How different afferent inputs control motoneuron discharge and the output of the motoneuron pool

Neuromodulation reduces interindividual variability of neuronal output

In similar states, neural circuits produce similar outputs across individuals despite substantial interindividual variability in neuronal ionic conductances and synapses. Circuit states are largely shaped by neuromodulators that tune ionic conductances. It is therefore possible that, in addition to producing flexible circuit output, neuromodulators also contribute to output similarity despite varying ion channel expression. We studied whether neuromodulation at saturating concentrations can increase the output similarity of a single identified neuron across individual animals. Using the LP neuron of the crab stomatogastric ganglion (STG), we compared the variability of f-I curves and rebound properties in the presence of neuropeptides. The two neuropeptides we used converge to activate the same target current, which increases neuronal excitability. Output variability was lower in the presence of the neuropeptides, regardless of whether the neuropeptides significantly changed the mean of the corresponding parameter or not. However, the addition of the second neuropeptide did not add further to the reduction of variability. With a family of computational LP-like models, we explored how increased excitability and target variability contribute to output similarity and found two mechanisms: Saturation of the responses and a differential increase in baseline activity. Saturation alone can reduce the interindividual variability only if the population shares a similar ceiling for the responses. In contrast, reduction of variability due to the increase in baseline activity is independent of ceiling effects.Significance StatementThe activity of single neurons and neural circuits can be very similar across individuals even though the ionic currents underlying activity are variable. The mechanisms that compensate for the underlying variability and promote output similarity are poorly understood but may involve neuromodulation. Using an identified neuron, we show that neuropeptide modulation of excitability can reduce interindividual variability of response properties at a single-neuron level in two ways. First, the neuropeptide increases baseline excitability in a differential manner, resulting in similar response thresholds. Second, the neuropeptide increases excitability towards a shared saturation level, promoting similar maximal firing rates across individuals. Such tuning of neuronal excitability could be an important mechanism compensating for interindividual variability of ion channel expression.

Ia EPSPs in rat spinal motoneurons are potentiated after a 5-week whole-body vibration

Whole-body vibration (WBV) is often applied as an alternative method for strength training or to prevent muscle force decrease. Previous studies indicated that WBV induced: 1) changes in the contractile parameters predominantly of fast motor units, 2) higher motoneuron excitability, and 3) higher motoneuron firing rates at lower stimulus intensities compared with the control. In this study, we evaluated the influence of WBV on Ia monosynaptic input from muscle spindles because the tonic vibration reflex is responsible for the enhancement of muscle activity observed after WBV. The aim was to answer the question of whether repeated activation of muscle spindles during WBV may result in altered synaptic excitation of motoneurons. WBV was performed on adult male Wistar rats, 5 days per week, for 5 weeks, and each daily session consisted of four 30-s runs of vibration at 50 Hz. Fast-type medial gastrocnemius motoneurons were investigated intracellularly in deeply anesthetized animals in the experimental (n=7, 34 motoneurons) and control (n=7, 32 motoneurons) groups. Monosynaptic Ia EPSPs were evoked by electrical stimulation of afferent fibers from the synergistic lateral gastrocnemius and soleus muscles. Data were analyzed using a mixed linear model. WBV induced an increase of the mean EPSP amplitude by 28% (P=0.025), correlated with the resting membrane potential and input resistance, and a shortening of the mean EPSP rise time by 11% (P=0.012). The potentiation of synaptic excitation of motoneurons indicates that WBV may support rehabilitation or training processes aimed at increasing muscle strength on the basis of increased motoneuronal drive.

Non-uniform Effects of Nociceptive Stimulation to Motoneurones during Experimental Muscle Pain

Nociceptive stimulation is predicted to uniformly inhibit motoneurone pools of painful muscles and those producing painful movements. Although reduced motoneurone discharge rate during pain provides some evidence, recent data show evidence of increased excitability of some motoneurones. These observations suggest non-uniform effects of nociception on motoneurone excitability. More direct measures are required, but this is difficult to assess as few measures enable in vivo evaluation of motoneurone excitability in humans. We investigated changes in motoneurone excitability during experimental pain using two methods in separate experiments: (i) estimation of the time-course of motoneurone afterhyperpolarization (AHP) from interval death rate analysis of interspike intervals of single motor unit discharge; and (ii) probability of early motoneurone discharge to a descending volley excited using transcranial magnetic stimulation (TMS). Tibialis anterior motor units were recorded with fine-wire electrodes before, during and after painful infusion of 5% hypertonic saline into the muscle. Activation of 17 units (16 participants) could be used for AHP analysis. Data show shortened (n = 11) and lengthened (n = 6) AHP time-course. Increased (n = 6) and decreased (n = 6) probability of early motoneurone discharge were observed in the TMS experiment. These convergent observations suggest non-uniform effects of nociceptive stimulation on motoneurone pools. This does not support the hypothesis that nociceptive input induces uniform inhibition of painful muscle. Instead, interpretation of results implies redistribution of activity between motor units, with possible benefit for unloading painful tissues. This finding supports an interpretation that differs from the generally accepted view in pain physiology regarding adaptation to motor function in pain.

Synaptic restoration by cAMP/PKA drives activity-dependent neuroprotection to motoneurons in ALS

Excessive excitation is hypothesized to cause motoneuron (MN) degeneration in amyotrophic lateral sclerosis (ALS), but actual proof of hyperexcitation in vivo is missing, and trials based on this concept have failed. We demonstrate, by in vivo single-MN electrophysiology, that, contrary to expectations, excitatory responses evoked by sensory and brainstem inputs are reduced in MNs of presymptomatic mutSOD1 mice. This impairment correlates with disrupted postsynaptic clustering of Homer1b, Shank, and AMPAR subunits. Synaptic restoration can be achieved by activation of the cAMP/PKA pathway, by either intracellular injection of cAMP or DREADD-Gs stimulation. Furthermore, we reveal, through independent control of signaling and excitability allowed by multiplexed DREADD/PSAM chemogenetics, that PKA-induced restoration of synapses triggers an excitation-dependent decrease in misfolded SOD1 burden and autophagy overload. In turn, increased MN excitability contributes to restoring synaptic structures. Thus, the decrease of excitation to MN is an early but reversible event in ALS. Failure of the postsynaptic site, rather than hyperexcitation, drives disease pathobiochemistry.

Glutamatergic input varies with phrenic motor neuron size

Like all skeletal muscles, the diaphragm muscle accomplishes a range of motor behaviors by recruiting different motor unit types in an orderly fashion. Recruitment of phrenic motor neurons (PhMNs) is generally assumed to be based primarily on the intrinsic properties of PhMNs with an equal distribution of descending excitatory inputs to all PhMNs. However, differences in presynaptic excitatory input across PhMNs of varying sizes could also contribute to the orderly recruitment pattern. In the spinal cord of Sprague-Dawley rats, we retrogradely labeled PhMNs using cholera toxin B (CTB) and validated a robust confocal imaging-based technique that utilizes semiautomated processing to identify presynaptic glutamatergic (Glu) terminals within a defined distance around the somal membrane of PhMNs of varying size. Our results revealed an ~10% higher density of Glu terminals at PhMNs in the lower tertile of somal surface area. These smaller PhMNs are likely recruited first to accomplish lower force ventilatory behaviors of the diaphragm as compared with larger PhMNs in the upper tertile that are recruited to accomplish higher force expulsive behaviors. These results suggest that differences in excitatory synaptic input to PhMNs may also contribute to the orderly recruitment of diaphragm motor units.NEW & NOTEWORTHY The distribution of excitatory glutamatergic synaptic input to phrenic motor neurons differs across motor neurons of varying size. These findings support the size principle of motor unit recruitment that underlies graded force generation in a muscle, which is based on intrinsic electrophysiological properties of motor neurons resulting from differences in somal surface area. A higher density of glutamatergic inputs at smaller, more excitable motor neurons substantiates the earlier and more frequent recruitment of these units.

Experimental investigations of control principles of involuntary movement: a comprehensive review of the Kohnstamm phenomenon

The Kohnstamm phenomenon refers to the observation that if one pushes the arm hard outwards against a fixed surface for about 30 s, and then moves away from the surface and relaxes, an involuntary movement of the arm occurs, accompanied by a feeling of lightness. Central, peripheral and hybrid theories of the Kohnstamm phenomenon have been advanced. Afferent signals may be irrelevant if purely central theories hold. Alternatively, according to peripheral accounts, altered afferent signalling actually drives the involuntary movement. Hybrid theories suggest afferent signals control a centrally-programmed aftercontraction via negative position feedback control or positive force feedback control. The Kohnstamm phenomenon has provided an important scientific method for comparing voluntary with involuntary movement, both with respect to subjective experience, and for investigating whether involuntary movements can be brought under voluntary control. A full review of the literature reveals that a hybrid model best explains the Kohnstamm phenomenon. On this model, a central adaptation interacts with afferent signals at multiple levels of the motor hierarchy. The model assumes that a Kohnstamm generator sends output via the same pathways as voluntary movement, yet the resulting movement feels involuntary due to a lack of an efference copy to cancel against sensory inflow. This organisation suggests the Kohnstamm phenomenon could represent an amplification of neuromotor processes normally involved in automatic postural maintenance. Future work should determine which afferent signals contribute to the Kohnstamm phenomenon, the location of the Kohnstamm generator, and the principle of feedback control operating during the aftercontraction.

The potential for understanding the synaptic organization of human motor commands via the firing patterns of motoneurons

Motoneurons are unique in being the only neurons in the CNS whose firing patterns can be easily recorded in human subjects. This is because of the one-to-one relationship between the motoneuron and muscle cell behavior. It has long been appreciated that the connection of motoneurons to their muscle fibers allows their action potentials to be amplified and recorded, but only recently has it become possible to simultaneously record the firing pattern of many motoneurons via array electrodes placed on the skin. These firing patterns contain detailed information about the synaptic organization of motor commands to the motoneurons. This review focuses on parameters in these firing patterns that are directly linked to specific features of this organization. It is now well established that motor commands consist of three components, excitation, inhibition, and neuromodulation; the importance of the third component has become increasingly evident. Firing parameters linked to each of the three components are discussed, along with consideration of potential limitations in their utility for understanding the underlying organization of motor commands. Future work based on realistic computer simulations of motoneurons may allow quantitative "reverse engineering" of human motoneuron firing patterns to provide good estimates of the relative amplitudes and temporal patterns of all three components of motor commands.

Posture interacts with arm weight support to modulate corticomotor excitability to the upper limb

The use of arm weight support (WS) to optimize movement quality may be an avenue for improved upper limb stroke rehabilitation; however, the underlying neurophysiological effects of WS are not well understood. Rehabilitation exercises may be performed when sitting or standing, but the interaction of posture with WS has not been examined until now. We explored the effect of posture with WS on corticomotor excitability (CME) in healthy adults. Thirteen participants performed static shoulder abduction in two postures (sitting and standing) at three levels of WS (0, 45, and 90 % of full support). Transcranial magnetic stimulation of primary motor cortex was used to elicit motor-evoked potentials (MEPs) in eight upper limb muscles. Stimulus-response (SR) curves were fitted to the MEP data using nonlinear regression. Whole-body posture interacted with WS to influence tonic activity and CME in all muscles examined. SR curve parameters revealed greater CME when standing compared to sitting for upper arm muscles, but lower CME to the shoulder, forearm, and hand. Distal to the shoulder, tonic activity and CME were modulated independent of any explicit differences in task requirements. Overall, these results support a model of integrated upper limb control influenced by whole-body posture and WS. These findings have implications for the application of WS in settings such as upper limb rehabilitation after stroke.

Intrinsic excitability of human motoneurons in biceps brachii versus triceps brachii

The intrinsic excitability of spinal motoneurons is mediated in part by the presence of persistent inward currents (PICs), which amplify synaptic input and promote self-sustained firing. Studies using animal models have shown that PICs are greater in extensor motoneurons over flexor motoneurons, but this difference has not yet been demonstrated in humans. The primary objective of this study was to determine whether a similar difference exists in humans by recording from motor units in biceps and triceps brachii during isometric contractions. We compared firing rate profiles of pairs of motor units, in which the firing rate of the lower-threshold "control" unit was used as an indicator of common drive to the higher-threshold "test" unit. The estimated contribution of the PIC was calculated as the difference in firing rate of the control unit at recruitment versus derecruitment of the test unit, a value known as the delta-F (ΔF). We found that ΔF values were significantly higher in triceps brachii (5.4 ± 0.9 imp/s) compared with biceps brachii (3.0 ± 1.4 imp/s; P < 0.001). This difference was still present even after controlling for saturation in firing rate of the control unit, rate modulation of the control unit, and differences in recruitment time between test and control units, which are known to contribute to ΔF variability. We conclude that human elbow flexor and extensor motor units exhibit differences in intrinsic excitability, contributing to different neural motor control strategies between muscle groups.

Intrinsic electrical properties of mammalian neurons and CNS function: a historical perspective

This brief review summarizes work done in mammalian neuroscience concerning the intrinsic electrophysiological properties of four neuronal types; Cerebellar Purkinje cells, inferior olivary cells, thalamic cells, and some cortical interneurons. It is a personal perspective addressing an interesting time in neuroscience when the reflex view of brain function, as the paradigm to understand global neuroscience, began to be modified toward one in which sensory input modulates rather than dictates brain function. The perspective of the paper is not a comprehensive description of the intrinsic electrical properties of all nerve cells but rather addresses a set of cell types that provide indicative examples of mechanisms that modulate brain function.

Modulation of the input-output function by GABAA receptor-mediated currents in rat oculomotor nucleus motoneurons

The neuronal input-output function depends on recruitment threshold and gain of the firing frequency-current (f-I) relationship. These two parameters are positively correlated in ocular motoneurons (MNs) recorded in alert preparation and inhibitory inputs could contribute to this correlation. Phasic inhibition mediated by γ-amino butyric acid (GABA) occurs when a high concentration of GABA at the synaptic cleft activates postsynaptic GABAA receptors, allowing neuronal information transfer. In some neuronal populations, low concentrations of GABA activate non-synaptic GABAA receptors and generate a tonic inhibition, which modulates cell excitability. This study determined how ambient GABA concentrations modulate the input-output relationship of rat oculomotor nucleus MNs. Superfusion of brain slices with GABA (100 μm) produced a GABAA receptor-mediated current that reduced the input resistance, increased the recruitment threshold and shifted the f-I relationship rightward without any change in gain. These modifications did not depend on MN size. In absence of exogenous GABA, gabazine (20 μm; antagonist of GABAA receptors) abolished spontaneous inhibitory postsynaptic currents and revealed a tonic current in MNs. Gabazine increased input resistance and decreased recruitment threshold mainly in larger MNs. The f-I relationship shifted to the left, without any change in gain. Gabazine effects were chiefly due to MN tonic inhibition because tonic current amplitude was five-fold greater than phasic. This study demonstrates a tonic inhibition in ocular MNs that modulates cell excitability depending on cell size. We suggest that GABAA tonic inhibition acting concurrently with glutamate receptors activation could reproduce the positive covariation between threshold and gain reported in alert preparation.

Glutamate modulates the firing rate in oculomotor nucleus motoneurons as a function of the recruitment threshold current

Studies in alert preparations have demonstrated that ocular motoneurons exhibit a phasic–tonic firing rate related to eye velocity and position, respectively. The slopes of these relationships are higher in motoneurons with higher recruitment threshold and have been proposed to depend upon synaptic input. To investigate this hypothesis, motoneurons of the rat oculomotor nucleus were recorded in a brain slice preparation in control conditions and during glutamate (5 μm) application to the bath. Glutamate did not affect membrane potential or input resistance, but produced a decrease in rheobase and depolarization voltage as a function of the current needed for generating a maintained repetitive discharge (recruitment threshold current). In addition, glutamate compressed the range of recruitment threshold current (0.1–0.4 nA) as compared to the control (0.15–0.7 nA). Glutamate exposed motoneurons showed an increase in the tonic frequency gain and the peak frequency. Such increments depended on the recruitment threshold current and the last recruited motoneurons almost doubled the tonic frequency gain (35.2 vs. 57.9 spikes s(−1) nA(−1)) and the peak frequency (52.4 vs. 102.6 spikes s(−1)). Finally, glutamate increased the spike frequency adaptation due to a significant increase in the phasic firing component as compared to the tonic one. In conclusion, glutamate modulates tonic and phasic discharge properties as a function of the recruitment threshold current and, presumably, motoneuron size. These findings contribute to understand the link between cellular functions and motoneuron discharge during oculomotor behaviour.

Changes in the Estimated Time Course of the Motoneuron Afterhyperpolarization Induced by Tendon Vibration

Group Ia afferents are activated vigorously with high-frequency tendon vibration and provide excitatory input to the agonist muscle and inhibitory input to the antagonist muscle group via inhibitory interneurons. The purpose of this experiment was to determine whether the afterhyperpolarization (AHP) time course in humans is altered in response to tendon vibration. The AHP time course is estimated using the interval death rate (IDR) analysis, a transform of the motor unit action potential train. Single motor units from tibialis anterior (TA) were recorded as subjects held low force dorsiflexor contractions for 600 s with and without vibration. The vibratory stimulus was superimposed on the low force contraction either to the tendon of the TA or the antagonist Achilles tendon. During TA tendon vibration, the time course of the AHP, as expressed by its time constant (τ), decreased from 35.5 ms in the previbration control condition to 31.3 ms during the vibration ( P = 0.003) and returned to 36.3 ms after the vibration was removed ( P = 0.002). The AHP τ during vibration of the antagonist Achilles tendon (38.6 ms) was greater than the previbration control condition (33.6 ms; P = 0.001). It is speculated that the reduction in AHP time constant with TA vibration may have resulted alone or in combination with a modulation of motoneuron gain, an alteration of persistent inward currents and/or the restructuring of synaptic noise. A decrease in firing probability, possibly reflecting Ia reciprocal inhibition, may have been responsible for the larger AHP time constant.

Oxytocin and vasopressin enhance synaptic transmission in the hypoglossal motor nucleus of young rats by acting on distinct receptor types

Hypoglossal (XII) motoneurons innervate extrinsic and intrinsic muscles of the tongue and control behaviors such as suckling, swallowing, breathing or chewing. In young rats, XII motoneurons express V1a vasopressin and oxytocin receptors. Previous studies have shown that activation of these receptors induces direct powerful excitation in XII motoneurons. In addition, by activating V1a receptors vasopressin can also enhance inhibitory synaptic transmission in the XII nucleus. In the present work, we have further characterized the effect of these neuropeptides on synaptic transmission in the XII nucleus. We have used brainstem slices of young rats and whole-cell patch clamp recordings. Oxytocin enhanced the frequency of spontaneous inhibitory postsynaptic currents by a factor of two and a half. GABAergic and glycinergic events were both affected. The oxytocin effect was mediated by uterine-type oxytocin receptors. Vasopressin and oxytocin also increased the frequency of excitatory synaptic currents, the enhancement being sixfold for the former and twofold for the latter compound. These effects were mediated by V1a and oxytocin receptors, respectively. Miniature synaptic events were unaffected by either vasopressin or oxytocin. This indicates that the peptide-dependent facilitation of synaptic currents was mediated by receptors located on the somatodendritic membrane of interneurons or premotor neurons, and not by receptors sited on axon terminals contacting XII motoneurons. Accordingly, recordings obtained from non-motoneurons located near the border of the XII nucleus showed that part of these cells possess functional V1a and oxytocin receptors whose activation leads to excitation. Some of these neurons could be antidromically activated following electrical stimulation of the XII nucleus, suggesting that they may act as premotor neurons. We propose that in young rats, oxytocin and vasopressin may function as neuromodulators in brainstem motor circuits responsible of tongue movements.

Recruitment in retractor bulbi muscle during eyeblink conditioning: EMG analysis and common-drive model

To analyze properly the role of the cerebellum in classical conditioning of the eyeblink and nictitating membrane (NM) response, the control of conditioned response dynamics must be better understood. Previous studies have suggested that the control signal is linearly related to the CR as a result of recruitment within the accessory abducens motoneuron pool, which acts to linearize retractor bulbi muscle and NM response mechanics. Here we investigate possible recruitment mechanisms. Data came from simultaneous recordings of NM position and multiunit electromyographic (EMG) activity from the retractor bulbi muscle of rabbits during eyeblink conditioning, in which tone and periocular shock act as conditional and unconditional stimuli, respectively. Action potentials (spikes) were extracted and classified by amplitude. Firing rates of spikes with different amplitudes were analyzed with respect to NM response temporal profiles and total EMG spike firing rate. Four main regularities were revealed and quantified: 1) spike amplitude increased with response amplitude; 2) smaller spikes always appeared before larger spikes; 3) subsequent firing rates covaried for spikes of different amplitude, with smaller spikes always firing at higher rates than larger ones; and 4) firing-rate profiles were approximately Gaussian for all amplitudes. These regularities suggest that recruitment does take place in the retractor bulbi muscle during conditioned NM responses and that all motoneurons receive the same command signal (common-drive hypothesis). To test this hypothesis, a model of the motoneuron pool was constructed in which motoneurons had a range of intrinsic thresholds distributed exponentially, with threshold linearly related to EMG spike amplitude. Each neuron received the same input signal as required by the common-drive assumption. This simple model reproduced the main features of the data, suggesting that conditioned NM responses are controlled by a common-drive mechanism that enables simple commands to determine response topography in a linear fashion.

Muscarinic modulation of recruitment threshold and firing rate in rat oculomotor nucleus motoneurons

Above recruitment threshold, ocular motoneurons (Mns) show a firing rate linearly related with eye position. Current hypothesis suggests that synaptic inputs are determinant for establishing the recruitment threshold and firing rate gain in these Mns. We investigated this proposal by studying the cholinergic modulation in oculomotor nucleus Mns by intracellular recordings in rat brain slice preparation. All recorded Mns were silent at their resting membrane potential. Bath application of carbachol (10 microm) produced a depolarization and a sustained firing that was not silenced on returning membrane potential to the precarbachol value via DC injection. In response to similar membrane depolarization or equal-current steps, carbachol-exposed Mns produced a higher firing rate and a shorter spike afterhyperpolarization phase with lower amplitude. The relationship between injected current and firing rate (I-F) was linear in control and carbachol-exposed Mns. The slope of these relationships (I-F gain) decreased with carbachol exposure. Bath application of agonist and antagonist of nicotinic and muscarinic acetylcholine receptors in addition to immunohistochemical studies support the notion that muscarinic receptors are primarily involved in the preceding responses. We conclude that muscarinic inputs play an important role in determining the recruitment threshold and firing rate gain observed in oculomotor Mns in vivo.

Effects of the physiological parameters on the signal-to-noise ratio of single myoelectric channel

Background

An important measure of the performance of a myoelectric (ME) control system for powered artificial limbs is the signal-to-noise ratio (SNR) at the output of ME channel. However, few studies illustrated the neuron-muscular interactive effects on the SNR at ME control channel output. In order to obtain a comprehensive understanding on the relationship between the physiology of individual motor unit and the ME control performance, this study investigates the effects of physiological factors on the SNR of single ME channel by an analytical and simulation approach, where the SNR is defined as the ratio of the mean squared value estimation at the channel output and the variance of the estimation.

Methods

Mathematical models are formulated based on three fundamental elements: a motoneuron firing mechanism, motor unit action potential (MUAP) module, and signal processor. Myoelectric signals of a motor unit are synthesized with different physiological parameters, and the corresponding SNR of single ME channel is numerically calculated. Effects of physiological multi factors on the SNR are investigated, including properties of the motoneuron, MUAP waveform, recruitment order, and firing pattern, etc.

Results

The results of the mathematical model, supported by simulation, indicate that the SNR of a single ME channel is associated with the voluntary contraction level. We showed that a model-based approach can provide insight into the key factors and bioprocess in ME control. The results of this modelling work can be potentially used in the improvement of ME control performance and for the training of amputees with powered prostheses.

Conclusion

The SNR of single ME channel is a force, neuronal and muscular property dependent parameter. The theoretical model provides possible guidance to enhance the SNR of ME channel by controlling physiological variables or conscious contraction level.

The excitability of lumbar motoneurones in the neonatal rat is increased by a hyperpolarization of their voltage threshold for activation by descending serotonergic fibres

Previous work has shown there is an increase in motoneurone excitability produced by hyperpolarization of the threshold potential at which an action potential is elicited (Vth) at the onset, and throughout brainstem-induced fictive locomotion in the decerebrate cat. This represents a transient facilitation in the membrane potential for activation dependent on the presence of fictive locomotion. The present study tests the hypothesis that a similar neuromodulatory mechanism facilitating neuronal recruitment also exists in the neonatal rat, and the endogenous pathway mediating the Vth hyperpolarization can be activated by electrical stimulation of the neonatal brainstem. Isolated brainstem-spinal cord preparations from 1- to 5-day-old neonatal rats, and whole-cell recording techniques were used to examine the patterns of ventral root (VR) activity produced, and the effect of electrical stimulation of the ventromedial medulla on lumbar spinal neurones. Hyperpolarization of Vth was seen in 10/11 (range -2 to -18 mV) neurones recorded during locomotor-like VR activity, and appeared analogous to the locomotor-dependent Vth hyperpolarization previously described in the cat. However, in the present study, Vth hyperpolarization was also seen during electrical brainstem stimulation that evoked alternating, rhythmic, or tonic VR activity, or failed to evoke VR activity. Thirty-six of 71 neurones were antidromically identified as lumbar motoneurones and 33/36 showed a hyperpolarization of Vth (-2 to -14 mV) during electrical brainstem stimulation. Of the unidentified lumbar ventral horn neurones, 31/35 also showed hyperpolarization of Vth (-2 to -20 mV) during brainstem stimulation. The hyperpolarization of Vth and VR activity induced by brainstem stimulation was reversibly blocked by cooling of the cervical cord, indicating it is mediated by descending fibres, and application of the serotonergic antagonist ketanserin to the spinal cord was effectively able to block the brainstem-evoked hyperpolarization of Vth. These results demonstrate a previously unknown action of the endogenous descending serotonergic system to facilitate spinal motoneuronal recruitment and firing by inducing a hyperpolarization of Vth. This modulatory process can be examined in the neonatal rat brainstem-spinal cord preparation without the requirement for ongoing locomotor activity.

Influence of active dendritic currents on input-output processing in spinal motoneurons in vivo

The extensive dendritic tree of the adult spinal motoneuron generates a powerful persistent inward current (PIC). We investigated how this dendritic PIC influenced conversion of synaptic input to rhythmic firing. A linearly increasing, predominantly excitatory synaptic input was generated in triceps ankle extensor motoneurons by slow stretch (duration: 2-10 s) of the Achilles tendon in the decerebrate cat preparation. The firing pattern evoked by stretch was measured by injecting a steady current to depolarize the cell to threshold for firing. The effective synaptic current (I(N), the net synaptic current reaching the soma of the cell) evoked by stretch was measured during voltage clamp. Hyperpolarized holding potentials were used to minimize the activation of the dendritic PIC and thus estimate stretch-evoked I(N) for a passive dendritic tree (I(N,PASS)). Depolarized holding potentials that approximated the average membrane potential during rhythmic firing allowed strong activation of the dendritic PIC and thus resulted in marked enhancement of the total stretch-evoked I(N) (I(N,TOT)). The net effect of the dendritic PIC on the generation of rhythmic firing was assessed by plotting stretch-evoked firing (strong PIC activation) versus stretch-evoked I(N,PASS) (minimal PIC activation). The gain of this input-output function for the neuron (I-O(N)) was found to be ~2.7 times as high as for the standard injected frequency current (F-I) function in low-input conductance neurons. However, about halfway through the stretch, firing rate tended to become constant, resulting in a sharp saturation in I-O(N) that was not present in F-I. In addition, the gain of I-O(N) decreased sharply with increasing input conductance, resulting in much lower stretch-evoked firing rates in high-input conductance cells. All three of these phenomena (high initial gain, saturation, and differences in low- and high-input conductance cells) were also readily apparent in the differences between stretch-evoked I(N,TOT) and I(N, PASS) and thus could be accounted for by the activation of the dendritic PIC. As a result, stretch-evoked I(N,TOT) and F-I provided an accurate prediction of the overall change in stretch-evoked firing. However, in about half of the low-input conductance cells, the rate of rise of firing in response to stretch was not smoothly graded but instead consisted of a rapid surge. Stretch-evoked I(N,TOT) was always smoothly graded. This suggests that although stretch-evoked I(N,TOT) can be used to predict the overall change in firing, prediction of the dynamics of firing may be less accurate.

Chapter 36 Modulation of sensorimotor performances and cognition abilities induced by RPMS: clinical and experimental investigations

The investigations presented in this chapter lead to the conclusion that proprioceptive afferent inflow to the CNS induced by RPMS elicits various modulatory effects in sensorimotor and cognitive systems. Since the build-up of the conditioning effects is delayed and the effects itself are long-lasting, it has to be assumed that these effects are caused via neuromodulators. Therefore, the presented approach is promising to improve sensorimotor and cognitive disturbances after lesions in the CNS, e.g. after a stroke, by facilitation of reorganization.

Effects of excitatory modulation on intrinsic properties of turtle motoneurons

The purpose of this study was to quantify the effects of excitatory modulation on the intrinsic properties of motoneurons (MNs) in slices of spinal cord taken from the adult turtle. Responses were noted following application of an excitatory modulator: serotonin (5-HT), muscarine, trans-1-amino-1,3-cyclopentane dicarboxylic acid (tACPD), or all three combined. A sample of 44 MNs was divided into 2 groups, on the basis of whether MNs did (28/44) or did not (16/44) demonstrate a nifedipine-sensitive acceleration of discharge during a 2-s, intracellularly injected stimulus pulse. Such acceleration indicates the development of a plateau potential (PP). Excitatory modulation lowered the MNs' resting potential, increased input resistance, decreased rheobase, reduced several afterhyperpolarization values, and shifted the conventional, one-phase stimulus current-spike frequency (I-f) relation to the left. For both MN groups, the relative efficacy of excitatory modulation on both non-PP and PP MNs was generally in the following order: combined application > 5-HT approximately muscarine > tACPD. In many instances, the effects of modulation differed significantly for non-PP versus PP MNs, the most pronounced being in their I-f relation. To describe this difference, it was necessary to measure a two-phase relation. In PP MNs, excitatory modulation considerably increased the slope of the first (initial) phase and flattened the second (later) phase of this relation. The latter result bore similarities to that obtained in a previous study, which addressed MN firing behavior during fictive locomotion of the high-decerebrate cat.

Effect of vasopressin on the input-output properties of rat facial motoneurons

Vasopressin can directly excite facial motoneurons in young rats and mice. It acts by generating a persistent inward current, which is Na(+)-dependent, tetrodotoxin-insensitive and voltage-gated. This peptide-evoked current is unaffected by Ca(++) or K(+) channel blockade and is modulated by extracellular divalent cations. In the present work, we determined how vasopressin alters the input-output properties of facial motoneurons. Whole-cell recordings were obtained from these neurons in the current clamp mode, in brainstem slices of young rats. Repetitive firing was evoked by injecting depolarizing current pulses. Steady-state frequency-current (f-I) relationships were constructed and the effect of vasopressin on these relationships was studied. We found that vasopressin caused a parallel shift to the left of the cell steady-state f-I relationship. This effect persisted in the presence of blockers of K(+) or Ca(++) channels. The peptide effect was distinct from that brought about by Ca(++) channel suppression or by apamin, a blocker of the mAHP. These latter manipulations resulted in an increase in the slope of the steady-state f-I relationship. We conclude that the vasopressin-induced modification of the input-output properties of facial motoneurons is probably exclusively caused by the sodium-dependent, voltage-modulated inward current elicited by the peptide, rather than being due to indirect effects of the peptide on Ca(++) channels, K(+) channels or Ca(++)-dependent K(+) channels. Computer simulation, based on a simple model of facial motoneurons, indicates that the introduction of a conductance having the properties of the vasopressin-dependent conductance can entirely account for the observed peptide-induced shift of the f-I relationship.

Input-output functions of mammalian motoneurons

Our intent in this review was to consider the relationship between the biophysical properties of motoneurons and the mechanisms by which they transduce the synaptic inputs they receive into changes in their firing rates. Our emphasis has been on experimental results obtained over the past twenty years, which have shown that motoneurons are just as complex and interesting as other central neurons. This work has shown that motoneurons are endowed with a rich complement of active dendritic conductances, and flexible control of both somatic and dendritic channels by endogenous neuromodulators. Although this new information requires some revision of the simple view of motoneuron input-output properties that was prevalent in the early 1980's (see sections 2.3 and 2.10), the basic aspects of synaptic transduction by motoneurons can still be captured by a relatively simple input-output model (see section 2.3, equations 1-3). It remains valid to describe motoneuron recruitment as a product of the total synaptic current delivered to the soma, the effective input resistance of the motoneuron and the somatic voltage threshold for spike initiation (equations 1 and 2). However, because of the presence of active channels activated in the subthreshold range, both the delivery of synaptic current and the effective input resistance depend upon membrane potential. In addition, activation of metabotropic receptors by achetylcholine, glutamate, noradrenaline, serotonin, substance P and thyrotropin releasing factor (TRH) can alter the properties of various voltage- and calcium-sensitive channels and thereby affect synaptic current delivery and input resistance. Once motoneurons are activated, their steady-state rate of repetitive discharge is linearly related to the amount of injected or synaptic current reaching the soma (equation 3). However, the slope of this relation, the minimum discharge rate and the threshold current for repetitive discharge are all subject to neuromodulatory control. There are still a number of unresolved issues concerning the control of motoneuron discharge by synaptic inputs. Under dynamic conditions, when synaptic input is rapidly changing, time- and activity-dependent changes in the state of ionic channels will alter both synaptic current delivery to the spike-generating conductances and the relation between synaptic current and discharge rate. There is at present no general quantitative expression for motoneuron input-output properties under dynamic conditions. Even under steady-state conditions, the biophysical mechanisms underlying the transfer of synaptic current from the dendrites to the soma are not well understood, due to the paucity of direct recordings from motoneuron dendrites. It seems likely that resolving these important issues will keep motoneuron afficiandoes well occupied during the next twenty years.

Amplification and linear summation of synaptic effects on motoneuron firing rate

The aim of this study was to measure the effects of synaptic input on motoneuron firing rate in an unanesthetized cat preparation, where activation of voltage-sensitive dendritic conductances may influence synaptic integration and repetitive firing. In anesthetized cats, the change in firing rate produced by a steady synaptic input is approximately equal to the product of the effective synaptic current measured at the resting potential (I(N)) and the slope of the linear relation between somatically injected current and motoneuron discharge rate (f-I slope). However, previous studies in the unanesthetized decerebrate cat indicate that firing rate modulation may be strongly influenced by voltage-dependent dendritic conductances. To quantify the effects of these conductances on motoneuron firing behavior, we injected suprathreshold current steps into medial gastrocnemius motoneurons of decerebrate cats and measured the changes in firing rate produced by superimposed excitatory synaptic input. In the same cells, we measured I(N) and the f-I slope to determine the predicted change in firing rate (Delta F = I(N) * f-I slope). In contrast to previous results in anesthetized cats, synaptically induced changes in motoneuron firing rate were greater-than-predicted. This enhanced effect indicates that additional inward current was present during repetitive firing. This additional inward current amplified the effective synaptic currents produced by two different excitatory sources, group Ia muscle spindle afferents and caudal cutaneous sural nerve afferents. There was a trend toward more prevalent amplification of the Ia input (14/16 cells) than the sural input (11/16 cells). However, in those cells where both inputs were amplified (10/16 cells), amplification was similar in magnitude for each source. When these two synaptic inputs were simultaneously activated, their combined effect was generally very close to the linear sum of their amplified individual effects. Linear summation is also observed in medial gastrocnemius motoneurons of anesthetized cats, where amplification is not present. This similarity suggests that amplification does not disturb the processes of synaptic integration. Linear summation of amplified input was evident for the two segmental inputs studied here. If these phenomena also hold for other synaptic sources, then the presence of active dendritic conductances underlying amplification might enable motoneurons to integrate multiple synaptic inputs and drive motoneuron firing rates throughout the entire physiological range in a relatively simple fashion.

Synaptic control of motoneuronal excitability

Movement, the fundamental component of behavior and the principal extrinsic action of the brain, is produced when skeletal muscles contract and relax in response to patterns of action potentials generated by motoneurons. The processes that determine the firing behavior of motoneurons are therefore important in understanding the transformation of neural activity to motor behavior. Here, we review recent studies on the control of motoneuronal excitability, focusing on synaptic and cellular properties. We first present a background description of motoneurons: their development, anatomical organization, and membrane properties, both passive and active. We then describe the general anatomical organization of synaptic input to motoneurons, followed by a description of the major transmitter systems that affect motoneuronal excitability, including ligands, receptor distribution, pre- and postsynaptic actions, signal transduction, and functional role. Glutamate is the main excitatory, and GABA and glycine are the main inhibitory transmitters acting through ionotropic receptors. These amino acids signal the principal motor commands from peripheral, spinal, and supraspinal structures. Amines, such as serotonin and norepinephrine, and neuropeptides, as well as the glutamate and GABA acting at metabotropic receptors, modulate motoneuronal excitability through pre- and postsynaptic actions. Acting principally via second messenger systems, their actions converge on common effectors, e.g., leak K(+) current, cationic inward current, hyperpolarization-activated inward current, Ca(2+) channels, or presynaptic release processes. Together, these numerous inputs mediate and modify incoming motor commands, ultimately generating the coordinated firing patterns that underlie muscle contractions during motor behavior.

Weak, stochastic temporal correlation of large scale synaptic input is A major determinant of neuronal bandwidth

We determine the bandwidth of a model neurone to large-scale synaptic input by assessing the frequency response between the outputs of a two-cell simulation that share a percentage of the total synaptic input. For temporally uncorrelated inputs, a large percentage of common inputs are required before the output discharges of the two cells exhibit significant correlation. In contrast, a small percentage (5%) of the total synaptic input that involves stochastic spike trains that are weakly correlated over a broad range of frequencies exert a clear influence on the output discharge of both cells over this range of frequencies. Inputs that are weakly correlated at a single frequency induce correlation between the output discharges only at the frequency of correlation. The strength of temporal correlation required is sufficiently weak that analysis of a sample pair of input spike trains could fail to reveal the presence of correlated input. Weak temporal correlation between inputs is therefore a major determinant of the transmission to the output discharge of frequencies present in the spike discharges of presynaptic inputs, and therefore of neural bandwidth.

Synaptic control of motoneuron excitability in rodents: from months to milliseconds

1. Motoneurons (MN) shape motor patterns by transforming inputs into action potential output. This transformation, excitability, is determined by an interaction between synaptic inputs and intrinsic membrane properties. Excitability is not static, but changes over multiple time scales. The purpose of the present paper is to review our recent data on synaptic factors important in the dynamic control of MN excitability over time scales ranging from weeks to milliseconds. 2. Developmental changes in modulation of MN excitability are well established. Noradrenergic potentiation of hypoglossal (XII) MN inspiratory activity in rhythmically active medullary slice preparations from rodents increases during the first two postnatal weeks. This is due to increasing alpha 1- and beta-adrenoceptor excitatory mechanisms and to a decreasing inhibitory mechanism mediated by alpha 2-adrenoceptors. Over a similar period, ATP potentiation of XII inspiratory activity does not change. 3. Motoneuron excitability may also change on a faster time scale, such as between different behaviours or different phases of a behaviour. Examination of this has been confounded by the fact that excitatory synaptic drives underlying behaviour can obscure smaller concurrent changes in excitability. Using the rhythmically active neonatal rat brain-stem-spinal cord preparation, we blocked excitatory inspiratory drive to phrenic MN (PMN) to reveal a reduction in PMN excitability specific to the inspiratory phase that: (i) arises from an inhibitory GABAergic input; (ii) is not mediated by recurrent pathways; and (iii) is proportional to and synchronous with the excitatory inspiratory input. We propose that the proportionality of the concurrent inhibitory and excitatory drives provides a means for phase-specific modulation of PMN gain. 4. Modulation across such diverse time scales emphasizes the active role that synaptic factors play in controlling MN excitability and shaping behaviour.

Ionic mechanisms underlying excitatory effects of serotonin on embryonic rat motoneurons in long-term culture

The actions of serotonin were investigated on motoneurons isolated from embryonic day 14 rat spinal cord and enriched by metrizamide density gradient centrifugation. Trophic support was provided by a spinal cord glial monolayer, ciliary neurotrophic factor and heat-inactivated serum. Cultures were maintained for 17-83 days and investigated using whole-cell patch-clamp recording. Serotonin evoked slow depolarizations (6.2+/-0.7 or 9.3+/-1.3 mV in the presence of 6-cyano-7-nitroquinoxaline-2,3-dione and strychnine, EC50 8.2 nM), which were reversibly blocked by 0.1 microM ketanserin. Serotonin generated synaptic potentials in motoneurons, lowered the threshold for repetitive firing and changed the slope of the current intensity-firing frequency relationship. The inward current evoked by serotonin (-147+/-15.2 pA) was ascribed to a complex ionic mechanism, which varied amongst neurons in the sampled population. It was due to closure of barium-sensitive potassium channels, effects on Ih and increase in a separate mixed cation current which comprised both transient voltage-sensitive and sustained components. We conclude that serotonergic responses develop in motoneurons cultured under these conditions in the absence of serotonergic input, sensory neurons or many interneurons.

Discharge properties of motoneurones: how are they matched to the properties and use of their muscle units?

A general survey is given of old as well as more recent findings concerning matches between electrophysiological properties of motoneurones and contractile properties of their muscle fibres. Mechanisms for creating and maintaining such matches are discussed. It is pointed out that it is not sufficient to describe the variation of functional motoneurone characteristics simply in terms of 'fast' or 'slow': all properties seem continuously graded and there is cytochemical evidence for several, seemingly independent parameters of functional specialisation.

Simulated recruitment of medial rectus motoneurons by abducens internuclear neurons: synaptic specificity vs. intrinsic motoneuron properties

Ocular motoneuron firing rate is linearly related to conjugate eye position with slope K above recruitment threshold theta. Within the population of ocular motoneurons K increases as theta increases. These differences in firing rate between motoneurons might be determined either by the intrinsic properties of the motoneurons, or by differences in synaptic input to them, or by a combination of the two. This question was investigated by simulating the input signal to medial rectus motoneurons (MR-MNs) from internuclear neurons of the abducens nucleus (INNs). INNs were represented as input nodes in a two-layer neural net, each with weighted connections to every output node representing an MR-MN. Individual simulated MR-MNs were assigned parameters corresponding to an intrinsic current threshold I(R) and an intrinsic frequency-current (f-I) slope gamma. Their firing rates were calculated from these parameters, together with the effective synaptic current produced by their synaptically weighted INN inputs, with the use of assumptions employed in computer simulations of spinal motoneuron pools. The experimentally observed firing rates of MR-MNs served as training data for the net. The following two training conditions were used: 1) synaptic weights were fixed and the intrinsic parameters of the MR-MNs were allowed to vary, corresponding to the situation in which each MR-MN receives a common synaptic drive and 2) intrinsic MR-MN properties were fixed and synaptic weights were allowed to vary. In each case, the varying quantities were trained with a form of gradient descent error reduction. The simulations revealed the following three problems with the common-drive model: 1) the recruitment of INNs produced nonlinear responses in MR-MNs with low thetas; 2) the range of I(R)s required to reproduce the observed range of theta were generally larger than those measured experimentally for cat ocular motoneurons; and 3) the intrinsic f-I slope gamma increased with I(R). Experimental data from cat indicate that gamma decreases with I(R). When synaptic weights were allowed to vary, all three problems with the common-drive model were overcome. This required MR-MNs receiving selective input from INNs with similar firing rate thresholds. These results suggest that the differences in firing rate properties among MR-MNs in relation to steady-state eye position cannot be derived from their intrinsic properties alone but result at least partly from differences in their synaptic inputs. An MR-MN's individual set of synaptic inputs constitutes, in effect, a premotor receptive field.

Effects of neurochemically excited group III-IV muscle afferents on motoneuron afterhyperpolarization

When humans voluntarily and maximally contract a muscle under isometric conditions, the average firing rate of motor units decreases from an initially high value over several tens of seconds. The mechanisms underlying the rate reduction are probably manifold. One mechanism could involve changes in the motoneuron afterhyperpolarization, another reflex effects of group III-IV muscle afferents that are excited during developing muscle fatigue. It appears possible that changes in motoneuron afterhyperpolarization are mediated by these afferent inputs. We therefore studied effects on motoneuron afterhyperpolarization of small-diameter muscle afferents excited by intra-arterially injected metabolites such as bradykinin and serotonin. In decerebrate and mostly spinalized cats, lumbosacral alpha-motoneurons were recorded intracellularly. Current pulses were injected to test for input resistance and elicit action potentials and afterhyperpolarizations. Afterhyperpolarizations were averaged from c. 10 successive stimulus repetitions. Measurements were taken of afterhyperpolarization amplitude, half-width and area; and exponential functions were fitted to the afterhyperpolarization decay phase to determine afterhyperpolarization decay time-constants. In selected cases, the entire afterhyperpolarization trajectory was fitted with a sum of two exponentials to assess more precisely changes in afterhyperpolarization trajectory. Small catheters were inserted into side-branches of the sural artery and the accompanying vein to apply substances like bradykinin, serotonin and KCl to the calf muscles. Concentrations were in the range of those used by other workers. Intra-arterial injection of bradykinin and serotonin usually decreased blood pressure, which may at times have affected mean motoneuron membrane potentials. Afterhyperpolarization amplitude usually changed with membrane potential in a way expected from ensuing changes in driving potential. Whenever excitation of group III-IV muscle afferents caused moderate to strong increases in motoneuron synaptic noise, afterhyperpolarization amplitudes were reduced, usually in parallel to decreases in input resistance. Afterhyperpolarization half-widths were mostly unaffected, but occasionally decreased. There was a significant trend for afterhyperpolarization decay time-constants to increase during increased synaptic noise, this increase being inversely correlated with the reduction in afterhyperpolarization amplitude. The reduction in input resistance was associated with a decrease in the membrane time-constant, which could therefore not account for the prolongation of the afterhyperpolarization decay time-constant. The afterhyperpolarization area decreased, indicating that the reduction of afterhyperpolarization amplitude outweighed the prolongation of afterhyperpolarization decay time-constant. During a prolonged fatiguing muscle contraction group III-IV afferents become increasingly excited, produce augmenting synaptic inputs in motoneurons, and will change afterhyperpolarization properties. On average, these changes per se tend to diminish the effect of afterhyperpolarization on motoneuron discharge.

Motor pool organization of the external gastrocnemius muscle in the turtle, Pseudemys (Trachemys) scripta elegans

The spinal cord of the adult turtle, Pseudemys (Trachemys) scripta elegans, is now considered a promising model for the study of the segmental motor system in the generalized tetrapod. To facilitate such studies we have examined the location, soma geometry, soma size, and number of motoneurons innervating the external gastrocnemius (EG) muscle in this species, as this muscle is ideally suited to the study of interrelations between the neuronal and muscular components of the segmental motor system. Motoneurons were retrogradely labeled following application of horseradish peroxidase to the EG muscle nerve. In both horizontal and transverse planes, labeled motoneurons innervating the EG muscle were concentrated in the S1 lumbosacral segment, and extended rostrally and caudally as far as the exists of the D10 and S2 spinal nerves, respectively. In the transverse plane, motoneurons were arranged in a longitudinal column which occupied the dorsolateral quadrant of the ventral horn. EG motoneurons are fusiform in shape and present their largest dimension in the transverse plane with their long axis oriented in the ventromedial to dorsolateral plane. The soma diameters of EG motoneurons were normally distributed, reflecting the absence of separate fusimotor innervation in reptilian species. In individual turtles, there was a two- to threefold range in soma diameter while soma surface area extended over a seven- to tenfold range. Based on cell counts from five animals, the EG motor pool was composed of approximately 75 motoneurons. Taken together, the results of this study provide valuable information for interpreting the results of future studies on the segmental motor system of this species under both normal and pathophysiological conditions.

Repetitive firing properties of developing rat brainstem motoneurones

1. The repetitive firing properties of neonatal and adult rat hypoglossal motoneurones (HMs) were investigated in a brainstem slice preparation. Neonatal HMs could be classified into two main groups: (1) neurones with a decrementing or adapting firing pattern (type D); exhibiting an early and a late phase; and (2) neurones with an incrementing or accelerating firing pattern (type I). 2. The pattern of repetitive firing changed markedly during development. While most HMs recorded from young rats (< postnatal day (P) 4) were type D, the majority of HMs recorded during the second postnatal week were type I. In adults (> P21), nearly all HMs had a decrementing firing pattern, characterized by a brief period of adaptation and high steady-state firing rates. 3. The calcium-dependent after-hyperpolarization (AHP) was shortest in type I neonatal HMs, and decreased in amplitude during trains of action potentials (APs). In type D neurones, these same trains caused a slight enhancement of AHP amplitude. In adult HMs, with a decrementing firing pattern, trains of APs also caused summation of the AHP. 4. Type D neonatal HMs showed a progressive prolongation of the AP during repetitive firing. In contrast, type I neonatal HMs had almost no change in AP duration. In adult HMs the AP was short and experienced only a modest increase in duration during fast repetitive firing. 5. The function relating steady-state firing frequency to injected current (f-I curve) was linear. The mean steady-state f-I slope was significantly higher in neonates than in adults (approximately 30 vs. approximately 20 Hz nA-1), and was weakly correlated with input resistance. The f-I slope was negatively correlated with AHP duration in neonatal HMs only. In addition, for a given AHP duration the slope was higher in neonatal HMs. 6. Two threshold behaviours were observed among neonatal HMs: (a) a progressive rhythmic firing threshold, and (b) a sudden transition from subthreshold to regular repetitive firing. Current threshold for repetitive firing was strongly correlated with cell input conductance. Type I neonatal HMs had higher minimal steady firing rates (fmin) than type D HMs. In neonates, fmin was strongly correlated with AHP duration. Adult HMs showed a weaker correlation between these two parameters, and fmin was higher than predicted by AHP duration. 7. In summary, HMs responded to depolarizing current pulses with different firing patterns during postnatal development.(ABSTRACT TRUNCATED AT 400 WORDS)

A commentary on the segmental motor system of the turtle: implications for the study of its cellular mechanisms and interactions

A commentary is provided on the segmental motor system of the turtle Pseudemys (Trachemys) scripta elegans with an emphasis on neuronal, neuromuscular, and muscular mechanisms that control the development of force under normal, fatiguing, and pathophysiological conditions. For the central neuronal component of the segmental motor system, it has recently been shown that intracellular analysis of the firing properties of motoneurons and interneurons can be undertaken for relatively long periods of time in in vitro slices of the lumbosacral spinal cord of the adult turtle. In other less reduced in vitro preparations, analyses are available on complex motor behaviors generated by the isolated spinal cord. These behaviors of spinal neuronal networks are analogous in key aspects to those generated by the isolated in vivo cord, and by the cord in intact preparations. These results suggest that the neuronal components of the segmental motor system can not be studied from the cellular/molecular level of analysis in in vitro slice preparations to the systems level in conscious, freely moving animals. The in vitro approach can also be used for the analysis of cellular mechanisms in suprasegmental brain structures, which contribute to the control of voluntary movement. For the peripheral neuromuscular component of the segmental motor system, information is now available on muscle fiber types and selected aspects of sensory innervation, and it is feasible to study the mechanical and biochemical properties of motor units. As such, the turtle presents a valuable model for exploring interrelations between the neuronal and mechanical components of the segmental motor system of the generalized tetrapod. A prominent feature of these recent developments is the extent to which they have been driven by findings that have emphasized an evolutionary conservation of motor-control mechanisms extending from ion channels, at the cellular level, to the control of multijointed movements at the systems level of analysis.

The anthroform biorobotic arm: a system for the study of spinal circuits

This paper reports the design, construction, and testing of a replica of the human arm, which aims to be dynamically as well as kinematically accurate. The arm model is actuated with McKibben pneumatic artificial muscles, and controlled by a special purpose digital signal processing system designed to simulate spinal neural networks in real time. An artificial muscle spindle has also been designed and tested. Design and test data are reviewed, and the paper describes how we hope to use the system to improve our understanding of the reflexive control of human movement and posture.

Neuromuscular frequency-coding and fatigue

In daily life, muscle fatigue often becomes noticeable as an apparent decline in the efficiency of force production by central commands, making it necessary to increase drive (or "effort") to produce a constant motor output. Such aspects of fatigue may be caused by changes in the way in which synaptic messages arriving at the motoneurons are translated into forces by the muscle fibers. Therefore, an understanding of these neuromuscular gradation mechanisms is essential for any analysis of motor fatigue. A brief general review is given of 1) how muscle fibers transduce motoneuronal discharge rates into force; 2) how synaptic currents are transduced into motoneuronal discharge rates; 3) how activity-dependent changes in the neuromuscular transduction mechanisms contribute to neuromuscular fatigue; and 4) how the matching between the transduction mechanisms of motoneurons and those of their muscle fibers may help to optimize neuromuscular gradation efficiency and decrease the severity of fatigue.