Exogenous neuromodulation of spinal neurons induces beta-band coherence during self-sustained discharge of hind limb motor unit populations

Abstract

The spontaneous or self-sustained discharge of spinal motoneurons can be observed in both animals and humans. Although the origins of this self-sustained discharge are not fully known, it can be generated by activation of persistent inward currents intrinsic to the motoneuron. If self-sustained discharge is generated exclusively through this intrinsic mechanism, the discharge of individual motor units will be relatively independent of one another. Alternatively, if increased activation of premotor circuits underlies this prolonged discharge of spinal motoneurons, we would expect correlated activity among motoneurons. Our aim is to assess potential synaptic drive by quantifying coherence during self-sustained discharge of spinal motoneurons. Electromyographic activity was collected from 20 decerebrate animals using a 64-channel electrode grid placed on the isolated soleus muscle before and following intrathecal administration of methoxamine, a selective α₁-noradrenergic agonist. Sustained muscle activity was recorded and decomposed into the discharge times of ~10–30 concurrently active individual motor units. Consistent with previous reports, the self-sustained discharge of motor units occurred at low mean discharge rates with low-interspike variability. Before methoxamine administration, significant low-frequency coherence (<2 Hz) was observed, while minimal coherence was observed within higher frequency bands. Following intrathecal administration of methoxamine, increases in motor unit discharge rates and strong coherence in both the low-frequency and 15- to 30-Hz beta bands were observed. These data demonstrate beta-band coherence among motor units can be observed through noncortical mechanisms and that neuromodulation of spinal/brainstem neurons greatly influences coherent discharge within spinal motor pools.

NEW & NOTEWORTHY The correlated discharge of spinal motoneurons is often used to describe the input to the motor pool. We demonstrate spinal/brainstem neurons devoid of cortical input can generate correlated motor unit discharge in the 15- to 30-Hz beta band, which is amplified through neuromodulation. Activity in the beta band is often ascribed to cortical drive in humans; however, these data demonstrate the capability of the mammalian segmental motor system to generate and modulate this coherent state of motor unit discharge.

INTRODUCTION

The spinal motoneuron and its associated muscle fibers compose the motor unit, the fundamental unit of force generation. Motoneurons can be depolarized through either synaptic activation or through changes in intrinsic excitability. Synaptic transmission through descending, afferent, or local interneuron sources serves to alter the membrane potential of spinal motoneurons. In addition to these sources of excitation and inhibition, descending serotonergic (5HT) and noradrenergic (NE) input from brainstem centers exerts a profound neuromodulatory control of spinal neurons (27).

The neuromodulatory effects of 5HT and NE occur in part through changes in the intrinsic properties of the motoneuron through several mechanisms including 1) increased input resistance (15, 32), 2) decreased afterhyperpolarization duration (32, 62, 64), and 3) hyperpolarization of the voltage threshold for discharge (16, 26, 51). Importantly, these neuromodulators also facilitate persistent inward currents (PICs) through the activation of 5HT₂ and NE_α1 receptors found on spinal motoneurons (29, 31, 41, 45) and interneurons (33). Activation of these receptors cause an increase in Ca²⁺ and Na⁺ conductances throughout the dendrites of spinal motoneurons. These channels have particularly long time courses for inactivation and can warm up with repeated activation (4). These intrinsic channels provide additional depolarizing currents to the motoneuron, in effect amplifying and prolonging synaptic input. A hallmark of PIC behavior is the spontaneous or self-sustained discharge of motoneurons, where motoneurons discharge in the absence of underlying synaptic input (29, 40). The brainstem thus has the ability to provide widespread regulation of the state of motoneuron excitability.

In addition to changes in motoneuron excitability, brainstem neuromodulation of spinal centers can generate rhythmic activation of motoneurons through activation of spinal networks. For example, exogenous application of 5HT can initiate or modify locomotion in a large number of animals including the lamprey (25), mouse (42), rat, rabbit (61), and cat (3). Further evidence for the role of endogenous brainstem modulation of spinal circuits includes the locomotor speed-dependent discharge of reticulospinal neurons (60) and increases in spinal concentrations of 5HT and NE during locomotion (50) in the cat. NE may be more responsible for promoting tonic, rather than rhythmic, motor output (40), although NE can induce locomotor patterns in spinal neurons (35). These data support the notion that brainstem neuromodulation also has a strong influence on the discharge of spinal interneurons and serves to pattern rhythmic motor output. Here, we focus on the self-sustained discharge of spinal motoneurons observed in vivo in the cat. This preparation has a relatively high level of PIC activity, and self-sustained discharge of spinal motoneurons is commonly observed, particularly after intrathecal administration of methoxamine, a NE_α1 agonist. Activation of the NE_α1 receptor activates Ca²⁺ channels intrinsic to the spinal motoneuron and causes prolonged increases in excitability across spinal motoneuron pools. As the intrathecal application of this drug initiates no explicit pattern of activity, and this drug has been shown to activate ion channels intrinsic to the motoneurons, our primary hypothesis is that the self-sustained discharge observed across spinal motoneurons will be relatively independent of one another. To assess this, we performed coherence analysis among motor unit discharge times before and following activation of the NE_α1 receptor.

MATERIALS AND METHODS

Ethical approval.

Data presented here are from 20 adult cats of either sex. Animals were obtained from a breeding establishment for scientific research and housed at Northwestern University’s Center for Comparative Medicine, an Association for Assessment and Accreditation of Laboratory Animal Care-accredited animal research program All procedures were approved by the Institutional Animal Care and Use Committee at Northwestern University and conform to the ethics policy of the Journal of Applied Physiology.

Surgical methods.

Anesthesia was induced with 4% isoflurane and a 1:3 mixture of N₂O and O₂. Following intubation, isoflurane was reduced to 0.5–2.5% for the remainder of surgery. Cannulation of the carotid artery and jugular vein allowed for the monitoring of blood pressure and administration of supplemental fluids. The animal was immobilized on a stereotaxic frame using a head clamp, spinal clamp on the L2 dorsal vertebral process, and bilateral hip pins at the iliac crest. The left hind limb was immobilized through pins at the knee and clamps at the ankle, and the right hind limb was secured using a clamp at the lower leg. The left soleus was dissected, and the distal tendon was attached to a customized voice coil. A cuff electrode was secured to the distal, cutaneous branch of the right superficial peroneal nerve.

In 11 experiments, a L4–S1 laminectomy was provided for intrathecal drug administration. An intrathecal catheter was constructed from polyethylene tubing inserted through a dorsal incision of the dura at L4. The catheter was routed along the dorsal surface of the cord to the L6 region. The dorsal and ventral roots remained intact.

A precollicular decerebration was performed; at this point the animals are considered to have a complete lack of sentience (55), and anesthesia was discontinued. A thermistor was placed in the esophagus, and with the use of heat lamps and hot pads, core temperature was maintained at 35–37°C throughout the experiment. At the end of the experiment, animals were euthanized using a bilateral thoracotomy in addition to intravenous delivery of 2 mmol/kg solution of KCl.

Electromyographic recordings.

Electromyographic (EMG) activity of the left soleus muscle was collected using a custom 64-channel array electrode placed on the surface of the exposed muscle in either monopolar or differential modes. A ground electrode was place on the back. When needed, a silver impregnated leather strap was placed on the upper right thigh and used as reference. Array data were filtered (100–900 Hz), amplified (1–2 k), and sampled at 5,120 Hz by a 12-bit A/D converter (EMG-USB 2, 256-channel EMG amplifier; OT Bioelettronica, Torino, Italy).

Experimental design and statistical analysis.

EMG data were collected during spontaneous discharge and during the self-sustained discharge occurring after two forms of reflex activation. First, tendon vibration was delivered at high frequencies (~130 Hz) and small amplitude (~80 μm) through a custom voice coil in series with the tendon. This provides potent and selective activation of Ia afferents (6) and activates the homonymous motoneuron pool through monosynaptic pathways. Second, a distal branch of the right contralateral superficial peroneal nerve was stimulated for several seconds through the nerve cuff using 1-ms pulses delivered at 1.1–2.0 times the motor threshold at 10–50 Hz using a Grass S88 stimulator and isolation unit. These stimuli would often evoke prolonged motor output, which persisted beyond the end of the stimulation by several tens of seconds. Figure 1 shows an example of motor unit discharge during and following contralateral cutaneous stimulation. To increase the activity of soleus motoneurons, during 11 experiments, methoxamine, a norepinephrine α₁-agonist, was applied to the spinal cord through the intrathecal catheter. Distilled water was used to create a 25- to 100-μL solution of methoxamine at a concentration of 100 mM. A 0.3-mL syringe was used to manually deliver the drug. At least 20 min was provided following drug administration before recordings commenced again. This approach is consistent with previous modes of intrathecal drug administration used in this laboratory (19, 37–41, 59).

Fig. 1.

Self-sustained motor unit discharge of the soleus muscle in the decerebrate cat. Muscle force and discharge times of 41 motor units from the left soleus during a single trial of crossed extension reflex evoked through 20 s of 50-Hz stimulation at 2.8 V of the right superficial peroneal nerve. Prolonged self-sustained discharge is observed in 22 of the recorded motor units (black).

Offline, all trials were visually assessed for epochs of self-sustained firing defined as >10 s of motor output starting at least 6 s after any form of reflex evoking input. This allowed us to isolate the spontaneous discharge of spinal motoneurons. From each epoch, specific channels of EMG were removed due to low signal to noise determined through visual estimation. Up to 64 channels of EMG activity were selected for decomposition. The EMG activity was decomposed into the underlying motor unit spike trains using the convolutive blind source separation approach (47). Only spike trains with a silhouette measure >0.9 were used for further analyses. Previous investigations have demonstrated the validity of motor unit-decomposition approach during self-sustained discharge with rate of agreement through two-source validation at 98% (59).

For each spike train, mean discharge rate was found by averaging the reciprocal of each interspike interval (ISI). The coefficient of variation (CoV) of the ISI was calculated by dividing the standard deviation of the ISI by the mean ISI. Within each trial, the discharge rate and CoV values were averaged across units. Additionally, the number of motor units detected and length of recorded discharge was calculated.

Within each trial, iterative coherence analysis was performed between the cumulative spike trains (CSTs) derived from two equally sized groups of up to 10 unique motor unit spike trains using Welch’s averaged periodogram method using 1-s nonoverlapping windows (46). Within each trail, the level of significance was defined as the peak of the coherence function for frequencies >1,000 Hz (2, 8, 36). The coherence functions were z-transformed to account for differing number of windows across trails (52) and averaged within each animal to provide a single coherence function for the control and/or postmethoxamine conditions for each animal. The grand average coherence function across animals for each condition was calculated. Likewise, for each animal and condition, a histogram was constructed of significant coherence across all trials.

All statistical analyses for the subsequent linear mixed models were conducted in IBM SPSS Statistics Version 25. The effect of methoxamine administration on each dependent variable [mean discharge rate, CoV of the ISI, number of motor units, length of discharge, mean discharge rate of the collapsed discharge times without respect to unit (i.e., the CST), and the average of the z-transformed coherence values in the 15- to 30-Hz beta band without respect to significance] was determined using separate linear mixed models. All of the models were constructed as follows. Independent variables in the model included drug (administered/not administered) and animal (1–20). Drug was included in the model as a fixed, repeated factor, and animal was included as a random factor with a random intercept and random slope over the effect of drug. A scaled identity covariance structure was assumed for each factor. Use of the linear mixed model allowed for the inclusion of all trials from each animal while accounting for the clustering of trials within each animal. Additionally, data from all 20 animals could be included in the model because the 9 animals that did not receive the drug could be included as having missing cases for the drug condition.

The discharge rate of a CST has been shown to be directly related to the magnitude of coherence calculated from that CST (46, 48). Therefore, if the discharge rate of the CST is affected by methoxamine administration, then the discharge rate needs to be controlled for when determining the effect of methoxamine on coherence. To this end, the same linear mixed model as above was constructed with the addition of discharge rate of the CST as a covariate (included as a random factor). Linear regressions were calculated using data from all trials from all animals for both the control and methoxamine conditions to confirm the effect of discharge rate on the magnitude of coherence that has been previously shown.

RESULTS

Discharge characteristics during self-sustained discharge.

Figure 1 demonstrates self-sustained motor unit activity following electrical stimulation of the contralateral cutaneous nerve. Such self-sustained discharge was observed in 19 animals. During the course of these experiments, 8 ± 2.5 (means ± SD) trials of tonic discharge were collected from each animal, ranging from one to 44 trials per animal. A summary of the average values for mean discharge rate, CoV of the ISI, number of motor units, length of discharge, mean discharge rate of the CST, and beta-band coherence is shown in Table 1. Figure 2 shows the discharge rates and CoV values averaged within each trial for each animal in the control state.

Table 1.

Effect of methoxamine administration on motor unit discharge characteristics

Variable	Control	Methoxamine	Effect of Methoxamine Administration
Mean discharge rate, per MU, pulse/s	7.23 ± 0.63	10.4 ± 0.75	P < 0.0001 F(1,32) = 15.1
Mean CoV of the ISI, per MU, %	16.36 ± 1.32	16.41 ± 1.57	P = 0.975 F(1,28) = 0.001
Number of motor units detected	16.59 ± 1.97	21.56 ± 2.22	P = 0.062 F(1,30) = 3.8
Length of discharge, s	40.29 ± 4.43	51.4 ± 5.1	P = 0.064 F(1,26) = 3.7
Mean discharge rate of the CST, pulse/s	393 ± 51.24	663.13 ± 58.85	P < 0.0001 F(1,30) = 16.4
Average 15- to 30-Hz coherence	1.23 ± 0.29	2.70 ± 0.33	P < 0.0001 F(1,33) = 15.0
Average 15- to 30-Hz coherence (controlled for CST discharge rate)	0.45 ± 0.13	1.1 ± 0.19	P < 0.0001 F(1,291) = 15.8
Dependent variable ~ drug × (drug|animal) + (1|animal)

Means ± SE values for the control (no methoxamine administered) and methoxamine conditions are displayed as estimates from the linear mixed model for each dependent variable. The effect of methoxamine administration on each dependent variable was determined using separate linear mixed models. Independent variables in the model included drug (administered/not administered) and animal (1–20). Drug was included in the model as a fixed, repeated factor, and animal was included as a random factor with a random intercept and random slope over the effect of drug. Results of each model related to the main effect of drug are shown. The general model is shown at bottom. MU, motor unit; CoV, coefficient of variation; CST, composite spike train; ISI, interspike interval.

Fig. 2.

Characteristics of soleus motor units during self-sustained discharge. A: histogram of all 867,763 interspike intervals (ISIs) without medication delivery (blue) and 1,517,289 ISIs following methoxamine delivery (red). B: for each trial, average motor unit discharge rate and coefficient of variation (CoV) of the ISI was found and then averaged within animals for each condition; data are presented as means ± SD; pps, pulse/s. Among the 19 animals without medication delivery (blue), the self-sustained discharge of spinal motor units occurred at a low-discharge frequency with relatively low CoV. Following methoxamine delivery (red), the mean discharge rate increases with little change in CoV values.

Within each trial, coherence among motor unit spike trains is used to quantify common synaptic input to the motor pool. Figure 3 shows motor unit discharge times and their underlying coherence from a single trial, where minimal coherence is observed in the control condition. Figure 4 demonstrates significant magnitude and occurrence of coherence during self-sustained discharge. In the control condition, over 50% of the coherence functions revealed common fluctuations in the <4-Hz delta bandwidth. Very little coherence was observed in the 4- to 7-Hz theta and 8- to 13-Hz alpha bands. Significant coherence was observed in the 15- to 30-Hz beta bandwidth, although this occurred <20% of the time.

Fig. 3.

Motor unit discharge during self-sustained motor output of the soleus before and following activation of the noradrenergic-α₁ (NE_α1) receptor. A: smoothed discharge times of individual motor unit discharges are shown during self-sustained motor output of the soleus muscle of the unanesthetized, unparalyzed decerebrate cat. This animal demonstrates low-discharge rates [9.8 ± 0.76 pulse/s (pps)] with low coefficient of variations (14.0 ± 2.0% CoV). B: a 5-s expansion of the interval pulse train underlying each of the motor units is shown (gray box in A). Detection of these well-defined, nonoverlapping peaks represents the discharge of the single motor unit and is denoted with a closed circle. C: coherence across these motor units is quite low, with significant coherence observed only at very low frequencies. D and E: following administration of the NE_α1 agonist methoxamine in this same animal, an increase in mean discharge rates (D; 14.1 ± 0.31 pps) and a slight decrease in CoV values (E; 10.5 ± 0.45%) are observed. F: increases in coherence in the low-frequency and 15- to 30-Hz bandwidth are observed following drug administration.

Fig. 4.

Intrathecal activation of the noradrenergic-α₁ (NE_α1) receptor increases beta-band coherence during the self-sustained discharge of soleus motor units in the cat. A and B: average coherence function (A) and histogram of significance coherence (B) from 19 animals before medication delivery (blue) reveal increased low-frequency coherence with <20% occurrence of coherence in the 15- to 30-Hz bandwidth. Following activation of the NE_α1 receptor (red) in 11 experiments, a 3-fold increase in both magnitude (A) and occurrence (B) of 15- to 30-Hz coherence is observed.

Discharge characteristics following intrathecal activation of the NE_α1 receptor.

Following administration of the NE_α1 receptor agonist, methoxamine, we were able to collect 13 ± 3.0 trials of self-sustained discharge in each of 11 animals. The mean values for each dependent variable following methoxamine administration are shown in Table 1. Methoxamine administration had a significant effect on mean discharge rate, 15- to 30-Hz coherence, and mean discharge rate of the CST at the P < 0.05 level (P < 0.0001 for each) and on number of motor units and length of discharge at the P < 0.075 level (P = 0.06 for each). There was no significant effect of methoxamine administration on CoV of the ISI (P = 0.975).

The stark increase in 15- to 30-Hz coherence following intrathecal activation of the NE_α1 receptor can be seen in Fig. 4. Across the 11 animals, over half of the trials demonstrated significant coherence in this bandwidth. Eight of the ten animals assessed in both conditions demonstrated increased 15- to 30-Hz coherence. Additionally, low-frequency coherence was observed at a higher frequency (80%) following NE_α1 receptor activation. Coherence in the 4- to 7-Hz and 8- and 13-Hz bandwidth also increased slightly following methoxamine. Figure 4 demonstrates theses changes in magnitude and occurrence of coherence during self-sustained discharge evoked via intrathecal activation of the NE_α1 receptor.

Controlling for differences in discharge rate.

As expected, a significant linear relationship was observed between the average discharge rate of the nonnormalized CST and the magnitude of beta-band coherence in both the control (n = 158, r = 0.657, P < 0.0001) and methoxamine (n = 139, r = 0.403, P < 0.0001) conditions (Fig. 5). This demonstrated that the increased sampling capacity of the motor unit spike trains amplified the ability to detect coherence.

Fig. 5.

Effects of discharge rate on the detection of coherence. The average discharge rate of the nonnormalized composite spike train is plotted against the average 15- to 30-Hz z-transformed coherence values for each trial in both the control (blue) and methoxamine (red) conditions; pps, pulse/s. A significant relationship between discharge frequency and beta-band coherence is observed in both conditions (both P < 0.0001).

Additionally, because of the substantial increase in mean discharge rate of the CST following methoxamine administration (+269.8 pulse/s, on average), it was necessary to include this variable as a covariate in the linear mixed model assessing the effect of drug administration on coherence. The effect of drug remained statistically significant after controlling for discharge rate of the CST [F(291.3) = 15.8; P < 0.0001]. This finding suggests the amplification of beta-band coherence following methoxamine administration is due to factors other than discharge rate associated considerations.

DISCUSSION

In this study, we have quantified the coherent discharge of soleus motor units before and following intrathecal activation of the NE_α1 receptor. Cat spinal motoneurons demonstrate persistent depolarization, which is regulated by neuromodulators and underlies self-sustained discharge. The actions of neuromodulators on synaptic input are less well understood. Our primary finding is that intrathecal activation of the NE_α1 receptor increases coherence of soleus motor units in the 15- to 30-Hz bandwidth during spontaneous discharge in the unparalyzed, unanesthetized, decerebrate cat. These data demonstrate that the mammalian central nervous system can generate beta-band coherence through noncortical mechanisms.

Discharge characteristics during self-sustained discharge.

The self-sustained discharge of soleus motoneurons is characterized by low mean discharge rates and relatively low CoV values. This is consistent with the activation of intrinsic PICs due to endogenous modulatory inputs from brainstem centers. Such PIC activation may provide relatively constant depolarization of the motoneurons, allowing for a discharge that is heavily constrained by the motoneuron afterhyperpolarization (30).

Low-frequency correlations during self-sustained discharge.

Low-frequency (<4 Hz) correlation was observed during self-sustained discharge of soleus motor units in the cat. This suggests that the intrinsic excitability of spinal motoneurons may not be the sole source of motoneuron depolarization. As described below, there may be several sources of this putative input to the motoneuron pool.

Because of the nature of the task, steady discharge is not truly observed. There is often a gradual decrease in discharge rate over several seconds due to either waning of neuromodulatory concentrations or inactivation of the Ca²⁺ channel, which has a particularly long time constant (54). This steady decrease in discharge may influence the low-frequency component of the coherence value. Additionally, known rhythmic inputs to spinal networks, such as those active during locomotion, are sensitive to neuromodulation (9, 23, 28, 35) and may provide common input in this frequency range. Intramuscular coherence, as described here using motor units, has not been assessed during locomotion in the cat, although intermuscular coherence during fictive locomotion in the cat has been shown to be rather limited and only apparent in close synergists (49). Lastly, descending synaptic inputs, such as respiratory drive potentials (66) or other brainstem inputs (18, 60), may contribute to low-frequency common synaptic input.

Discharge characteristics following intrathecal activation of the NE_α1 receptor.

Consistent with previous work, intrathecal application of methoxamine resulted in increased activation of spinal motoneurons (10, 40, 41). An increase in duration, number of motor units, and mean rate of soleus motor unit discharge was observed following intrathecal application of methoxamine. This is consistent with the increases in PIC observed from intracellular recordings of spinal motoneurons in paralyzed preparations. Little change was observed in CoV measures following methoxamine administration. Similar CoV measures before and following methoxamine may indicate similar synaptic noise (7) and forms of synaptic input (53, 56) between conditions, consistent with intrinsic PIC activation.

Intrathecal activation of the NE_α1 receptor induces beta-band coherence.

Methoxamine administration produced an increase in the magnitude and occurrence of motor unit coherence, most notably in the beta band. Coherence in such frequency bands may be from descending inputs from the brainstem as described below; however, due to the local intrathecal delivery of microliter amounts of drug, it is likely that such changes occur at the spinal level instead of at the brainstem. These changes may be due to either changes in the excitability of spinal motoneurons and/or changes in the segmental inputs to spinal motoneurons. First, activation of the NE_α1 receptor may serve to amplify a constant background synaptic drive. There is a small amount of beta-band coherence observed in the control state (Fig. 4). It is possible this low-level synaptic drive in the beta frequencies does not change in response to medication delivery, but increased excitability of spinal motoneurons serves to amplify our ability to detect this consistent input (57). The methoxamine trials do contain more information than the nonmethoxamine trials: following methoxamine administration an additional five spike trains are detected per trial, these units are recorded for an additional 10 s, and the units are discharging at an additional 2 pulse/s. Although coherence analyses are less sensitive to discharge characteristics than some traditional time domain measures (46), we do observe significant correlations between discharge rate and coherence. However, our results show that the difference in beta-band coherence was higher after methoxamine administration even after controlling for mean discharge rate of the CST. This suggests that the increases in coherence we observe following intrathecal administration of methoxamine is due to factors other than discharge rate.

Second, as our preparation is unparalyzed, the increased motor output following methoxamine administration will likely alter the reflexive drive to the motor pool. Increased motor unit recruitment and rate coding will generate more muscle force, and this may cause changes in reflexive drive, such as from force sensitive Ib afferents (14). Furthermore, neuromodulators may alter transmission of group II afferents to gamma motoneurons (22). Potential increases in patterned reflex loop activity may serve to pattern the common input and evoke coherence. In the human, such reflex activity is often ascribed to power in the alpha band (12). However, the soleus comprises particularly slow motor units with long time courses for both the afterhyperpolarization of the motoneuron and muscle twitch (13, 24). This contributes to the low-pass characteristics of the muscle at 5–10 Hz and would likely limit the ability to generate a reflex loop activation at the 15- to 30-Hz frequencies observed here.

Third, the increased beta-band coherence may be evoked by the neuromodulation itself. Both endogenous and exogenous neuromodulation would have the potential to provide a common source of input to the motor pool. It may be possible that patterns of this input may impact our results; however, descending monoaminergic neurons from brainstem nuclei discharge at particularly low rates. Serotonergic neurons from the rostral raphe nucleus discharge at low rates, ~3 Hz, whereas the more caudal raphe nuclei, which target spinal neurons, may discharge a bit higher, nearing 8 Hz. These nuclei discharge in an activity dependent manner (18, 60). The discharge of descending NE neurons is not well described in the cat; investigations in the rat and monkey have shown NE neurons in the locus coeruleus discharge at similar low frequencies (2 Hz), with peak discharge rates at 8–15 Hz during high levels of salient arousal (17). It remains unclear what effect decerebration may have on these discharge characteristics, but if similar to the intact condition, such physiological activation of brainstem nuclei may evoke coherence among spinal motoneurons well below the 15- to 30-Hz coherence observed here. Notwithstanding, the intrathecal application of neuromodulators to the lumbar cord used here has no explicit temporal pattern and may not rely on these endogenous parameters.

Fourth, the increased beta-band activity may be evoked via increases in synaptic drive to spinal motoneurons from spinal interneurons. Spinal interneurons demonstrate complex patterns of activation in response to neuromodulators. Pharmacological activation of the NE_α1 receptor has been shown to induce slow (<1 Hz) rhythms in the cervical (44) and lumbosacral (11, 20) circuitry in the neonate. The NE_α1 receptor demonstrates a particularly wide distribution throughout the spinal cord, with particular high densities in lamina II, IX, and X (21). As such, multiple candidate subpopulations may contribute including lamina II interneurons (68), group II interneurons in dorsal horn (5, 34), V2a interneurons (33), and ventral horn interneurons (58). The use of intraspinal microelectrode arrays may be able to aid in delineating a potential interneuronal subpopulation that may be driving this behavior (1).

Relationship to human movement.

Human motor output is thought to be consistent with these animal preparations, in that spinal neurons are sensitive to such neuromodulators. For example, both endogenous and exogenous increases in neuromodulatory drive can increases the gain of spinal motoneurons (63). It is not known at this point if neuromodulatory medications can alter coherence among motor units in humans. However, in the absence of exogenous inputs, the beta-band coherence observed in humans is often ascribed to cortical mechanisms (43). It is not clear if beta-band coherence could be generated by similar mechanisms in the intact cat. Pyramidal tract neurons in the cat do discharge at 15- to 30-Hz frequencies during quiet stance but can reach discharge rates of ~100 Hz during locomotion or a reaching task (65, 67). Irrespective, our current data demonstrate that noncortical mechanisms may generate beta-band coherence and this activity is augmented though neuromodulatory drive in the mammalian spinal cord.

GRANTS

This work was supported by a Craig H. Neilsen Foundation Postdoctoral Fellowship (C. K. Thompson), National Instiitutes of Health (NIH) Grants T32-HD-007418 (C. K. Thompson and M. D. Johnson), T32-EB-009406 (L. M. McPherson), R01-NS-109552 (C. J. Heckman), and R01-NS-085331 (R. K. Powers and C. J. Heckman), the European Union’s Horizon 2020 Research and Innovation Programme under Marie Skłodowska-Curie Grant agreement No. 702491 (NeuralCon; F. Negro) and by the European Research Council Advanced Grant contract No. 810346 (NaturalBionicS; D. Farina).

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

C.K.T., F.N., D.F., and C.J.H. conceived and designed research; C.K.T. and M.D.J. performed experiments; C.K.T., F.N., and L.M.M. analyzed data; C.K.T., F.N., D.F., and C.J.H. interpreted results of experiments; C.K.T. prepared figures; C.K.T. drafted manuscript; C.K.T., M.D.J., F.N., L.M.M., D.F., and C.J.H. edited and revised manuscript; C.K.T., M.D.J., F.N., L.M.M., D.F., and C.J.H. approved final version of manuscript.