossed PT actions on interneurones were to a great extent relayed via reticulospinal neurones with axons in the ipsilateral medial longitudinal fascicle (MLF), as indicated by occlusion and mutual facilitation of actions evoked by PT and MLF stimulation. However, PT actions were also relayed by other supraspinal or spinal neurones, as some remained after MLF lesions. Mutual facilitation and occlusion of actions evoked from the ipsilateral and contralateral PTs lead to the conclusion that the same midlumbar interneurones in pathways from group I or II muscle afferents may relay uncrossed and crossed PT actions.

Developmental reorganization of the output of a GABAergic interneuronal circuit

Locally projecting inhibitory interneurons play a crucial role in the patterning and timing of network activity. However, because of their relative inaccessibility, little is known about their development or incorporation into circuits. In this report we demonstrate that the GABAergic R-interneuron circuit undergoes a reorganization in the chick embryo spinal cord between embryonic days 8 and 15 (E8 and E15). R-interneurons receive synaptic input from and project back to motoneurons. By stimulating motoneurons projecting in one ventral root and recording the disynaptic response from motoneurons in adjacent segments, we show that the output of the R-interneuron circuit is reorganized during development. After stimulation of the LS2 ventral root, disynaptic responses observed in whole cell recordings became more common and stronger for LS3 motoneurons and less common for the more distant LS4 motoneurons from E8 to E10. Optical studies demonstrated that R-interneurons activated by LS2 stimulation were restricted to the LS2 segment and had a small glutamatergic component at both E8 and E10, but that more R-interneurons were activated within the segment by E10. The recruitment of more LS2 R-interneurons at E10 is likely to contribute to stronger projections to LS3 motoneurons, but the fact that fewer LS4 motoneurons receive this input is more consistent with a functional refinement of the more distant projection of the GABAergic R-interneuron. Interestingly, this pattern of reorganization was not observed throughout the rostrocaudal extent of the cord, introducing the possibility that refinement could serve to remove connections between functionally unrelated interneurons and motoneurons.

Pathways and genes differentially expressed in the motor cortex of patients with sporadic amyotrophic lateral sclerosis

BACKGROUND

Amyotrophic lateral sclerosis (ALS) is a fatal disorder caused by the progressive degeneration of motoneurons in brain and spinal cord. Despite identification of disease-linked mutations, the diversity of processes involved and the ambiguity of their relative importance in ALS pathogenesis still represent a major impediment to disease models as a basis for effective therapies. Moreover, the human motor cortex, although critical to ALS pathology and physiologically altered in most forms of the disease, has not been screened systematically for therapeutic targets.

RESULTS

By whole-genome expression profiling and stringent significance tests we identify genes and gene groups de-regulated in the motor cortex of patients with sporadic ALS, and interpret the role of individual candidate genes in a framework of differentially expressed pathways. Our findings emphasize the importance of defense responses and cytoskeletal, mitochondrial and proteasomal dysfunction, reflect reduced neuronal maintenance and vesicle trafficking, and implicate impaired ion homeostasis and glycolysis in ALS pathogenesis. Additionally, we compared our dataset with publicly available data for the SALS spinal cord, and show a high correlation of changes linked to the diseased state in the SALS motor cortex. In an analogous comparison with data for the Alzheimer's disease hippocampus we demonstrate a low correlation of global changes and a moderate correlation for changes specifically linked to the SALS diseased state.

CONCLUSION

Gene and sample numbers investigated allow pathway- and gene-based analyses by established error-correction methods, drawing a molecular portrait of the ALS motor cortex that faithfully represents many known disease features and uncovers several novel aspects of ALS pathology. Contrary to expectations for a tissue under oxidative stress, nuclear-encoded mitochondrial genes are uniformly down-regulated. Moreover, the down-regulation of mitochondrial and glycolytic genes implies a combined reduction of mitochondrial and cytoplasmic energy supply, with a possible role in the death of ALS motoneurons. Identifying candidate genes exclusively expressed in non-neuronal cells, we also highlight the importance of these cells in disease development in the motor cortex. Notably, some pathways and candidate genes identified by this study are direct or indirect targets of medication already applied to unrelated illnesses and point the way towards the rapid development of effective symptomatic ALS therapies.

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.

Sprouting, regeneration and circuit formation in the injured spinal cord: factors and activity

Central nervous system (CNS) injuries are particularly traumatic, owing to the limited capabilities of the mammalian CNS for repair. Nevertheless, functional recovery is observed in patients and experimental animals, but the degree of recovery is variable. We review the crucial characteristics of mammalian spinal cord function, tract development, injury and the current experimental therapeutic approaches for repair. Regenerative or compensatory growth of neurites and the formation of new, functional circuits require spontaneous and experimental reactivation of developmental mechanisms, suppression of the growth-inhibitory properties of the adult CNS tissue and specific targeted activation of new connections by rehabilitative training.

Genetic basis of Joubert syndrome and related disorders of cerebellar development

Over three decades have passed since Marie Joubert described the original proband for Joubert syndrome, a rare neurological disorder featuring absence of the cerebellar vermis (i.e. midline). Efforts at deciphering the molecular basis for this disease have been complicated by the clinical and genetic heterogeneity as well as extensive phenotypic overlap with other syndromes. However, progress has been made in recent years with the mapping of three genetic loci and the identification of mutations in two genes, AHI1 and NPHP1. These genes encode proteins with some shared functional domains, but their role in brain development is unclear. Clues may come from studies of related syndromes, including Bardet-Biedl syndrome and nephronophthisis, for which all of the encoded proteins localize to primary cilia. The data suggest a tantalizing connection between intraflagellar transport in cilia and brain development.

Postnatal development of vibrissae motor output following neonatal infraorbital nerve manipulation

Using the model of infraorbital nerve (IoN) injury, we have studied the role IoN signals have on the developing vibrissal motor system. To this end, in ten rats, the IoN was severed on the day of birth: in five rats, the IoN was repaired to promote axon regeneration (Reinnervated group) while axon regeneration was prevented in the remaining five rats (Deafferented group). In another five rats, the isolated IoN was left intact (Sham group) and still another group of five rats was left untouched (Control group). After these rats had reached adulthood, the compound muscle action potential (MAP) was recorded from the vibrissa muscle and intracortical microstimulation (ICMS)-evoked movements were mapped in the frontal cortex contralateral to the operated side. We found: (i) no difference between Control, Sham and Reinnervated groups in the integrated MAPs and in the size and excitability of the M1 vibrissal representation. (ii) the Deafferented group showed a 42.9% decrease in the integrated MAP plus a 47.2% and 36.9% reduction, respectively, in the size and excitability of the M1 vibrissae representation. We conclude that, during perinatal life, IoN signals regulate the development of both the peripheral and central vibrissal motor system and that IoN reinnervation restores sensory signals able to stabilize normal development of the vibrissal motor system.

How can corticospinal tract neurons contribute to ipsilateral movements? A question with implications for recovery of motor functions

In this review, the authors discuss some recent findings that bear on the issue of recovery of function after corticospinal tract lesions. Conventionally the corticospinal tract is considered to be a crossed pathway, in keeping with the clinical findings that damage to one hemisphere, for example, in stroke, leads to a contralateral paresis and, if the lesion is large, a paralysis. However, there has been great interest in the possibility of compensatory recovery of function using the undamaged hemisphere. There are several substrates for this including ipsilaterally descending corticospinal fibers and bilaterally operating neuronal networks. Recent studies provide important evidence bearing on both of these issues. In particular, they reveal networks of neurons interconnecting two sides of the gray matter at both brainstem and spinal levels, as well as intrahemispheric transcallosal connections. These may form "detour circuits" for recovery of function, and here the authors will consider some possibilities for exploiting these networks for motor control, even though their analysis is still at an early stage.

Quantitative assessment of right and left reaching movements in infants: A longitudinal study from 6 to 36 months

This longitudinal study aimed to explore the early presence and developmental pattern of laterality in reaching kinematics and its relationship to side use. In order to do so, 3-D kinematic measurements as well as 2-D video recordings of right-left reaching movements were successively carried out for 17 infants at the ages of 6, 9, 12, and 36 months. Additional investigations of hand preference were made at 36 months. As four infants were prematurely born, their outcomes were compared to those of the fullterm participants. While most of the infants in the early ages showed a rather inconsistent preference in terms of frequency and distributions of right-left side use, the analyses of reaching kinematics revealed a more consistent pattern of fewer movements units (MUs) and straighter right-sided reaching for the majority of infants at all tested ages. However, reaching kinematics from the preterm infants were generally more variable and less side consistent. It is proposed that the development of human handedness originates from an early right arm rather than hand preference in that representations of asymmetry in bilateral projections (involved in arm movements) developmentally precede contralateral projections (involved in refined hand/finger movements).

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.

Transgenic labeling of the corticospinal tract for monitoring axonal responses to spinal cord injury

The rodent corticospinal tract (CST) has been used extensively to investigate regeneration and remodeling of central axons after injury. CST axons are currently visualized after injection of tracer dye, which is invasive, incomplete and prone to variation, and often does not show functionally crucial but numerically minor tract components. Here, we characterize transgenic mice in which CST fibers are specifically and completely labeled by yellow fluorescent protein (YFP). Using these CST-YFP mice, we show that minor CST components are responsible for most monosynaptic contacts onto motoneurons. Lesions of the main dorsal CST lead to extension of new collaterals, some of them originating from large, heavily myelinated axons within the minor dorsolateral and ventral CST components. Some of these new collaterals form additional direct synapses onto motoneurons. We propose that CST-YFP mice will be useful for evaluating strategies designed to maximize such remodeling and to promote regeneration.

Motor but not sensory representation in motor cortex depends on postsynaptic activity during development and in maturity

The movement representation in the primary motor cortex (M1) of the cat develops between postnatal weeks 7-12. The somatosensory representation in motor cortex is present by the age that the motor map begins to develop. In this study we examined the role of neural activity in development and maintenance of the M1 movement and somatosensory representations. We blocked activity of M1 neurons unilaterally for one month by intracortical infusion of the GABA agonist muscimol during the motor map development period in kittens and in mature cats. After the drug effects were no longer present, we used microstimulation and multiunit recording in the forelimb areas of M1 to determine the motor and somatosensory representations in the infused and noninfused sides. In both kittens and adults, there was a severe reduction or elimination of sites where microstimulation evoked a motor response in the inactivated compared with the control side. In contrast, there was no difference in the percentage, topography or receptive field modality of sites receiving somatosensory inputs on the inactivated and control sides. Moreover, the pattern of somatosensory input to M1 was similar before and after inactivation. This suggests that somatosensory input to M1 is stable after the connections initially develop. Since activity blockade had the same effects on the motor representation of kittens and adult cats, M1 neuronal activity, while possibly important in map development, is equally necessary for map maintenance.