Scholarly article on topic 'Cortical and subcortical compensatory mechanisms after spinal cord injury in monkeys'

Cortical and subcortical compensatory mechanisms after spinal cord injury in monkeys Academic research paper on "Clinical medicine"

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{"Corticospinal tract" / Injury / "Precision grip" / "Propriospinal neurons" / Recovery / Hand / Monkey / "Positron emission tomography" / GAP-43}

Abstract of research paper on Clinical medicine, author of scientific article — Yukio Nishimura, Tadashi Isa

Abstract This is a review of our investigations into the neuronal mechanisms of functional recovery after spinal cord injury (SCI) in a non-human primate model. In primates, the lateral corticospinal tract (l-CST) makes monosynaptic connections with spinal motoneurons. The existence of direct cortico-motoneuronal (CM) connections has been thought to be the basis of dexterous digit movements, such as precision gripping. However, recent studies have shown that after lesion of the direct CM connections, by a l-CST lesion at the C4/C5 level, precision gripping is initially impaired, but shows remarkable recovery with training within several weeks. Plastic changes of the neural circuits underlying the recovery occur at various levels of the central nervous system. In the subcortical networks, intracellular recordings from the motoneurons in anesthetized animals demonstrated that transmission through the disynaptic pathways from the CST was enhanced, presumably mediated by the propriospinal neurons in the mid-cervical segments. The γ-band musculo-muscular coherence (MMC), with a peak frequency around 30Hz, appeared over a wide range of forelimb muscles and was strengthened in parallel to the recovery of the precision grip. Appearance of the γ-band MMC also paralleled the change in the activation pattern of forelimb muscles; muscles which were antagonists before the lesion showed co-activation after recovery. Such γ-band MMC is thought to originate in the subcortical network, presumably in the brainstem or spinal cord. In the cortical networks, a combination of positron emission tomography and reversible inactivation techniques has shown that the bilateral primary motor cortex (M1) and ventral premotor cortex (PMv) have different contributions to functional recovery depending on the recovery stage; the bilateral M1 plays a major role in early stage recovery (<1month), whereas the contralateral M1 and bilateral PMv are the prominent contributors to the later stages (3–4months). Such changes in cortical activity in M1 and PMv have been shown to accompany changes in the expressions of plasticity-related genes, such as GAP-43. Changes in the dynamic properties of neural circuits, both at the cortical and subcortical levels, are time-dependent. Multidisciplinary studies to clarify how the changes in the dynamic properties of individual components of the large-scaled networks are coordinated during recovery will help to develop effective therapeutic strategies to recovery from SCI.

Academic research paper on topic "Cortical and subcortical compensatory mechanisms after spinal cord injury in monkeys"

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Experimental Neurology

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Review

Cortical and subcortical compensatory mechanisms after spinal cord injury in monkeys

Yukio Nishimura a,b,c, Tadashi Isa ^^

a Department of Developmental Physiology, National Institute for Physiological Sciences, Myodaiji, Okazaki 444-8585, Japan b The Graduate University for Advanced Studies (SOKENDAI), Hayama, Japan

c Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan

ARTICLE INFO

ABSTRACT

Article history:

Received 29 November 2010 Revised 27 July 2011 Accepted 12 August 2011 Available online 22 August 2011

Keywords: Corticospinal tract Injury

Precision grip

Propriospinal neurons

Recovery

Monkey

Positron emission tomography GAP-43

This is a review of our investigations into the neuronal mechanisms of functional recovery after spinal cord injury (SCI) in a non-human primate model. In primates, the lateral corticospinal tract (l-CST) makes monosynaptic connections with spinal motoneurons. The existence of direct cortico-motoneuronal (CM) connections has been thought to be the basis of dexterous digit movements, such as precision gripping. However, recent studies have shown that after lesion of the direct CM connections, by a l-CST lesion at the C4/C5 level, precision gripping is initially impaired, but shows remarkable recovery with training within several weeks. Plastic changes of the neural circuits underlying the recovery occur at various levels of the central nervous system. In the subcortical networks, intracellular recordings from the motoneurons in anesthetized animals demonstrated that transmission through the disynaptic pathways from the CST was enhanced, presumably mediated by the propriospinal neurons in the mid-cervical segments. The Y-band musculo-muscular coherence (MMC), with a peak frequency around 30 Hz, appeared over a wide range of forelimb muscles and was strengthened in parallel to the recovery of the precision grip. Appearance of the Y-band MMC also paralleled the change in the activation pattern of fore-limb muscles; muscles which were antagonists before the lesion showed co-activation after recovery. Such Y-band MMC is thought to originate in the subcortical network, presumably in the brainstem or spinal cord. In the cortical networks, a combination of positron emission tomography and reversible inactivation techniques has shown that the bilateral primary motor cortex (M1) and ventral premotor cortex (PMv) have different contributions to functional recovery depending on the recovery stage; the bilateral M1 plays a major role in early stage recovery (< 1 month), whereas the contralateral M1 and bilateral PMv are the prominent contributors to the later stages (3-4 months). Such changes in cortical activity in M1 and PMv have been shown to accompany changes in the expressions of plasticity-related genes, such as GAP-43. Changes in the dynamic properties of neural circuits, both at the cortical and subcortical levels, are time-dependent. Multidisciplinary studies to clarify how the changes in the dynamic properties of individual components of the large-scaled networks are coordinated during recovery will help to develop effective therapeutic strategies to recovery from SCI.

© 2011 Elsevier Inc. All rights reserved.

Contents

Introduction................................................................................................................................153

Basic anatomy; direct and indirect cortico-motoneuronal pathways in macaques................................................................153

Functional recovery of finger dexterity after lesion of the direct CM pathway....................................................................154

Subcortical mechanisms of recovery..........................................................................................................154

Enhancement of the indirect CM transmission............................................................................................154

Formation of Y-band coherent oscillatory network........................................................................................154

Cortical mechanisms of recovery ............................................................................................................156

Functional brain imaging study..........................................................................................................156

Effects of reversible functional blockade..................................................................................................157

Changes in gene expression..................................................................................................................157

* Corresponding author at: Department of Developmental Physiology, National Institute for Physiological Sciences, Myodaiji, Okazaki 444-8585, Japan. Fax: +81 564 55 7766. E-mail address: tisa@nips.ac.jp (T. Isa).

0014-4886/$ - see front matter © 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.expneurol.2011.08.013

Concluding remarks and future direction .................................................. 159

Acknowledgments............................................................. 160

References ................................................................ 160

Introduction

Symptoms of spinal cord injury (SCI) in human patients are complicated; they usually result from accidents in which the primary damage is to the spinal cord itself. But many other complications arise, such as damage to afferent pathways, secondary expansion of damage by vascular dysfunction, edema, ischemia, excitotoxicity, electrolyte shifts, free radical production, inflammation, and delayed apoptotic cell death (Rowland et al., 2008). It is often difficult to delineate the neuroanatomical extent of the damage, which varies considerably from patient to patient. Moreover, the pre-lesional status of motor abilities is often difficult to trace. To overcome such difficulties and obtain deeper insight into the neuronal mechanisms of functional recovery after SCI, it is important to dissociate these symptoms into different factors and develop therapeutic strategies against each of these. Animal models of partial SCI have several advantages for such investigations: (1) the extent of the lesion can be controlled by the researcher, (2) the motor ability of the subject can be recorded and analyzed at a variety of pre- and post-lesional stages, and (3) the amount of pre- and post-lesional training can also be controlled. Because the brain and body structures of macaques are close to those of humans, they are ideal models for examining SCIs (Courtine et al., 2007; Darian-Smith, 2007; Isa et al., 2007; Lemon and Griffiths, 2005; Nishimura and Isa, 2009). In macaque models of SCI, careful lesioning with a fine forceps minimizes the secondary expansion of damage, which makes it possible to dissect the components of neural plasticity underlying functional recovery, and may identify targets of future therapeutic strategies. Rodent models are of course valuable, especially for studying the molecular mechanisms of functional recovery (Bareyre and Schwab, 2003; Courtine et al., 2009). However, there are considerable differences in the structure and function of the corticospinal tracts (CST), a major output pathway for the control of voluntary movements, of rodents and primates. In macaque monkeys, the CST passes mainly in the dorsolateral funiculus and makes monosynaptic connections to spinal motoneurons (Armand et al., 1994, 1997; Bortoff and Strick, 1993; Cheema et al., 1984; Dum and Strick, 1996; Kuypers, 1962; Lacroix et al., 2004; Liu and Chambers, 1964; Ralston and Ralston, 1985; Rosenzweig et al., 2009; Yoshino-Saito et al., 2010). A considerable number of CST fibers are uncrossed at the pyramidal decussation and descend either in the dorsolateral funiculus or ventromedial funiculus on the ipsilateral side, and many of the crossed CST fibers re-cross the midline at the cervical spinal cord level. In contrast, the CST in rodents passes through the dorsal funiculus but does not make direct connection to spinal motoneurons (Alstermark et al., 2004; Alstermark and Ogawa, 2004; Casale et al., 1988; Umeda et al., 2010). In addition, the number of re-crossing axons in rodents is much smaller than in primates. Moreover, the relative contribution of the cortical and subcortical centers to some motor repertories appears to be different between the two species. For instance, damage to the rodent motor cortex does not cause severe impairment to locomotion, but in primates it causes paralysis (Zweckberger et al., 2003).

Deficiency in hand movements is one of the most serious issues in tetraplegic patients (Anderson, 2004). Galea and Darian-Smith (1997a, b) studied the recovery of grasping after spinal hemisection between C3 and C6, but in these studies, performance after the lesion demonstrated poor recovery of precision gripping. Rouiller and colleagues (Freund et al., 2006; Schmidlin et al., 2004) and Tsuzynski and colleagues (Rosenzweig et al., 2010) showed partial recovery of

hand dexterity after hemisection at the lower cervical segments. On the other hand, we, and others, have established a macaque model of partial SCI, in which lesioning is limited to the lateral CST (l-CST) particularly affecting the direct cortico-motoneuronal (CM) connection (Higo et al., 2009; Nishimura et al., 2007b, 2009; Sasaki et al., 2004). In these studies, through training, the ability to perform dexterous finger movements was taken over by indirect pathways from the CST to motoneurons, resulting in near-complete functional recovery in several weeks. Here we review our studies using this animal model and our multidisciplinary analyses of the neuronal mechanisms of functional recovery. The way in which compensation occurs may vary depending on the rostro-caudal level and the extent of the lesion. In this review, we also compared our lesion model with other studies using different lesion models and discussed which pathways play a major role in the recovery in each model.

Basic anatomy; direct and indirect cortico-motoneuronal pathways in macaques

In primates, the CST makes monosynaptic connections with spinal motoneurons. The direct CM pathway is thought to be involved in the control of relatively independent finger movements (Lemon, 2008), because impairment of these CM connections results in deficiencies in hand dexterity (Lawrence and Kuypers, 1968a, 1968b; Tower, 1940), and activity of a population of corticospinal neurons is specified for the control of precision grip (Muir and Lemon, 1983). By contrast, in lower vertebrates the CST does not make direct connections with motoneurons. The shortest pathway to forelimb motoneurons in the cat, for example, is disynaptic, with a substantial portion of the disynaptic excitation being mediated by propriospinal neurons with cell bodies located in the mid-cervical segments, C3-C4 (C3-C4 propriospinal neurons; C3-C4 PNs) (Alstermark and Lundberg, 1992; Illert et al., 1977). The existence of such PNs in primates has been questioned, primarily because disynaptic excitation of motoneurons was rarely observed in either intracellular recordings from motoneurons in anesthetized monkeys (Maier et al., 1998; Nakajima et al., 2000) or in single motor unit recordings from hand muscles in awake monkeys (Olivier et al., 2001) following stimulation of the CST or the motor cortex. However, it has been shown that disynaptic corticospinal excitation mediated by C3-C4 PNs, became evident after glycinergic inhibition was reduced by intravenous injection of strychnine (Alstermark et al., 1999, Fig. 1). Moreover, the existence of C3-C4 PNs, which receive monosynaptic pyramidal excitation and transmit the excitation to forelimb motoneurons, has been demonstrated in anesthetized monkeys (Isa et al., 2006). The disynaptic excitation was not observed without strychnine because feedforward inhibition from the CST to the C3-C4 PNs was so potent that stimulation of the CST caused substantial disynaptic inhibition, which prevented the PNs from firing in response to the CST stimulation; the monosynaptic excitation was sharply curtailed by the succeeding disynaptic inhibition (Isa et al., 2006, Fig. 1A). The C3-C4 PNs also exist in humans, and similar potential feedforward inhibition from the CST has been demonstrated (Burke et al., 1994; Malmgren and Pierrot-Deseilligny, 1988; Nicolas et al., 2001). Thus, the existence of C3-C4 PNs has been shown in electrophysiological experiments in anesthetized monkeys, but only after application of strychnine. Their functional significance needed to be clarified in behavioral studies. The experiments described in the following sections were initiated to examine the functional significance of the PNs in monkeys.

Fig. 1. Disynaptic corticospinal excitation of motoneurons (MN) revealed after intravenous injection of strychnine in macaque. A: Diagram of the circuits and experimental arrangement used for intracellular recordings from forelimb MNs in the C6-T1 segments of the cord following stimulation of the pyramidal tract. Interneurons mediating the feed-forward inhibition to the C3-C4 propriospinal neurons (PNs, red) are indicated as black circles. B: Records from a MN in the intact spinal cord. Electrical stimulation was applied to the contralateral pyramidal (co-Pyr) tract with a train of 4 pulses. The top traces are intracellular recordings and the bottom traces are surface recordings from the cord dorsum. (a) Pyramidal stimulation before intravenous injection of strychnine. (b) 2 min after the injection (0.1 mg/kg). C: Records from another MN after CST lesion at C5. Disynaptic EPSPs evoked after injection of strychnine. D: Records from another MN after CST lesion at C2. Figure was modified, with permission, from Alstermark et al. (1999).

Functional recovery of finger dexterity after lesion of the direct CM pathway

To demonstrate the functional role of the indirect CM pathway mediated by the C3-C4 PNs in monkeys, the direct CM connection was transected at the C4/C5 border (Fig. 2A), leaving most of the PN axons intact, and reaching and grasping behaviors were tested (Alstermark et al., 2011; Nishimura et al., 2007b, 2009; Sasaki et al., 2004). In this model, the dexterity of finger movements, such as precision gripping and the independency of individual fingers, were restored within one to three months after lesion (Fig. 2B and C). One month after the lesion, the success rate of precision gripping was higher than 90%, however the performance appeared to be still unstable (early recovery stage), while 3 months after the lesion, the performance was stabilized in all the monkeys (late recovery stage). Completeness of the CST lesion was confirmed both electrophysiologi-cally, by observing the synaptic potentials in the motor nucleus or the conduction volley of the CST, and histologically, with CaMKll immuno-histochemistry or labeling of the CST axons by biotinylated dextran amine (BDA) injected into the motor cortex. In these animals, not only the direct CM pathway, but also the rubrospinal pathway located in the dorsolateral funiculus, was damaged. When a similar lesion was made at the C2 level, the recovery of precision gripping was limited (Alstermark et al., 2011, and unpublished observation). Therefore, it was postulated that the functions of the direct CM pathway had been taken over by remaining neuronal systems, such as the PNs located in the C3-C4 segments (Alstermark et al., 1999; Isa et al., 2006). Detailed descriptions of the indirect pathways from the motor cortex to hand/arm motoneurons are partly given in recent reviews (Alstermark et al., 2007; Isa et al., 2007; Pettersson et al., 2007) and fully in Fig. 7 of this review (see below).

Subcortical mechanisms of recovery

Enhancement of the indirect CM transmission

Transmission through the indirect CM pathway was studied by making intracellular recordings in forelimb motoneurons; the effects of electrical stimulation of the contralateral medullary pyramid (coPyr) were investigated in monkeys under anesthesia after recovery from the l-CST lesion (Fig. 2A and D). In these monkeys, stimulation of the coPyr evoked monosynaptic EPSPs in all motoneurons on the intact side and

disynaptic EPSPs in about half of those on the lesioned side (Fig. 2Da-f). The disynaptic EPSPs were not observed at all after additional lesion of the CST at the C2 level during the experiments, suggesting that the disynaptic EPSPs were mediated by the C3-C4 PNs. Such disynaptic pyramidal EPSPs were rarely observed in intact monkeys (Alstermark et al., 1999; Maier etal., 1998); they were observed only after reducing glycinergic inhibition by intravenous application of strychnine (Alstermark et al., 1999). These results suggested that transmission through the disynaptic pathways from the motor cortex to forelimb motoneurons was somehow facilitated during the recovery course, either by strengthening of the excitatory transmission through the pathway or by reduction in the feedforward inhibition to the C3-C4 PNs (Fig. 2A).

Formation of y-band coherent oscillatory network

To further explore this change in the dynamic properties of processing in the motor-related network during recovery, we recorded local field potentials (LFPs) in the M1 and electromyographic (EMG) activity of a variety of forelimb muscles, from proximal to distal, in 2 monkeys during a force-tracking precision grip task, and studied the functional coupling between the cortical and muscle activities (Nishimura et al., 2009). Before the lesion, the cortical LFP and EMG of distal hand muscles exhibited coherent activity with a peak at 17 Hz ((3-band) (data not shown). Such (3-band cortico-muscular coherence (CMC) has been reported both in monkeys and humans (Baker et al., 1997; Mima and Hallett, 1999). After lesion of the CST, the CMC completely disappeared and never recovered even three months after the lesion (data not shown), when near-complete recovery was observed in precision gripping. Such results represent conclusive evidence for the hypothesis that the (3-band CMC depends on the direct CM connection, as has been suggested by human studies (Mima et al., 2001). On the other hand, marked change was observed in the EMG activity. As shown in Fig. 3Aa, the adductor pollicis (ADP) and extensor digitorum 2 and 3 (ED23) muscles functioned as antagonists during the force tracking precision gripping before the lesion; however, about 1 month after the lesion, they exhibited co-activation (Fig. 3Ba). This may reflect the fact that the monkeys changed their behavioral strategy from a reciprocal pattern of muscle activation to a co-activation pattern, presumably because it was necessary for them to increase joint stiffness to compensate for the loss of force. Such a global co-activation pattern was prominent during the early recovery phase (about 1 month

Fig. 2. Animal model of spinal cord injury in the macaque monkey. A: Diagram of the presumed circuits after transection of the corticospinal tract (CST). The direct cortico-motoneuronal (CM) connection via the CST was transected at the border between the C4 and C5 segments of the spinal cord, rostral to the segments where motoneurons of hand muscles are located ("C4/C5 CST lesion"). A large proportion of the indirect CM pathways, except for those mediated by segmental interneurons, remained intact in the present preparation. B: CST lesion at the caudal C4 (Nissl staining) showing the extent of the smallest lesion area in the recent study. Dotted line indicates the border between the lesioned and intact tissue. C: Recovery of precision gripping. Representative frames (10 frames/s) showing the retrieval of a small piece of food before the lesion (Preop) and 7,14, and 99 days post-lesion. D: Pyramidal effects on forelimb motoneurons (MNs) before and after spinal cord injury. a, b: the effects of contralateral medullary pyramidal (coPyr) stimulation, delivered once (a) or three times (b), on the intracellular^ recorded activity in a deep radial (DR) MN (top) on the intact side. Bottom traces: records from the cord dorsum in the same segment as the MN, showing the direct CST volleys followed by synaptic volleys. c, d: the effects of coPyr stimulation in another deep radial MN recorded intracellularly (top) on the lesioned side of the monkey. The cord dorsum recordings show the absence of the direct CST volleys but the presence of the synaptic volleys. e: Averages of records shown in (a) and (c). The dashed line indicates the onset of the disynaptic IPSP in (a) and the disynaptic EPSP in (c). The double arrows show the latency difference between the monosynaptic and disynaptic EPSPs recorded from the intact and lesioned sides, respectively. f: histogram of latencies of pyramidal EPSPs (from the third volley) in forelimb MNs recorded in the C6-C8 segments. Latency measurements were made with respect to the incoming direct CST volleys on the intact side. Figures were modified, with permission, from Sasaki et al. (2004) and Nishimura et al. (2007b).

postoperatively); however, it gradually decreased during the late recovery phase (about 3 months). Furthermore, it was found that the EMG activities of the two muscles exhibited synchronous oscillations, yielding musculo-muscular coherence (MMC) with a peak between 30 and 46 Hz (Y-band, Fig. 3Bc and d). Such Y-band MMC appeared about 1 month

after the SCI, grew in parallel to the functional recovery in the precision gripping task in both monkeys, and became distributed over a wide range of muscles, from proximal to distal. Neither the (3-band CMC nor a Y-band oscillation was observed in the motor cortex after the CST lesion, suggesting that a subcortical Y-band oscillator commonly recruits

Fig. 3. Emergence of Y-band functional coupling between the subcortical system and spinal motoneurons after recovering from spinal cord injury. A: Intact, B: After recovery (3 months). a: EMG activity from ED23 (top) and ADP (second row) muscles and force trajectory of thumb (bottom row). b: Cross-correlogram between ED23 and ADP EMG activities. The dotted vertical lines represent zero-lag time in the cross-correlogram. c: Oscillatory coupling of muscle activities. d: Coherence between ED23 and ADP. e: Schematic illustrations of the proposed mechanisms underlying functional recovery after the l-CST lesion. Ae: In the intact state, a direct CM connection (black) or peripheral feedback (green) contributes to generate the cortico-muscular coupling at the frequency of the p-band. Be: During the recovery from l-CST lesion, subcortical neural systems (red) that mediate cortical command (black) or peripheral feedback (green) to motoneurons might be involved in generating the 30-46-Hz musculo-muscular coherence (Y-band) that emerged in a variety of hand/arm muscles. Dotted lines indicate polysynaptic connections. Figures were modified, with permission, from Nishimura et al. (2009). Abbreviations: ADP: adductor pollicis, ED23: extensor digitorum 2 and 3.

hand/arm muscles via remaining pathways, such as the reticulospinal and/or propriospinal tracts, independent of cortical oscillation, and contributes to functional recovery (Fig. 3Ae and Be). The causal relationship of the Y-band MMC to the functional recovery remains unclear; however, it is possible that the synchronous oscillation is a useful way to effectively transmit signals in the limited resource of neural circuits after injury, and that it is also involved in causing the synaptic plasticity that give rise to shifts from the early recovery stage to the late recovery stage.

Cortical mechanisms of recovery

Functional brain imaging study

It has been documented that, not only the primary motor cortex, but also higher motor related area, has several descending pathways that provide it with direct and indirect access to the motoneuronal pools of hand muscles (Borra et al., 2010; Dum and Strick, 1991; Dum and Strick, 1996; He et al., 1995). Especially, the ventral premo-tor area F5 projects to the reticular formation (Borra et al., 2010) and intermediate zone in upper cervical segment where C3-C4 PN are located (Borra et al., 2010; Dum and Strick, 1991; He et al., 1995). These pathways can be involved in functional recovery after CST lesion. A number of brain imaging studies have been performed in patients after stroke and spinal cord injury, and changes in cortical activity have been reported (Dong et al., 2006; Marshall et al., 2000; Roelcke

et al., 1997; Ward et al., 2003). Thus, plastic changes in neural circuits might occur, not only in the spinal cord, but also in higher order structures after the SCI. However, in patients, brain imaging over longer periods of time is often difficult. Therefore, we conducted a brain imaging study at different times before and after lesion with positron emission tomography (PET) in three monkeys with similar lesions and recovery time courses.

We conducted PET scanning using H^O to study changes in cortical activities related to precision gripping tasks during early (1-2 months) and late (3-4 months) stages of recovery after C4/C5 l-CST lesions. Fig. 4A-C shows the regions of the brain with significant increases in activity associated with the visually guided reach and grasp movements before the SCI. Task-related increases in activity were observed in the banks of the intraparietal sulcus and both rostral and caudal banks of the central sulcus, corresponding to the hand/arm regions of the S1, M1, and premotor cortex (PM) on the contralateral hemisphere during all recovery stages (Nishimura et al., 2007a).

To identify the cortical regions that showed recovery-related increases in activity, we compared the regional cerebral blood flow (r-CBF) during the postoperative stages with that during the preoperative stage (Nishimura et al., 2007b). Increased activity was observed in the bilateral M1 during the early stage (Fig. 4D-F) and in the contrale-sional M1 (co-M1; Fig. 4G and H) and ipsilesional ventral premotor cortex (PMv, Fig. 4I) during the late recovery stage. The area of the co-M1 with increased activity was expanded during the late recovery stage compared with that during the early recovery stage (compare Fig. 4D

Intact

Contra Ipsi Contra Ipsi

2.34 15.00

Early > Intact

2.34 4.36

Fig. 4. Brain activation after spinal cord injury. A-C: Brain areas activated during the precision grip task at the preoperative stage were determined by increased regional cerebral blood flow using positron emission tomography. D-I: Increased brain activation related to functional recovery. Activations during early (D-F) and late (G-l) recovery stages were compared with that during the preoperative stage. A, D and G: top view, B, E and H: view from the contralesional hemisphere, C, F and l: view from the ipsile-sional hemisphere. Abbreviations: M1 = primary motor cortex; PMv = ventral premo-tor cortex. Contra = contralesional hemisphere; lpsi = ipsilesional hemisphere. Figures were modified, with permission, from Nishimura et al. (2007b).

and G, G and H), and extended into the contralesional PMv (Fig. 4H). Thus, we observed a time-dependent change in activation of cortical networks during the functional recovery of finger dexterity after SCI.

Effects of reversible functional blockade

Activation of the ipsilesional motor cortices has been reported in several human case studies of stroke (Dong et al., 2006; Marshall et al., 2000; Ward et al., 2003) and SCI (Roelcke et al., 1997). However, it has also been reported that the activity on the ipsilesional side is negatively correlated with the progress of recovery (Ward et al., 2003, see review by Hallett, 2001). Such negative correlation can be interpreted in two ways: the ipsilesional activation impedes the recovery, or it is necessary to drive spinal motoneurons, because the damage is severe. It is difficult to determine which of these two possibilities is correct, because the imaging studies only demonstrate the correlation between brain activity and behaviors, and do not provide information about the causality. To overcome this limitation of the imaging study and to clarify whether the increased activity of those cortical regions observed in the PET study causally contributed to the functional recovery, we performed focal reversible inactivation of individual cortical regions using microinjections of muscimol, a Y-aminobutyric acid type A (GABAa) receptor agonist, at various recovery stages and observed the

effects on the performance of precision gripping with the affected hand in two monkeys (Nishimura et al., 2007b). Because we observed increases in the activity of the M1 and PMv in both hemispheres during the early and late recovery stages in the PET study (Fig. 4D-I), we chose the digit areas of these cortical regions as targets of the muscimol injection.

During the preoperative trials, inactivation of the digit area of the contralesional M1 (co-M1, Fig. 5A) resulted in clumsy finger movements, accompanied by a loss of the independent control of each digit as shown in Fig. 5A (Mus, Intact). Both monkeys reached for the food piece but were not able to achieve a precision grip. (Fig. 5C, Intact). Precision gripping during the early stage of recovery was severely impaired by inactivation of the co-M1. One monkey showed total paresis of the hand. Another monkey could move its digits, but the thumb and index finger could not be inserted into the slit (Fig. 5A, Mus, Early). The success rate for retrieval remained zero in both monkeys (Fig. 5C, Early). During the late recovery stage, the effect from inactivating co-M1 was reduced, even in comparison with the preoperative trials (Fig. 5A and C, Late). This was interpreted to mean that partial blockade of the expanded region of the digit area of co-M1 (Fig. 4G and H) might have been largely compensated at this stage by surrounding regions or other regions, such as the bilateral PMv.

In contrast to co-M1, inactivation of the ipsilesional M1 (ip-M1) in the preoperative trials, (Fig. 5B Mus and 5D, Intact) did not impair digit movements as reported previously (Fogassi et al., 2001). Interestingly, after inactivation of the ip-M1 during the early recovery stage, the ability to retrieve the food piece with precision gripping was impaired (Fig. 5B, Early). The success rates in the two monkeys decreased by 33 and 15%, from that before the inactivation (Fig. 5D, Early). Even in successful trials, both monkeys achieved the grip not with the pads of the index finger and thumb, but with the pad of the index finger and the nail of the thumb. ln contrast to the early stage, inactivation of the ip-M1 during the late recovery stage did not impair digit movement (Fig. 5B, Cont and Mus, Late). Schmidlin et al. (2004) also tested muscimol injection into the M1 in monkeys with subhemisections at the C7/C8, but they did not observe digital movement impairments. This may be because they made the injection during postoperative months 3-5, and thus their results are consistent with our present finding.

Inactivation of the co-PMv in the preoperative trials as well as during the late stage of recovery did not impair digit movements in our study. During the early recovery stage, inactivation impaired the capacity to retrieve the food piece, and digit movements became clumsy in one monkey but not in the other.

lnactivation of the ip-PMv in the preoperative trials also did not impair digit movements. During the early stage of recovery, the capacity to retrieve the food piece was unimpaired in one monkey and impaired in the other, who showed clumsiness in their digit movements. During the late stage of recovery, inactivation led to a marked slowing of movements in both monkeys. All these results suggested a time-dependent change in the contribution of a variety of cortical motor-related areas to the functional recovery after SCl.

Changes in gene expression

As described above, a substantial change in the activation of the sensori-motor cortices occurred during the recovery from SCI. However, it is not clear whether such changes in the activation pattern accompany plastic structural changes in the cortical circuits. To answer this question and to explore the molecular basis of such changes at the cortical level, we investigated the expression of growth-associated protein 43 (GAP-43), a protein related to neurite extension, by in situ hybridization before and after SCI (Higo et al., 2009). The expression of GAP-43 mRNA was enhanced in laminae II/III in S1, M1, PMd, and PMv. Expression also increased in the medium-large sized pyramidal cells in layer V of the

Fig. 5. Effect of reversible inactivation on finger dexterity before and after spinal cord injury. Representative photo frames comparing precision gripping before (Cont) and after (Mus) muscimol inactivation of the contralesional M1 (co-M1) (A) and ipsilesional M1 (ip-M1) (B) on food retrieval before the lesion (intact) and during the early and late stages of recovery. C-D: The success rates for target retrievals obtained before (Cont, black bars) and 2 h after inactivating co-M1 (C) and ip-M1 (D) with muscimol (Mus, white bars) in two monkeys (S and C) preoperatively (Intact) and during the early and late stages of recovery. Figures were modified, with permission, from Nishimura et al. (2007b).

bilateral M1 (Fig. 6A-C), to the origin of subcortical projections, such as the corticothalamic and corticostriatal projections (Jones and Wise, 1977; Higo et al., 2007). Furthermore, expression also increased in the large-sized pyramidal cells in layer V, especially in the ip-M1 (Fig. 6C), which contribute to corticospinal projections (Murray and Coulter, 1981; Toyoshima and Sakai, 1982). The results fit surprisingly well with the distribution of areas that exhibited increased activity in the

PET study, and suggest that neurite extension accompanied by reorganization of neural circuits may have occurred in S1, M1, and PM as well as along the association network connecting these cortical regions and the descending tract from the M1, as schematically summarized in Fig. 6D.

The summarized results suggest a possible reorganization of the topographical map of M1 and PMv after the lesion of the motor pathway, which was also suggested by Rouiller and colleagues (Schmidlin

Fig. 6. Expression of growth-associated protein-43 (GAP-43) mRNA in cerebral cortex before and after spinal cord injury. A-C: Laminar expression of GAP-43 mRNA in the forelimb region of the primary motor area. A: Intact monkey. B and C: CST-lesioned monkey in the early recovery stage, showing contralateral (B, co-M1) and ipsilateral (C, ip-M1) hemispheres to the CST lesion. Scale bar = 200 |jmin A-C. D: Schematic showing the implications of the present data regarding increased expression of GAP-43 mRNA after CST lesioning. ®: Increased expression in large pyramidal cells in layer V of the ipsilesional M1 indicates increasing GAP-43 mRNA expression in the descending corticospinal tracts. ®: Increased GAP-43 mRNA expression was also observed in the medium-sized pyramidal cells in layer V of the bilateral M1, suggesting an increase in GAP-43 expression in the cells that project to subcortical structures, such as the striatum, thalamus, and brainstem. ©, ®:Increased GAP-43 mRNA expression in the excitatory neurons of layers II-III of the bilateral M1 and the contralesional PMv and S1 implies a corresponding increase in GAP-43 mRNA expression in the callosal projections (©) and/or in projections to other cortical areas within the hemisphere (®). The distal body representations in M1 and S1 have very sparse callosal connectivity (Rouiller et al., 1994), and therefore, interaction between the ipsi- and contralateral M1 in the recovery of finger movements might be via indirect connections (dotted line). Figures were modified, with permission, from Higo et al. (2009).

et al., 2004). Reorganization of the sensory topography has already been reported in several lines of studies of spinal cord injury or in the rhizotomy model (Darian-Smith and Brown, 2000; Kaas et al., 2008). Possible reorganization of the motor map has thus been suggested as well, however the details are still elusive.

Concluding remarks and future direction

The experimental results summarized here showed both cortical and subcortical mechanisms underlying the functional recovery after l-CST lesions at the mid-cervical level. A surprising finding was that finger dexterity considerably recovered even after the direct CM pathway was completely transected. The results of our PET imaging study indicated that brain areas responsible for functional recovery change depending on the recovery stage. However, it should also be noted that the strategy of muscle activation may also have changed, from a reciprocal to a co-activation pattern (Nishimura et al., 2009). Although it is still speculative, we would like to propose the following hypothesis to explain the above observations, based on pathways schematically diagramed in Fig. 7.

In normal animals, hand/arm movements are primarily controlled by descending pathways from the contralateral M1 via the direct CM pathway and, presumably, partly via indirect pathways, such as those through the C3-C4 PNs and reticulospinal neurons (RSNs). In Lawrence and Kuypers (1968a, 1968b), lesions were made at the brainstem

(lesion 1 in Fig. 7A) that caused permanent impairment of precision gripping, while partial recovery was mediated by brainstem systems, such as the red nucleus (RNm) and reticulospinal neurons (RSNs). Contribution of the indirect pathway through PNs in intact animals is still controversial, but it is supported by our recent comparison of hand movements immediately after transection of the l-CST at C5 vs at C2 (Alstermark et al., 2011 and unpublished observation). In contrast to the C5 lesion, the C2 lesion (lesion 2 in Fig. 7A) caused serious impairment of precision gripping (lesion 3 in Fig. 7A). Contribution of this indirect pathway is also supported by a single unit study recording from the C3-C4 PNs by Perlmutter and colleagues (Niwa et al., 2004). In addition, there may be indirect pathways from the ipsilateral M1 to moto-neurons, either through descending projections via the re-crossed corticospinal axons from the ipsilesional motor cortex (tract <4> in Fig. 7B) or through reticulospinal pathways (tracts <5>,<6> and <7> in Fig. 7B) (Davidson et al., 2007; Davidson and Buford, 2006; Jankowska et al., 2006; Riddle et al., 2009), but these pathways might be under inhibitory regulation and not be very functional in the intact state. However, when the CST is damaged, in monkeys (and perhaps humans) inhibition may be reduced and the remaining indirect pathways from the ipsilateral M1 would compensate for the lost function. At this stage, mirror movements are often observed in the hand of the intact side when the animal tries to grasp with the affected hand. Such mirror movements might reflect the activation of the ipsilesional M1, as has been shown in our recent study on mirror movements

Fig. 7. Descending pathways that might be involved in the control of hand/arm motoneurons in the cervical enlargement ("MN"). A. Descending pathways from the contralateral motor cortex (Cx). Neurons in the magnocellular division of the red nucleus (RNm) receive inputs from the Cxand cerebellum (Cb); their axons descend through the contralateral dorsolateral funiculus (DLF) and are connected with MNs either directly or presumably via the propriospinal neurons (PN) or segmental interneurons (SIN). Reticulospinal neurons (RSN) on the ipsilateral side to MNs directly mediate the cortical inputs to MNs, and those on the contralateral side (<3>) mediate them indirectly via the commissural spinal interneurons ("C"). RSN axons descend through the ventrolateral or ventral funiculi (VLF/VF). Uncrossed corticospinal fibers descend either in the DLF (< 1>) orVF(<2>) to the cervical segments. Recent studies (Rosenzweig et al., 2010; Yoshino-Saito et al., 2010) showed that those in the DLF directly cross the midline and might control the contralateral MN (<1>). On the other hand, those in the VF are terminated in lamina VIII and might control the contralateral MNs via the commissural neurons ("C"). Red dotted lines 0, ©, ©, ® indicate the lesions in previous studies as indicated in the Concluding remarks. (1): Lawrence and Kuypers (1968a, b),(2): Alstermark et al. (2011),(3): Sasaki et al. (2004), Nishimura et al. (2007b, 2009), Alstermark et al. (2011); (4):Galea and Darian-Smith (1997a, b), Schmidlin et al. (2004), and Rosenzweig et al. (2010). B. Descending pathways from the ipsilateral motor cortex based on previous reports by Jankowska et al. (2006), Davidson and Buford (2006), Davidson et al. (2007); and Riddle et al. (2009). Re-crossed axons of the crossed CST might regulate the MNs, but has not yet been clearly demonstrated (<4>). Crossed (<5>) or uncrossed (<6>) RSN pathways mediate the cortical inputs to commissural neurons ("C"). The ipsilateral uncrossed RSN pathway mediates the cortical inputs to MN (< 7>). The existence of some of these pathways has not been demonstrated in monkeys (drawn as dotted line), but they are extrapolated from previous studies in cats.

caused by temporal inactivation of the M1 (Tsuboi et al., 2010). As the recovery progresses, the digit area of the contralesional M1 expands and the PMv on both sides is recruited for more stable control; which are accompanied by plastic changes in the neural circuits. Then, at later stages of recovery, disinhibition of the ipsilesional M1 is reduced. Actually, the mirror movements disappeared at the late stage.

By combining the results of the brain imaging and focal reversible inactivation studies, we demonstrated the contribution of brain regions found to be activated in PET studies to the recovery process. Among these, demonstration of a positive function of the ipsilesional M1 is especially noteworthy, although there is no doubt that contribution of the contralesional M1 is more important than other regions. As described above, the plastic changes in signal transmission through the cortical and subcortical networks are both time-dependent and may be coordinated with one another, but at this moment it is not known how. Clarifying the normal recovery process should contribute to the evaluation of the recovery status and to setting goals of rehabilitative training in human subjects.

On the other hand, it is important to clarify whether similar mechanisms function in the recovery from other types of lesions caused by brain stroke and other types of SCI. In the present model of SCI (lesion (3) in Fig. 7A), the PNs might play a major role in taking over the impaired function of the CST in precision gripping, and the PNs reticu-lospinal neurons may also be involved in the control of reaching. However, the compensatory mechanism might vary depending on the extent of the SCI. Several series of studies based on the hemisec-tion model at various cervical segments (Galea and Darian-Smith, 1997a, b; Rosenzweig et al., 2010; Schmidlin et al., 2004), the axons of the PNs and reticulospinal neurons descending in the ventromedial part of the white matter appear to be mostly damaged (lesion 4 in Fig. 7A). In such cases, recovery might be mediated via axons descending in the contralateral spinal half, such as uncrossed CST originating from the contralesional motor cortex; the crossed CST originating from the ipsilesional motor cortex, either in the dorsolat-eral funiculus or ventromedial funiculus (tracts < 1> and <2> in Fig. 7A); or reticulospinal tracts, which terminate on the commissural spinal interneurons (cell "C" in Fig. 7A).

For the future, therapies accompanying interventions might be developed, based on the summarized findings, to treat patients with severe spinal-cord injury or stroke. Interventions might include brain stimulation and gene therapy. For the latter purpose, studying gene expression during recovery might lead to the development of effective gene therapy to facilitate functional recovery. Along this line, we showed that expression of GAP-43 mRNA increased in the cortical areas involved in the functional recovery (Higo et al., 2009). However, more systematic analysis of gene expression is necessary to obtain a whole picture of gene expression changes. In this regard, we have recently revealed the gene expression of the M1, premotor, and prefrontal cortices in macaque monkeys using gene chip analysis (Sato et al., 2007). Such a comprehensive analysis of changes in the gene expression pattern in these cortical areas might accelerate research in this direction. In summary, the multidisciplinary approaches, including behavioral, electrophysiological, neuroanatomical, pharmacological, brain imaging, and molecular genetic approaches, are converging in the macaque model of spinal-cord injury/stroke, which will surely lead to the development of more effective neuro-rehabilitation therapy.

Acknowledgments

This research was supported by the Human Frontier Science Program, grants from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT, Grant for Higher Priority Area - Integrative Brain Research - project No. 18200027), Core Research for Evolutionary Science and Technology (CREST) by the Japan Science and Technology Corporation (JST) to T.I., and the Strategic Research Program for Brain

Sciences (SRPBS) of the MEXT and Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST) to Y.N.

References

Alstermark, B., Lundberg, A., 1992. The C3-C4 propriospinal system: target-reaching and food-taking. In: Jami, L., Pierrot-Deseilligny, E., Zytnicki, D. (Eds.), IBRO Symposium, Paris 1991, Muscle Afferents and Spinal Control of Movement. Pergamon Press, Oxford, pp. 327-354.

Alstermark, B., Ogawa, J., 2004. In vivo recordings of bulbospinal excitation in adult mouse forelimb motoneurons. J. Neurophysiol. 92,1958-1962.

Alstermark B., Isa, T., Ohki, Y., Saito, Y., 1999. Disynaptic pyramidal excitation in forelimb motoneurons mediated via C3-C4 propriospinal neurons in the Macaca fuscata. J. Neurophysiol. 82, 3580-3585.

Alstermark B., Ogawa, J., Isa, T., 2004. Lack of monosynaptic corticomotoneuronal excitation in the adult rat: fast disynaptic excitation is mediated via reticulospinal neurones and slow polysynaptic excitation via segmental interneurons. J. Neurophysiol. 91, 1832-1839.

Alstermark, B., Isa, T., Pettersson, L.-G., Sasaki, S., 2007. The C3-C4 propriospinal system in the cat and monkey: a spinal pre-motoneuronal centre for voluntary motor control. Acta Physiol (Oxf) 189,123-140.

Alstermark, B., Pettersson, L.G., Nishimura, Y., Yoshino-Saito, K., Tsuboi, F., Takahashi, M., Isa, T., 2011. Motor command for precision grip in the macaque monkey can be mediated by spinal interneurons. J Neurophysiol. 106,122-126.

Anderson, K.D., 2004. Targeting recovery: priorities of the spinal cord-injured population. J. Neurotrauma. 21,1371-1383.

Armand, J., Edgley, S.A., Lemon, R.N., Olivier, E., 1994. Protracted postnatal development of corticospinal projections from the primary motor cortex to hand motoneurones in the macaque monkey. Exp. Brain Res. 101,178-182.

Armand, J., Olivier, E., Edgley, S.A., Lemon, R.N., 1997. Postnatal development of corti-cospinal projections from motor cortex to the cervical enlargement in the macaque monkey. J. Neurosci. 17, 251-266.

Baker, S.N., Olivier, E., Lemon, R.N., 1997. Coherent oscillations in monkey motor cortex and hand muscle EMG show task-dependent modulation. J Physiol (Lond). 501, 25-41.

Bareyre, F.M., Schwab, M.E., 2003. Inflammation, degeneration and regeneration in the injured spinal cord: insights from DNA microarrays. Trends Neurosci. 26,555-563.

Borra, E., Belmalih, A., Gerbella, M., Rozzi, S., Luppino, G., 2010. Projections of the hand field of the macaque ventral premotor area F5 to the brainstem and spinal cord. J Comp Neurol. 518 (13), 2570-2591.

Bortoff, G.A., Strick, P.L., 1993. Corticospinal terminations in two new-world primates: further evidence that corticomotoneuronal connections provide part of the neural substrate for manual dexterity. J. Neurosci. 13, 5105-5118.

Burke, D., Gracies, J.M., Mazevet, D., Meunier, S., Pierrot-Deseilligny, E., 1994. Non-monosynaptic transmission of the cortical command for voluntary movement in man. J. Physiol (Lond). 480,191-202.

Casale, E.J., Light, A.R., Rustioni, A., 1988. Direct projection of the corticospinal tract to the superficial laminae of the spinal cord in the rat. J. Comp. Neurol. 278,275-286.

Cheema, S.S., Rustioni, A., Whitsel, B.L., 1984. Light and electron microscopic evidence for a direct corticospinal projection to superficial laminae of the dorsal horn in cats and monkeys. J. Comp. Neurol. 225, 276-290.

Courtine, G., Bunge, M.B., Fawcett, J.W., Grossman, R.G., Kaas, J.H., Lemon, R., Maier, I., Martin, J., Nudo, R.J., Ramon-Cueto, A., Rouiller, E.M., Schnell, L., Wannier, T., Schwab, M.E., Edgerton, V.R., 2007. Can experiments in nonhuman primates expedite the translation of treatments for spinal cord injury in humans? Nat. Med. 13, 561-566.

Courtine, G., Gerasimenko, Y., van den Brand, R., Yew, A., Musienko, P., Zhong, H., Song, B., Ao, Y., Ichiyama, R.M., Lavrov, I., Roy, R.R., Sofroniew, M.V., Edgerton, V.R., 2009. Transformation of nonfunctional spinal circuits into functional states after the loss of brain input. Nat. Neurosci. 12,1333-1342.

Darian-Smith, C., 2007. Monkey models of recovery of voluntary hand movement after spinal cord and dorsal root injury. ILAR J. 48, 396-410.

Darian-Smith, C., Brown, S., 2000. Functional changes at periphery and cortex following dorsal root lesions in adult monkeys. Nat Neurosci 3,476-481.

Davidson, A.G., Buford, J.A., 2006. Bilateral actions of the reticulospinal tract on arm and shoulder muscles in the monkey: stimulus triggered averaging. Exp. Brain Res. 173, 25-39.

Davidson, A.G., Schieber, M.H., Buford, JA, 2007. Bilateral spike-triggered average effects in arm and shoulder muscles from the monkey pontomedullary reticular formation. J. Neurosci. 27, 8053-8058.

Dong, Y., Dobkin, B.H., Cen, S.Y., Wu, A.D., Winstein, C.J., 2006. Motor cortex activation during treatment may predict therapeutic gains in paretic hand function after stroke. Stroke. 37,1552-1555.

Dum, R.P., Strick, P.L., 1991. The origin of corticospinal projections from the premotor areas in the frontal lobe. J Neurosci 11, 667-689.

Dum, R.P., Strick, P.L., 1996. Spinal cord terminations of the medial wall motor areas in macaque monkeys. J. Neurosci. 16, 6513-6525.

Fogassi, L., Gallese, V., Buccino, G., Craighero, L., Fadiga, L., Rizzolatti, G., 2001. Cortical mechanism for the visual guidance of hand grasping movements in the monkey: a reversible inactivation study. Brain. 124, 571-586.

Freund, P., Schmidlin, E., Wannier, T., Bloch, J., Mir, A., Schwab, M.E., Rouiller, E.M., 2006. Nogo-A-specific antibody treatment enhances sprouting and functional recovery after cervical lesion in adult primates. Nat. Med. 12, 790-792.

Galea, M.P., Darian-Smith, I., 1997a. Corticospinal projection patterns following unilateral section of the cervical spinal cord in the newborn and juvenile macaque monkey. J Comp Neurol. 381, 282-306.

Galea, M.P., Darian-Smith, I., 1997b. Manual dexterity and corticospinal connectivity following unilateral section of the cervical spinal cord in the macaque monkey. J Comp Neurol. 381, 307-319.

Hallett, M., 2001. Functional reorganization after lesions of the human brain: studies with transcranial magnetic stimulation. Rev Neurol (Paris) 157, 822-826.

He, S.Q., Dum, R.P., Strick P.L., 1995. Topographic organization of corticospinal projections from the frontal lobe: motor areas on the medial surface of the hemisphere. J Neurosci. 15,3284-3306.

Higo, N., Oishi, T., Yamashita, A., Murata, Y., Matsuda, K., Hayashi, M., 2007. Expression of protein kinase-C substrate mRNA in the motor cortex of adult and infant macaque monkeys. Brain Res. 1171, 30-41.

Higo, N., Nishimura, Y., Murata, Y., Oishi, T., Yoshino-Saito, K., Takahashi, M., Tsuboi, F., Isa, T., 2009. Increased expression ofthe growth-associated protein 43 gene in the senso-rimotor cortex of the macaque monkey after lesioning the lateral corticospinal tract. J. Comp. Neurol. 516,493-506.

Illert, M., Lundberg, A., Tanaka, R., 1977. Integration in descending motor pathways controlling the forelimb in the cat. 3. Convergence on propriospinal neurones transmitting disynaptic excitation from the corticospinal tract and other descending tracts. Exp Brain Res 9, 323-346.

Isa, T., Ohki, Y., Seki, K., Alstermark B., 2006. Properties of propriospinal neurons in the C3-C4 segments mediating disynaptic pyramidal excitation to forelimb motoneu-rons in the macaque monkey. J. Neurophysiol. 95, 3674-3685.

Isa, T., Ohki, Y., Alstermark, B., Pettersson, L.-G., Sasaki, S., 2007. Direct and indirect cortico-motoneuronal pathways and control of hand/arm movements. Physiology (Bethesda). 22,145-152.

Jankowska, E., Stecina, K., Cabaj, A., Pettersson, L.G., Edgley, S.A., 2006. Neuronal relays in double crossed pathways between feline motor cortex and ipsilateral hindlimb motoneurones. J Physiol (Lond) 575, 527-541.

Jones, E.G., Wise, S.P., 1977. Size, laminar and columnar distribution of efferent cells in the sensory-motor cortex of monkeys. J. Comp. Neurol. 175, 391-438.

Kaas, J.H., Qi, H.X., Burish, M.J., Gharbawie, OA, Onifer, S.M., Massey, J.M., 2008. Cortical and subcortical plasticity in the brains of humans, primates, and rats after damage to sensory afferents in the dorsal columns of the spinal cord. Exp Neurol. 209,407-416.

Kuypers, H.G., 1962. Corticospinal connections: postnatal development in the rhesus monkey. Science 138, 678-680.

Lacroix, S., Havton, L.A., McKay, H., Yang, H., Brant, A., Roberts, J., Tuszynski, M.H., 2004. Bilateral corticospinal projections arise from each motor cortex in the macaque monkey: a quantitative study. J. Comp. Neurol. 473,147-161.

Lawrence, D.G., Kuypers, H.G., 1968a. The functional organization of the motor system in the monkey. I. The effect of bilateral pyramidal lesions. Brain 91,1-14.

Lawrence, D.G., Kuypers, H.G., 1968b. The functional organization of the motor system in the monkey. II. The effects of lesions of the descending brain-stem pathways. Brain 91,15-36.

Lemon, R.N., 2008. Descending pathways in motor control. Ann. Rev. Neurosci. 31, 195-218.

Lemon, R.N., Griffiths, J., 2005. Comparing the function of the corticospinal system in different species: organizational differences for motor specialization? Muscle Nerve 32, 261-279.

Liu, C.N., Chambers, W.W., 1964. An experimental study of the corticospinal system in the monkey (Macaca mulatta).J. Comp. Neurol. 123, 257-283.

Maier, M.A., Illert, M., Kirkwood, P.A., Nielsen, J., Lemon, R.N., 1998. Does a C3-C4 pro-priospinal system transmit corticospinal excitation in the primate? An investigation in the macaque monkey. J Physiol (Lond). 511,191-212.

Malmgren, K., Pierrot-Deseilligny, E., 1988. Evidence for non-monosynaptic Ia excitation of human wrist flexor motoneurones, possibly via propriospinal neurones. J. Physiol (Lond). 405, 747-764.

Marshall, R.S., Perera, G.M., Lazar, R.M., Krakauer, J.W., Constantine, R.C., DeLaPaz, R.L., 2000. Evolution of cortical activation during recovery from corticospinal tract infarction. Stroke 31, 656-661.

Mima, T., Hallett, M., 1999. Electroencephalograph«: analysis of cortico-muscular coherence: reference effect, volume conduction and generator mechanism. Clin Neurophysiol. 110, 1892-1899.

Mima, T., Toma, K., Koshy, B., Hallett, M., 2001. Coherence between cortical and muscular activities after subcortical stroke. Stroke. 32, 2597-2601.

Muir, R.B., Lemon, R.N., 1983. Corticospinal neurons with a special role in precision grip. Brain Res. 261, 312-316.

Murray, E.A., Coulter, J.D., 1981. Organization of corticospinal neurons in the monkey. J. Comp. Neurol. 195, 339-365.

Nakajima, K., Maier, M.A., Kirkwood, P.A., Lemon, R.N., 2000. Striking differences in transmission ofcorticospinal excitation to upper limb motoneurons in two primate species. J. Neurophysiol. 84, 698-709.

Nicolas, G., Marchand-Pauvert, V., Burke, D., Pierrot-Deseilligny, E., 2001. Corticospinal excitation of presumed cervical propriospinal neurones and its reversal to inhibition in humans. J Physiol (Lond). 533,903-919.

Nishimura, Y., Isa, T., 2009. Compensatory changes at the cerebral cortical level after spinal cord injury. Neuroscientist. 5, 436-444.

Nishimura, Y., Onoe, H., Morichika, Y., Tsukada, H., Isa, T., 2007a. Activation of parieto-frontal stream during reaching and grasping studied by positron emission tomography in monkeys. Neurosci. Res. 59, 243-250.

Nishimura, Y., Onoe, H., Morichika, Y., Perfiliev, S., Tsukada, H., Isa, T., 2007b. Time-dependent central compensatory mechanisms of finger dexterity after spinal cord injury. Science 318,1150-1155.

Nishimura, Y., Morichika, Y., Isa, T., 2009. A subcortical oscillatory network contributes to recovery of hand dexterity after spinal cord injury. Brain 132, 709-721.

Niwa, M., Fadok, J.P., Fetz, E.E., Perlmutter, S.I., 2004. C3-C4 interneurons in behaving monkeys: activity related to reach-and-grasp and wrist movements. Abstract, Society for Neuroscience 656.6.

Olivier, E., Baker, S.N., Nakajima, K., Brochier, T., Lemon, R.N., 2001. Investigation into non-monosynaptic corticospinal excitation of macaque upper limb single motor units. J Neurophysiol. 86, 1573-1586.

Pettersson, L.G., Alstermark, B., Blavovechtchenski, E., Isa, T., Sasaki, S., 2007. Skilled digit movements in feline and primate — recovery after selective spinal cord lesions. Acta Physiol (Oxf). 189,141-154.

Ralston, D.D., Ralston III, H.J., 1985. The terminations of corticospinal tract axons in the macaque monkey. J. Comp. Neurol. 242, 325-337.

Riddle, C.N., Edgley, S.A., Baker, S.N., 2009. Direct and indirect connections with upper limb motoneurons from the primate reticulospinal tract. J Neurosci. 29, 4993-4999.

Roelcke, U., Curt, A., Otte, A., Missimer, J., Maguire, R.P., Dietz, V., Leenders, K.L., 1997. Influence of spinal cord injury on cerebral sensorimotor systems: a PET study. J. Neurol. Neurosurg. Psychiatry 62, 61 -65.

Rosenzweig, E.S., Brock, J.H., Culbertson, M.D., Lu, P., Moseanko, R., Edgerton, V.R., Hav-ton, L.A., Tuszynski, M.H., 2009. Extensive spinal decussation and bilateral termination of cervical corticospinal projections in rhesus monkeys. J. Comp. Neurol. 513, 151-163.

Rosenzweig, E.S., Courtine, G., Jindrich, D.L., Brock, J.H., Ferguson, A.R., Strand, S.C., Nout, Y.S., Roy, R.R., Miller, D.M., Beattie, M.S., Havton, L.A., Bresnahan, J.C., Edge-rton, V.R., Tuszynski, M.H., 2010. Extensive spontaneous plasticity of corticospinal projections after primate spinal cord injury. Nat Neurosci 13,1505-1510.

Rouiller, E.M., Babalian, A., Kazennikov, O., Moret, V., Yu, X.H., Wiesendanger, M., 1994. Transcallosal connections of the distal forelimb representations of the primary and supplementary motor cortical areas in macaque monkeys. Exp. Brain Res. 102, 227-243.

Rowland, J.W., Hawryluk, G.W., Kwon, B., Fehlings, M.G., 2008. Current status of acute spinal cord injury pathophysiology and emerging therapies: promise on the horizon. Neurosurg. Focus 25:E2.

Sasaki, S., Isa, T., Pettersson, L.G., Alstermark, B., Naito, K., Yoshimura, K., Seki, K., Ohki, Y., 2004. Dexterous finger movements in primate without monosynaptic cortico-motoneuronal excitation. J. Neurophysiol. 92, 3142-3147.

Sato, A., Nishimura, Y., Oishi, T., Higo, N., Murata, Y., Onoe, H., Saito, K., Tsuboi, F., Taka-hashi, M., Isa, T., Kojima, T., 2007. Differentially expressed genes among motor and prefrontal areas of macaque neocortex. Biochem. Biophys. Res. Commun. 362, 665-669.

Schmidlin, E., Wannier, T., Bloch, J., Rouiller, E.M., 2004. Progressive plastic changes in the hand representation of the primary motor cortex parallel incomplete recovery from a unilateral section of the corticospinal tract at cervical level in monkeys. Brain Res. 1017,172-183.

Tower, S.S., 1940. Pyramidal lesion in the monkey. Brain 63, 36-90.

Toyoshima, K., Sakai, H., 1982. Exact cortical extent of the origin of the corticospinal tract (CST) and the quantitative contribution to the CST in different cytoarchitec-tonic areas. A study with horseradish peroxidase in the monkey. J. Hirnforsch. 23, 257-269.

Tsuboi, F., Nishimura, Y., Yoshino-Saito, K., Isa, T., 2010. Neuronal mechanism of mirror movements caused by dysfunction of the motor cortex. Eur. J. Neurosci. 32, 1397-1406.

Umeda, T., Takahashi, M., Isa, K., Isa, T., 2010. Formation of descending pathways mediating cortical command to ipsilateral forelimb motoneurons in rats with neonatal hemidecortication. J. Neurophysiol. 104,1707-1716.

Ward, N.S., Brown, M.M., Thompson, A.J., Frackowiak, R.S., 2003. Neural correlates of motor recovery after stroke: a longitudinal fMRI study. Brain 126, 2476-2496.

Yoshino-Saito, K., Nishimura, Y., Oishi, T., Isa, T., 2010. Quantitative inter-segmental and inter-laminar comparison of corticospinal projection from forelimb area of primary motor cortex of macaque monkeys. Neurosci. 171,1164-1179.

Zweckberger, K., Stoffel, M., Baethmann, A., Plesnila, N., 2003. Effect of decompression craniotomy on increase of contusion volume and functional outcome after controlled cortical impact in mice. J. Neurotrauma 20, 1307-1314.