Midbrain circuits that set locomotor speed and gait selection.

Locomotion is a fundamental motor function common to the animal kingdom. It is implemented episodically and adapted to behavioural needs, including exploration, which requires slow locomotion, and escape behaviour, which necessitates faster speeds. The control of these functions originates in brainstem structures, although the neuronal substrate(s) that support them have not yet been elucidated. Here we show in mice that speed and gait selection are controlled by glutamatergic excitatory neurons (GlutNs) segregated in two distinct midbrain nuclei: the cuneiform nucleus (CnF) and the pedunculopontine nucleus (PPN). GlutNs in both of these regions contribute to the control of slower, alternating-gait locomotion, whereas only GlutNs in the CnF are able to elicit high-speed, synchronous-gait locomotion. Additionally, both the activation dynamics and the input and output connectivity matrices of GlutNs in the PPN and the CnF support explorative and escape locomotion, respectively. Our results identify two regions in the midbrain that act in conjunction to select context-dependent locomotor behaviours.

Global gene expression analysis of rodent motor neurons following spinal cord injury associates molecular mechanisms with development of postinjury spasticity.

Spinal cord injury leads to severe problems involving impaired motor, sensory, and autonomic functions. After spinal injury there is an initial phase of hyporeflexia followed by hyperreflexia, often referred to as spasticity. Previous studies have suggested a relationship between the reappearance of endogenous plateau potentials in motor neurons and the development of spasticity after spinalization. To unravel the molecular mechanisms underlying the increased excitability of motor neurons and the return of plateau potentials below a spinal cord injury we investigated changes in gene expression in this cell population. We adopted a rat tail-spasticity model with a caudal spinal transection that causes a progressive development of spasticity from its onset after 2 to 3 wk until 2 mo postinjury. Gene expression changes of fluorescently identified tail motor neurons were studied 21 and 60 days postinjury. The motor neurons undergo substantial transcriptional regulation in response to injury. The patterns of differential expression show similarities at both time points, although there are 20% more differentially expressed genes 60 days compared with 21 days postinjury. The study identifies targets of regulation relating to both ion channels and receptors implicated in the endogenous expression of plateaux. The regulation of excitatory and inhibitory signal transduction indicates a shift in the balance toward increased excitability, where the glutamatergic N-methyl-d-aspartate receptor complex together with cholinergic system is up-regulated and the gamma-aminobutyric acid type A receptor system is down-regulated. The genes of the pore-forming proteins Cav1.3 and Nav1.6 were not up-regulated, whereas genes of proteins such as nonpore-forming subunits and intracellular pathways known to modulate receptor and channel trafficking, kinetics, and conductivity showed marked regulation. On the basis of the identified changes in global gene expression in motor neurons, the present investigation opens up for new potential targets for treatment of motor dysfunction following spinal cord injury.

Characterization of reliability of spike timing in spinal interneurons during oscillating inputs.

The spike timing in rhythmically active interneurons in the mammalian spinal locomotor network varies from cycle to cycle. We tested the contribution from passive membrane properties to this variable firing pattern, by measuring the reliability of spike timing, P, in interneurons in the isolated neonatal rat spinal cord, using intracellular injection of sinusoidal command currents of different frequencies (0.325-31.25 Hz). P is a measure of the precision of spike timing. In general, P was low at low frequencies and amplitudes (P = 0-0.6; 0-1.875 Hz; 0-30 pA), and high at high frequencies and amplitudes (P = 0.8-1; 3.125-31.25 Hz; 30-200 pA). The exact relationship between P and amplitude was difficult to describe because of the well-known low-pass properties of the membrane, which resulted in amplitude attenuation of high-frequency compared with low-frequency command currents. To formalize the analysis we used a leaky integrate and fire (LIF) model with a noise term added. The LIF model was able to reproduce the experimentally observed properties of P as well as the low-pass properties of the membrane. The LIF model enabled us to use the mathematical theory of nonlinear oscillators to analyze the relationship between amplitude, frequency, and P. This was done by systematically calculating the rotational number, N, defined as the number of spikes divided by the number of periods of the command current, for a large number of frequencies and amplitudes. These calculations led to a phase portrait based on the amplitude of the command current versus the frequency-containing areas [Arnold tongues (ATs)] with the same rotational number. The largest ATs in the phase portrait were those where N was a whole integer, and the largest areas in the ATs were seen for middle to high (>3 Hz) frequencies and middle to high amplitudes (50-120 pA). This corresponded to the amplitude- and frequency-evoked increase in P. The model predicted that P would be high when a cell responded with an integer and constant N. This prediction was confirmed by comparing N and P in real experiments. Fitting the result of the LIF model to the experimental data enabled us to estimate the standard deviation of the internal neuronal noise and to use these data to simulate the relationship between N and P in the model. This simulation demonstrated a good correspondence between the theoretical and experimental values. Our data demonstrate that interneurons can respond with a high reliability of spike timing, but only by combining fast and slow oscillations is it possible to obtain a high reliability of firing during rhythmic locomotor movements. Theoretical analysis of the rotation number provided new insights into the mechanism for obtaining reliable spike timing.

5-HT modulation of multiple inward rectifiers in motoneurons in intact preparations of the neonatal rat spinal cord.

This study introduces novel aspects of inward rectification in neonatal rat spinal motoneurons (MNs) and its modulation by serotonin (5-HT). Whole cell tight-seal recordings were made from MNs in an isolated lumbar spinal cord preparation from rats 1-2 days of age. In voltage clamp, hyperpolarizing step commands were generated from holding potentials of -50 to -40 mV. Discordant with previous reports involving slice preparations, fast inward rectification was commonly expressed and in 44% of the MNs co-existed with a slow inward rectification related to activation of I(h). The fast inward rectification is likely caused by an I(Kir). Thus it appeared around E(K) and was sensitive to low concentrations (100-300 microM) of Ba2+ but not to ZD 7288, which blocked I(h). Both I(Kir) and I(h) were inhibited by Cs2+ (0.3-1.5 mM). Extracellular addition of 5-HT (10 microM) reduced the instantaneous conductance, most strongly at membrane potentials above E(K). Low [Ba2+] prevented the 5-HT-induced instantaneous conductance reduction below, but not that above, E(K). This suggests that 5-HT inhibits I(Kir), but also other instantaneous conductances. The biophysical parameters of I(h) were evaluated before and under 5-HT. The maximal I(h) conductance, G(max), was 12 nS, much higher than observed in slice preparations. G(max) was unaffected by 5-HT. In contrast, 5-HT caused a 7-mV depolarizing shift in the activation curve of I(h). Double-exponential fits were generally needed to describe I(h) activation. The fast and slow time constants obtained by these fits differed by an order of magnitude. Both time constants were accelerated by 5-HT, the slow time constant to the largest extent. We conclude that spinal neonatal MNs possess multiple forms of inward rectification. I(h) may be carried by two spatially segregated channel populations, which differ in kinetics and sensitivity to 5-HT. 5-HT increases MN excitability in several ways, including inhibition of a barium-insensitive leak conductance, inhibition of I(Kir), and enhancement of I(h). The quantitative characterization of these effects should be useful for further studies seeking to understand how neuromodulation prepares vertebrate MNs for concerted behaviors such as locomotor activity.

Motor coordination without action potentials in the mammalian spinal cord.

Coordination of neuronal activity to produce movement is generally thought to depend on spike activity in premotor interneuronal networks. Here we show that even without such activity, the neonatal rat spinal cord could produce a stable motor rhythm mediated by the synchronization of motor neuron oscillations across gap junctions. These rhythms, however, were not coordinated between motor pools in different parts of the spinal cord. We further showed that neural coordination through gap junctions contributed to the fundamental organization and function of spinal motor systems. These results suggest that neural coordination across gap junctions is important in motor-pattern generation in the mammalian spinal cord.

Spike coding during locomotor network activity in ventrally located neurons in the isolated spinal cord from neonatal rat.

To characterize spike coding in spinal neurons during rhythmic locomotor activity, we recorded from individual cells in the lumbar spinal cord of neonatal rats by using the on-cell patch-clamp technique. Locomotor activity was induced by N-methyl-D aspartate (NMDA) and 5-hydroxytryptamine (5-HT) and monitored by ventral root recording. We made an estimator based on the assumption that the number of spikes arriving during two halves of the locomotor cycle could be a code used by the neuronal network to distinguish between the halves. This estimator, termed the spike contrast, was calculated as the difference between the number of spikes in the most and least active half of an average cycle. The root activity defined the individual cycles and the positions of the spikes were calculated relative to these cycles. By comparing the average spike contrast to the spike contrast in noncyclic, randomized spike trains we found that approximately one half the cells (19 of 42) contained a significant spike contrast, averaging 1.25 +/- 0.23 (SE) spikes/cycle. The distribution of spike contrasts in the total population of cells was exponential, showing that weak modulation was more typical than strong modulation. To investigate if this low spike contrast was misleading because a higher spike contrast averaged out by occurring at different positions in the individual cycles we compared the spike contrast obtained from the average cycle to its maximal value in the individual cycles. The value was larger (3.13 +/- 0.25 spikes) than the spike contrast in the average cycle but not larger than the spike contrast in the individual cycles of a random, noncyclic spike trains (3.21 +/- 0.21 spikes). This result suggested that the important distinction between cyclic and noncyclic cells was only the repeated cycle position of the spike contrast and not its magnitude. Low spike frequencies (5.2 +/- 0.82 spikes/cycle, that were on average 3.5 s long) and a minimal spike interval of 100-200 ms limited the spike contrast. The standard deviation (SD) of the spike contrast in the individual neurons was similar to the average spike contrasts and was probably stochastic because the SDs of the simulated, noncyclic spike trains were also similar. In conclusion we find a highly distributed and variable locomotor related cyclic signal that is represented in the individual neurons by very few spikes and that becomes significant only because the spike contrast is repeated at a preferred phase of the locomotor cycle.

Population reconstruction of the locomotor cycle from interneuron activity in the mammalian spinal cord.

Lesion studies have shown that neuronal networks in the ventromedial regions of the neonatal rat spinal cord are critical for the production of locomotion. We examined whether the locomotor cycle could be accurately predicted based on the activity recorded in a population of spinal interneurons located in these regions during pharmacologically induced locomotion. We used a Bayesian probabilistic reconstruction procedure to predict the most likely phase of locomotion given the observed activity in the neuronal population. The population reconstruction was able to predict the correct locomotor phase with high accuracy using a relatively small number of neurons. This result demonstrates that although the spike activity of individual spinal interneurons in the ventromedial region is weak and varies from cycle to cycle, the locomotor phase can be accurately predicted when information from the population is combined. This result is consistent with the proposed involvement of interneurons within these regions of the spinal cord in the production of locomotion.

Activity of hindlimb motor units during locomotion in the conscious rat.

This paper compares the activity of hindlimb motor units from muscles mainly composed of fast-twitch muscle fibers (medial and lateral gastrocnemius: MG/LG, tibialis anterior: TA) to motor units from a muscle mainly composed of slow-twitch muscle fibers (soleus: SOL) during unrestrained walking in the conscious rat. Several differences in the activation profiles of motor units from these two groups of muscles were observed. For example, motor units from fast muscles (e.g., MG/LG and TA) fired at very high mean frequencies of discharge, ranging from 60 to 100 Hz, and almost always were recruited with initial doublets or triplets, i.e., initial frequencies >/=100 Hz. In contrast, the majority of SOL units fired at much lower mean rates of discharge, approximately 30 Hz, and had initial frequencies of only 30-60 Hz (i.e., there were no initial doublets/triplets >/=100 Hz). Thus the presence of initial doublet or triplets was dependent on the intrinsic properties of the motor unit, i.e., faster units were recruited with a doublet/triplet more often than slower units. Moreover, in contrast to units from the slow SOL muscle, the activity of single motor units from the fast MG/LG muscle, especially units recruited midway or near the end of a locomotor burst, was unrelated to the activity of the remainder of the motoneuron pool, as measured by the corresponding gross-electromyographic (EMG) signal. This dissociation of activity was suggested to arise from a compartmentalized recruitment of the MG/LG motoneuron pool by the rhythm-generating networks of the spinal cord. In contrast, when comparing the rate modulation of simultaneously recorded motor units within a single LG muscle compartment, the frequency profiles of unit pairs were modulated in a parallel fashion. This suggested that the parent motoneurons were responsive to changes in synaptic inputs during unrestrained walking, unlike the poor rate modulation that occurs during locomotion induced from brain stem stimulation. In summary, data from this study provide evidence that the firing behavior of motor units during unrestrained walking is influenced by both the intrinsic properties of the parent motoneuron and by synaptic inputs from the locomotor networks of the spinal cord. In addition, it also provides the first extensive description of motor-unit activity from different muscles during unrestrained walking in the conscious rat.

Contributions of intrinsic motor neuron properties to the production of rhythmic motor output in the mammalian spinal cord.

Motor neurons are endowed with intrinsic and conditional membrane properties that may shape the final motor output. In the first half of this paper we present data on the contribution of I(h), a hyperpolarization-activated inward cation current, to phase-transition in motor neurons during rhythmic firing. Motor neurons were recorded intracellularly during locomotion induced with a mixture of N-methyl-D-aspartate (NMDA) and serotonin, after pharmacological blockade of I(h). I(h) was then replaced by using dynamic clamp, a computer program that allows artificial conductances to be inserted into real neurons. I(h) was simulated with biophysical parameters determined in voltage clamp experiments. The data showed that electronic replacement of the native I(h) caused a depolarization of the average membrane potential, a phase-advance of the locomotor drive potential, and increased motor neuron spiking. Introducing an artificial leak conductance could mimic all of these effects. The observed effects on phase-advance and firing, therefore, seem to be secondary to the tonic depolarization; i.e., I(h) acts as a tonic leak conductance during locomotion. In the second half of this paper we discuss recent data showing that the neonatal rat spinal cord can produce a stable motor rhythm in the absence of spike activity in premotor interneuronal networks. These coordinated motor neuron oscillations are dependent on NMDA-evoked pacemaker properties, which are synchronized across gap junctions. We discuss the functional relevance for such coordinated oscillations in immature and mature spinal motor systems.

Coding of locomotor phase in populations of neurons in rostral and caudal segments of the neonatal rat lumbar spinal cord.

Several experiments have demonstrated that rostral segments of the vertebrate lumbar spinal cord produce a rhythmic motor output more readily and of better quality than caudal segments. Here we examine how this rostrocaudal gradient of rhythmogenic capability is reflected in the spike activity of neurons in the rostral (L(2)) and caudal (L(5)) lumbar spinal cord of the neonatal rat. The spike activity of interneurons in the ventromedial cord, a region necessary for the production of locomotion, was recorded intracellularly with patch electrodes and extracellularly with tetrodes during pharmacologically induced locomotion. Both L(2) and L(5) neurons tended to be active in phase with their homologous ventral root. L(5) neurons, however, had a wider distribution of their preferred phases of activity throughout the locomotor cycle than L(2) neurons. The strength of modulation of the activity of individual L(2) neurons was also larger than that of L(5) neurons. These differences resulted in a stronger rhythmic signal from the L(2) neuronal population than from the L(5) population. These results demonstrate that the rhythmogenic capability of each spinal segment was reflected in the activity of interneurons located in the same segment. In addition to paralleling the rostrocaudal gradient of rhythmogenic capability, these results further suggest a colocalization of motoneurons and their associated interneurons involved in the production of locomotion.

Activation patterns of hindlimb motor units in the awake rat and their relation to motoneuron intrinsic properties.

The activity of hindlimb motor units from the lateral gastrocnemius and tibialis anterior muscles in the awake rat was compared during locomotion and during slow, sinusoidal muscle stretch. The majority of units were activated with high initial frequencies and often commenced firing with an initial doublet or triplet, even when activated by slow muscle stretch. The high firing rates at recruitment occurred without jumps in the firing rates of other concurrently activated units, the firing rate profiles of which were used as a measure of the net synaptic drive onto the motoneuronal pool. This suggested that the sharp recruitment jumps were not due to an abrupt increase in synaptic drive but rather due to intrinsic properties of the motoneuron. In addition, motor units that were activated phasically by the muscle stretch fired more action potentials during muscle shortening than during muscle lengthening, resulting in rightwardly skewed, asymmetrical firing profiles. In contrast, when the same units fired tonically during the imposed muscle stretch, the frequency profiles were modulated symmetrically and no nonlinearities were observed. Tonically firing units were modulated symmetrically throughout a wide range of firing frequencies, and discrete jumps in rate (i.e., bistable firing) were not observed. The sharp recruitment jumps during locomotion and muscle stretch are proposed to have resulted from the additional depolarization produced by the activation of plateau potentials at recruitment. Likewise, the sustained activation of plateaus subsequent to recruitment may have produced the prolonged firing of the motor units during sinusoidal muscle stretch.

Effects of noradrenaline on locomotor rhythm-generating networks in the isolated neonatal rat spinal cord.

We have studied the effects of the biogenic amine noradrenaline (NA) on motor activity in the isolated neonatal rat spinal cord. The motor output was recorded with suction electrodes from the lumbar ventral roots. When applied on its own, NA (0.5-50 microM) elicited either no measurable root activity, or activity of a highly variable nature. When present, the NA-induced activity consisted of either low levels of unpatterned tonic discharges, or an often irregular, slow rhythm that displayed a high degree of synchrony between antagonistic motor pools. Finally, in a few cases, NA induced a slow locomotor-like rhythm, in which activity alternated between the left and right sides, and between rostral and caudal roots on the same side. As shown previously, stable locomotor activity could be induced by bath application of N-methyl-D-aspartate (NMDA; 4-8.5 microM) and/or serotonin (5-HT; 4-20 microM). NA modulated this activity by decreasing the cycle frequency and increasing the ventral root burst duration. These effects were dose dependent in the concentration range 1-5 microM. In contrast, at no concentration tested did NA have consistent effects on burst amplitudes or on the background activity of the ongoing rhythm. Moreover, NA did not obviously affect the left/right and rostrocaudal alternation of the NMDA/5-HT rhythm. The NMDA/5-HT locomotor rhythm sometimes displayed a time-dependent breakdown in coordination, ultimately resulting in tonic ventral root activity. However, the addition of NA to the NMDA/5-HT saline could reinstate a well-coordinated locomotor rhythm. We conclude that exogenously applied NA can elicit tonic activity or can trigger a slow, irregular and often synchronous motor pattern. When NA is applied during ongoing locomotor activity, the amine has a distinct slowing effect on the rhythm while preserving the normal coordination between flexors and extensors. The ability of NA to "rescue" rhythmic locomotor activity after its time-dependent deterioration suggests that the amine may be important in the maintenance of rhythmic motor activity.

Characterization of commissural interneurons in the lumbar region of the neonatal rat spinal cord.

Neurons with axons that extend to the contralateral side of the spinal cord--commissural interneurons (CINs)--coordinate left/right alternation during locomotion. Little is known about the organization of CINs in the mammalian spinal cord. To determine the numbers, distribution, dendritic morphologies, axonal trajectories, and termination patterns of CINs located in the lumbar spinal cord of the neonatal rat, several different retrograde and anterograde axonal tracing paradigms were performed with fluorescent dextran amines and the lipophilic tracer 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI). CINs with ascending (aCINs) and descending (dCINs) axons were labeled independently. The aCINs and dCINs occupied different but overlapping domains within the transverse plane. The aCINs were clustered into four recognizable groups, and the dCINs were clustered into two recognizable groups. All dCINs and most aCINs were located within the gray matter, with somata ranging from 10-30 microm in diameter and with large, multipolar dendritic trees. One group of aCINs was located outside the gray matter along the dorsal and dorsolateral margin and had dendrites that were nearly confined to the dorsolateral surface. All CIN axons traversed the ventral commissure at right angles to the midline. CIN axons coursed up to six or seven segments rostrally and/or caudally in the ventral and ventrolateral white matter and gave off collaterals over a shorter range, predominantly to the ventral gray matter. These findings show that the lumbar spinal cord of the neonatal rat contains substantial numbers of CINs with axon projections and collateral ranges spanning several segments and that CINs projecting rostrally vs. caudally have different distributions in the transverse plane. The study provides an anatomical framework for future electrophysiological studies of the spinal neuronal circuits underlying locomotion in mammals.

Synaptic signaling in an active central network only moderately changes passive membrane properties.

The membrane resistance of mammalian central neurons may be dramatically reduced by synaptic events during network activity, thereby changing their integration properties. We have used the isolated neonatal rat spinal cord to provide measurements of the effect of synaptic signaling on passive membrane properties during network activity. Synaptic signaling could take place during fictive locomotor activity with only modest (on average 35%) reduction of the input resistance (Rin) and of the cell's charging time constant (tauin). Individual synaptic signals, however, often introduced a peak conductance that was greater than the input conductance (Gin = 1/Rin) without synaptic activity. The combination of moderate average synaptic conductance and large conductance of individual synaptic signals suggests that individual presynaptic neurons have large but short-lasting influence on the integration properties of postsynaptic neurons.

Distribution of central pattern generators for rhythmic motor outputs in the spinal cord of limbed vertebrates.

Neuronal networks in the spinal cord are capable of producing rhythmic movements, such as walking and swimming, when the spinal cord itself is isolated from the brain and sensory inputs. These spinal networks, also called central pattern generators or CPGs, serve as relatively simple model systems for our understanding of brain functions. In this paper we concentrate on spinal CPGs in limbed vertebrates and in particular address the question: Where in the spinal cord, in the longitudinal and transverse planes, are they located? We will review the use of lesions to isolate the rhythm and pattern-generating parts of the CPG network, indirect methods like activity-dependent labeling with [14C]-2-deoxyglucose, c-fos, sulforhodamine 101, and WGA-HRP, which label presumed rhythmically active neurons en bloc, and direct methods such as calcium-imaging, extracellular and intracellular recordings, which identify rhythmically active cells directly. With this review we hope to highlight the scientific disagreements and the consensus, which have emerged from these studies with regard to the distribution of the CPG networks in the spinal cord.

Functional role of plateau potentials in vertebrate motor neurons.

The expression of plateau potentials in spinal motor neurons is regulated by neuromodulatory substances. Recent experiments have shed new light on this regulation at the cellular level. It is now possible to evaluate the existence of plateau potentials in intact organisms, including humans, and to address the functional role of plateau potentials in motor control, as well as in information transfer in the brain.

Analysis of EPSCs and IPSCs carrying rhythmic, locomotor-related information in the isolated spinal cord of the neonatal rat.

To understand better the synaptic language used by neurons in active networks, we have analyzed postsynaptic currents (PSCs) received by interneurons in the isolated spinal cord from neonatal rats during 5-hydroxytryptamine- and N-methyl--aspartate-induced fictive locomotion. Using a computer algorithm, we identified PSCs in rhythmically active interneurons in laminae VII and X. To test whether the PSCs actually participated in the transmission of the cyclic, locomotor-related signal, we constructed an analytic current trace based on only the identified events. Each identified PSC was fitted by a mathematical function, and the shape of this function was added to a baseline with time delays given by the time positions of the identified PSCs. By averaging the resulting analytic current trace over several cycles, we showed that the identified PSCs built a cyclic signal locked to the rhythmic activity recorded from the ventral roots. Furthermore, subtraction of the analytic from the original current trace reduced the amplitude of the cyclic signal received by these cells. Thus the identified PSCs contributed to the cyclic information, allowing us to analyze how they built the compound cyclic signal. Most often there was an inverse relationship between the contribution from excitatory and inhibitory PSCs during the cyclic modulation, indicating that there was a reciprocal regulation of the presynaptic inhibitory and excitatory cells. Comparing the most inhibitory and most excitatory halves of the locomotor related cycle, there was a considerably larger modulation of the frequency of PSCs than of their amplitude. The small and sometimes insignificant modulation of PSC amplitude suggests that facilitation and depression had little importance for the information transfer. The modest amplitude modification also suggests that the large range of available PSC amplitudes seen in these neurons was not used very efficiently to code the cyclic information.

Development in neonatal rats of the sensory resetting of the locomotor rhythm induced by NMDA and 5-HT.

Developmental changes in the effects of quadriceps (Q) nerve stimulation on the locomotor rhythm induced by a mixture of N-methyl-D-aspartic acid and 5-hydroxytryptamine were examined using in vitro preparations from neonatal rats at postnatal days (P) 1-6. The effects of such stimulation on the rhythm were dependent both on stimulus strength and on the age of the animal. Low-intensity stimulation (< or =3.0 x T, where T=threshold for the monosynaptic reflex) during the flexor phase reset the rhythm via a prolongation of the flexor burst in most rats at P1-3, but via flexor burst truncation at P4-6. At any age, low-intensity stimulation during the extensor phase had no consistent effect on the ongoing rhythm. Activation of muscle afferents evoked via isometric contraction of the Q muscle caused effects similar to those obtained on low-intensity electrical stimulation in all age groups. In all age groups, high-intensity stimulation (> or =5.0 x T) caused resetting when delivered during the flexor phase via a prolongation of the flexor burst and during the extensor phase via a truncation of the extensor burst. These results suggest that the type of resetting evoked from low-threshold muscle afferents changes drastically during postnatal weekl, while effects evoked from high-threshold afferents remain unchanged.

Crossed rhythmic synaptic input to motoneurons during selective activation of the contralateral spinal locomotor network.

To investigate the cellular mechanisms underlying locomotor-related left-right coordination, we monitored the crossed synaptic input to lumbar motoneurons during contralateral ventral root rhythmicity in the neonatal rat spinal cord in vitro. Using a longitudinal split-bath setup, one hemicord was kept in normal solution, whereas the contralateral hemicord was exposed to 5-HT and NMDA. With this approach, rhythmic bursting could be induced in the ventral roots on the agonist-exposed side, whereas the ventral roots on the agonist-free side remained silent. Intracellular recordings were made from L1-L3 motoneurons on the silent agonist-free side during rhythmic activity in the contralateral ventral roots. At the resting membrane potential, the typical crossed synaptic input was a rhythmic barrage of depolarizing IPSPs. This input modulated the frequency of spikes induced with depolarizing direct current by inhibiting firing in phase with the contralateral bursts. Intracellular chloride loading increased the amplitude of the IPSPs, suggesting that they were chloride-dependent. Strychnine but not bicuculline generally blocked the rhythmic inhibitory input when added to the agonist-free side during contralateral rhythmicity. APV and CNQX on the agonist-free side abolished the rhythmic inhibitory input in most motoneurons but not in all. We suggest that rat spinal motoneurons receive a mainly glycinergic rhythmic inhibition from the contralateral half of the locomotor network. Unlike in simpler vertebrates, the crossed inhibition often appears to be at least disynaptic, involving inhibitory premotor neurons located on the same side as the receiving motoneurons. These premotor neurons are rhythmically excited via a crossed pathway that depends on glutamatergic transmission.

Prolonged firing in motor units: evidence of plateau potentials in human motoneurons?

Serotonin (5-HT) and norepinephrine-dependent plateau potentials are found in spinal motoneurons in reduced turtle and cat preparations. Triggering the plateau potential by short-lasting synaptic excitation causes a prolonged self-sustained firing, which can be terminated by short-lasting synaptic inhibition. The presence of plateau potentials can also allow neurons to fire in a bistable manner, i.e., shifting between stable low and high firing frequencies. Such a bistable firing behavior has been found in soleus motor units in unrestrained rats. In the present study single motor-unit activity was recorded from low-threshold units in human soleus and tibialis anterior muscles to evaluate whether a bistable firing behavior and/or prolonged firing could be evoked. Vibration of the homonymous muscle tendon (30-100 Hz, 2-10 s) was used as excitatory input to the motoneuron pool. Brief excitation while the muscle was electrically silent induced firing during the vibration and sometimes recruited units into prolonged stable firing outlasting the vibratory stimulus. However, a bistable firing behavior, i.e., vibration-induced maintained shifts between two stable levels of firing, could not be convincingly demonstrated. The reason for this was twofold. First, low-threshold human motor units tended to jump to a "preferred firing range" shortly after voluntary recruitment. This firing range was the same as when units were recruited from silence into prolonged firing by vibration. Below the preferred firing range, maintained firing was unstable and usually only possible when subjects were listening to the spike potentials or had visual force-feedback. Second, vibration when units were firing in the preferred firing range caused a transient increase in firing frequency but no maintained frequency shifts. Recordings from pairs of motor units showed that short-lasting vibration could recruit one unit into prolonged firing, while a second unit, which already fired in its preferred firing range, only transiently increased its firing rate during the vibration. This suggests that the prolonged firing was not the result of an increase in the common synaptic drive to the motoneuron pool. We conclude that a bistable firing behavior as seen in intact rats is probably absent in human low-threshold motor units, but that prolonged firing could be seen in response to short-lasting excitation. This latter phenomenon is compatible with the existence of plateau potentials, which have to have a threshold close to the threshold for sodium spike generation.