Contribution of outward currents to spike-frequency adaptation in hypoglossal motoneurons of the rat

Enhanced availability of serotonin limits muscle activation during high-intensity, but not low-intensity, fatiguing contractions

Serotonin (5-HT) modulates motoneuron excitability during muscle contractions, where the release of 5-HT in the central nervous system (CNS) is linked to the intensity of physical activity. Although there is evidence that enhanced availability of 5-HT can exacerbate fatigue, these effects on the development of fatigue during different contraction intensities are largely unknown. The purpose of this study was to investigate how enhanced 5-HT availability affects voluntary muscle activation and corticospinal excitability during fatigue-inducing contractions. Two experiments were performed. In the first experiment (n = 11), twelve isometric elbow flexions at 20% maximal voluntary contractions (MVC) were performed for 2-min each with 40-s rest periods. In the second experiment (n = 14), twelve maximal isometric elbow flexions were held for 10-s each with 40-s rest periods. In both experiments, the selective serotonin reuptake inhibitor (20 mg paroxetine), or a placebo, was administered in a two-way crossover-design. Muscle responses to transcranial magnetic stimulation (TMS) of the motor cortex (both experiments 1 and 2), as well as motor point stimulation of the elbow flexors (experiment 2) were assessed. Paroxetine reduced both motor cortical (p = 0.018) and motor point voluntary activation (p = 0.036) during the maximal contraction protocol. Paroxetine also reduced exercise-induced lengthening of the TMS silent period during the submaximal (p = 0.037) and maximal (p = 0.002) contraction protocols. Activation of inhibitory 5-HT_1A receptors on motoneurons likely exacerbated exercise-induced reductions in voluntarily drive to the elbow flexors. However, 5-HT modulation of motor activity also appeared at the supraspinal level.

Axonal Computations

Axons functionally link the somato-dendritic compartment to synaptic terminals. Structurally and functionally diverse, they accomplish a central role in determining the delays and reliability with which neuronal ensembles communicate. By combining their active and passive biophysical properties, they ensure a plethora of physiological computations. In this review, we revisit the biophysics of generation and propagation of electrical signals in the axon and their dynamics. We further place the computational abilities of axons in the context of intracellular and intercellular coupling. We discuss how, by means of sophisticated biophysical mechanisms, axons expand the repertoire of axonal computation, and thereby, of neural computation.

Postnatal development of persistent inward currents in rat XII motoneurons and their modulation by serotonin, muscarine and noradrenaline

KEY POINTS

Persistent inward currents (PICs) in spinal motoneurons are long-lasting, voltage-dependent currents that increase excitability; they are dramatically potentiated by serotonin, muscarine, and noradrenaline (norepinephrine). Loss of these modulators (and the PIC) during sleep is hypothesized as a major contributor to REM sleep atonia. Reduced excitability of XII motoneurons that drive airway muscles and maintain airway patency is causally implicated in obstructive sleep apnoea (OSA), but whether XII motoneurons possess a modulator-sensitive PIC that could be a factor in the reduced airway tone of sleep is unknown. Whole-cell recordings from rat XII motoneurons in brain slices indicate that PIC amplitude increases ∼50% between 1 and 23 days of age, when potentiation of the PIC by 5HT₂ , muscarinic, or α₁ noradrenergic agonists peaks at <50%, manyfold lower than the potentiation observed in spinal motoneurons. α₁ noradrenergic receptor activation produced changes in XII motoneuron firing behaviour consistent with PIC involvement, but indicators of strong PIC activation were never observed; in vivo experiments are needed to determine the role of the modulator-sensitive PIC in sleep-dependent reductions in airway tone.

ABSTRACT

Hypoglossal (XII) motoneurons play a key role in maintaining airway patency; reductions in their excitability during sleep through inhibition and disfacilitation, i.e. loss of excitatory modulation, is implicated in obstructive sleep apnoea. In spinal motoneurons, 5HT₂ , muscarinic and α₁ noradrenergic modulatory systems potentiate persistent inward currents (PICs) severalfold, dramatically increasing excitability. If the PICs in XII and spinal motoneurons are equally sensitive to modulation, loss of the PIC secondary to reduced modulatory tone during sleep could contribute to airway atonia. Modulatory systems also change developmentally. We therefore characterized developmental changes in magnitude of the XII motoneuron PIC and its sensitivity to modulation by comparing, in neonatal (P1-4) and juvenile (P14-23) rat brainstem slices, the PIC elicited by slow voltage ramps in the absence and presence of agonists for 5HT₂ , muscarinic, and α₁ noradrenergic receptors. XII motoneuron PIC amplitude increased developmentally (from -195 ± 12 to -304 ± 19 pA). In neonatal XII motoneurons, the PIC was only potentiated by α₁ receptor activation (5 ± 4%). In contrast, all modulators potentiated the juvenile XII motoneurons PIC (5HT₂ , 5 ± 5%; muscarine, 22 ± 11%; α₁ , 18 ± 5%). These data suggest that the influence of the PIC and its modulation on XII motoneuron excitability will increase with postnatal development. Notably, the modulator-induced potentiation of the PIC in XII motoneurons was dramatically smaller than the 2- to 6-fold potentiation reported for spinal motoneurons. In vivo measurements are required to determine if the modulator-sensitive, XII motoneuron PIC is an important factor in sleep-state dependent reductions in airway tone.

Dramatically Amplified Thoracic Sympathetic Postganglionic Excitability and Integrative Capacity Revealed with Whole-Cell Patch-Clamp Recordings

Thoracic paravertebral sympathetic postganglionic neurons (tSPNs) comprise the final integrative output of the distributed sympathetic nervous system controlling vascular and thermoregulatory systems. Considered a non-integrating relay, what little is known of tSPN intrinsic excitability has been determined by sharp microelectrodes with presumed impalement injury. We thus undertook the first electrophysiological characterization of tSPN cellular properties using whole-cell recordings and coupled results with a conductance-based model to explore the principles governing their excitability in adult mice of both sexes. Recorded membrane resistance and time constant values were an order of magnitude greater than values previously obtained, leading to a demonstrable capacity for synaptic integration in driving recruitment. Variation in membrane resistivity was the primary determinant controlling cell excitability with vastly lower currents required for tSPN recruitment. Unlike previous microelectrode recordings in mouse which observed inability to sustain firing, all tSPNs were capable of repetitive firing. Computational modeling demonstrated that observed differences are explained by introduction of a microelectrode impalement injury conductance. Overall, tSPNs largely linearly encoded injected current magnitudes over a broad frequency range with distinct subpopulations differentiable based on repetitive firing signatures. Thus, whole-cell recordings reveal tSPNs have more dramatically amplified excitability than previously thought, with greater intrinsic capacity for synaptic integration and with the ability for maintained firing to support sustained actions on vasomotor tone and thermoregulatory function. Rather than acting as a relay, these studies support a more responsive role and possible intrinsic capacity for tSPNs to drive sympathetic autonomic function.

Retracing your footsteps: developmental insights to spinal network plasticity following injury

During development of the spinal cord, a precise interaction occurs between descending projections and sensory afferents, with spinal networks that lead to expression of coordinated motor output. In the rodent, during the last embryonic week, motor output first occurs as regular bursts of spontaneous activity, progressing to stochastic patterns of episodes that express bouts of coordinated rhythmic activity perinatally. Locomotor activity becomes functionally mature in the 2nd postnatal wk and is heralded by the onset of weight-bearing locomotion on the 8th and 9th postnatal day. Concomitantly, there is a maturation of intrinsic properties and key conductances mediating plateau potentials. In this review, we discuss spinal neuronal excitability, descending modulation, and afferent modulation in the developing rodent spinal cord. In the adult, plastic mechanisms are much more constrained but become more permissive following neurotrauma, such as spinal cord injury. We discuss parallel mechanisms that contribute to maturation of network function during development to mechanisms of pathological plasticity that contribute to aberrant motor patterns, such as spasticity and clonus, which emerge following central injury.

Altered neurobiological function of brainstem hypoglossal neurons in DiGeorge/22q11.2 Deletion Syndrome

DiGeorge/22q11.2 Deletion Syndrome (22q11DS) is a common genetic microdeletion syndrome that underlies several neurodevelopmental disorders including autism, attention deficit/hyperactivity disorder, and schizophrenia. In addition to cognitive impairments, those with 22q11DS have disrupted feeding and swallowing from birth onward. This perinatal dysphagia significantly compromises nutritional status, impairs appropriate weight gain, and can lead to life threatening aspiration-based infections. Appropriately timed excitation and inhibition of brainstem hypoglossal motor neurons, which innervate tongue muscles, is essential for proper feeding and swallowing. In this study we have examined changes in hypoglossal motor neuron function in the LgDel mouse model of 22q11DS. Hypoglossal motor neurons from LgDel mouse pups have action potentials with afterhyperpolarizations, mediated by a large conductance charybdotoxin-sensitive Ca-activated K current, that are significantly shorter in duration and greater in magnitude than those in wild-type pups. In addition, the amplitude, but not frequency, of glutamatergic excitatory glutamatergic postsynaptic currents (EPSCs) is diminished, and GABAergic, but not glycinergic, neurotransmission to hypoglossal motor neurons was reduced in LgDel animals. These observations provide a foundation for understanding the neurological changes in hypoglossal motor neuron function and their contribution to swallowing abnormalities that occur in DiGeorge/22q11.2 Deletion Syndrome.

Effects of self-coupling and asymmetric output on metastable dynamical transient firing patterns in arrays of neurons with bidirectional inhibitory coupling

Metastable dynamical transient patterns in arrays of bidirectionally coupled neurons with self-coupling and asymmetric output were studied. First, an array of asymmetric sigmoidal neurons with symmetric inhibitory bidirectional coupling and self-coupling was considered and the bifurcations of its steady solutions were shown. Metastable dynamical transient spatially nonuniform states existed in the presence of a pair of spatially symmetric stable solutions as well as unstable spatially nonuniform solutions in a restricted range of the output gain of a neuron. The duration of the transients increased exponentially with the number of neurons up to the maximum number at which the spatially nonuniform steady solutions were stabilized. The range of the output gain for which they existed reduced as asymmetry in a sigmoidal output function of a neuron increased, while the existence range expanded as the strength of inhibitory self-coupling increased. Next, arrays of spiking neuron models with slow synaptic inhibitory bidirectional coupling and self-coupling were considered with computer simulation. In an array of Class 1 Hindmarsh-Rose type models, in which each neuron showed a graded firing rate, metastable dynamical transient firing patterns were observed in the presence of inhibitory self-coupling. This agreed with the condition for the existence of metastable dynamical transients in an array of sigmoidal neurons. In an array of Class 2 Bonhoeffer-van der Pol models, in which each neuron had a clear threshold between firing and resting, long-lasting transient firing patterns with bursting and irregular motion were observed.

Distinguishing intrinsic from extrinsic factors underlying firing rate saturation in human motor units

During voluntary contraction, firing rates of individual motor units (MUs) increase modestly over a narrow force range beyond which little additional increase in firing rate is seen. Such saturation of MU discharge may be a consequence of extrinsic factors that limit net synaptic excitation acting on motor neurons (MNs) or may be due to intrinsic properties of the MNs. Two sets of experiments involving recording of human biceps brachii MUs were carried out to evaluate saturation. In the first set, the extent of saturation was quantified for 136 low-threshold MUs during isometric ramp contractions. Firing rate-force data were best fit by a saturating function for 90% of MUs recorded with a maximum rate of 14.8 ± 2.0 impulses/s. In the second set of experiments, to distinguish extrinsic from intrinsic factors underlying saturation, we artificially augmented descending excitatory drive to biceps MNs by activation of muscle spindle afferents through tendon vibration. We examined the change in firing rate caused by tendon vibration in 96 MUs that were voluntarily activated at rates below and at saturation. Vibration had little effect on the discharge of MUs that were firing at saturation frequencies but strongly increased firing rates of the same units when active at lower frequencies. These results indicate that saturation is likely caused by intrinsic mechanisms that prevent further increases in firing rate in the presence of increasing synaptic excitation. Possible intrinsic cellular mechanisms that limit firing rates of motor units during voluntary effort are discussed.

Physical and cognitive consequences of fatigue: A review

Fatigue is a common worrying complaint among people performing physical activities on the basis of training or rehabilitation. An enormous amount of research articles have been published on the topic of fatigue and its effect on physical and physiological functions. The goal of this review was to focus on the effect of fatigue on muscle activity, proprioception, and cognitive functions and to summarize the results to understand the influence of fatigue on these functions. Attaining this goal provides evidence and guidance when dealing with patients and/or healthy individuals in performing maximal or submaximal exercises.

Influence of developmental nicotine exposure on spike-timing precision and reliability in hypoglossal motoneurons

Smoothly graded muscle contractions depend in part on the precision and reliability of motoneuron action potential generation. Whether or not a motoneuron generates spikes precisely and reliably depends on both its intrinsic membrane properties and the nature of the synaptic input that it receives. Factors that perturb neuronal intrinsic properties and/or synaptic drive may compromise the temporal precision and the reliability of action potential generation. We have previously shown that developmental nicotine exposure (DNE) alters intrinsic properties and synaptic transmission in hypoglossal motoneurons (XIIMNs). Here we show that the effects of DNE also include alterations in spike-timing precision and reliability, and spike-frequency adaptation, in response to sinusoidal current injection. Current-clamp experiments in brainstem slices from neonatal rats show that DNE lowers the threshold for spike generation but increases the variability of spike-timing mechanisms. DNE is also associated with an increase in spike-frequency adaptation and reductions in both peak and steady-state firing rate in response to brief, square wave current injections. Taken together, our data indicate that DNE causes significant alterations in the input-output efficiency of XIIMNs. These alterations may play a role in the increased frequency of obstructive apneas and altered suckling strength and coordination observed in nicotine-exposed neonatal humans.

Context-dependent coding in single neurons

The linear-nonlinear cascade model (LN model) has proven very useful in representing a neural system's encoding properties, but has proven less successful in reproducing the firing patterns of individual neurons whose behavior is strongly dependent on prior firing history. While the cell's behavior can still usefully be considered as feature detection acting on a fluctuating input, some of the coding capacity of the cell is taken up by the increased firing rate due to a constant "driving" direct current (DC) stimulus. Furthermore, both the DC input and the post-spike refractory period generate regular firing, reducing the spike-timing entropy available for encoding time-varying fluctuations. In this paper, we address these issues, focusing on the example of motoneurons in which an afterhyperpolarization (AHP) current plays a dominant role regularizing firing behavior. We explore the accuracy and generalizability of several alternative models for single neurons under changes in DC and variance of the stimulus input. We use a motoneuron simulation to compare coding models in neurons with and without the AHP current. Finally, we quantify the tradeoff between instantaneously encoding information about fluctuations and about the DC.

Recovery of neuronal and network excitability after spinal cord injury and implications for spasticity

The state of areflexia and muscle weakness that immediately follows a spinal cord injury (SCI) is gradually replaced by the recovery of neuronal and network excitability, leading to both improvements in residual motor function and the development of spasticity. In this review we summarize recent animal and human studies that describe how motoneurons and their activation by sensory pathways become hyperexcitable to compensate for the reduction of functional activation of the spinal cord and the eventual impact on the muscle. Specifically, decreases in the inhibitory control of sensory transmission and increases in intrinsic motoneuron excitability are described. We present the idea that replacing lost patterned activation of the spinal cord by activating synaptic inputs via assisted movements, pharmacology or electrical stimulation may help to recover lost spinal inhibition. This may lead to a reduction of uncontrolled activation of the spinal cord and thus, improve its controlled activation by synaptic inputs to ultimately normalize circuit function. Increasing the excitation of the spinal cord with spared descending and/or peripheral inputs by facilitating movement, instead of suppressing it pharmacologically, may provide the best avenue to improve residual motor function and manage spasticity after SCI.

Concomitant changes in afterhyperpolarization and twitch following repetitive stimulation of fast motoneurones and motor units

The study aimed at determining changes in a course of motoneuronal afterhyperpolarization (AHP) and in contractile twitches of motor units (MUs) during activity evoked by increasing number of stimuli (from 1 to 5), at short interspike intervals (5 ms). The stimulation was applied antidromically to spinal motoneurones or to isolated axons of MUs of the medial gastrocnemius muscle within two separate series of experiments on anesthetized rats. Alterations in the amplitude and time parameters of the AHP of successive spikes were compared to changes in force and time course of successive twitches obtained by mathematical subtraction of tetanic contractions evoked by one to five stimuli. The extent of changes of the studied parameters depended on a number of applied stimuli. The maximal modulation of the AHP and twitch parameters (a prolongation and an increase in the AHP and twitch amplitudes) was typically observed after the second pulse, while higher number of pulses at the same frequency did not induce so prominent changes. One may conclude that changes observed in parameters of action potentials of motoneurons are concomitant to changes in contractile properties of MU twitches. This suggests that both modulations of the AHP and twitch parameters reflect mechanisms leading to force development at the beginning of MU activity.

Expression of postsynaptic Ca2+-activated K+ (SK) channels at C-bouton synapses in mammalian lumbar -motoneurons

Small-conductance calcium-activated potassium (SK) channels mediate medium after-hyperpolarization (AHP) conductances in neurons throughout the central nervous system. However, the expression profile and subcellular localization of different SK channel isoforms in lumbar spinal α-motoneurons (α-MNs) is unknown. Using immunohistochemical labelling of rat, mouse and cat spinal cord, we reveal a differential and overlapping expression of SK2 and SK3 isoforms across specific types of α-MNs. In rodents, SK2 is expressed in all α-MNs, whereas SK3 is expressed preferentially in small-diameter α-MNs; in cats, SK3 is expressed in all α-MNs. Function-specific expression of SK3 was explored using post hoc immunostaining of electrophysiologically characterized rat α-MNs in vivo. These studies revealed strong relationships between SK3 expression and medium AHP properties. Motoneurons with SK3-immunoreactivity exhibit significantly longer AHP half-decay times (24.67 vs. 11.02 ms) and greater AHP amplitudes (3.27 vs. 1.56 mV) than MNs lacking SK3-immunoreactivity. We conclude that the differential expression of SK isoforms in rat and mouse spinal cord may contribute to the range of medium AHP durations across specific MN functional types and may be a molecular factor distinguishing between slow- and fast-type α-MNs in rodents. Furthermore, our results show that SK2- and SK3-immunoreactivity is enriched in distinct postsynaptic domains that contain Kv2.1 channel clusters associated with cholinergic C-boutons on the soma and proximal dendrites of α-MNs. We suggest that this remarkably specific subcellular membrane localization of SK channels is likely to represent the basis for a cholinergic mechanism for effective regulation of channel function and cell excitability.

The sodium-potassium pump controls the intrinsic firing of the cerebellar Purkinje neuron

In vitro, cerebellar Purkinje cells can intrinsically fire action potentials in a repeating trimodal or bimodal pattern. The trimodal pattern consists of tonic spiking, bursting, and quiescence. The bimodal pattern consists of tonic spiking and quiescence. It is unclear how these firing patterns are generated and what determines which firing pattern is selected. We have constructed a realistic biophysical Purkinje cell model that can replicate these patterns. In this model, Na(+)/K(+) pump activity sets the Purkinje cell's operating mode. From rat cerebellar slices we present Purkinje whole cell recordings in the presence of ouabain, which irreversibly blocks the Na(+)/K(+) pump. The model can replicate these recordings. We propose that Na(+)/K(+) pump activity controls the intrinsic firing mode of cerbellar Purkinje cells.

Activity-dependent depression of the recurrent discharge of human motoneurones after maximal voluntary contractions

Despite maximal voluntary effort, the output of human motoneurone pools diminishes during fatigue. To assess motoneurone behaviour, we measured recurrent discharges evoked antidromically by supramaximal nerve stimulation after isometric maximal voluntary contractions (MVCs).They were measured as F-waves in the electromyographic activity (EMG). Supramaximal stimuli to the common peroneal and ulnar nerves evoked F-waves at rest before and after MVCs in tibialis anterior (TA) and abductor digit minimi (ADM), respectively. F-waves were depressed immediately after a sustained MVC. For TA, the size and time course of depression of the F-wave area (26 ± 13%; mean ± SD; P =0.007) and persistence (∼20%) were similar after a 10-s or 1-min MVC. For ADM, the decline in F-wave area (39.8 ± 19.6%; P <0.01) was similar after the two contractions but the decline in persistence (probability of occurrence) of the F-wave differed (14.6 ± 10.5% and 32.5 ± 17.1% after 10-s and 1-min MVCs respectively). Comparison of a very long (2-min) with a very short (2-s)MVC in ADM showed that the depression of F-wave area, as well as persistence, was greater after the longer contraction. This suggests, at least for ADM, that the depression is related to the duration of voluntary activity and that the decrease in F-waves could contribute to central fatigue. To examine whether changes in motor axon excitability caused the depression, we measured compound muscle action potentials (M-waves) to submaximal stimulation of the ulnar nerve after a 2-s and 2-min MVC. Submaximal M-waves were not depressed after a 2-s MVC. They were depressed by a 2-min MVC, but the time course of depression of the F- and M-waves differed. Thus, depression of F-waves does not simply reflect reduced excitability of peripheral motor axons.Hence, we propose that activity-dependent changes at the soma or the initial segment depress the recurrent discharge of human motoneurones and that this may contribute to central fatigue.

Gain of spinal motoneurons measured from square and ramp current pulses

The gain of motoneurons (MNs) characterizes how variations in synaptic input are transformed in to variations in output firing and muscle contraction. Experimentally gain is often defined as the frequency-current relation observed in response to injected suprathreshold square current pulses or current ramps during intracellular recording. The gain of MNs is strongly affected by adaptation: transient gain in response to depolarization is usually higher than steady state gain measured during sustained depolarization. The transient and the stationary gain of neurons are separate entities that can be selectively modified. Here we investigated how the transient and the stationary gain of spinal MNs obtained from responses to square current pulses are related to gain estimated from the responses to the current ramps. We found, that the gain in response to current ramps is identical to the steady state gain during sustained depolarization. Therefore, gain modulation is more fully characterized with square current pulses than with current ramps.

Effects of persistent inward currents, accommodation, and adaptation on motor unit behavior: a simulation study

Motor neurons are often assumed to generate spikes in proportion to the excitatory synaptic input received. There are, however, many intrinsic properties of motor neurons that might affect this relationship, such as persistent inward currents (PICs), spike-threshold accommodation, or spike-frequency adaptation. These nonlinear properties have been investigated in reduced animal preparation but have not been well studied during natural motor behaviors because of the difficulty in characterizing synaptic input in intact animals. Therefore, we studied the influence of each of these intrinsic properties on spiking responses and muscle force using a population model of motor units that simulates voluntary contractions in human subjects. In particular, we focused on the difference in firing rate of low-threshold motor units when higher threshold motor units were recruited and subsequently derecruited, referred to as ΔF. Others have used ΔF to evaluate the extent of PIC activation during voluntary behavior. Our results showed that positive ΔF values could arise when any one of these nonlinear properties was included in the simulations. Therefore, a positive ΔF should not be considered as exclusive evidence for PIC activation. Furthermore, by systematically varying contraction duration and speed in our simulations, we identified a means that might be used experimentally to distinguish among PICs, accommodation, and adaptation as contributors to ΔF.

Behaviour of the motoneurone pool in a fatiguing submaximal contraction

During fatigue caused by a sustained maximal voluntary contraction (MVC), motoneurones become markedly less responsive when tested during the silent period following transcranial magnetic stimulation (TMS). To determine whether this reduction depends on the repetitive activation of the motoneurones, responses to TMS (motor evoked potentials, MEPs) and to cervicomedullary stimulation (cervicomedullary motor evoked potentials, CMEPs) were tested during a sustained submaximal contraction at a constant level of electromyographic activity (EMG). In such a contraction, some motoneurones are repetitively activated whereas others are not active. On four visits, eight subjects performed a 10 min maintained-EMG elbow flexor contraction of 25% maximum. Test stimuli were delivered with and without conditioning by TMS given 100 ms prior. Test responses were MEPs or CMEPs (two visits each, small responses evoked by weak stimuli on one visit and large responses on the other). During the sustained contraction, unconditioned CMEPs decreased ∼20% whereas conditioned CMEPs decreased ∼75 and 30% with weak and strong stimuli, respectively. Conditioned MEPs were reduced to the same extent as CMEPs of the same size. The data reveal a novel decrease in motoneurone excitability during a submaximal contraction if EMG is maintained. Further, the much greater reduction of conditioned than unconditioned CMEPs shows the critical influence of voluntary drive on motoneurone responsiveness. Strong test stimuli attenuate the reduction of conditioned CMEPs which indicates that low-threshold motoneurones active in the contraction are most affected. The equivalent reduction of conditioned MEPs and CMEPs suggests that, similar to findings with a sustained MVC, impaired motoneurone responsiveness rather than intracortical inhibition is responsible for the fatigue-related impairment of the MEP during a sustained submaximal contraction.

Reversal of the late phase of spike frequency adaptation in cat spinal motoneurons during fictive locomotion

In spinal motoneurons, late spike frequency adaptation (SFA) is defined as the slowing of the firing rate over tens of seconds and can be seen during sustained or intermittent current injection. Although the function of late SFA is not known, it may result in a decrease in force production over time, or muscle fatigue. Because locomotion can persist for long periods of time without fatigue, late SFA was studied using intracellular recordings from adult cat motoneurons during fictive locomotion. Of eight lumbar motoneurons studied, all showed late adaptation during control conditions, but none demonstrated late adaptation during locomotor activity. The most consistent properties that correlated with the presence or absence of late SFA were those related to availability of fast, inactivating sodium channels, particularly action potential rate of rise. Evidence of the reversal of late SFA during locomotion was present for several minutes following locomotor trials, consistent with the suggestion that SFA is modulated through slow metabotropic pathways. The abolition of late adaptation in spinal motoneurons during fictive locomotion is an example of a state-dependent change in the "intrinsic" properties of mammalian motoneurons. This change contributes to increased excitability of motoneurons during locomotion and results in robust firing during sustained locomotion.

The response to paired motor cortical stimuli is abolished at a spinal level during human muscle fatigue

During maximal exercise, supraspinal fatigue contributes significantly to the decline in muscle performance but little is known about intracortical inhibition during such contractions. Long-interval inhibition is produced by a conditioning motor cortical stimulus delivered via transcranial magnetic stimulation (TMS) 50-200 ms prior to a second test stimulus. We aimed to delineate changes in this inhibition during a sustained maximal voluntary contraction (MVC). Eight subjects performed a 2 min MVC of elbow flexors. Single test and paired (conditioning-test interval of 100 ms) stimuli were delivered via TMS over the motor cortex every 7-8 s throughout the effort and during intermittent MVCs in the recovery period. To determine the role of spinal mechanisms, the protocol was repeated but the TMS test stimulus was replaced by cervicomedullary stimulation which activates the corticospinal tract. TMS motor evoked potentials (MEPs) and cervicomedullary motor evoked potentials (CMEPs) were recorded from biceps brachii. Unconditioned MEPs increased progressively with fatigue, whereas CMEPs increased initially but returned to the control value in the final 40 s of contraction. In contrast, both conditioned MEPs and CMEPs decreased rapidly with fatigue and were virtually abolished within 30 s. In recovery, unconditioned responses required <30 s but conditioned MEPs and CMEPs required 90 s to return to control levels. Thus, long-interval inhibition increased markedly as fatigue progressed. Contrary to expectations, subcortically evoked CMEPs were inhibited as much as MEPs. This new phenomenon was also observed in the first dorsal interosseous muscle. Tested with a high intensity conditioning stimulus during a fatiguing maximal effort, long-interval inhibition of MEPs was increased primarily by spinal rather than motor cortical mechanisms. The spinal mechanisms exposed here may contribute to the development of central fatigue in human muscles.

Functional imaging, spatial reconstruction, and biophysical analysis of a respiratory motor circuit isolated in vitro

We combined real-time calcium-based neural activity imaging with whole-cell patch-clamp recording techniques to map the spatial organization and analyze electrophysiological properties of respiratory neurons forming the circuit transmitting rhythmic drive from the pre-Bötzinger complex (pre-BötC) through premotoneurons to hypoglossal (XII) motoneurons. Inspiratory pre-BötC neurons, XII premotoneurons (preMNs), and XII motoneurons (MNs) were retrogradely labeled with Ca(2+)-sensitive dye in neonatal rat in vitro brainstem slices. PreMN cell bodies were arrayed dorsomedially to pre-BötC neurons with little spatial overlap; axonal projections to MNs were ipsilateral. Inspiratory MNs were distributed in dorsal and ventral subnuclei of XII. Voltage-clamp recordings revealed that two currents, persistent sodium current (NaP) and K(+)-dominated leak current (Leak), primarily contribute to preMN/MN subthreshold current-voltage relationships. NaP or Leak conductance densities in preMNs and MNs were not significantly different. We quantified preMN and MN action potential time course and spike frequency-current (f-I) relationships and found no significant differences in repetitive spiking dynamics, steady-state f-I gains, and afterpolarizing potentials. Rhythmic synaptic drive current densities were similar in preMNs and MNs. Our results indicate that, despite topographic and morphological differences, preMNs and MNs have some common intrinsic membrane, synaptic integration, and spiking properties that we postulate ensure fidelity of inspiratory drive transmission and conversion of synaptic drive into (pre)motor output. There also appears to be a common architectonic organization for some respiratory drive transmission circuits whereby many preMNs are spatially segregated from pre-BötC rhythm-generating neurons, which we hypothesize may facilitate downstream integration of convergent inputs for premotor pattern formation.

A comparison of central aspects of fatigue in submaximal and maximal voluntary contractions

Magnetic and electrical stimulation at different levels of the neuraxis show that supraspinal and spinal factors limit force production in maximal isometric efforts ("central fatigue"). In sustained maximal contractions, motoneurons become less responsive to synaptic input and descending drive becomes suboptimal. Exercise-induced activity in group III and IV muscle afferents acts supraspinally to limit motor cortical output but does not alter motor cortical responses to transcranial magnetic stimulation. "Central" and "peripheral" fatigue develop more slowly during submaximal exercise. In sustained submaximal contractions, central fatigue occurs in brief maximal efforts even with a weak ongoing contraction (<15% maximum). The presence of central fatigue when much of the available motor pathway is not engaged suggests that afferent inputs contribute to reduce voluntary activation. Small-diameter muscle afferents are likely to be activated by local activity even in sustained weak contractions. During such contractions, it is difficult to measure central fatigue, which is best demonstrated in maximal efforts. To show central fatigue in submaximal contractions, changes in motor unit firing and force output need to be characterized simultaneously. Increasing central drive recruits new motor units, but the way this occurs is likely to depend on properties of the motoneurons and the inputs they receive in the task. It is unclear whether such factors impair force production for a set level of descending drive and thus represent central fatigue. The best indication that central fatigue is important during submaximal tasks is the disproportionate increase in subjects' perceived effort when maintaining a low target force.

Facilitation of somatic calcium channels can evoke prolonged tail currents in rat hypoglossal motoneurons

Voltage-dependent persistent inward currents (PICs) make an important contribution to the input-output properties of alpha motoneurons. PICs are thought to be mediated by membrane channels located primarily on the dendrites as evidenced by prolonged tail currents following the termination of a voltage step and by a clockwise hysteresis in the whole cell inward currents recorded in response to depolarizing then repolarizing voltage ramp commands. We report here, however, that voltage-clamp currents with these same features can be generated in isolated somatic membrane patches from rat hypoglossal motoneurons. Long-lasting (200-800 ms) tail currents after 1-s voltage-clamp pulses were observed in nucleated patches from 16 of 23 cells. Further, these somatic PICs display "facilitation" in response to conditioning depolarization as previously observed in whole cell recordings from intact neurons. Pharmacological tests suggest that the PICs were primarily mediated by Cav1 channels. Our results show that many of the features of persistent calcium currents recorded from intact motoneurons do not necessarily reflect a remote dendritic origin but can also be ascribed to the intrinsic properties of their Cav1 channels.

Does motoneuron adaptation contribute to muscle fatigue?

To help reduce the gap between the cellular physiology of motoneurons (MNs) as studied "bottom-up" in animal preparations and the "top-down" study of the firing patterns of human motor units (MUs), this article addresses the question of whether motoneuron adaptation contributes to muscle fatigue. Findings are reviewed on the intracellularly recorded electrophysiology of spinal MNs as studied in vivo and in vitro using animal preparations, and the extracellularly recorded discharge of MUs as studied in conscious humans. The latter "top-down" approach, combined with kinetic measurements, has provided most of what is currently known about the neurobiology of muscle fatigue, including its task and context dependencies. It is argued that although the question addressed is still open, it should now be possible to design new "bottom-up" research paradigms using animal preparations that take advantage of what has been learned with the use of relatively noninvasive quantitative procedures in conscious humans.

Spike frequency adaptation of rat hindlimb motoneurons

The objective of our study was to resolve two issues pertaining to motoneuron (MN) spike frequency adaptation (SFA): 1) to develop an index of SFA that is sensitive to a wide range of adaptation patterns and would correlate well with MN excitability and 2) to determine whether SFA pattern is stimulus current dependent. Sprague-Dawley rats (250–350 g) were anesthetized (ketamine-xylazine) before electrophysiological properties from sciatic nerve MNs located in the lumbar spinal cord were recorded. SFA was measured by 30-s square-wave current injections at 1.5, 3.0, and 5.0 nA above estimated rhythmic firing threshold. Discharges per second were significantly ( P < 0.001) higher for 5-nA than for 1.5- and 3-nA currents > rhythmic firing threshold in the first 2 s. SFA was quantified by using ratios of the final to initial number of discharges with 1-, 2-, and 5-s bins. The best index of SFA was the percent decline in the number of spikes fired in the fifth 5-s bin relative to the first 5-s bin [1 − ( bin 5/ bin 1)]. With the use of this index, we found that SFA was significantly correlated with several measures of MN excitability, including estimated persistent inward current amplitude ( r = −0.76) and rheobase current ( r = 0.71), and tended to correlate with input resistance ( r = −0.43) and frequency-current slope ( r = −0.57). This index also showed the widest range of SFA among MNs. In conclusion, an SFA pattern can be ascertained for each MN and becomes more pronounced as MN excitability decreases. Finally, for the first time, we report evidence of a relationship between persistent inward current and SFA.

Repetitive firing of a model motoneuron: Inhibitory effect of a Ca2+-activated potassium conductance on the slope of the frequency–current relationship

We developed a novel motoneuron model to examine the role of voltage-independent, Ca(2+)-activated potassium conductance (AHP conductance, or gAHP) in regulating repetitive firing. In addition to gAHP, the model also includes five voltage-gated conductances and a system that can reproduce Ca(2+) dynamics in the cytoplasm. Conductance kinetics were based on empirical data, and the model reproduced the piecewise linear, steady-state frequency-current relationship (f-I curve). The model revealed that gAHP has a "braking effect" that suppresses spike generation and thereby reduces firing frequency; the magnitude of the reduction in firing frequency is proportional to the temporal average of gAHP activation; and the level of activation depends on the magnitude of the injected current. Moreover, plotting the activation level as a function of injected current produced a bell-shaped curve, and this relationship was essential to the transition of the f-I curve. In conclusion, our study confirms the importance of gAHP to repetitive firing and presents a novel explanation for the piecewise linear f-I curve of motoneurons.

Compartmental models of mammalian motoneurons of types S, FR and FF and their computer simulation

Mathematical models of motoneurons (MNs) of types S, FR and FF were developed based on cat MN data. Each of the three models has an initial segment, a soma and a dendritic tree. The initial segment and the soma include models of several types of ionic currents, including a calcium-dependent slow potassium current. The dendritic tree is modeled as a series association of several electrically passive cylinders. Afterhyperpolarization parameters, current to frequency relation and the responses to input current steps, ramps and sinusoids were used for model validation. The effects of sinusoidally varying synaptic inputs at different levels of the dendritic tree were studied by computer simulation. The corresponding frequency response functions resulted of lowpass type with cutoff frequencies from 10 to 40 Hz, for synapses occurring more distally or more proximally, respectively. The nonlinear effects caused by two sinusoidally varying synaptic conductances (at 7 and 11 Hz), acting at different dendritic segments, were quantified by spectral analysis of the current reaching the soma. The simulations pointed to two main nonlinear effects: (i) harmonics of the two input frequencies (e.g., 14 Hz) and (ii) intermodulation terms (e.g., 4 Hz). When the two synaptic inputs occurred on more distal dendritic compartments the nonlinear effects were more pronounced.

Multiple time scales of temporal response in pyramidal and fast spiking cortical neurons

Neural dynamic processes correlated over several time scales are found in vivo, in stimulus-evoked as well as spontaneous activity, and are thought to affect the way sensory stimulation is processed. Despite their potential computational consequences, a systematic description of the presence of multiple time scales in single cortical neurons is lacking. In this study, we injected fast spiking and pyramidal (PYR) neurons in vitro with long-lasting episodes of step-like and noisy, in-vivo-like current. Several processes shaped the time course of the instantaneous spike frequency, which could be reduced to a small number (1-4) of phenomenological mechanisms, either reducing (adapting) or increasing (facilitating) the neuron's firing rate over time. The different adaptation/facilitation processes cover a wide range of time scales, ranging from initial adaptation (<10 ms, PYR neurons only), to fast adaptation (<300 ms), early facilitation (0.5-1 s, PYR only), and slow (or late) adaptation (order of seconds). These processes are characterized by broad distributions of their magnitudes and time constants across cells, showing that multiple time scales are at play in cortical neurons, even in response to stationary stimuli and in the presence of input fluctuations. These processes might be part of a cascade of processes responsible for the power-law behavior of adaptation observed in several preparations, and may have far-reaching computational consequences that have been recently described.

Fatigue-sensitive afferents inhibit extensor but not flexor motoneurons in humans

The role of group III and IV muscle afferents in controlling the output from human muscles is poorly understood. We investigated the effects of these afferents from homonymous or antagonist muscles on motoneuron pools innervating extensor and flexor muscles of the elbow. In study 1, subjects (n = 8) performed brief maximal voluntary contractions (MVCs) of elbow extensors before and after a 2 min MVC of the extensors. During MVCs, electromyographic responses from triceps were evoked by stimulation of the corticospinal tracts [cervicomedullary motor evoked potentials (CMEPs)]. The same subjects repeated the protocol, but input from fatigue-sensitive afferents was prolonged after the fatiguing contraction by maintained muscle ischemia. In study 2, CMEPs were evoked in triceps during brief extensor MVCs before and after a 2 min sustained flexor MVC (n = 7) or in biceps during brief flexor MVCs before and after a sustained extensor MVC (n = 7). Again, ischemia was maintained after the sustained contractions. During sustained MVCs of the extensors, CMEPs in triceps decreased by approximately 35%. Without muscle ischemia, CMEPs recovered within 15 s, but with maintained ischemia, they remained depressed (by approximately 28%; p < 0.001). CMEPs in triceps were also depressed (by approximately 20%; p < 0.001) after fatiguing flexor contractions, whereas CMEPs in biceps were facilitated (by approximately 25%; p < 0.001) after fatiguing extensor contractions. During fatigue, inputs from group III and IV muscle afferents from homonymous or antagonist muscles depress extensor motoneurons but facilitate flexor motoneurons. The more pronounced inhibitory influence of these afferents on extensors suggests that these muscles may require greater cortical drive to generate force during fatigue.

Beginning at the end: repetitive firing properties in the final common pathway

Since the early 20th century, it has been recognized that motoneurons must fire repetitive trains of action potentials to produce muscle contraction. In 1932, Sir John Eccles, together with Hebbel Hoff, found that action potential spike trains in motor axons were produced by "rhythmic centres", which were within the motoneurons themselves. Two decades later, Eccles attended a Cold Spring Harbor Symposium in NY, USA entitled "The Neuron". Two of the many notable presentations at this symposium were juxtaposed: one by Eccles from the University of Otago, Dunedin, NZL, and the other by J. Walter Woodbury and Harry Patton from the University of Washington, Seattle, USA. Both presentations included data obtained using sharp microelectrodes to study the intracellularly recorded potentials of cat motoneurons. In this review, I discuss some of the events leading up to and surrounding this jointly accomplished advance and proceed to discussion of subsequent studies over 5+ decades that have made use of intracellular recordings from motoneurons to study their repetitive firing behavior. This begins with early descriptions of primary and secondary range firing, and continues to the discovery of dendritic persistent inward currents and their relation to plateau potentials, synaptic amplification, and motoneuronal firing. Following a brief description of the possible mechanisms underlying spike frequency adaptation, I discuss the modulation of repetitive firing properties during various motor behaviors. It has become increasingly clear that the central nervous system has exquisite control of the repetitive firing of motoneurons. Eccles' work laid the foundation for the present-day study of these processes.

Mechanisms underlying the early phase of spike frequency adaptation in mouse spinal motoneurones

Spike frequency adaptation (SFA) is a fundamental property of repetitive firing in motoneurones (MNs). Early SFA (occurring over several hundred milliseconds) is thought to be important in the initiation of muscular contraction. To date the mechanisms underlying SFA in spinal MNs remain unclear. In the present study, we used both whole-cell patch-clamp recordings of MNs in lumbar spinal cord slices prepared from motor functionally mature mice and computer modelling of spinal MNs to investigate the mechanisms underlying SFA. Pharmacological blocking agents applied during whole-cell recordings in current-clamp mode demonstrated that the medium AHP conductance (apamin), BK-type Ca2+ -dependent K+ channels (iberiotoxin), voltage-activated Ca2+ channels (CdCl2), M-current (linopirdine) and persistent Na+ currents (riluzole) are all unnecessary for SFA. Measurements of Na+ channel availability including action potential amplitude, action potential threshold and maximum depolarization rate of the action potential were found to correlate with instantaneous firing frequency suggesting that the availability of fast, inactivating Na+ channels is involved in SFA. Characterization of this Na+ conductance in voltage-clamp mode demonstrated that it undergoes slow inactivation with a time course similar to that of SFA. When experimentally measured parameters for the fast, inactivating Na+ conductance (including slow inactivation) were incorporated into a MN model, SFA could be faithfully reproduced. The removal of slow inactivation from this model was sufficient to remove SFA. These data indicate that slow inactivation of the fast, inactivating Na+ conductance is likely to be the key mechanism underlying early SFA in spinal MNs.

Perinatal development of respiratory motoneurons

Breathing movements require the coordinated recruitment of cranial and spinal motoneurons innervating muscles of the upper airway and ribcage. A significant part of respiratory motoneuron development and maturation occurs prenatally to support the generation of fetal breathing movements in utero and sustained breathing at birth. Postnatally, motoneuron properties are further refined and match changes in the maturing respiratory musculoskeletal system. In this review, we outline developmental changes in key respiratory motoneuronal populations occurring from the time of motoneuron birth in the embryo through the postnatal period. We will also bring attention to major deficiencies in the current knowledge of perinatal respiratory motoneuron development. To date, our understanding of processes occurring during the prenatal period comes primarily from analysis of phrenic motoneurons (PMNs), whereas information about postnatal development derives largely from studies of PMN and hypoglossal motoneuron properties.

Measurement and nature of firing rate adaptation in turtle spinal neurons

There is sparse literature on the profile of action potential firing rate (spike-frequency) adaptation of vertebrate spinal motoneurons, with most of the work undertaken on cells of the adult cat and young rat. Here, we provide such information on adult turtle motoneurons and spinal ventral-horn interneurons. We compared adaptation in response to intracellular injection of 30-s, constant-current stimuli into high-threshold versus low-threshold motoneurons and spontaneously firing versus non-spontaneously-firing interneurons. The latter were shown to possess some adaptive properties that differed from those of motoneurons, including a delayed initial adaptation and more predominant reversal of adaptation attributable to plateau potentials. Issues were raised concerning the interpretation of changes in the action potentials' afterhyperpolarization shape parameters throughout spike-frequency adaptation. No important differences were demonstrated in the adaptation of the two motoneuron and two interneuron groups. Each of these groups, however, was modeled by its own unique combination of action potential shape parameters for the simulation of its 30-s duration of spike-frequency adaptation. Also, for a small sample of the very highest-threshold versus lowest-threshold motoneurons, the former group had significantly more adaptation than the latter. This finding was like that shown previously for cat motoneurons supplying fast- versus slow twitch motor units.

Ionic current model of a hypoglossal motoneuron

We have developed a single-compartment, electrophysiological, hypoglossal motoneuron (HM) model based primarily on experimental data from neonatal rat HMs. The model is able to reproduce the fine features of the HM action potential: the fast afterhyperpolarization, the afterdepolarization, and the medium-duration afterhyperpolarization (mAHP). The model also reproduces the repetitive firing properties seen in neonatal HMs and replicates the neuron's response to pharmacological experiments. The model was used to study the role of specific ionic currents in HM firing and how variations in the densities of these currents may account for age-dependent changes in excitability seen in HMs. By varying the density of a fast inactivating calcium current, the model alternates between accelerating and adapting firing patterns. Modeling the age-dependent increase in H current density accounts for the decrease in mAHP duration observed experimentally, but does not fully account for the decrease in input resistance. An increase in the density of the voltage-dependent potassium currents and the H current is required to account for the decrease in input resistance. These changes also account for the age-dependent decrease in action potential duration.

Contribution of Persistent Sodium Currents to Spike-Frequency Adaptation in Rat Hypoglossal Motoneurons

In response to constant current inputs, the firing rates of motoneurons typically show a continuous decline over time. The biophysical mechanisms underlying this process, called spike-frequency adaptation, are not well understood. Spike-frequency adaptation normally exhibits a rapid initial phase, followed by a slow, later phase that continues throughout the duration of firing. One possible mechanism mediating the later phase might be a reduction in the persistent sodium current ( INaP) that has been shown to diminish the capacity of cortical pyramidal neurons and spinal motoneurons to sustain repetitive firing. In this study, we used the anticonvulsant phenytoin to reduce the INaP of juvenile rat hypoglossal motoneurons recorded in brain stem slices, and we examined the consequences of a reduction in INaP on the magnitude and time course of spike-frequency adaptation. Adding phenytoin to the bathing solution (≥50 μM) generally produced a marked reduction in the persistent inward currents (PICs) recorded at the soma in response to slow, voltage-clamp triangular ramp commands (−70 to 0 mV and back). However, the same concentrations of phenytoin appeared to have no significant effect on spike-frequency adaptation even though the phenytoin often augmented the reduction in action potential amplitude that occurs during repetitive firing. The surprising finding that the reduction of a source of sustained inward current had no appreciable effect on the pattern of spike generation suggests that several types of membrane channels must act cooperatively to insure that these motoneurons can generate the sustained repetitive firing required for long-lasting motor behaviors.

Preceding muscle activity influences motor unit discharge and rate of torque development during ballistic contractions in humans

To investigate the effect of initial conditions on the modulation of motor unit discharge during fast voluntary contractions, we compared ballistic isometric contractions of the ankle dorsiflexor muscles that were produced from either a resting state or superimposed on a sustained contraction. The torque of the dorsiflexors and the surface and intramuscular EMGs from the tibialis anterior were recorded. The results showed that the performance of a ballistic contraction from a sustained contraction ( approximately 25% maximal voluntary contraction (MVC)) had a negative effect on the maximal rate of torque development. Although the electromechanical delay was shortened, the EMG activity during the ballistic contraction was less synchronized. These observations were associated with a significant decline in the average discharge rate of single motor units (89.8 +/- 3.8 versus 115 +/- 5.8 Hz) and in the percentage of units (6.2 versus 15.5% of the whole sample) that exhibited double discharges at brief intervals (= 5 ms). High-threshold units that were not recruited during the sustained contraction displayed the same activation pattern, which indicates that the mechanisms responsible for the decline in discharge rate were not restricted to previously activated units, but appear to influence the entire motor unit pool. When a premotor silent period (SP) was observed at the transition from the sustained muscular activity to the ballistic contraction (19% of the trials), these adjustments in motor unit activity were not present, and the ballistic contractions were similar to those performed from a resting state. Together, these results indicate that initial conditions can influence the capacity for motor unit discharge rate and hence the performance of a fast voluntary contraction.

Contributions of the input signal and prior activation history to the discharge behaviour of rat motoneurones

The principal computational operation of neurones is the transformation of synaptic inputs into spike train outputs. The probability of spike occurrence in neurones is determined by the time course and magnitude of the total current reaching the spike initiation zone. The features of this current that are most effective in evoking spikes can be determined by injecting a Gaussian current waveform into a neurone and using spike-triggered reverse correlation to calculate the average current trajectory (ACT) preceding spikes. The time course of this ACT (and the related first-order Wiener kernel) provides a general description of a neurone's response to dynamic stimuli. In many different neurones, the ACT is characterized by a shallow hyperpolarizing trough followed by a more rapid depolarizing peak immediately preceding the spike. The hyperpolarizing phase is thought to reflect an enhancement of excitability by partial removal of sodium inactivation. Alternatively, this feature could simply reflect the fact that interspike intervals that are longer than average can only occur when the current is lower than average toward the end of the interspike interval. Thus, the ACT calculated for the entire spike train displays an attenuated version of the hyperpolarizing trough associated with the long interspike intervals. This alternative explanation for the characteristic shape of the ACT implies that it depends upon the time since the previous spike, i.e. the ACT reflects both previous stimulus history and previous discharge history. The present study presents results based on recordings of noise-driven discharge in rat hypoglossal motoneurones that support this alternative explanation. First, we show that the hyperpolarizing trough is larger in ACTs calculated from spikes preceded by long interspike intervals, and minimal or absent in those based on short interspike intervals. Second, we show that the trough is present for ACTs calculated from the discharge of a threshold-crossing neurone model with a postspike afterhyperpolarization (AHP), but absent from those calculated from the discharge of a model without an AHP. We show that it is possible to represent noise-driven discharge using a two-component linear model that predicts discharge probability based on the sum of a feedback kernel and a stimulus kernel. The feedback kernel reflects the influence of prior discharge mediated by the AHP, and it increases in amplitude when AHP amplitude is increased by pharmacological manipulations. Finally, we show that the predictions of this model are virtually identical to those based on the first-order Wiener kernel. This suggests that the Wiener kernels derived from standard white-noise analysis of noise-driven discharge in neurones actually reflect the effects of both stimulus and discharge history.

Afterhyperpolarization-firing rate relation of turtle spinal neurons

This study addressed the afterhyperploarization-firing rate relationship of unanesthetized turtle spinal motoneurons and interneurons. The afterhyperploarization of their solitary action potential at rheobase was compared to that during the cells' minimum and maximum firing rates. Like previous mammalian findings, afterhyperpolarization duration and area at rheobase were 32 and 19% less for high- versus low-threshold motoneurons. Contrariwise, maximum firing rate was two times less for the high-threshold group. Other new findings were that for high- versus low-threshold interneurons, afterhyperpolarization duration and area were 25 and 95% less, and maximum firing rate 21% higher for the high-threshold group. For combined motoneurons versus interneurons, there were no differences in afterhyperpolarization duration and area at rheobase, whereas maximum firing rate was 265% higher for the interneurons. For high-threshold motoneurons alone, there were significant associations between minimum firing rate and afterhyperpolarization duration and area measured at rheobase. In summary, this study showed that (1) the afterhyperploarization values of both turtle spinal motoneurons and interneurons at rheobase provided little indication of their corresponding values at the cells' minimum and maximum firing states, and (2) the evolution of afterhyperploarization from rheobase to maximum firing state differed both qualitatively and quantitatively for motoneurons versus interneurons.

Single-neuron discharge properties and network activity in dissociated cultures of neocortex

Cultures of neurons from rat neocortex exhibit spontaneous, temporally patterned, network activity. Such a distributed activity in vitro constitutes a possible framework for combining theoretical and experimental approaches, linking the single-neuron discharge properties to network phenomena. In this work, we addressed the issue of closing the loop, from the identification of the single-cell discharge properties to the prediction of collective network phenomena. Thus, we compared these predictions with the spontaneously emerging network activity in vitro, detected by substrate arrays of microelectrodes. Therefore, we characterized the single-cell discharge properties to Gauss-distributed noisy currents, under pharmacological blockade of the synaptic transmission. Such stochastic currents emulate a realistic input from the network. The mean (m) and variance (s(2)) of the injected current were varied independently, reminiscent of the extended mean-field description of a variety of possible presynaptic network organizations and mean activity levels, and the neuronal response was evaluated in terms of the steady-state mean firing rate (f). Experimental current-to-spike-rate responses f(m, s(2)) were similar to those of neurons in brain slices, and could be quantitatively described by leaky integrate-and-fire (IF) point neurons. The identified model parameters were then used in numerical simulations of a network of IF neurons. Such a network reproduced a collective activity, matching the spontaneous irregular population bursting, observed in cultured networks. We finally interpret such a collective activity and its link with model details by the mean-field theory. We conclude that the IF model is an adequate minimal description of synaptic integration and neuronal excitability, when collective network activities are considered in vitro.

Responses of human motoneurons to corticospinal stimulation during maximal voluntary contractions and ischemia

The discharge frequency of human motoneurons declines during a sustained isometric maximal voluntary contraction (MVC) of elbow flexor muscles, but the cause is unresolved. We aimed to determine whether motoneurons were inhibited during a sustained fatiguing contraction of the elbow flexor muscles and whether this inhibition was caused by the discharge of group III and IV muscle afferents. Subjects performed brief MVCs before and after a fatiguing 2 min MVC. During maximal efforts, electromyographic responses recorded from the elbow flexor muscles were evoked by stimulation of the corticospinal tracts at the cervicomedullary level [cervicomedullary motor evoked potentials (CMEPs)] and by supramaximal stimulation over the brachial plexus (Mmax). This revealed a novel decrease in the size of the muscle response to corticospinal tract stimulation during fatigue. During the sustained MVCs, the size of CMEPs decreased to 81 +/- 15 and 78 +/- 15% of the control value for brachioradialis and biceps brachii, respectively (mean +/- SEM; n = 8). This recovered within 15 sec after the fatiguing contraction. In a second set of studies, input from group III and IV muscle afferents was prolonged after the end of the fatiguing contraction by holding the muscle ischemic with a cuff inflated above arterial pressure. Despite the maintained discharge of group III and IV afferents, the CMEPs again recovered within 15 sec of the end of the sustained contraction. These results show a diminished output of spinal motoneurons to stimulation of corticospinal tracts during a fatiguing MVC; however, the mechanisms responsible for this decline are not attributable to activity in group III and IV muscle afferents.

Neocortical pyramidal cells respond as integrate-and-fire neurons to in vivo-like input currents

In the intact brain neurons are constantly exposed to intense synaptic activity. This heavy barrage of excitatory and inhibitory inputs was recreated in vitro by injecting a noisy current, generated as an Ornstein-Uhlenbeck process, into the soma of rat neocortical pyramidal cells. The response to such in vivo-like currents was studied systematically by analyzing the time development of the instantaneous spike frequency, and when possible, the stationary mean spike frequency as a function of both the mean and the variance of the input current. All cells responded with an in vivo-like action potential activity with stationary statistics that could be sustained throughout long stimulation intervals (tens of seconds), provided the frequencies were not too high. The temporal evolution of the response revealed the presence of mechanisms of fast and slow spike frequency adaptation, and a medium duration mechanism of facilitation. For strong input currents, the slow adaptation mechanism made the spike frequency response nonstationary. The minimal frequencies that caused strong slow adaptation (a decrease in the spike rate by more than 1 Hz/s), were in the range 30-80 Hz and depended on the pipette solution used. The stationary response function has been fitted by two simple models of integrate-and-fire neurons endowed with a frequency-dependent modification of the input current. This accounts for all the fast and slow mechanisms of adaptation and facilitation that determine the stationary response, and proved necessary to fit the model to the experimental data. The coefficient of variability of the interspike interval was also in part captured by the model neurons, by tuning the parameters of the model to match the mean spike frequencies only. We conclude that the integrate-and-fire model with spike-frequency-dependent adaptation/facilitation is an adequate model reduction of cortical cells when the mean spike-frequency response to in vivo-like currents with stationary statistics is considered.

Spike frequency adaptation and signaling properties of identified neurons in rodent deep spinal dorsal horn

Using whole cell recordings, I analyzed the intrinsic discharge properties for 285 neurons in Rexed's laminae III-V of isolated hamster spinal cord preparations. Neurons were characterized by their responses to step-wise and ramp-hold depolarizing current applied through the recording pipettes. Tonic cells (133/285; 47%) fired repetitively during step-wise current application. Firing decayed linearly (-0.14 to -4.3 imp . s(-1) . s(-1)) or was bimodal, with an initial exponential phase (tau approximately 450 ms) followed by a linear decline (-0.02 to -6.3 imp . s(-1) . s(-1)); discharge frequency was unrelated to current trajectory. Phasic-firing cells (108/285; 38%) responded with a burst discharge having an initial rapid, exponential decrease (tau approximately 30 ms) and subsequent linear decline (-1 to -78 imp . s(-1) . s(-1)). Phasic cells were activated preferentially by fast current ramps (slope, 70 pA/s-2.2 nA/s) with the number and frequency of impulses increasing with current slope. Delayed-firing cells (44/285; 15%), responded to current steps with an accelerating firing following a substantial latent period (0.5-4 s) and discharged during current ramps with slopes less than approximately 100 pA/s. Intracellular staining revealed a significant association between electrophysiological profile and neuronal morphology. A majority of presumed projection cells (22/30; 73%) exhibited tonic firing to step-wise activation. The preponderance of phasic and delayed firing cells, 93% (42/45) and 71% (12/17), respectively, were interneurons with local or intersegmental terminations. Differential sensitivity to static and time-varying components of membrane current suggest differences in neuronal signaling properties that may have important implications for integration of mechanosensory information in the deep spinal dorsal horn.

Sustaining a constant effort by the tongue and hand: effects of acute fatigue

A constant-effort task was used previously as a potential assessment technique for fatigue. Participants sustained submaximal target effort levels with the tongue and hand against soft air-filled bulbs. For 80% of all trials, pressure decreased exponentially to a positive asymptote. In addition, pressure decreased faster when the muscles were fatigued than when they were rested. This study attempted to replicate the previous findings with new participants and to extend the findings to include surface electromyographic (EMG) data. Pressure and surface EMG signals were collected simultaneously while 10 neurologically normal young adults performed the constant-effort task at 50% of maximum pressure with the tongue and the hand. Eighty-one percent of the pressure data were modeled by a negative exponential equation with a nonzero asymptote. Seventy-three percent of the corresponding EMG data also fit this mathematical model. The pressure signals decayed more slowly than the corresponding EMG signals, particularly for the hand. After participants fatigued the tongue and hand with repeated brief maximal voluntary contractions, the time constants were reduced (rate of decay increased) for the tongue but not the hand. These results corroborate the previous finding that the time constant, determined from an exponential curve-fitting procedure, is a replicable measure. Furthermore, the reduction in the time constant after inducing acute fatigue in the tongue was replicated, although this same relationship was not replicated for the hand. The EMG data suggest that decreases in neuromuscular drive, including increased early adaptation, motor unit derecruitment, and motor unit desynchronization, contributed to the decrease in pressure during the constant-effort task, especially after acute fatigue was induced. These observations support the hypothesis that the task reflects, at least in part, central fatigue processes.