Dynamic behaviour of alpha-motoneurons subjected to recurrent inhibition and reflex feedback via muscle spindles

Spatial and Temporal Arrangement of Recurrent Inhibition in the Primate Upper Limb

Renshaw cells mediate recurrent inhibition between motoneurons within the spinal cord. The function of this circuit is not clear; we previously suggested based on computational modeling that it may cancel oscillations in muscle activity around 10 Hz, thereby reducing physiological tremor. Such tremor is especially problematic for dexterous hand movements, yet knowledge of recurrent inhibitory function is sparse for the control of the primate upper limb, where no direct measurements have been made to date. In this study, we made intracellular penetrations into 89 motoneurons in the cervical enlargement of four terminally anesthetized female macaque monkeys, and recorded recurrent IPSPs in response to antidromic stimulation of motor axons. Recurrent inhibition was strongest to motoneurons innervating shoulder muscles and elbow extensors, weak to wrist and digit extensors, and almost absent to the intrinsic muscles of the hand. Recurrent inhibitory connections often spanned joints, for example from motoneurons innervating wrist and digit muscles to those controlling the shoulder and elbow. Wrist and digit flexor motoneurons sometimes inhibited the corresponding extensors, and vice versa. This complex connectivity presumably reflects the flexible usage of the primate upper limb. Using trains of stimuli to motor nerves timed as a Poisson process and coherence analysis, we also examined the temporal properties of recurrent inhibition. The recurrent feedback loop effectively carried frequencies up to 100 Hz, with a coherence peak around 20 Hz. The coherence phase validated predictions from our previous computational model, supporting the idea that recurrent inhibition may function to reduce tremor.

SIGNIFICANCE STATEMENT We present the first direct measurements of recurrent inhibition in primate upper limb motoneurons, revealing that it is more flexibly organized than previous observations in cat. Recurrent inhibitory connections were relatively common between motoneurons controlling muscles that act at different joints, and between flexors and extensors. As in the cat, connections were minimal for motoneurons innervating the most distal intrinsic hand muscles. Empirical data are consistent with previous modeling: temporal properties of the recurrent inhibitory feedback loop are compatible with a role in reducing physiological tremor by suppressing oscillations around 10 Hz.

Physical therapy examination and management of a 48-year-old male with vertigo, cephalalgia, and cervicalgia secondary to unilateral vestibular hypofunction

PURPOSE

This case report presents evidence-based physical therapy assessments and interventions for a patient with unilateral vestibular hypofunction (UVH). UVH is the result of peripheral vestibular dysfunction in the inner ear. Case Description : The patient was a 48-year-old male with symptoms of dizziness, cephalalgia, and cervicalgia. The examination and treatment were focused on impaired cervical proprioception, which is a vital component of balance training in addition to visual, vestibular, and somatosensory re-education for patients with dizziness. Toward the end of the physical therapy episode of care, the patient was medically diagnosed with Chiari malformation, a congenital cerebellar tonsillar herniation. Outcomes : The patient made significant strides on the Dizziness Handicap Inventory, Ten Meter Walk Test, Single Leg Stance, Balance Error Scoring System, Fukuda Stepping Test, Cervical Joint Position Error Sense Test, Convergence Distance, Global Rate of Change, and cervical range of motion assessments. The patient did not demonstrate comparable improvements on the Dynamic Visual Acuity Test. Conclusion : This case report demonstrates a physical therapy program for a patient with peripheral UVH-related symptoms. This approach may also be applicable for patients with the central cause of dizziness such as Chiari malformation. Future directions for research and clinical practice are also suggested in this report.

"The Fly": a new clinical assessment and treatment method for deficits of movement control in the cervical spine: reliability and validity

STUDY DESIGN

Test-retest and case-control study designed to detect accuracy of cervical spine movements by comparing 3 incrementally difficult movement patterns. An asymptomatic group, a nontrauma neck pain group, and a group with whiplash-associated disorders, Grade II, were tested (n = 18 in each group).

OBJECTIVE

To determine the test-retest reliability and the discriminative validity of the new Fly method.

SUMMARY OF BACKGROUND DATA

A lack of reliable and valid measures for grading the deficits of movement control in the cervical spine makes it impossible to prescribe treatment appropriate to each patient's respective impairment level.

METHODS

Head tracking of a moving fly which appeared on a computer screen. Easy, medium, and difficult patterns, each of which was repeated 3 times in random order, were tested. Amplitude accuracy (deviation of movements), directional accuracy (time on target, undershoots vs. overshoots) were compared across patterns and groups on 2 occasions, 1 week apart.

RESULTS

The intraclass correlation coefficient(2,1) ranged from 0.53 to 0.82 for both variables, except for the subvariable "overshoots" (0.14-0.42). The limits of agreement (LOA) were progressively wider across patterns (easy-medium-difficult) and groups (asymptomatic-nontrauma-whiplash-associated disorder). Analysis of variance with repeated measures revealed significant differences between patterns within each group and between groups respectively for both outcome variables (P < 0.001).

CONCLUSION

The Fly method provides reliable and valid measures for movement control of the cervical spine. Higher means and wider LOA across patterns and subject groups are reasoned to be inherent in the new Fly method and the subject groups tested. The wide LOA in the symptomatic groups supports the development of a normative database. The new Fly method can be used both as an assessment and a treatment method and ensures gradual progression in the treatment for deficits of movement control in patients with neck pain.

Sensorimotor function and dizziness in neck pain: implications for assessment and management

SYNOPSIS

The term sensorimotor describes all the afferent, efferent, and central integration and processing components involved in maintaining stability in the postural control system through intrinsic motor-control properties. The scope of this paper is to highlight the sensorimotor deficits that can arise from altered cervical afferent input. From a clinical orthopaedic perspective, the peripheral mechanoreceptors are the most important in functional joint stability; but in the cervical region they are also important for postural stability, as well as head and eye movement control. Consequently, conventional musculoskeletal intervention approaches may be sufficient only for patients with neck pain and minimal sensorimotor proprioceptive disturbances. Clinical experience and research indicates that significant sensorimotor cervical proprioceptive disturbances might be an important factor in the maintenance, recurrence, or progression of various symptoms in some patients with neck pain. In these cases, more specific and novel treatment methods are needed which progressively address neck position and movement sense, as well as cervicogenic oculomotor disturbances, postural stability, and cervicogenic dizziness. In this commentary we review the most relevant theoretical and practical knowledge on this matter and implications for clinical assessment and management, and we propose future directions for research.

LEVEL OF EVIDENCE

Level 5.

Muscle proprioceptive feedback and spinal networks

This review revolves primarily around segmental feedback systems established by muscle spindle and Golgi tendon organ afferents, as well as spinal recurrent inhibition via Renshaw cells. These networks are considered as to their potential contributions to the following functions: (i) generation of anti-gravity thrust during quiet upright stance and the stance phase of locomotion; (ii) timing of locomotor phases; (iii) linearization and correction for muscle nonlinearities; (iv) compensation for muscle lever-arm variations; (v) stabilization of inherently unstable systems; (vi) compensation for muscle fatigue; (vii) synergy formation; (viii) selection of appropriate responses to perturbations; (ix) correction for intersegmental interaction forces; (x) sensory-motor transformations; (xi) plasticity and motor learning. The scope will at times extend beyond the narrow confines of spinal circuits in order to integrate them into wider contexts and concepts.

Effects of spinal recurrent inhibition on motoneuron short-term synchronization

Spinal recurrent inhibition linking skeleto- motoneurons (alpha-MNs) via Renshaw cells (RCs) has been variously proposed to increase or decrease tendencies toward synchronous discharges between alpha-MNs. This controversy is not easy to settle experimentally in animal or human paradigms because RCs receive, in addition to excitatory input from alpha-MNs, many other modulating influences which may change their mode of operation. Computer simulations help to artificially isolate the recurrent inhibitory circuit and thus to study its effects on alpha-MN synchronization under conditions not achievable in natural experiments. We present here such a study which was designed to specifically test the following hypothesis. Since many alpha-MNs excite any particular Renshaw cell, which in turn inhibits many alpha-MNs, this convergence-divergence pattern establishes a random network whose random discharge patterns inject uncorrelated noise into alpha-MNs, and this noise counteracts any synchronization potentially arising from other sources, e.g., common inputs (Adam et al. in Biol Cybern 29:229-235, 1978). We investigated the short-term synchronization of alpha-MNs with two types of excitatory input signals to alpha-MNs (random and sinusoidally modulated random patterns). The main results showed that, while recurrent inhibitory inputs to different alpha-MNs were indeed different, recurrent inhibition (1) exerted rather small effects on the modulation of alpha-MN discharge, (2) tended to increase the short-term synchronization of alpha-MN discharge, and (3) did not generate secondary peaks in alpha-MN-alpha-MN cross-correlograms associated with alpha-MN rhythmicity.

Common modulation of motor unit pairs during slow wrist movement in man

1. The activity of 36 pairs of single motor units were recorded with intramuscular wire electrodes from m. extensor carpi radialis while subjects performed slow wrist extension and flexion movements. Periods of steady position holding were interposed between movements. 2. The discharge trains from pairs of motor units were analysed statistically in the time and frequency domains. During extension movements, when the muscle recorded from was the agonist, coherence between motor units was significant below 12 Hz, with a peak at 6-12 Hz in 30 of 36 pairs (83 %). The magnitude of coherence decreased during position holding compared to movements in 26 pairs, while the difference in average firing rate was small. 3. During movements, but not during position holding, coherence estimates between single motor units and acceleration showed a significant peak at 6-12 Hz in 56 out of 62 motor units, suggesting that a modulation of motor unit discharge contributed to angular acceleration at these frequencies. Common motor unit modulation was present at 3 Hz as well, although the coupling between motor unit activity was weaker than at 6-12 Hz. 4. It is concluded that a 6-12 Hz common modulation of agonist motor units is a distinguishing feature of slow voluntary wrist movements, extending the previously established notion of an 8-10 Hz rhythmic organization of slow finger movements to more proximal limb segments. It is suggested that the 6-12 Hz input is specific for movements and is normally absent or much weaker during steady maintenance of position or force.

Single motor unit activity in relation to pulsatile motor output in human finger movements

1. Forty-six single motor units in the common finger extensor, superficial finger flexor, and first dorsal interosseus muscles were recorded with intramuscular wire electrodes while subjects made voluntary flexion and extension finger movements at a single metacarpo-phalangeal joint. 2. Motor unit firing was analysed in relation to the 8-10 Hz discontinuities which previously have been shown to characterize these movements. Statistical time- and frequency-domain analyses of the activity of individual motor units in relation to the discontinuities showed that when the muscle was the agonist, all motor units in the common finger extensor muscle, and all units except one in the flexor muscles exhibited significant frequency modulation of their discharge in close temporal association with the joint acceleration. On the other hand, motor unit firing rate was not related to the frequency of the discontinuities. When the muscle recorded from was the antagonist, 21 of the 25 active units exhibited a similar frequency modulation. 3. When angular movement velocity was increased from 4 to 25 deg s-1, the strength of motor unit frequency modulation increased. Peak coherence between motor unit activity and acceleration increased by 74 %, on average, in the common finger extensor units. 4. The findings rule out a tentative mechanism attributing the discontinuities to newly recruited motor units firing at circa 8-10 Hz. Instead, a coherent 8-10 Hz input to the agonist and antagonist motoneurone pools is implied.

Functional roles of the proprioceptive system in the control of goal-directed movement

This article explored functional roles of the proprioceptive system during the control of goal-directed movements. Proprioceptive information contributes to the control of movement through both reflex and central connections. Spinal and transcortical reflex loops establish a servomechanism which provides automatic corrections of unexpected changes in muscle length and allows compensation for undesirable irregularities in the mechanical properties of muscles by modulating limb stiffness at the subconscious level. Central connections provide the control system with information about peripheral states which is used in voluntary components of movement control. Before the initiation of movement, proprioceptive information about initial limb orientation becomes a basis for the programming of motor commands. During a movement, proprioceptive input about velocities and angular displacements of a limb is used to regulate movement by triggering planned sequences of muscle activation and modulating motor commands. After movement, feedback produced by responses is compared with previously stored information, verifying the quality of the movement. Considering potential roles of the reflex and central connections, the proprioceptive system seems to constitute an important aspect of motor control mechanisms, providing the control system with efficiency and flexibility in the regulation of goal-directed movements.

On the role of recurrent inhibitory feedback in motor control

This article reviews presumed roles of recurrent inhibition in motor control, that have been proposed over the past five decades. The discussion is structured in an order of increasing complexity. It starts out with the simplest and earliest circuit, that is recurrent self-inhibition of skeleto-motoneurons, and related functions. It soon becomes clear that in order to understand recurrent inhibition, we must look beyond the simple self-inhibitory CNS circuit. First, recurrent inhibition must be seen in the context of other neural circuits. Second, some quantitative features appear to be correlated with features of the neuromusculo-skeletal periphery. Third, the aspect of lateral inhibition between different members of a motoneuron pool as well as between different motoneuron pools points to the essential multiple input-multiple output structure of recurrent inhibition that again can be understood only by correlating it with features of the neuromusculo-skeletal periphery. Another extension results from the discovery that recurrent inhibition affects not only skeleto-motoneurons, but also gamma-motoneurons, Ia inhibitory interneurons mediating reciprocal inhibition between antagonist motoneurons, other Renshaw cells and cells of origin of the ventral spinocerebellar tract (VSCT). Then the view broadens again, investigating the potential role that recurrent inhibition plays in two far-ranging theories of motor control, the inverse-dynamics approach and the equilibrium-point hypothesis. Finally, the present author tries to formulate, in broad strokes, a personal functional interpretation of recurrent inhibition. All the functional considerations, right or wrong, should yield ideas for new experiments, and this then is the last objective of this review.

Pulsatile motor output in human finger movements is not dependent on the stretch reflex

1. Stretch perturbations were delivered during slow voluntary finger movements with the aim of exploring the role of the stretch reflex in generating the 8-10 Hz discontinuities that characterize these movements. Afferent activity from muscle spindle primary endings in the finger extensor muscles was recorded from the radial nerve, along with the EMG activity of these muscles, and kinematics of the relevant metacarpo-phalangeal joint. 2. Perturbations elicited a distinct response from the muscle spindles appearing at the recording electrode after 13 ms, and weak reflex responses from the muscle with peak values at 53 and 63 ms during flexion and extension, respectively. 3. The time relations between kinematics, spindle firing and modulations of EMG activity elicited by the perturbations were compared with those of the self-generated discontinuities. These analyses indicate that stretch reflex mechanisms cannot account for the modulations of EMG activity that give rise to successive 8-10 Hz discontinuities. 4. A comparison of the reflex responses to perturbations with the EMG modulations during self-generated movements indicates that the reflex was too weak to account for the pulsatile motor output during voluntary movements. 5. By inference it was concluded that the 8-10 Hz discontinuities during self-generated movements are probably generated by mechanisms within the central nervous system.

Control variables and proprioceptive feedback in fast single-joint movement

Sensorimotor mechanisms were studied on the basis of kinematic and electromyographic data as well as the static torque developed by the muscles as a function of joint angle. The latter relationship is known as the torque/angle characteristic. Fast single-joint movement may result from a shift in this characteristic and a change in its slope. Such movements were studied at the wrist in 9 normal and 1 deafferented subject. After training to flex the wrist to a target, subjects repeated the same movements but in random test trials movements were opposed by the load generated by linear position feedback to a torque motor. At the end of the loaded trials, the load was suddenly removed. In the second experiment, subjects made wrist movements to the target that were opposed by the load and, on random test trials, the movements were not loaded. In these test trials, the wrist arrived in a static position outside the target zone. In both experiments, subjects were instructed not to correct errors. The final torque/angle characteristics specified in the movements were reconstructed on the basis of the static wrist positions and torques before and after unloading. Normal subjects made movements by shifting the position of the torque/angle characteristic and by increasing its slope. If subjects indeed maintained the same pattern of control variables (descending commands), the same final position of the characteristic would be reproduced from trial to trial regardless of load perturbations. This assumption of equifinality was tested by comparing the final position of the wrist in nonloaded movements with that after removal of the load in loaded movements. Equifinality was observed in normal subjects. Movements in the deafferented subject were also associated with a shift of the torque/angle characteristic and a change in its slope. However, she was unable to consistently reproduce its final position. In spite of muscle coactivation, her maximal stiffness was lower than in normal subjects. In the absence of vision, the subject made movements with the load by increasing the slope of the characteristic instead of by shifting its position far enough. Load perturbation affected her final wrist position (inequifinality), which may reflect the presence of a significant hysteresis of the characteristic as a result of the absence of stretch reflexes. The deficits following deafferentation presumably result from the destruction of biomechanical and sensorimotor mechanisms including the ability of control variables to specify the positional frame of reference for afferent and descending systems.

Frequency response method in pharmacokinetics

The paper presents the demonstration of applicability of the frequency response method in a bioavailability study. The frequency response method, common in system engineering, is based on an approximation of the frequency response of a linear dynamic system, calculated from input-output measurements, by a frequency model of the system transfer function in the frequency domain. In general, the influence of the system structure on the form of the system frequency response is much more distinct than on the form of the system output. This is of great advantage in modeling the system frequency response instead of the system output, commonly used in pharmacokinetics. After a brief theoretical section, the method is demonstrated on the estimation of the rate and extent of gentamicin bioavailability after intratracheal administration to guinea pigs. The optimal frequency model of the system describing the gentamicin pathway into the systemic circulation and point estimates of its parameters were selected by the approximation of the system frequency response in the frequency domain, using a noniterative algorithm. Two similar estimates of the system weighing function were independently obtained: the weighting function of the selected frequency model and the weighting function estimated by the numerical deconvolution procedure. Neither of the estimates of the weighting function does decrease monotonously after the maximum of about 2.2-2.5 unit of dose hr-1 recorded approximately 0.1 hr after drug administration. Both estimates show a marked additional peak approximately at 0.3 hr after administration and possible peaks in the further time period. We hypothesized that the loop found in the frequency response calculated and in the selected optimal frequency model, the high-order of this model, and several peaks identified in the estimates of the system weighting function indicated the complexity of the system and the presence of time delays. Three estimates of the extent of gentamicin intratracheal bioavailability obtained by the three different ways: directly from the calculated frequency response, calculated using the selected frequency model, and by the deconvolution method were 0.950, 0.934, and 0.907 respectively. Thus the conclusion can be made that gentamicin injected intratracheally to guinea pigs is almost completely available.

Signal transmission from motor axons to group Ia muscle spindle afferents: frequency responses and second-order non-linearities

Spinal recurrent inhibition via Renshaw cells and proprioceptive feedback via skeletal muscle and muscle spindle afferents have been hypothesized to constitute a compound feedback system [Windhorst (1989) Afferent Control of Posture and Locomotion; Windhorst (1993) Robots and Biological Systems--Towards a New Bionics]. To assess their detailed functions, it is necessary to know their dynamic characteristics. Previously we have extensively described the properties of signal transmission from motor axons to Renshaw cells using random motor axon stimulation and data analysis methods based thereupon. Using the same methods, we here compare these properties, in the cat, with those between motor axons and group Ia muscle spindle afferents in terms of frequency responses and nonlinear features. The frequency responses depend on the mean rate (carrier rate) of activation of motor axons and on the strength of coupling between motor units and spindles. In general, they are those of a second-order low-pass system with a cut-off at fairly low frequencies. This contrasts with the dynamics of motor axon-Renshaw cell couplings which are those of a much broader band-pass with its peak in the range of c. 2-15 Hz [Christakos (1987) Neuroscience 23, 613-623]. The second-order non-linearities in motor unit-muscle spindle signal lines are much more diverse than those in motor axon-Renshaw cell couplings. Although the average strength of response declines with mean stimulus rate in both subsystems, there is no systematic relationship between the amount of non-linearity and the average response in the former, whilst there is in the latter. The qualitative appearance of motor unit-muscle spindle non-linearities was complicated as was the average response to motor unit twitches. Thus, whilst Renshaw cells appear to dynamically reflect motor output rather faithfully, muscle spindles seem to signal local muscle fibre length changes and their dynamics. This would be consistent with the hypothesis that the two feedback pathways monitor different state variables determining the production of muscle force: neural input and length and its changes. Specifically, the dynamic properties of both subsystems may combine favourably to decrease the risk of instability (tremor) in the motoneuron-muscle spindle loop.