Variable impact of tizanidine on the medium latency reflex of upper and lower limbs

Secondary endings of muscle spindles: Structure, reflex action, role in motor control and proprioception

NEW FINDINGS

What is the topic of this review? We describe the structure and function of secondary sensory endings of muscle spindles, their reflex action and role in motor control and proprioception. What advances does it highlight? In most mammalian skeletal muscles, secondary endings of spindles are more or much more numerous than primary endings but are much less well studied. By focusing on secondary endings in this review, we aim to redress the balance, draw attention to what is not known and stimulate future research.

ABSTRACT

Kinaesthesia and the control of bodily movement rely heavily on the sensory input from muscle spindles. Hundreds of these sensory structures are embedded in mammalian muscles. Each spindle has one or more sensory endings and its own complement of small muscle fibres that are activated by the CNS via fusimotor neurons, providing efferent control of sensory responses. Exactly how the CNS wields this influence remains the subject of much fascination and debate. There are two types of sensory endings, primary and secondary, with differing development, morphology, distribution and responsiveness. Spindle primary endings have received more attention than secondaries, although the latter usually outnumber them. This review focuses on the secondary endings. Their location within the spindle, their response properties, the projection of their afferents within the CNS and their reflex actions all suggest that secondaries have certain separate roles from the primaries in proprioception and motor control. Specifically, spindle secondaries seem more adapted than primaries to signalling slow and maintained changes in the relative position of bodily segments, thereby contributing to position sense, postural control and static limb positioning. By highlighting, in this way, the roles of secondary endings, a final aim of the review is to broaden understanding of muscle spindles more generally and of the important contributions they make to both sensory and motor mechanisms.

Long-latency Responses to a Mechanical Perturbation of the Index Finger Have a Spinal Component

In an uncertain external environment, the motor system may need to respond rapidly to an unexpected stimulus. Limb displacement causes muscle stretch; the corrective response has multiple activity bursts, which are suggested to originate from different parts of the neuraxis. The earliest response is so fast, it can only be produced by spinal circuits; this is followed by slower components thought to arise from primary motor cortex (M1) and other supraspinal areas.

In an uncertain external environment, the motor system may need to respond rapidly to an unexpected stimulus. Limb displacement causes muscle stretch; the corrective response has multiple activity bursts, which are suggested to originate from different parts of the neuraxis. The earliest response is so fast, it can only be produced by spinal circuits; this is followed by slower components thought to arise from primary motor cortex (M1) and other supraspinal areas. Spinal cord (SC) contributions to the slower components are rarely considered. To address this, we recorded neural activity in M1 and the cervical SC during a visuomotor tracking task, in which 2 female macaque monkeys moved their index finger against a resisting motor to track an on-screen target. Following the behavioral trial, an increase in motor torque rapidly returned the finger to its starting position (lever velocity >200°/s). Many cells responded to this passive mechanical perturbation (M1: 148 of 211 cells, 70%; SC: 67 of 119 cells, 56%). The neural onset latency was faster for SC compared with M1 cells (21.7 ± 11.2 ms vs 25.5 ± 10.7 ms, respectively, mean ± SD). Using spike-triggered averaging, some cells in both regions were identified as likely premotor cells, with monosynaptic connections to motoneurons. Response latencies for these cells were compatible with a contribution to the muscle responses following the perturbation. Comparable fractions of responding neurons in both areas were active up to 100 ms after the perturbation, suggesting that both SC circuits and supraspinal centers could contribute to later response components.

SIGNIFICANCE STATEMENT Following a limb perturbation, multiple reflexes help to restore limb position. Given conduction delays, the earliest part of these reflexes can only arise from spinal circuits. By contrast, long-latency reflex components are typically assumed to originate from supraspinal centers. We recorded from both spinal and motor cortical cells in monkeys responding to index finger perturbations. Many spinal interneurons, including those identified as projecting to motoneurons, responded to the perturbation; the timing of responses was compatible with a contribution to both short- and long-latency reflexes. We conclude that spinal circuits also contribute to long-latency reflexes in distal and forearm muscles, alongside supraspinal regions, such as the motor cortex and brainstem.

Increased human stretch reflex dynamic sensitivity with height-induced postural threat

KEY POINTS

Threats to standing balance (postural threat) are known to facilitate soleus tendon-tap reflexes, yet the mechanisms driving reflex changes are unknown. Scaling of ramp-and-hold dorsiflexion stretch reflexes to stretch velocity and amplitude were examined as indirect measures of changes to muscle spindle dynamic and static function with height-induced postural threat. Overall, stretch reflexes were larger with threat. Furthermore, the slope (gain) of the stretch-velocity vs. short-latency reflex amplitude relationship was increased with threat. These findings are interpreted as indirect evidence for increased muscle spindle dynamic sensitivity, independent of changes in background muscle activity levels, with a threat to standing balance. We argue that context-dependent scaling of stretch reflexes forms part of a multisensory tuning process where acquisition and/or processing of balance-relevant sensory information is continuously primed to facilitate feedback control of standing balance in challenging balance scenarios.

ABSTRACT

Postural threat increases soleus tendon-tap (t-) reflexes. However, it is not known whether t-reflex changes are a result of central modulation, altered muscle spindle dynamic sensitivity or combined spindle static and dynamic sensitization. Ramp-and-hold dorsiflexion stretches of varying velocities and amplitudes were used to examine velocity- and amplitude-dependent scaling of short- (SLR) and medium-latency (MLR) stretch reflexes as an indirect indicator of spindle sensitivity. t-reflexes were also performed to replicate previous work. In the present study, we examined the effects of postural threat on SLR, MLR and t-reflex amplitude, as well as SLR-stretch velocity scaling. Forty young-healthy adults stood with one foot on a servo-controlled tilting platform and the other on a stable surface. The platform was positioned on a hydraulic lift. Threat was manipulated by having participants stand in low (height 1.1 m; away from edge) then high (height 3.5 m; at the edge) threat conditions. Soleus stretch reflexes were recorded with surface electromyography and SLRs and MLRs were probed with fixed-amplitude variable-velocity stretches. t-reflexes were evoked with Achilles tendon taps using a linear motor. SLR, MLR and t-reflexes were 11%, 9.5% and 16.9% larger, respectively, in the high compared to low threat condition. In 22 out of 40 participants, SLR amplitude was correlated to stretch velocity at both threat levels. In these participants, the gain of the SLR-velocity relationship was increased by 36.1% with high postural threat. These findings provide new supportive evidence for increased muscle spindle dynamic sensitivity with postural threat and provide further support for the context-dependent modulation of human somatosensory pathways.

Neuromodulatory Inputs to Motoneurons Contribute to the Loss of Independent Joint Control in Chronic Moderate to Severe Hemiparetic Stroke

In chronic hemiparetic stroke, increased shoulder abductor activity causes involuntary increases in elbow, wrist, and finger flexor activation, an abnormal muscle coactivation pattern known as the flexion synergy. Recent evidence suggests that flexion synergy expression may reflect recruitment of contralesional cortico-reticulospinal motor pathways following damage to the ipsilesional corticospinal tract. However, because reticulospinal motor pathways produce relatively weak post-synaptic potentials in motoneurons, it is unknown how preferential use of these pathways could lead to robust muscle activation. Here, we hypothesize that the descending neuromodulatory component of the ponto-medullary reticular formation, which uses the monoaminergic neurotransmitters norepinephrine and serotonin, serves as a gain control mechanism to facilitate motoneuron responses to reticulospinal motor commands. Thus, inhibition of the neuromodulatory component would reduce flexion synergy expression by disfacilitating spinal motoneurons. To test this hypothesis, we conducted a pre-clinical study utilizing two targeted neuropharmacological probes and inert placebo in a cohort of 16 individuals with chronic hemiparetic stroke. Test compounds included Tizanidine (TIZ), a noradrenergic α₂ agonist and imidazoline ligand selected for its ability to reduce descending noradrenergic drive, and Isradipine, a dihyropyridine calcium-channel antagonist selected for its ability to post-synaptically mitigate a portion of the excitatory effects of monoamines on motoneurons. We used a previously validated robotic measure to quantify flexion synergy expression. We found that Tizanidine significantly reduced expression of the flexion synergy. A predominantly spinal action for this effect is unlikely because Tizanidine is an agonist acting on a baseline of spinal noradrenergic drive that is likely to be pathologically enhanced post-stroke due to increased reliance on cortico-reticulospinal motor pathways. Although spinal actions of TIZ cannot be excluded, particularly from Group II pathways, our finding is consistent with a supraspinal action of Tizanidine to reduce descending noradrenergic drive and disfacilitate motoneurons. The effects of Isradipine were not different from placebo, likely related to poor central bioavailability. These results support the hypothesis that the descending monoaminergic component of the ponto-medullary reticular formation plays a key role in flexion synergy expression in chronic hemiparetic stroke. These results may provide the basis for new therapeutic strategies to complement physical rehabilitation.

Altered Neuromodulatory Drive May Contribute to Exaggerated Tonic Vibration Reflexes in Chronic Hemiparetic Stroke

Exaggerated stretch-sensitive reflexes are a common finding in elbow flexors of the contralesional arm in chronic hemiparetic stroke, particularly when muscles are not voluntarily activated prior to stretch. Previous investigations have suggested that this exaggeration could arise either from an abnormal tonic ionotropic drive to motoneuron pools innervating the paretic limbs, which could bring additional motor units near firing threshold, or from an increased influence of descending monoaminergic neuromodulatory pathways, which could depolarize motoneurons and amplify their responses to synaptic inputs. However, previous investigations have been unable to differentiate between these explanations, leaving the source(s) of this excitability increase unclear. Here, we used tonic vibration reflexes (TVRs) during voluntary muscle contractions of increasing magnitude to infer the sources of spinal motor excitability in individuals with chronic hemiparetic stroke. We show that when the paretic and non-paretic elbow flexors are preactivated to the same percentage of maximum prior to vibration, TVRs remain significantly elevated in the paretic arm. We also show that the rate of vibration-induced torque development increases as a function of increasing preactivation in the paretic limb, even though the amplitude of vibration-induced torque remains conspicuously unchanged as preactivation increases. It is highly unlikely that these findings could be explained by a source that is either purely ionotropic or purely neuromodulatory, because matching preactivation should control for the effects of a potential ionotropic drive (and lead to comparable tonic vibration reflex responses between limbs), while a purely monoaminergic mechanism would increase reflex magnitude as a function of preactivation. Thus, our results suggest that increased excitability of motor pools innervating the paretic limb post-stroke is likely to arise from both ionotropic and neuromodulatory mechanisms.