Locomotor-related propriospinal V3 neurons produce primary afferent depolarization and modulate sensory transmission to motoneurons

Modulation of Sensory Input to the Spinal Cord by Peripheral Afferent Fibres via GABAergic Astrocytes

A long-lasting GABA-dependent increase in the excitability of afferent fibres, and thus modulation of the sensory input to the spinal cord, may be evoked by epidural polarization. However, the direct effects of fibre polarization are short-lasting and the sustained increase in their excitability appears to be secondary to the release of GABA from nearby astrocytes. We have now investigated whether the modulation of spinal sensory input by stimulation of a peripheral nerve, previously attributed to synaptically evoked intraspinal field potentials, is evoked in a similar way. However, as neither its dependence on GABA nor its relays have been investigated, we addressed the question of whether the increase in the excitability of epidurally stimulated afferent fibres following a peripheral nerve stimulation does or does not depend on GABA and whether it might be mediated by astrocytes. The effects of conditioning stimulation of the tibial nerve were evaluated from changes in the excitability of both Group I and II muscle afferents, estimated from action potentials recorded in peripheral nerves and in field potentials recorded in the dorsal horn respectively in acute experiments on deeply anaesthetized rats. The excitability of the afferents was increased by stimulation of Group II and/or cutaneous but not Group I muscle afferents. The effects were significantly weakened by blocking GABA channels by gabazine and by astrocyte toxin L-alpha-aminoadipic acid (L-AAA), indicating that the excitability of both Group I and II afferent fibres may be modulated by GABAergic astrocytes, the new role played by astrocytes.

Widespread innervation of motoneurons by spinal V3 neurons globally amplifies locomotor output in mice

While considerable progress has been made in understanding the neuronal circuits that underlie the patterning of locomotor behaviors, less is known about the circuits that amplify motoneuron output to adjust muscle force. Here, we demonstrate that propriospinal V3 neurons (Sim1+) account for ∼20% of excitatory input to motoneurons across hindlimb muscles. V3 neurons also form extensive connections among themselves and with other excitatory premotor neurons, such as V2a neurons. Optical activation of V3 neurons in a single segment rapidly amplifies locomotor-related motoneuron output at all lumbar segments in in vitro spinal cord and the awake adult mouse. Despite similar innervation from V3 neurons to flexor and extensor motoneuron pools, V3 neurons preferentially activate extensor muscles. Genetically or optogenetically silencing V3 neurons leads to slower and weaker mice with a reduced ability to adjust extensor muscle force. Thus, V3 neurons serve as global command neurons that amplify locomotion intensity.

Medial and lateral vestibulospinal projections to the cervical spinal cord of the squirrel monkey

Introduction

The brainstem vestibular nuclei neurons receive synaptic inputs from inner ear acceleration-sensing hair cells, cerebellar output neurons, and ascending signals from spinal proprioceptive-related neurons. The lateral (LVST) and medial (MVST) vestibulospinal (VS) tracts convey their coded signals to the spinal circuits to rapidly counter externally imposed perturbations to facilitate stability and provide a framework for self-generated head movements.

Methods

The present study describes the morphological characteristics of intraaxonally recorded and labeled VS neurons monosynaptically connected to the 8th nerve. The visualization of axon location in the descending medial longitudinal fasciculus (MLF) differentiated ipsi- (i) and contralateral (c)-projecting MVST neurons. Vestibuloocular collic (VOC) neurons were comparably typed as cMVST cells but were also antidromically activated from the rostral MLF. Cervical-only LVST neurons projected ipsilaterally in the lateral to ventrolateral funiculi. Targets of VS axons, such as central cervical nucleus neurons, sternocleidomastoid, trapezius, and splenius motoneurons, were identified using anti- and orthodromic electrical stimuli and intra-somatically labeled to describe their local spinal morphology.

Results

Thirty-five VS neurons (26% of the 134 attempted samples) were successfully labeled to permit a moderate to (near) complete reconstruction of their trajectories and synaptic innervations. VOC neurons exhibited a prolific innervation of caudal brainstem nuclei, extensively innervated laminae VII and VIII, and, to a lesser extent, lateral and ventromedial lamina IX, from C1 to C8, and on average issued 15 branches along their trajectory with 92 terminal and en passant boutons per branch. The VOC innervation was either uniformly distributed among the cervical segments, indicating a more global control of head and neck movement, or restricted specific spinal segments, indicating a more precise motor control strategy. The innervation pattern of iMVST axons resembled that of VOC and cMVST axons but was less extensive and supplied mostly the upper two cervical segments. LVST and cMVST neurons exhibited a predominantly equally weighted innervation of separate and joint moto- and inter-neuronal spinal circuits along their cervical trajectory.

Discussion

Their extensive axon branching distribution in the ventral horn provides a redundant and variable synaptic input to spinal cell groups. This suggests a common and site-specific control of the head and neck reflexes.

Modulation of sensory input to the spinal cord: Contribution of focal epidural polarization and of GABA released by interneurons and glial cells

Modulation of input from primary afferent fibres has long been examined at the level of the first relays of these fibres. However, recent studies reveal that input to the spinal cord may also be modulated at the level of the very entry of afferent fibres to the spinal grey matter before action potentials in intraspinal collaterals of afferent fibres reach their target neurons. Such modulation greatly depends on the actions of GABA via extrasynaptic membrane receptors. In the reported study we hypothesized that the increase in excitability of afferent fibres following epidural polarization close to the site where collaterals of afferent fibres leave the dorsal columns is due to the release of GABA from two sources: not only GABAergic interneurons but also glial cells. We present evidence, primo, that GABA released from both these sources contributes to a long-lasting increase in the excitability and a shortening of the refractory period of epidurally stimulated afferent fibres and, secondo, that effects of epidural polarization on the release of GABA are more critical for these changes than direct effects of DC on the stimulated fibres. The experiments were carried out in deeply anaesthetized rats in which changes in compound action potentials evoked in hindlimb peripheral nerves by dorsal column stimulation were used as a measure of the excitability of afferent fibres. The study throws new light on the modulation of input to spinal networks but also on mechanisms underlying the restoration of spinal functions.

Presynaptic inhibition selectively suppresses leg proprioception in behaving Drosophila

Controlling arms and legs requires feedback from proprioceptive sensory neurons that detect joint position and movement. Proprioceptive feedback must be tuned for different behavioral contexts, but the underlying circuit mechanisms remain poorly understood. Using calcium imaging in behaving Drosophila, we find that the axons of position-encoding leg proprioceptors are active across behaviors, whereas the axons of movement-encoding leg proprioceptors are suppressed during walking and grooming. Using connectomics, we identify a specific class of interneurons that provide GABAergic presynaptic inhibition to the axons of movement-encoding proprioceptors. The predominant synaptic inputs to these interneurons are descending neurons, suggesting they are driven by predictions of leg movement originating in the brain. Calcium imaging from both the interneurons and their descending inputs confirmed that their activity is correlated with self-generated but not passive leg movements. Overall, our findings elucidate a neural circuit for suppressing specific proprioceptive feedback signals during self-generated movements.

Skin and not dorsal root stimulation reduces hypertonus in thoracic motor complete spinal cord injury: a single case report

On demand and localized treatment for excessive muscle tone after spinal cord injury (SCI) is currently not available. Here, we examine the reduction in leg hypertonus in a person with mid-thoracic, motor complete SCI using a commercial transcutaneous electrical stimulator (TES) applied at 50 or 150 Hz to the lower back and the possible mechanisms producing this bilateral reduction in leg tone. Hypertonus of knee extensors without and during TES, with both cathode (T11-L2) and anode (L3-L5) placed over the spinal column (midline, MID) or 10 cm to the left of midline (lateral, LAT) to only active underlying skin and muscle afferents, was simultaneously measured in both legs with the pendulum test. Spinal reflexes mediated by proprioceptive (H-reflex) and cutaneomuscular reflex (CMR) afferents were examined in the right leg opposite to the applied LAT TES. Hypertonus disappeared in both legs but only during thoracolumbar TES, and even during LAT TES. The marked reduction in tone was reflected in the greater distance both lower legs first dropped to after being released from a fully extended position, increasing by 172.8% and 94.2% during MID and LAT TES, respectively, compared with without TES. Both MID and LAT (left) TES increased H-reflexes but decreased the first burst, and lengthened the onset of subsequent bursts, in the cutaneomuscular reflex of the right leg. Thoracolumbar TES is a promising method to decrease leg hypertonus in chronic, motor complete SCI without activating spinal cord structures and may work by facilitating proprioceptive inputs that activate excitatory interneurons with bilateral projections that in turn recruit recurrent inhibitory neurons.NEW & NOTEWORTHY We present proof of concept that surface stimulation of the lower back can reduce severe leg hypertonus in a participant with motor complete, thoracic spinal cord injury (SCI) but only during the applied stimulation. We propose that activation of skin and muscle afferents from thoracolumbar transcutaneous electrical stimulation (TES) may recruit excitatory spinal interneurons with bilateral projections that in turn recruit recurrent inhibitory networks to provide on demand suppression of ongoing involuntary motoneuron activity.

Widespread innervation of motoneurons by spinal V3 neurons globally amplifies locomotor output in mice

While considerable progress has been made in understanding the neuronal circuits that underlie the patterning of locomotor behaviours such as walking, less is known about the circuits that amplify motoneuron output to enable adaptable increases in muscle force across different locomotor intensities. Here, we demonstrate that an excitatory propriospinal neuron population (V3 neurons, Sim1 + ) forms a large part of the total excitatory interneuron input to motoneurons (∼20%) across all hindlimb muscles. Additionally, V3 neurons make extensive connections among themselves and with other excitatory premotor neurons (such as V2a neurons). These circuits allow local activation of V3 neurons at just one segment (via optogenetics) to rapidly depolarize and amplify locomotor-related motoneuron output at all lumbar segments in both the in vitro spinal cord and the awake adult mouse. Interestingly, despite similar innervation from V3 neurons to flexor and extensor motoneuron pools, functionally, V3 neurons exhibit a pronounced bias towards activating extensor muscles. Furthermore, the V3 neurons appear essential to extensor activity during locomotion because genetically silencing them leads to slower and weaker mice with a poor ability to increase force with locomotor intensity, without much change in the timing of locomotion. Overall, V3 neurons increase the excitability of motoneurons and premotor neurons, thereby serving as global command neurons that amplify the locomotion intensity.

Tapping into the human spinal locomotor centres with transspinal stimulation

Human locomotion is controlled by spinal neuronal networks of similar properties, function, and organization to those described in animals. Transspinal stimulation affects the spinal locomotor networks and is used to improve standing and walking ability in paralyzed people. However, the function of locomotor centers during transspinal stimulation at different frequencies and intensities is not known. Here, we document the 3D joint kinematics and spatiotemporal gait characteristics during transspinal stimulation at 15, 30, and 50 Hz at sub-threshold and supra-threshold stimulation intensities. We document the temporal structure of gait patterns, dynamic stability of joint movements over stride-to-stride fluctuations, and limb coordination during walking at a self-selected speed in healthy subjects. We found that transspinal stimulation (1) affects the kinematics of the hip, knee, and ankle joints, (2) promotes a more stable coordination at the left ankle, (3) affects interlimb coordination of the thighs, and (4) intralimb coordination between thigh and foot, (5) promotes greater dynamic stability of the hips, (6) increases the persistence of fluctuations in step length variability, and lastly (7) affects mechanical walking stability. These results support that transspinal stimulation is an important neuromodulatory strategy that directly affects gait symmetry and dynamic stability. The conservation of main effects at different frequencies and intensities calls for systematic investigation of stimulation protocols for clinical applications.

Tapping Into the Human Spinal Locomotor Centres With Transspinal Stimulation

Human locomotion is controlled by spinal neuronal networks of similar properties, function, and organization to those described in animals. Transspinal stimulation affects the spinal locomotor networks and is used to improve standing and walking ability in paralyzed people. However, the function of locomotor centers during transspinal stimulation at different frequencies and intensities is not known. Here, we document the 3D joint kinematics and spatiotemporal gait characteristics during transspinal stimulation at 15, 30, and 50 Hz at sub-threshold and supra-threshold stimulation intensities. We document the temporal structure of gait patterns, dynamic stability of joint movements over stride-to-stride fluctuations, and limb coordination during walking at a self-selected speed in healthy subjects. We found that transspinal stimulation 1) affects the kinematics of the hip, knee, and ankle joints, 2) promotes a more stable coordination at the left ankle, 3) improves interlimb coordination of the thighs, 4) improves intralimb coordination between thigh and foot, 5) promotes greater dynamic stability of the hips, and lastly 6) affects the mechanical stability of the joints. These results support that transspinal stimulation is an important neuromodulatory strategy that directly affects gait symmetry and dynamic stability. The conservation of main effects at different frequencies and intensities calls for systematic investigation of stimulation protocols for clinical applications.

Post-activation depression from primary afferent depolarization (PAD) produces extensor H-reflex suppression following flexor afferent conditioning

Suppression of the extensor H-reflex by flexor afferent conditioning is thought to be produced by a long-lasting inhibition of extensor Ia afferent terminals via GABAA receptor-activated primary afferent depolarization (PAD). Given the recent finding that PAD does not produce presynaptic inhibition of Ia afferent terminals, we examined in 28 participants if H-reflex suppression is instead mediated by post-activation depression of the extensor Ia afferents triggered by PAD-evoked spikes and/or by a long-lasting inhibition of the extensor motoneurons. A brief conditioning vibration of the flexor tendon suppressed both the extensor soleus H-reflex and the tonic discharge of soleus motor units out to 150 ms following the vibration, suggesting that part of the H-reflex suppression during this period was mediated by postsynaptic inhibition of the extensor motoneurons. When activating the flexor afferents electrically to produce conditioning, the soleus H-reflex was also suppressed but only when a short-latency reflex was evoked in the soleus muscle by the conditioning input itself. In mice, a similar short-latency reflex was evoked when optogenetic or afferent activation of GABAergic (GAD2+ ) neurons produced a large enough PAD to evoke orthodromic spikes in the test Ia afferents, causing post-activation depression of subsequent monosynaptic EPSPs. The long duration of this post-activation depression and related H-reflex suppression (seconds) was similar to rate-dependent depression that is also due to post-activation depression. We conclude that extensor H-reflex inhibition by brief flexor afferent conditioning is produced by both post-activation depression of extensor Ia afferents and long-lasting inhibition of extensor motoneurons, rather than from PAD inhibiting Ia afferent terminals. KEY POINTS: Suppression of extensor H-reflexes by flexor afferent conditioning was thought to be mediated by GABAA receptor-mediated primary afferent depolarization (PAD) shunting action potentials in the Ia afferent terminal. In line with recent findings that PAD has a facilitatory role in Ia afferent conduction, we show here that when large enough, PAD can evoke orthodromic spikes that travel to the Ia afferent terminal to evoke EPSPs in the motoneuron. These PAD-evoked spikes also produce post-activation depression of Ia afferent terminals and may mediate the short- and long-lasting suppression of extensor H-reflexes in response to flexor afferent conditioning. Our findings highlight that we must re-examine how changes in the activation of GABAergic interneurons and PAD following nervous system injury or disease affects the regulation of Ia afferent transmission to spinal neurons and ultimately motor dysfunction in these disorders.