Spinal branching of corticospinal axons in the cat

Axonal projection of the medullary expiratory neurons in the feline thoracic spinal cord

Expiratory neurons in the caudal ventral respiratory group extend descending axons to the lumbar and sacral spinal cord, and they possess axon collaterals, the distribution of which has been well-documented. Likewise, these expiratory neurons extend axons to the thoracic spinal cord and innervate thoracic expiratory motoneurons. These axons also give rise to collaterals, and their distribution may influence the strength of synaptic connectivity between the axons and the thoracic expiratory motoneurons. We investigated the distribution of axon collaterals in the thoracic spinal cord using a microstimulation technique. This study was performed on cats; one cat was used to make an anatomical atlas and six were used in the experiment. Extracellular spikes of expiratory neurons were recorded in artificially ventilated cats. The thoracic spinal gray matter was microstimulated from dorsal to ventral sites at 100-μm intervals using a glass-insulated tungsten microelectrode with a current of 150-250 μA. The stimulation tracks were made at 1 mm intervals along the spinal cord in segments Th9 to Th13, and the effective stimulating sites of antidromic activation in axon collaterals were systematically mapped. The effective stimulating sites in the contralateral thoracic spinal cord with expiratory neurons in the caudal ventral respiratory group (cVRG) occupied 14.4% of the total length of the thoracic spinal cord examined. The mean percentage of effective stimulating tracks per unit was 18.6 ± 4.4%. The distribution of axon collaterals of expiratory neurons in the feline thoracic spinal cord indeed resembled that reported in the upper lumbar spinal cord. We propose that a single medullary expiratory neuron exerts excitatory effects across multiple segments of the thoracic spinal cord via its collaterals.

Brainstem Neural Circuits Triggering Vertical Saccades and Fixation

Neurons in the nucleus raphe interpositus have tonic activity that suppresses saccadic burst neurons (BNs) during eye fixations, and that is inhibited before and during saccades in all directions (omnipause neurons, OPNs). We have previously demonstrated via intracellular recording and anatomical staining in anesthetized cats of both sexes that OPNs are inhibited by BNs in the medullary reticular formation (horizontal inhibitory BNs, IBNs). These horizontal IBNs receive monosynaptic input from the caudal horizontal saccade area of the superior colliculus (SC), and then produce monosynaptic inhibition in OPNs, providing a mechanism to trigger saccades. However, it is well known that the neural circuits driving horizontal components of saccades are independent from the circuits driving vertical components. Thus, our previous results are unable to explain how purely vertical saccades are triggered. Here, we again apply intracellular recording to show that a disynaptic vertical IBN circuit exists, analogous to the horizontal circuit. Specifically, we show that stimulation of the SC rostral vertical saccade area produces disynaptic inhibition in OPNs, which is not abolished by midline section between the horizontal IBNs. This excludes the possibility that horizontal IBNs could be responsible for the OPN inhibition during vertical saccades. We then show that vertical IBNs in the interstitial nucleus of Cajal, which receive monosynaptic input from rostral SC, are responsible for the disynaptic inhibition of OPNs. These results indicate that a similarly functioning SC-IBN-OPN circuit exists for both the horizontal and vertical oculomotor pathways. These two IBN-mediated circuits are capable of triggering saccades in any direction.Significance Statement Saccades shift gaze to objects of interest, moving their image to the central retina, where it is maintained for detailed examination (fixation). During fixation, high gain saccade burst neurons (BNs) are tonically inhibited by omnipause neurons (OPNs). Our previous study showed that medullary horizontal inhibitory BNs (IBNs) activated from the caudal superior colliculus (SC) inhibit tonically active OPNs in order to initiate horizontal saccades. The present study addresses the source of OPN inhibition for vertical saccades. We find that OPNs monosynaptically inhibit vertical IBNs in the interstitial nucleus of Cajal during fixation. Those same vertical IBNs are activated by the rostral SC, and inhibit OPN activity to initiate vertical saccades.

Corticospinal populations broadcast complex motor signals to coordinated spinal and striatal circuits

Many models of motor control emphasize the role of sensorimotor cortex in movement, principally through the projections that corticospinal neurons (CSNs) make to the spinal cord. Additionally, CSNs possess expansive supraspinal axon collaterals, the functional organization of which is largely unknown. Using anatomical and electrophysiological circuit-mapping techniques in the mouse, we reveal dorsolateral striatum as the preeminent target of CSN collateral innervation. We found that this innervation is biased so that CSNs targeting different striatal pathways show biased targeting of spinal cord circuits. Contrary to more conventional perspectives, CSNs encode not only individual movements, but also information related to the onset and offset of motor sequences. Furthermore, similar activity patterns are broadcast by CSN populations targeting different striatal circuits. Our results reveal a logic of coordinated connectivity between forebrain and spinal circuits, where separate CSN modules broadcast similarly complex information to downstream circuits, suggesting that differences in postsynaptic connectivity dictate motor specificity.

Morphometry of cervical spinal cord in cat using design-based stereology

The spinal cord harbours nerve fibres that facilitate reflex actions and that transmit impulses to and from the brain. The cervical spinal cord is an area of particular interest in medicine and veterinary due to frequent pathologic alterations in this region. This study describes the morphometric features of the cervical spinal cord in cat using design-unbiased stereological methods. The cervical spinal cords of four male cats were dissected and samples were taken according to systematic uniform random sampling. Each sample was embedded in agar and cut into 60-µm thick sections and stained with cresyl violet 0.1% for stereological estimations. The total cervical spinal cord volume obtained by the Cavalieri estimator was 2,321.21 ± 285.5 mm³ . The relative volume of grey matter and white matter was 23.8 ± 1.3% and 76.1 ± 1.3%. The dorsal horn and ventral horn volume were 12.3 ± 1.2% and 11.4 ± 0.7% of the whole cervical spinal cord. The volume of central canal was estimated to 3.8 ± 1 mm³ . The total number of neurons was accounted 3,405,366.2 ± 267,469.4 using the optical disector/fractionator method. The number of motoneurons and interneurons was estimated to be 1,120,433.2 ± 174,796.7 and 2,284,932.9 ± 127,261.5, respectively. The average volume of the motoneurons and interneurons was estimated to 1980 µm³ and 680 µm³ , respectively, using the spatial rotator method. This knowledge of cat spinal cord findings may serve as a foundation as a translational model in spinal cord experimental research and provide basic findings for diagnosis and treatment of spinal cord disorders.

Does the cerebellum shape the spatiotemporal organization of muscle patterns? Insights from subjects with cerebellar ataxias

The role of the cerebellum in motor control has been investigated extensively, but its contribution to the muscle pattern organization underlying goal-directed movements is still not fully understood. Muscle synergies may be used to characterize multimuscle pattern organization irrespective of time (spatial synergies), in time irrespective of the muscles (temporal synergies), and both across muscles and in time (spatiotemporal synergies). The decomposition of muscle patterns as combinations of different types of muscle synergies offers the possibility to identify specific changes due to neurological lesions. In this study, we recorded electromyographic activity from 13 shoulder and arm muscles in subjects with cerebellar ataxias (CA) and in age-matched healthy subjects (HS) while they performed reaching movements in multiple directions. We assessed whether cerebellar damage affects the organization of muscle patterns by extracting different types of muscle synergies from the muscle patterns of each HS and using these synergies to reconstruct the muscle patterns of all other participants. We found that CA muscle patterns could be accurately captured only by spatial muscle synergies of HS. In contrast, there were significant differences in the reconstruction R² values for both spatiotemporal and temporal synergies, with an interaction between the two synergy types indicating a larger difference for spatiotemporal synergies. Moreover, the reconstruction quality using spatiotemporal synergies correlated with the severity of impairment. These results indicate that cerebellar damage affects the temporal and spatiotemporal organization, but not the spatial organization, of the muscle patterns, suggesting that the cerebellum plays a key role in shaping their spatiotemporal organization.NEW & NOTEWORTHY In recent studies, the decomposition of muscle activity patterns has revealed a modular organization of the motor commands. We show, for the first time, that muscle patterns of subjects with cerebellar damage share with healthy controls spatial, but not temporal and spatiotemporal, modules. Moreover, changes in spatiotemporal organization characterize the severity of the subject's impairment. These results suggest that the cerebellum has a specific role in shaping the spatiotemporal organization of the muscle patterns.

Brain Stem Neural Circuits of Horizontal and Vertical Saccade Systems and their Frame of Reference

Sensory signals for eye movements (visual and vestibular) are initially coded in different frames of reference but finally translated into common coordinates, and share the same final common pathway, namely the same population of extraocular motoneurons. From clinical studies in humans and lesion studies in animals, it is generally accepted that voluntary saccadic eye movements are organized in horizontal and vertical Cartesian coordinates. However, this issue is not settled yet, because neural circuits for vertical saccades remain unidentified. We recently determined brainstem neural circuits from the superior colliculus to ocular motoneurons for horizontal and vertical saccades with combined electrophysiological and neuroanatomical techniques. Comparing well-known vestibuloocular pathways with our findings of commissural excitation and inhibition between both superior colliculi, we proposed that the saccade system uses the same frame of reference as the vestibuloocular system, common semicircular canal coordinate. This proposal is mainly based on marked similarities (1) between output neural circuitry from one superior colliculus to extraocular motoneurons and that from a respective canal to its innervating extraocular motoneurons, (2) of patterns of commissural reciprocal inhibitions between upward saccade system on one side and downward system on the other, and between anterior canal system on one side and posterior canal system on the other, and (3) between the neural circuits of saccade and quick phase of vestibular nystagmus sharing brainstem burst neurons. In support of the proposal, commissural excitation of the superior colliculi may help to maintain Listing's law in saccades in spite of using semicircular canal coordinate.

Neural Activity during Voluntary Movements in Each Body Representation of the Intracortical Microstimulation-Derived Map in the Macaque Motor Cortex

In order to accurately interpret experimental data using the topographic body map identified by conventional intracortical microstimulation (ICMS), it is important to know how neurons in each division of the map respond during voluntary movements. Here we systematically investigated neuronal responses in each body representation of the ICMS map during a reach-grasp-retrieval task that involves the movements of multiple body parts. The topographic body map in the primary motor cortex (M1) generally corresponds to functional divisions of voluntary movements; neurons at the recording sites in each body representation with movement thresholds of 10 μA or less were differentially activated during the task, and the timing of responses was consistent with the movements of the body part represented. Moreover, neurons in the digit representation responded differently for the different types of grasping. In addition, the present study showed that neural activity depends on the ICMS current threshold required to elicit body movements and the location of the recording on the cortical surface. In the ventral premotor cortex (PMv), no correlation was found between the response properties of neurons and the body representation in the ICMS map. Neural responses specific to forelimb movements were often observed in the rostral part of PMv, including the lateral bank of the lower arcuate limb, in which ICMS up to 100 μA evoked no detectable movement. These results indicate that the physiological significance of the ICMS-derived maps is different between, and even within, areas M1 and PMv.

Axon diameters and conduction velocities in the macaque pyramidal tract

Small axons far outnumber larger fibers in the corticospinal tract, but the function of these small axons remains poorly understood. This is because they are difficult to identify, and therefore their physiology remains obscure. To assess the extent of the mismatch between anatomic and physiological measures, we compared conduction time and velocity in a large number of macaque corticospinal neurons with the distribution of axon diameters at the level of the medullary pyramid, using both light and electron microscopy. At the electron microscopic level, a total of 4,172 axons were sampled from 2 adult male macaque monkeys. We confirmed that there were virtually no unmyelinated fibers in the pyramidal tract. About 14% of pyramidal tract axons had a diameter smaller than 0.50 μm (including myelin sheath), most of these remaining undetected using light microscopy, and 52% were smaller than 1 μm. In the electrophysiological study, we determined the distribution of antidromic latencies of pyramidal tract neurons, recorded in primary motor cortex, ventral premotor cortex, and supplementary motor area and identified by pyramidal tract stimulation (799 pyramidal tract neurons, 7 adult awake macaques) or orthodromically from corticospinal axons recorded at the mid-cervical spinal level (192 axons, 5 adult anesthetized macaques). The distribution of antidromic and orthodromic latencies of corticospinal neurons was strongly biased toward those with large, fast-conducting axons. Axons smaller than 3 μm and with a conduction velocity below 18 m/s were grossly underrepresented in our electrophysiological recordings, and those below 1 μm (6 m/s) were probably not represented at all. The identity, location, and function of the majority of corticospinal neurons with small, slowly conducting axons remains unknown.

Convergent synaptic inputs from the caudal fastigial nucleus and the superior colliculus onto pontine and pontomedullary reticulospinal neurons

The caudal fastigial nucleus (FN) is known to be related to the control of eye movements and projects mainly to the contralateral reticular nuclei where excitatory and inhibitory burst neurons for saccades exist [the caudal portion of the nucleus reticularis pontis caudalis (NRPc), and the rostral portion of the nucleus reticularis gigantocellularis (NRG) respectively]. However, the exact reticular neurons targeted by caudal fastigioreticular cells remain unknown. We tried to determine the target reticular neurons of the caudal FN and superior colliculus (SC) by recording intracellular potentials from neurons in the NRPc and NRG of anesthetized cats. Neurons in the rostral NRG received bilateral, monosynaptic excitation from the caudal FNs, with contralateral predominance. They also received strong monosynaptic excitation from the rostral and caudal contralateral SC, and disynaptic excitation from the rostral ipsilateral SC. These reticular neurons with caudal fastigial monosynaptic excitation were not activated antidromically from the contralateral abducens nucleus, but most of them were reticulospinal neurons (RSNs) that were activated antidromically from the cervical cord. RSNs in the caudal NRPc received very weak monosynaptic excitation from only the contralateral caudal FN, and received either monosynaptic excitation only from the contralateral caudal SC, or monosynaptic and disynaptic excitation from the contralateral caudal and ipsilateral rostral SC, respectively. These results suggest that the caudal FN helps to control also head movements via RSNs targeted by the SC, and these RSNs with SC topographic input play different functional roles in head movements.

Motor cortical regulation of sparse synergies provides a framework for the flexible control of precision walking

We have previously described a modular organization of the locomotor step cycle in the cat in which a number of sparse synergies are activated sequentially during the swing phase of the step cycle (Krouchev et al., 2006). Here, we address how these synergies are modified during voluntary gait modifications. Data were analysed from 27 bursts of muscle activity (recorded from 18 muscles) recorded in the forelimb of the cat during locomotion. These were grouped into 10 clusters, or synergies, during unobstructed locomotion. Each synergy was comprised of only a small number of muscles bursts (sparse synergies), some of which included both proximal and distal muscles. Eight (8/10) of these synergies were active during the swing phase of locomotion. Synergies observed during the gait modifications were very similar to those observed during unobstructed locomotion. Constraining these synergies to be identical in both the lead (first forelimb to pass over the obstacle) and the trail (second limb) conditions allowed us to compare the changes in phase and magnitude of the synergies required to modify gait. In the lead condition, changes were observed particularly in those synergies responsible for transport of the limb and preparation for landing. During the trail condition, changes were particularly evident in those synergies responsible for lifting the limb from the ground at the onset of the swing phase. These changes in phase and magnitude were adapted to the size and shape of the obstacle over which the cat stepped. These results demonstrate that by modifying the phase and magnitude of a finite number of muscle synergies, each comprised of a small number of simultaneously active muscles, descending control signals could produce very specific modifications in limb trajectory during locomotion. We discuss the possibility that these changes in phase and magnitude could be produced by changes in the activity of neurones in the motor cortex.

Robustness of muscle synergies underlying three-dimensional force generation at the hand in healthy humans

Previous studies using advanced matrix factorization techniques have shown that the coordination of human voluntary limb movements may be accomplished using combinations of a small number of intermuscular coordination patterns, or muscle synergies. However, the potential use of muscle synergies for isometric force generation has been evaluated mostly using correlational methods. The results of such studies suggest that fixed relationships between the activations of pairs of muscles are relatively rare. There is also emerging evidence that the nervous system uses independent strategies to control movement and force generation, which suggests that one cannot conclude a priori that isometric force generation is accomplished by combining muscle synergies, as shown in movement control. In this study, we used non-negative matrix factorization to evaluate the ability of a few muscle synergies to reconstruct the activation patterns of human arm muscles underlying the generation of three-dimensional (3-D) isometric forces at the hand. Surface electromyographic (EMG) data were recorded from eight key elbow and shoulder muscles during 3-D force target-matching protocols performed across a range of load levels and hand positions. Four synergies were sufficient to explain, on average, 95% of the variance in EMG datasets. Furthermore, we found that muscle synergy composition was conserved across biomechanical task conditions, experimental protocols, and subjects. Our findings are consistent with the view that the nervous system can generate isometric forces by assembling a combination of a small number of muscle synergies, differentially weighted according to task constraints.

Modules in the brain stem and spinal cord underlying motor behaviors

Previous studies using intact and spinalized animals have suggested that coordinated movements can be generated by appropriate combinations of muscle synergies controlled by the central nervous system (CNS). However, which CNS regions are responsible for expressing muscle synergies remains an open question. We address whether the brain stem and spinal cord are involved in expressing muscle synergies used for executing a range of natural movements. We analyzed the electromyographic (EMG) data recorded from frog leg muscles before and after transection at different levels of the neuraxis-rostral midbrain (brain stem preparations), rostral medulla (medullary preparations), and the spinal-medullary junction (spinal preparations). Brain stem frogs could jump, swim, kick, and step, while medullary frogs could perform only a partial repertoire of movements. In spinal frogs, cutaneous reflexes could be elicited. Systematic EMG analysis found two different synergy types: 1) synergies shared between pre- and posttransection states and 2) synergies specific to individual states. Almost all synergies found in natural movements persisted after transection at rostral midbrain or medulla but not at the spinal-medullary junction for swim and step. Some pretransection- and posttransection-specific synergies for a certain behavior appeared as shared synergies for other motor behaviors of the same animal. These results suggest that the medulla and spinal cord are sufficient for the expression of most muscle synergies in frog behaviors. Overall, this study provides further evidence supporting the idea that motor behaviors may be constructed by muscle synergies organized within the brain stem and spinal cord and activated by descending commands from supraspinal areas.

Spinal interneuron axons spontaneously regenerate after spinal cord injury in the adult feline

It is well established that long, descending axons of the adult mammalian spinal cord do not regenerate after a spinal cord injury (SCI). These axons do not regenerate because they do not mount an adequate regenerative response and growth is inhibited at the injury site by growth cone collapsing molecules, such as chondroitin sulfate proteoglycans (CSPGs). However, whether axons of axotomized spinal interneurons regenerate through the inhibitory environment of an SCI site remains unknown. Here, we show that cut axons from adult mammalian spinal interneurons can regenerate through an SCI site and form new synaptic connections in vivo. Using morphological and immunohistochemical analyses, we found that after a midsagittal transection of the adult feline spinal cord, axons of propriospinal commissural interneurons can grow across the lesion despite a close proximity of their growth cones to CSPGs. Furthermore, using immunohistochemical and electrophysiological analyses, we found that the regenerated axons conduct action potentials and form functional synaptic connections with motoneurons, thus providing new circuits that cross the transected commissures. Our results show that interneurons of the adult mammalian spinal cord are capable of spontaneous regeneration after injury and suggest that elucidating the mechanisms that allow these axons to regenerate may lead to useful new therapeutic strategies for restoring function after injury to the adult CNS.

On the nature of the intrinsic connectivity of the cat motor cortex: evidence for a recurrent neural network topology

The details and functional significance of the intrinsic horizontal connections between neurons in the motor cortex (MCx) remain to be clarified. To further elucidate the nature of this intracortical connectivity pattern, experiments were done on the MCx of three cats. The anterograde tracer biocytin was ejected iontophoretically in layers II, III, and V. Some 30-50 neurons within a radius of approximately 250 microm were thus stained. The functional output of the motor cortical point at which biocytin was injected, and of the surrounding points, was identified by microstimulation and electromyographic recordings. The axonal arborizations of the stained neurons were traced under camera lucida. The axon collaterals were extensive, reaching distances of <or=7 mm from the injection site. More importantly, the axonal branches were studded all along their course with boutons. The vast majority of boutons formed synaptic contacts on the target cells as identified by electron microscopy. The majority of these boutons made asymmetric (type I, excitatory) synapses mainly on dendritic spines. The bouton density decreased approximately monotonically with distance from the center of the injection. Cluster analysis, lagged covariance analysis, and eigenvalue decomposition showed the bouton distribution map to be unimodal. Superposition of the synaptic bouton distribution map and the motor output map revealed that motor cortical neurons don't make point-to-point connections but rather bind together the representations of a variety of muscles within a large neighborhood. This recurrent-network type connectivity strongly supports the hypothesis that the MCx controls the musculature in an integrated manner.

Pyramidal tract stimulation restores normal corticospinal tract connections and visuomotor skill after early postnatal motor cortex activity blockade

Motor development depends on forming specific connections between the corticospinal tract (CST) and the spinal cord. Blocking CST activity in kittens during the critical period for establishing connections with spinal motor circuits results in permanent impairments in connectivity and function. The changes in connections are consistent with the hypothesis that the inactive tract is less competitive in developing spinal connections than the active tract. In this study, we tested the competition hypothesis by determining whether activating CST axons, after previous silencing during the critical period, abrogated development of aberrant corticospinal connections and motor impairments. In kittens, we inactivated motor cortex by muscimol infusion between postnatal weeks 5 and 7. Next, we electrically stimulated CST axons in the medullary pyramid 2.5 h daily, between weeks 7 and 10. In controls (n = 3), CST terminations were densest within the contralateral deeper, premotor, spinal layers. After previous inactivation (n = 3), CST terminations were densest within the dorsal, somatic sensory, layers. There were more ipsilateral terminations from the active tract. During visually guided locomotion, there was a movement endpoint impairment. Stimulation after inactivation (n = 6) resulted in significantly fewer terminations in the sensory layers and more in the premotor layers, and fewer ipsilateral connections from active cortex. Chronic stimulation reduced the current threshold for evoking contralateral movements by pyramidal stimulation, suggesting strengthening of connections. Importantly, stimulation significantly improved stepping accuracy. These findings show the importance of activity-dependent processes in specifying CST connections. They also provide a strategy for harnessing activity to rescue CST axons at risk of developing aberrant connections after CNS injury.

A method to estimate the spatial extent of activation in thalamic deep brain stimulation

OBJECTIVE

The goal of this study was to develop, evaluate, and apply a method to quantify the unknown spatial extent of activation in deep brain stimulation (DBS) of the ventral intermedius nucleus (Vim) of the thalamus.

METHODS

The amplitude-distance relationship and the threshold amplitudes to elicit clinical responses were combined to estimate the unknown amplitude-distance constant and the distance between the electrode and the border between the Vim and the ventrocaudal nucleus (Vc) of the thalamus. We tested the sensitivity of the method to errors in the input parameters, and subsequently applied the method to estimate the amplitude-distance constant from clinically-measured threshold amplitudes.

RESULTS

The method enabled estimation of the amplitude-distance constant with a median squared error of 0.07-0.23V/mm2 and provided an estimate of the distance between the electrode and the Vc/Vim border with a median squared error of 0.01-0.04mm. Application of the method to clinically-measured threshold amplitudes to elicit paresthesias estimated the amplitude-distance constant to be 0.22V/mm2.

CONCLUSIONS

The method enabled robust quantification of the spatial extent of activation in thalamic DBS and predicted that stimulation amplitudes of 1-3.5V would produce a mean effective radius of activation of 2.0-3.9mm.

SIGNIFICANCE

Knowing the spatial extent of activation may improve methods of electrode placement and stimulation parameter selection in DBS.

Muscle synergies during locomotion in the cat: a model for motor cortex control

It is well established that the motor cortex makes an important contribution to the control of visually guided gait modifications, such as those required to step over an obstacle. However, it is less clear how the descending cortical signal interacts with the interneuronal networks in the spinal cord to ensure that precise changes in limb trajectory are appropriately incorporated into the base locomotor rhythm. Here we suggest that subpopulations of motor cortical neurones, active sequentially during the step cycle, may regulate the activity of small groups of synergistic muscles, likewise active sequentially throughout the step cycle. These synergies, identified by a novel associative cluster analysis, are defined by periods of muscle activity that are coextensive with respect to the onset and offset of the EMG activity. Moreover, the synergies are sparse and are frequently composed of muscles acting around more than one joint. During gait modifications, we suggest that subpopulations of motor cortical neurones may modify the magnitude and phase of the EMG activity of all muscles contained within a given synergy. Different limb trajectories would be produced by differentially modifying the activity in each synergy thus providing a flexible substrate for the control of intralimb coordination during locomotion.

Commissural mirror-symmetric excitation and reciprocal inhibition between the two superior colliculi and their roles in vertical and horizontal eye movements

The functional roles of commissural excitation and inhibition between the two superior colliculi (SCs) are not yet well understood. We previously showed the existence of strong excitatory commissural connections between the rostral SCs, although commissural connections had been considered to be mainly inhibitory. In this study, by recording intracellular potentials, we examined the topographical distribution of commissural monosynaptic excitation and inhibition from the contralateral medial and lateral SC to tectoreticular neurons (TRNs) in the medial or lateral SC of anesthetized cats. About 85% of TRNs examined projected to both the ipsilateral Forel's field H and the contralateral inhibitory burst neuron region where the respective premotor neurons for vertical and horizontal saccades reside. Medial TRNs received strong commissural excitation from the medial part of the opposite SC, whereas lateral TRNs received excitation mainly from its lateral part. Injection of wheat germ agglutinin-horseradish peroxidase into the lateral or medial SC retrogradely labeled many larger neurons in the lateral or medial part of the contralateral SC, respectively. These results indicated that excitatory commissural connections exist between the medial and medial parts and between the lateral and lateral parts of the rostral SCs. These may play an important role in reinforcing the conjugacy of upward and downward saccades, respectively. In contrast, medial SC projections to lateral SC TRNs and lateral SC projections to medial TRNs mainly produce strong inhibition. This shows that regions representing upward saccades inhibit contralateral regions representing downward saccades and vice versa.

Neural Organization of the Pathways From the Superior Colliculus to Trochlear Motoneurons

The neural organization of the pathways from the superior colliculus (SC) to trochlear motoneurons was analyzed in anesthetized cats using intracellular recording and transneuronal labeling techniques. Stimulation of the ipsilateral or contralateral SC evoked excitation and inhibition in trochlear motoneurons with latencies of 1.1–2.3 and 1.1–3.8 ms, respectively, suggesting that the earliest components of excitation and inhibition were disynaptic. A midline section between the two SCs revealed that ipsi- and contralateral SC stimulation evoked disynaptic excitation and inhibition in trochlear motoneurons, respectively. Premotor neurons labeled transneuronally after application of wheat germ agglutinin-conjugated horseradish peroxidase into the trochlear nerve were mainly distributed ipsilaterally in the Forel's field H (FFH) and bilaterally in the interstitial nucleus of Cajal (INC). Consequently, we investigated these two likely intermediaries between the SC and trochlear nucleus electrophysiologically. Stimulation of the FFH evoked ipsilateral mono- and disynaptic excitation and contralateral disynaptic inhibition in trochlear motoneurons. Preconditioning stimulation of the ipsilateral SC facilitated FFH-evoked monosynaptic excitation. Stimulation of the INC evoked ipsilateral monosynaptic excitation and inhibition, and contralateral monosynaptic inhibition in trochlear motoneurons. Preconditioning stimulation of the contralateral SC facilitated contralateral INC-evoked monosynaptic inhibition. These results revealed a reciprocal input pattern from the SCs to vertical ocular motoneurons in the saccadic system; trochlear motoneurons received disynaptic excitation from the ipsilateral SC via ipsilateral FFH neurons and disynaptic inhibition from the contralateral SC via contralateral INC neurons. These inhibitory INC neurons were considered to be a counterpart of inhibitory burst neurons in the horizontal saccadic system.

Chapter 2 Comparative anatomy and physiology of the corticospinal system

The corticospinal tract provides the most direct pathway over which the cerebral cortex controls movement. In rodents and marsupials this influence is exerted largely upon interneurons in the dorsal horn of the spinal gray matter. However, ascending the phylogenetic scale through carnivores and primates, the number of corticospinal axons grows and corticospinal terminations shift progressively toward the interneurons of the intermediate zone and ventral horn, ultimately forming increasing numbers of synaptic terminations directly on the motoneurons themselves. Based on this phylogenetic trend, humans are believed to have more direct corticomotoneuronal synapses than any other species, consistent with observations that humans suffer more extensive loss of motility from lesions of the corticospinal tract than do other mammals. Beyond this phylogenetic trend, studies of the corticospinal system in animals have provided insight into the motor abnormalities that result from corticospinal lesions in humans. Corticospinal lesions impair many functionally related muscles and movements in parallel, both because of the divergent output from single corticomotoneuronal cells to multiple motoneuron pools, and because of the convergent input to different motoneuron pools from large, overlapping cortical territories. Furthermore, the weakness, slowness and inflexible, stereotyped movements that remain after corticospinal lesions reflect the loss of input to spinal interneurons and motoneurons from corticospinal neurons, the discharge frequency of which varies with the force, direction and speed of both gross and fine movements. That these deficits resulting from corticospinal lesions are more prominent in humans than in animals indicates, moreover, that animals make greater use of additional descending pathways to control movement. Animal studies have shown that although the bulk of the corticospinal tract arises from the primary motor cortex, this projection is not the only route via which the brain controls movement. Adjacent areas in the frontal and parietal lobes also contribute axons to the corticospinal tract, as well as having corticocortical connections with the motor cortex. Furthermore, the motor cortex and premotor cortex both project to the red nucleus and to the pontomedullary reticular formation, from which the rubrospinal and reticulospinal tracts arise. However, given the limitations on experimental studies in humans, comparative animal studies of the distributed descending system through which the brain controls movement continue to provide deeper understanding and insight into the deficits resulting from human corticospinal lesions, whether caused by stroke, tumor, multiple sclerosis, trauma or ALS.

Direct and indirect activation of cortical neurons by electrical microstimulation

Electrical microstimulation has been used to elucidate cortical function. This review discusses neuronal excitability and effective current spread estimated by using three different methods: 1) single-cell recording, 2) behavioral methods, and 3) functional magnetic resonance imaging (fMRI). The excitability properties of the stimulated elements in neocortex obtained using these methods were found to be comparable. These properties suggested that microstimulation activates the most excitable elements in cortex, that is, by and large the fibers of the pyramidal cells. Effective current spread within neocortex was found to be greater when measured with fMRI compared with measures based on single-cell recording or behavioral methods. The spread of activity based on behavioral methods is in close agreement with the spread based on the direct activation of neurons (as opposed to those activated synaptically). We argue that the greater activation with imaging is attributed to transynaptic spread, which includes subthreshold activation of sites connected to the site of stimulation. The definition of effective current spread therefore depends on the neural event being measured.

Three channels of corticothalamic communication during locomotion

We studied the flow of corticothalamic (CT) information from the motor cortex of the cat during two types of locomotion: visually guided (cortex dependent) and unguided. Spike trains of CT neurons in layers V (CT5s) and VI (CT6s) were examined. All CT5s had fast-conducting axons (<2 ms conduction time), and nearly all showed step-phase-related activity (94%), sensory receptive fields (100%), and spontaneous activity (100%). In contrast, conduction times along CT6 axons were much slower, with bimodal peaks occurring at 6 and 32 ms. Remarkably, almost none of the slowest conducting CT6s showed step-related activity, sensory receptive fields, or spontaneous activity. As a group, these enigmatic neurons were all but silent. Some of the CT6s with moderately conducting axons showed step-related behavior (35%), and this response was more precisely timed than that of the CT5s. We propose distinct functional roles for these diverse corticothalamic populations.

Long descending motor tract axons and their control of neck and axial muscles

It has been tacitly assumed that a long descending motor tract axon consists of a private line connecting the cell of origin to a single muscle, as a motoneuron innervates a single muscle. However, this notion of a long descending motor tract referred to as a private line is no longer tenable, since recent studies have showed that axons of all major long descending motor tracts send their axon collaterals to multiple spinal segments, suggesting that they may exert simultaneous influences on different groups of spinal interneurons and motoneurons of multiple muscles. The long descending motor systems are divided into two groups, the medial and the lateral systems including interneurons and motoneurons. In this chapter, we focus mainly on the medial system (vestibulospinal, reticulospinal and tectospinal systems) in relation to movement control of the neck, describe the intraspinal morphologies of single long descending motor tract axons that are stained with intracellular injection of horseradish peroxidase, and provide evidence that single long motor-tract neurons are implicated in the neural implementation of functional synergies for head movements.

Commissural excitation and inhibition by the superior colliculus in tectoreticular neurons projecting to omnipause neuron and inhibitory burst neuron regions

Previous electrophysiological studies have shown that the commissural connections between the two superior colliculi are mainly inhibitory with fewer excitatory connections. However, the functional roles of the commissural connections are not well understood, so we sought to clarify the physiology of tectal commissural excitation and inhibition of tectoreticular neurons (TRNs) in the "fixation " and "saccade " zones of the superior colliculus (SC). By recording intracellular potentials, we identified TRNs by their antidromic responses to stimulation of the omnipause neuron (OPN) and inhibitory burst neuron (IBN) regions and analyzed the effects of stimulation of the contralateral SC on these TRNs in anesthetized cats. TRNs in the caudal SC (saccade neurons) projected to the IBN region, and received mono- or disynaptic inhibition from the entire rostrocaudal extent of the contralateral SC. In contrast, TRNs in the rostral SC projected to the OPN or IBN region and received monosynaptic excitation from the most rostral level of the contralateral SC, and mono- or disynaptic inhibition from its entire rostrocaudal extent. Among the rostral TRNs with commissural excitation, IBN-projecting TRNs also projected to Forel's field H (vertical gaze center), suggesting that they were most likely saccade neurons related to vertical saccades. In contrast, TRNs projecting only to the OPN region were most likely fixation neurons. Most putative inhibitory neurons in the rostral SC had multiple axon branches throughout the rostrocaudal extent of the contralateral SC, whereas excitatory commissural neurons, most of which were rostral TRNs, distributed terminals to a discrete region in the rostral SC.

Response characterstics of spinothalamic tract neurons that project to the posterior thalamus in rats

A sizeable number of spinothalamic tract axons terminate in the posterior thalamus. The functional roles and precise areas of termination of these axons have been a subject of recent controversy. The goals of this study were to identify spinothalamic tract neurons (STT) within the cervical enlargement that project to this area, characterize their responses to mechanical and thermal stimulation of their receptive fields, and use microantidromic tracking methods to determine the nuclei in which their axons terminate. Forty-seven neurons were antidromically activated using low-amplitude (< or =30 microA) current pulses in the contralateral posterior thalamus. The 51 points at which antidromic activation thresholds were lowest were surrounded by ineffective tracks indicating that the surrounded axons terminated within the posterior thalamus. The areas of termination were located primarily in the posterior triangular, medial geniculate, posterior and posterior intralaminar, and suprageniculate nuclei. Recording points were located in the superficial and deep dorsal horn. The mean antidromic conduction velocity was 6.4 m/s, a conduction velocity slower than that of other projections to the thalamus or hypothalamus in rats. Cutaneous receptive fields appeared to be smaller than those of neurons projecting to other areas of the thalamus or to the hypothalamus. Each of the examined neurons responded exclusively or preferentially to noxious stimuli. These findings indicate that the STT carries nociceptive information to several target nuclei within the posterior thalamus. We discuss the evidence that this projection provides nociceptive information that plays an important role in fear conditioning.

Physiological Characterization of Synaptic Inputs to Inhibitory Burst Neurons From the Rostral and Caudal Superior Colliculus

The caudal superior colliculus (SC) contains movement neurons that fire during saccades and the rostral SC contains fixation neurons that fire during visual fixation, suggesting potentially different functions for these 2 regions. To study whether these areas might have different projections, we characterized synaptic inputs from the rostral and caudal SC to inhibitory burst neurons (IBNs) in anesthetized cats. We recorded intracellular potentials from neurons in the IBN region and identified them as IBNs based on their antidromic activation from the contralateral abducens nucleus and short-latency excitation from the contralateral caudal SC and/or single-cell morphology. IBNs received disynaptic inhibition from the ipsilateral caudal SC and disynaptic inhibition from the rostral SC on both sides. Stimulation of the contralateral IBN region evoked monosynaptic inhibition in IBNs, which was enhanced by preconditioning stimulation of the ipsilateral caudal SC. A midline section between the IBN regions eliminated inhibition from the ipsilateral caudal SC, but inhibition from the rostral SC remained unaffected, indicating that the latter inhibition was mediated by inhibitory interneurons other than IBNs. A transverse section of the brain stem rostral to the pause neuron (PN) region eliminated inhibition from the rostral SC, suggesting that this inhibition is mediated by PNs. These results indicate that the most rostral SC inhibits bilateral IBNs, most likely via PNs, and the more caudal SC exerts monosynaptic excitation on contralateral IBNs and antagonistic inhibition on ipsilateral IBNs via contralateral IBNs. The most rostral SC may play roles in maintaining fixation by inhibition of burst neurons and facilitating saccadic initiation by releasing their inhibition.

The integrated nature of motor cortical function

Recent studies on the functional organization and operational principles of motor cortical function, taken together, strongly support the notion that the motor cortex controls the muscle activities subserving movements in an integrated manner. For example, during pointing the shoulder, elbow and wrist muscles appear to be controlled as a coupled functional system, rather than individually and separately. The pattern of intrinsic connections between motor cortical points is likely part of the explanation of this operational principle. So too is the manner in which muscles and muscle synergies are represented in the motor cortex. However, selection of movement-related muscle synergies is likely a dynamic process involving the functional linking of a variety of motor cortical points, rather than the selection of fixed patterns embedded in the motor cortical circuitry. One of the mechanisms that may be involved in the functional linking of motor cortical points is disinhibition. Thus, motor cortical points are recruited into action by selected excitation as well as by selected release from inhibition. The incoordination of limb movements in patients after a stroke may be understood, at least in part, as a disruption of the connections between motor cortical points and of the neural mechanisms involved in their functional linking.

Functional synergies among neck muscles revealed by branching patterns of single long descending motor-tract axons

In this chapter, we describe our recent work on the divergent properties of single, long descending motor-tract neurons in the spinal cord, using the method of intra-axonal staining with horseradish peroxidase, and serial-section, three-dimensional reconstruction of their axonal trajectories. This work provides evidence that single motor-tract neurons are implicated in the neural implementation of functional synergies for head movements. Our results further show that single medial vestibulospinal tract (MVST) neurons innervate a functional set of multiple neck muscles, and thereby implement a canal-dependent, head-movement synergy. Additionally, both single MVST and reticulospinal axons may have similar innervation patterns for neck muscles, and thereby control the same functional sets of neck muscles. In order to stabilize redundant control systems in which many muscles generate force across several joints, the CNS routinely uses a combination of a control hierarchy and sensory feedback. In addition, in the head-movement system, the elaboration of functional synergies among neck muscles is another strategy, because it helps to decrease the degrees of freedom in this particularly complicated control system.

Postnatal development of corticospinal postsynaptic action

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

Integrated motor cortical control of task-related muscles during pointing in humans

A large body of compelling but indirect evidence suggests that the motor cortex controls the different forelimb segments as a whole rather than individually. The purpose of this study was to obtain physiological evidence in behaving human subjects on the mode of operation of the primary motor cortex during coordinated movements of the forelimb. We approached this problem by studying a pointing movement involving the shoulder, elbow, wrist, and index finger as follows. Focal transcranial magnetic stimulation (TMS) was used to measure the input-output (I/O) curves-a measure of the corticospinal pathway excitability-of proximal (anterior deltoid, AD, and triceps brachii, TB) and distal muscles (extensor carpi radialis, ECR, and first dorsal interosseus, 1DI) during isolated contraction of one of these muscles or during selective co-activation with other muscles involved in pointing. Compared to an isolated contraction of the ECR, the plateau-level of the ECR sigmoid I/O curve increased markedly during co-activation with the AD while pointing. In contrast, the I/O curve of AD was not influenced by activation of the more distal muscles involved in pointing. Moreover, the 1DI I/O curve was not influenced by activation of the more proximal muscles. Three arguments argue for a cortical site of facilitation of ECR motor potentials. First, ECR motor potentials evoked by a near threshold TMS stimulus were facilitated when the AD and ECR were co-activated during pointing but not those in response to a near threshold anodal electrical stimulus. Second, the ECR H reflex was not found to be task dependent, indicating that the recruitment gain of the ECR alpha-motoneuron pool did not differ between tasks. Finally, in comparison with an isolated ECR contraction, intracortical inhibition tested at the ECR cortical site was decreased during pointing. These results suggest that activation of shoulder, elbow, and wrist muscles involved in pointing appear to involve, at least in part, common motor cortical circuits. In contrast, at least in the pointing task, the motor cortical circuits involved in activation of the 1DI appear to act independently.

Optical approaches to embryonic development of neural functions in the brainstem

The ontogenetic approach to physiological events is a useful strategy for understanding the functional organization/architecture of the vertebrate brainstem. However, conventional electrophysiological techniques are difficult or impossible to employ in the early embryonic central nervous system. Optical techniques using voltage-sensitive dyes have made it possible to monitor neural activities from multiple regions of living systems, and have proven to be a useful tool for analyzing the embryogenetic expression of brainstem neural function. This review describes recent progress in optical studies made on embryonic chick and rat brainstems. Several technical issues concerning optical recording from the embryonic brainstem preparations are discussed, and characteristics of the optical signals evoked by cranial nerve stimulation or occurring spontaneously are described. Special attention is paid to the chronological analyses of embryogenetic expression of brainstem function and to the spatial patterning of the functional organization/architecture of the brainstem nuclei. In addition, optical analyses of glutamate, GABA, and glycine receptor functions during embryogenesis are described in detail for the chick nucleus tractus solitarius. This review also discusses intrinsic optical signals associated with neuronal depolarization. Some emphases are also placed on the physiological properties of embryonic brainstem neurons, which may be of interest from the viewpoint of developmental neurobiology.

Electrophysiological assessment of the cutaneous arborization of Adelta-fiber nociceptors

Little is known about the relationship between the branching structure and function of physiologically identified cutaneous nociceptor terminals. The axonal arborization itself, however, has an impact on the afferent signal that is conveyed along the parent axon to the CNS. We therefore developed electrophysiological techniques to investigate the branching structure of cutaneous nociceptors. Single-fiber recordings were obtained from physiologically identified nociceptors that innervated the hairy skin of the monkey. Electrodes for transcutaneous stimulation were fixed at two separate locations inside the receptive field. For 32 Adelta-fiber nociceptors, distinct steps in latency of the recorded action potential were observed as the intensity of the transcutaneous electrical stimulus increased, indicating discrete sites for action potential initiation. The number of discrete latencies at each stimulation location ranged from 1 to 9 (3.7 +/- 0. 2; mean +/- SE) and the mean size of the latency step was 9.9 +/- 1. 0 ms (range: 0.4-89.1 ms). For seven Adelta fibers, collision techniques were used to locate the position of the branch point where the daughter fibers that innervated the two locations within the receptive field join the parent axon. To correct for changes in electrical excitability at the peripheral terminals, collision experiments between the two skin locations and between each skin location and a nerve trunk electrode were necessary. Nine branch points were studied in the seven Adelta fibers; the mean propagation time from the action potential initiation site to the branch point was 31 +/- 5 ms corresponding to a distance of 54 +/- 10 mm. Almost half of the daughter branches were unmyelinated. These results demonstrate that collision techniques can be used to study the functional anatomy of physiologically identified nociceptive afferent terminals. Furthermore these results indicate that some nociceptive afferents branch quite proximal to their peripheral receptive field. Occlusion of action potential activity can occur in these long branches such that the shorter branches dominate in the response to natural stimuli.

Neural organization from the superior colliculus to motoneurons in the horizontal oculomotor system of the cat

The neural organization of the superior colliculus (SC) projection to horizontal ocular motoneurons was analyzed in anesthetized cats using intracellular recording and transneuronal labeling. Intracellular responses to SC stimulation were analyzed in lateral rectus (LR) and medial rectus (MR) motoneurons and internuclear neurons in the abducens nucleus (AINs). LR motoneurons and AINs received excitation from the contralateral SC and inhibition from the ipsilateral SC. The shortest excitation (0.9-1.9 ms) and inhibition (1.4-2.4 ms) were mainly disynaptic from the SC and were followed by tri- and polysynaptic responses evoked with increasing stimuli or intensity. All MR motoneurons received excitation from the ipsilateral SC, whereas none of them received any short-latency inhibition from the contralateral SC, but some received excitation. The latency of the ipsilateral excitation in MR motoneurons (1.7-2.8 ms) suggested that this excitation was trisynaptic via contralateral AINs, because conditioning SC stimulation spatially facilitated trisynaptic excitation from the ipsilateral vestibular nerve. To locate interneurons mediating the disynaptic SC inputs to LR motoneurons, last-order premotor neurons were labeled transneuronally after injecting wheat germ agglutinin-conjugated horseradish peroxidase into the abducens nerve, and tectoreticular axon terminals were labeled after injecting dextran-biotin into the ipsilateral or contralateral SC in the same preparations. Transneuronally labeled neurons were mainly distributed ipsilaterally in the paramedian pontine reticular formation (PPRF) rostral to retrogradely labeled LR motoneurons and the vestibular nuclei, and contralaterally in the paramedian pontomedullary reticular formation (PPMRF) caudomedial to the abducens nucleus and the vestibular nuclei. Among the last-order premotor neuron areas, orthogradely labeled tectoreticular axon terminals were observed only in the PPRF and the PPMRF contralateral to the injected SC and seemed to make direct contacts with many of the labeled last-order premotor neurons in the PPRF and the PPMRF. These morphological results confirmed that the main excitatory and inhibitory connections from the SC to LR motoneurons are disynaptic and that the PPRF neurons that receive tectoreticular axon terminals from the contralateral SC terminate on ipsilateral LR motoneurons, whereas the PPMRF neurons that receive tectoreticular axon terminals from the contralateral SC terminate on contralateral LR motoneurons.

Topographical organization of projections to cat motor cortex from nucleus interpositus anterior and forelimb skin

1. The activation of the motor cortex from focal electrical stimulation of sites in the forelimb area of cerebellar nucleus interpositus anterior (NIA) was investigated in barbiturate-anaesthetized cats. Using a microelectrode, nuclear sites were identified by the cutaneous climbing fibre receptive fields of their afferent Purkinje cells. These cutaneous receptive fields can be identified by positive field potentials reflecting inhibition from Purkinje cells activated on natural stimulation of the skin. Thereafter, the sites were microstimulated and the evoked responses were systematically recorded over the cortical surface with a ball-tipped electrode. The topographical organization in the motor cortex of responses evoked by electrical stimulation of the forelimb skin was also analysed. 2. Generally, sites in the forelimb area of NIA projected to the lateral part of the anterior sigmoid gyrus (ASG). Sites in the hindlimb area of NIA also projected to lateral ASG and in addition to a more medial region. Sites in the face area of NIA, however, projected mainly to the middle part of the posterior sigmoid gyrus (PSG). 3. For sites in the forelimb area of NIA, the topographical organization and strength of the projections varied specifically with the cutaneous climbing fibre receptive field of the site. The largest cortical responses were evoked from sites with receptive fields on the distal or ventral skin of the forelimb. 4. Microelectrode recordings in the depth of the motor cortex revealed that responses evoked by cerebellar nuclear stimulation were due to an excitatory process in layer III. 5. Short latency surface responses evoked from the forelimb skin were found in the caudolateral part of the motor cortex. At gradually longer latencies, responses appeared in sequentially more rostromedial parts of the motor cortex. Since the responses displayed several temporal peaks that appeared in specific cortical regions for different areas of the forelimb skin, several somatotopic maps were seen. 6. The cerebellar and cutaneous projections activated mainly different cortical regions and had topographical organizations that apparently were constant between animals. Their patterns of activation may constitute a frame of reference for investigations of the functional organization of the motor cortex.

Branching and/or collateral projections of spinal dorsal horn neurons

Branching and/or collateral projections of spinal dorsal horn neurons is a common phenomenon. Evidence is presented for the existence of STTm/STTl, STTc/STTi, STT/SMT, STT/SRT, SCT/DCPS, SST/DCPS, SCT/SST, STT/SHT, STeT/SHT, STeTs and other doubly or multiply projecting spinal neurons that have been anatomically and physiologically identified and named based on the locations of the cells of origin and their terminations in the brain. These newly discovered spinal projection neurons are characterized by a single cell body and branched axons and/or collaterals that project to two or more target areas in the brain. These novel populations of neurons seem to be a fuzzy set of spinal projection neurons that function as an intersection set of the corresponding single projection spinal neurons and to be at an intermediate stage phylogenetically. Identification strategies are discussed, and general concluding remarks are made in this review.

Computer simulation of post-spike facilitation in spike-triggered averages of rectified EMG

When the spikes of a motor cortical cell are used to compile a spike-triggered average (STA) of rectified electromyographic (EMG) activity, a post-spike facilitation (PSF) is sometimes seen. This is generally thought to be indicative of direct corticomotoneuronal (CM) connections. However, it has been claimed that a PSF could be caused by synchronization between CM and non-CM cells. This study investigates the generation of PSF using a computer model. A population of cortical cells was simulated, some of which made CM connections to a pool of 103 motoneurons. Motoneurons were simulated using a biophysically realistic model. A subpopulation of the cortical cells was synchronized together. After a motoneuron discharge, a motor unit action potential was generated; these were summed to produce an EMG output. Realistic values were used for the corticospinal and peripheral nerve conduction velocity distribution, for slowing of impulse conduction in CM terminal axons, and for the amount of cortical synchrony. STA of the rectified EMG from all cortical neurons showed PSF; however, these were qualitatively different for CM versus non-CM cells. Using an epoch analysis to determine reliability in a quantitative manner, it was shown that the onset latency of PSF did not distinguish the two classes of cells after 10,000 spikes because of high noise in the averages. The time of the PSF peak and the peak width at half-maximum (PWHM) could separate CM from synchrony effects. However, only PWHM was robust against changes in motor unit action-potential shape and duration and against changes in the width of cortical synchrony. The amplitude of PSF from a CM cell could be doubled by the presence of synchrony. It is proposed that, if a PSF has PWHM < 7 ms, this reliably indicates that the trigger is a CM cell projecting to the muscle whose EMG is averaged. In an analysis of experimental data where macaque motor cortical cells facilitated hand and forearm muscle EMG, 74% of PSFs fulfilled this criterion. The PWHM criterion could be applied to other STA studies in which it is important to exclude the effects of synchrony.

Neocortical efferent neurons with very slowly conducting axons: strategies for reliable antidromic identification

Although simple in concept, reliable antidromic identification of efferent populations poses numerous technical challenges and is subject to a host of sampling biases, most of which select against the detection of the neurons with slowly conducting axons. This problem is particularly acute in studies of the neocortex. Many neocortical efferent systems have large sub-populations with very slowly conducting, nonmyelinated axons and these elements have been relatively neglected in antidromic studies of neocortical neurons. The present review attempts to redress this problem by analyzing the steps that must necessarily precede antidromic identification and the sampling biases associated with each of these steps. These steps include (1) initial recognition that the microelectrode is near a neuron; (2) activation of the efferent axon via the stimulating electrode; (3) conduction of the antidromic impulse from stimulation site to soma; (4) detection of the antidromic spike in the extracellular record and (5) discriminating antidromic from synaptic activation. Experimental strategies are suggested for minimizing the sampling biases associated with each of these steps; most of which can be reduced or eliminated by appropriate experimental procedures. Careful attention to such procedures will make it possible to better understand the nature and function of the information flow along the very slowly conducting axonal systems of the neocortex.

Lumbar collaterals of neurons of the C6 segment projecting to sacral segments of the cat spinal cord

Electrophysiological investigations of neurons of the C6 segment of the spinal cord were made in alpha-chloralose anesthetized animals. It was established in the experiments that a part of long descending propriospinal neurons originating in the sixth cervical segment (C6) that projected to sacral segments (S1/S2) gave off collateral branches at the level of the fourth lumbar segment (L4). Several types of neurons were distinguished according to the ipsilateral, contralateral or bilateral course of axons at the thoracic level as well as their lumbar or sacral projections. The cell bodies of 58 identified neurons were distributed in Rexed's laminae VII and VIII of the gray matter. Axons descended in lateral funiculi and their conduction velocities varied from 50 to 85 m/s. The existence of collaterals to various segments of the lumbosacral enlargement indicates that the same information conveyed by long descending propriospinal neurons can reach separate motor centers controlling various muscles of the hindlimbs.

Force and the motor cortex

The relation between the activity of cells in the motor cortex and static force has been studied extensively. Most studies have concentrated on the relation to the magnitude of force; this relation is more or less monotonic. The slope of the relation, however, shows considerable variation among different studies and seems to be inversely associated with the range of forces over which the cell activity has been studied. Cells in the motor cortex also show variation in activity with the direction of static force. When both the direction and the magnitude of static force are allowed to vary, a majority of cells show significant changes in activity with direction of force alone, an intermediate number relate to both direction and magnitude, while a small number relate purely to the magnitude. This suggests that the direction of static force can be controlled independently of its magnitude and that this directional signal is especially prominent in the motor cortex. In general, it has been more difficult to study the relations to dynamic force. There is a correlation between motor cortex cell activity and the rate of change of force. The direction of dynamic force is also an important determinant of cell activity. When both static and dynamic force output are required (for example, with arm movement in the presence of gravity) it is the dynamic signal that is most clearly reflected in motor cortex activity. The relations between motor cortex activity and static or dynamic force are not invariant, but may be modified by the behavioral context of the motor output.

Force and the motor cortex

The relation between the activity of cells in the motor cortex and static force has been studied extensively. Most studies have concentrated on the relation to the magnitude of force; this relation is more or less monotonic. The slope of the relation, however, shows considerable variation among different studies and seems to be inversely associated with the range of forces over which the cell activity has been studied. Cells in the motor cortex also show variation in activity with the direction of static force. When both the direction and the magnitude of static force are allowed to vary, a majority of cells show significant changes in activity with direction of force alone, an intermediate number relate to both direction and magnitude, while a small number relate purely to the magnitude. This suggests that the direction of static force can be controlled independently of its magnitude and that this directional signal is especially prominent in the motor cortex. In general, it has been more difficult to study the relations to dynamic force. There is a correlation between motor cortex cell activity and the rate of change of force. The direction of dynamic force is also an important determinant of cell activity. When both static and dynamic force output are required (for example, with arm movement in the presence of gravity) it is the dynamic signal that is most clearly reflected in motor cortex activity. The relations between motor cortex activity and static or dynamic force are not invariant, but may be modified by the behavioral context of the motor output.

Primary motor cortex influences on the descending and ascending systems

The motor cortex plays a crucial role in the co-ordination of movement and posture. This is possible because the pyramidal tract fibres have access both directly and through collateral branches to structures governing eye, head, neck trunk and limb musculature. Pyramidal tract axons also directly reach the dorsal laminae of the spinal cord and the dorsal column nuclei, thus aiding in the selection of the sensory ascendant transmission. No other neurones in the brain besides pyramidal tract cells have such a wide access to different structures within the central nervous system. The majority of the pyramidal tract fibres that originate in the motor cortex and that send collateral branches to multiple supraspinal structures do not reach the spinal cord. Also, the great majority of the corticospinal neurones that emit multiple intracraneal collateral branches terminate at the cervical spinal cord level. The pyramidal tract fibres directed to the dorsal column nuclei that send collateral branches to supraspinal structures also show a clear tendency to terminate at supraspinal and cervical cord levels. These facts suggest that a substantial co-ordination between descending and ascending pathways might be produced by the same motor cortex axons at both supraspinal and cervical spinal cord sites. This may imply that the motor cortex co-ordination will be mostly directed to motor responses involving eye-neck-forelimb muscle synergies. The review makes special emphasis in the available evidence pointing to the role of the motor cortex in co-ordinating the activities of both descending and ascending pathways related to somatomotor integration and control. The motor cortex may function to co-operatively select a unique motor command by selectively filter sensory information and by co-ordinating the activities of the descending systems related to the control of distal and proximal muscles.

Spinohypothalamic tract neurons in the cervical enlargement of rats: locations of antidromically identified ascending axons and their collateral branches in the contralateral brain

Antidromic activation was used to determine the locations of ascending spinohypothalamic tract (SHT) axons and their collateral projections within C1, medulla, pons, midbrain, and caudal thalamus. Sixty-four neurons in the cervical enlargement were antidromically activated initially by stimulation within the contralateral hypothalamus. All but one of the examined SHT neurons responded either preferentially or specifically to noxious mechanical stimuli. A total of 239 low-threshold points was classified as originating from 64 ascending (or parent) SHT axons. Within C1, 38 ascending SHT axons were antidromically activated. These were located primarily in the dorsal half of the lateral funiculus. Within the medulla, the 29 examined ascending SHT axons were located ventrolaterally, within or adjacent to the lateral reticular nucleus or nucleus ambiguus. Within the pons, the 25 examined ascending SHT axons were located primarily surrounding the facial nucleus and the superior olivary complex. Within the caudal midbrain, the 23 examined SHT ascending axons coursed dorsally in a position adjacent to the lateral lemniscus. Within the anterior midbrain, SHT axons traveled rostrally near the brachium of the inferior colliculus. Within the posterior thalamus, all 17 examined SHT axons coursed rostrally through the posterior nucleus of thalamus. A total of 114 low-threshold points was classified as collateral branch points. Sixteen collateral branches were seen in C1; these were located primarily int he deep dorsal horn. Forty-five collateral branches were located in the medulla. These were primarily in or near the medullary reticular nucleus, nucleus ambiguus, lateral reticular nucleus, parvocellular reticular nucleus, gigantocellular reticular nucleus, cuneate nucleus, and the nucleus of the solitary tract. Twentysix collateral branches from SHT axons were located in the pons. These were in the pontine reticular nucleus caudalis, gigantocellular reticular nucleus, parvocellular reticular nucleus, and superior olivary complex. Twenty-three collateral branches were located in the midbrain. These were in or near the mesencephalic reticular nucleus, brachium of the inferior colliculus, cuneiform nucleus, superior colliculus, central gray, and substantia nigra. Int he caudal thalamus, two branches were in the posterior thalamic nucleus and two were in the medial geniculate. These results indicate that SHT axons ascend toward the hypothalamus in a clearly circumscribed projection in the lateral brain stem and posterior thalamus. In addition, large numbers of collaterals from SHT axons appears to project to a variety of targets in C1, the medulla, pons, midbrain, and caudal thalamus. Through its widespread collateral projections, the SHT appears to be capable of providing nociceptive input to many areas that are involved in the production of multifaceted responses to noxious stimuli.

An electrophysiological demonstration of axonal projections of single ventral inspiratory neurons to the phrenic nucleus of the cat

Axonal branching patterns of single inspiratory (I) neurons of the nucleus retroambigualis (NRA) were studied electrophysiologically in cat phrenic nucleus (C4-C6). Experiments were performed on Nembutal anesthetized, artificially ventilated cats, and extracellular spikes of I neurons were recorded. The cervical spinal gray matter was microstimulated from dorsal to ventral sites at 100 microns intervals with an intensity of 150-250 microA using a glass insulated tungsten microelectrode. The stimulations were made at 1 mm intervals rostrocaudally along the spinal cord, and effective stimulating sites of antidromic activation in axonal collaterals were systematically mapped. I neurons examined (n = 8) descending contralaterally distributed multiple collaterals in the phrenic nucleus. These collaterals were found throughout the rostrocaudal phrenic nucleus. An I neuron (n = 1) descending ipsilaterally also distributed collaterals in the ipsilateral phrenic nucleus. Axonal collaterals in the contralateral phrenic nucleus occupied 44.2% of the total length of the cervical spinal cord examined. To determine the detailed trajectory of collaterals in the cervical gray matter, microstimulation was performed in and around the collateral arborizations at the maximum intensity of 50 microA. The descending stem axons could be localized in the lateral funiculus in four I neurons and in the ventral funiculus in one I neuron. I neurons distributed axonal collaterals within the phrenic nucleus. Some part of the collaterals ran to the medial region of the gray matter, re-crossed the midline under the central canal and reached the phrenic nucleus ipsilateral to the I neuron. Re-crossed collaterals arborized in the phrenic nucleus, but did not extend to the gray matter more lateral than the phrenic nucleus. Rostrocaudal extension of the re-crossed collaterals was found to be narrow.

Modular organization of motor behavior in the frog's spinal cord

The complex issue of translating the planning of arm movements into muscle forces is discussed in relation to the recent discovery of structures in the spinal cord. These structures contain circuitry that, when activated, produce precisely balanced contractions in groups of muscles. These synergistic contractions generate forces that direct the limb toward an equilibrium point in space. Remarkably, the force outputs, produced by activating different spinal-cord structures, sum vectorially. This vectorial combination of motor outputs might be a mechanism for producing a vast repertoire of motor behaviors in a simple manner.

Anatomy, development and lesion-induced plasticity of rodent corticospinal tract

In this review the current knowledge of the anatomy, development and plasticity of the rodent corticospinal tract is summarised. Recent technical advancements, especially in neuronal tracing methods, have provided much new data concerning the anatomy of the corticospinal tract. The rodent corticospinal axons project to the subcortical nuclei via collateral branches. These collateral branches of corticospinal axons are formed by delayed interstitial budding during early postnatal periods. Corticospinal neurons are generated in the ventricular zone during a short time lag, migrate into the cortical plate, and settle in layer V of the cerebral cortex. The migration of corticospinal neurons is experimentally deranged by prenatal exposure to alcohol or genetically affected by the reeler genetic locus (rl), resulting in generation of ectopic corticospinal neurons. Such experimentally or genetically induced ectopic corticospinal neurons are a good model for examining whether target recognition and path finding are affected by the intracortical position of corticospinal neurons. Some chemical molecules (e.g. L1 and B-50/GAP43) are transiently expressed in the corticospinal tract during the perinatal period, while others (e.g. protein kinase C gamma subspecies and alpha CaM kinase II) are permanently expressed in the adult corticospinal tract. The only chemical marker specific for layer V corticofugal neurons is an antibody to a soluble protein, protein 35. Since the corticospinal tract in the rodent is an easily identified group of fibers situated in the most ventral portion of the dorsal funiculus of the spinal cord and exhibits considerable postnatal development, it has often been utilized in the neurological studies on plasticity and regenerative capacity of the lesioned central nervous system. Recently, it has been clarified that growing corticospinal fibers have the ability to penetrate and traverse across the lesion sites under certain special conditions.