Physiology and clinical applications of hand muscle reflexes

Excitatory and inhibitory responses to cervical root magnetic stimulation in healthy subjects

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We studied excitatory and inhibitory responses to cervical root magnetic stimulation.

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CRMS elicited direct and reflex responses in hand muscles.

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CRMS is painless and well tolerated and, therefore, applicable to clinical studies.

Objectives

To characterize direct and reflex hand muscle responses to cervical root magnetic stimulation (CRMS) in healthy volunteers during sustained voluntary contraction.

Methods

In 18 healthy volunteers, we recorded from the first dorsal interosseous (FDI) muscle the responses to CRMS of progressively increasing intensity and level of muscle contraction. The compound muscle action potential (CMAP) and the silent period (SP) were compared to those obtained with plexus, midarm and wrist stimulation. Additionally, in a smaller number of subjects, we obtained the peristimulus time histogram (psth) of single motor unit firing in the FDI, examined the effects of vibration and recorded the modulation of sustained EMG activity in muscles of the lower limbs.

Results

Increasing CRMS intensity led to larger CMAP with no relevant changes in SP1 or SP2, except for lower amplitude of the burst interrupting the silent period (BISP). Increasing the level of muscle contraction led to reduced CMAP, shorter SP duration and increased BISP amplitude. The psth analysis showed the underlying changes in the motor unit firing frequency that corresponded to the changes seen in the CMAP and the SP with surface recordings. Progressively distal stimulation led to CMAPs of shorter latency and increased amplitude, SPs of longer latency and shorter duration, and a BISP of longer latency. Vibration led to reduction of the SP. CRMS induced SPs in muscles of the lower limb.

Conclusions

CRMS induces excitatory and inhibitory responses in hand muscles, fitting with the expected behavior of mixed nerve stimulation at very proximal sites.

Significance

Characterization of the effects of CRMS on hand muscles is of physiological and potentially clinical applicability, as it is a painless and reliable procedure.

Evaluation of movement and brain activity

Clinical neurophysiology studies can contribute important information about the physiology of human movement and the pathophysiology and diagnosis of different movement disorders. Some techniques can be accomplished in a routine clinical neurophysiology laboratory and others require some special equipment. This review, initiating a series of articles on this topic, focuses on the methods and techniques. The methods reviewed include EMG, EEG, MEG, evoked potentials, coherence, accelerometry, posturography (balance), gait, and sleep studies. Functional MRI (fMRI) is also reviewed as a physiological method that can be used independently or together with other methods. A few applications to patients with movement disorders are discussed as examples, but the detailed applications will be the subject of other articles.

Principles of Electrophysiological Assessments for Movement Disorders

Electrophysiological studies can provide objective and quantifiable assessments of movement disorders. They are useful in the diagnosis of hyperkinetic movement disorders, particularly tremors and myoclonus. The most commonly used measures are surface electromyography (sEMG), electroencephalography (EEG) and accelerometry. Frequency and coherence analyses of sEMG signals may reveal the nature of tremors and the source of the tremors. The effects of voluntary tapping, ballistic movements and weighting of the limbs can help to distinguish between organic and functional tremors. The presence of Bereitschafts-potentials and beta-band desynchronization recorded by EEG before movement onset provide strong evidence for functional movement disorders. EMG burst durations, distributions and muscle recruitment orders may identify and classify myoclonus to cortical, subcortical or spinal origins and help in the diagnosis of functional myoclonus. Organic and functional cervical dystonia can potentially be distinguished by EMG power spectral analysis. Several reflex circuits, such as the long latency reflex, blink reflex and startle reflex, can be elicited with different types of external stimuli and are useful in the assessment of myoclonus, excessive startle and stiff person syndrome. However, limitations of the tests should be recognized, and the results should be interpreted together with clinical observations.

Automatic gain control of neural coupling during cooperative hand movements

Cooperative hand movements (e.g. opening a bottle) are controlled by a task-specific neural coupling, reflected in EMG reflex responses contralateral to the stimulation site. In this study the contralateral reflex responses in forearm extensor muscles to ipsilateral ulnar nerve stimulation was analyzed at various resistance and velocities of cooperative hand movements. The size of contralateral reflex responses was closely related to the level of forearm muscle activation required to accomplish the various cooperative hand movement tasks. This indicates an automatic gain control of neural coupling that allows a rapid matching of corrective forces exerted at both sides of an object with the goal 'two hands one action'.

The stretch reflex and the contributions of C David Marsden

The stretch reflex or myotatic reflex refers to the contraction of a muscle in response to its passive stretching by increasing its contractility as long as the stretch is within physiological limits. For ages, it was thought that the stretch reflex was of short latency and it was synonymous with the tendon reflex, subserving the same spinal reflex arc. However, disparities in the status of the two reflexes in certain clinical situations led Marsden and his collaborators to carry out a series of experiments that helped to establish that the two reflexes had different pathways. That the two reflexes are dissociated has been proved by the fact that the stretch reflex and the tendon reflex, elicited by stimulation of the same muscle, have different latencies, that of the stretch reflex being considerably longer. They hypothesized that the stretch reflex had a transcortical course before it reached the spinal motor neurons for final firing. Additionally, the phenomenon of stimulus-sensitive cortical myoclonus lent further evidence to the presence of the transcortical loop where the EEG correlate preceded the EMG discharge. This concept has been worked out by later neurologists in great detail, and the general consensus is that indeed, the stretch reflex is endowed with a conspicuous transcortical component.

The utility of electrodiagnostic tests for the assessment of medically unexplained weakness and sensory deficit

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Medically unexplained symptoms (MUS) are challenge for electrodiagnostic testing.

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Weakness and sensory deficit can be manifestations of psychogenic disorders.

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This is a review of electrodiagnostic methods used for the assessment of MUS.

Patients with suspected medically unexplained symptoms or psychogenic disorders are frequently requested to undergo an EMG exam. However, the suspected diagnosis is not always told to the electromyography practitioner, who must be able to recognize such a condition to avoid false positive diagnosis without dismissing the possibility to uncover any true dysfunction. There are many clinical manoeuvers to assess the consistency of the patients’ reported weakness or sensory deficit. The electrodiagnostic practitioner should be aware of those clinical tricks and interpret the electrodiagnostic findings in the clinical context. There are many electrodiagnostic tests that the practitioner can use for the assessment of motor and sensory functions but these tests have also important drawbacks and limitations. Only after a good clinical evaluation would the practitioner be able to give his/her opinion on the clinical relevance of the electrodiagnostic findings. Here we review some of the tests that can help the practitioner to define the electrophysiological characteristics of a suspected functional disorder presenting with weakness or sensory deficit.

[Clinical, neurophysiological and neuroimaging study of tremor in multiple sclerosis]

OBJECTIVE

To identify clinical types of tremor in multiple sclerosis (MS) and clarify their pathophysiological mechanisms.

MATERIAL AND METHODS

We examined 124 patients with MS, including 58 patients with tremor, using clinical (digital spiralography), neurophysiological (tremor electromyography, visual and sensory evoked potentials, transcranial magnetic stimulation with tremor resetting, long latency reflexes, electroencephalography) and neuroimaging (MRI, morphometry) methods.

RESULTS AND CONCLUSION

Five main variants of tremor were identified: distal postural and postural-intention (variant 1), distal intention (variant 2), proximal and distal intention and postural-intention (variant 3), Holmes (variant 4), axial (variant 5). Postural tremor (variants 1, 3) and rest tremor (variant 4) are caused by the central oscillators. Intention tremor (variants 2, 3), postural-intention tremor (variant 4), axial (variant 5) are caused by the pathology of cerebellar feedback loops. Clarification of mechanisms for the development of tremor in MS allowed to develop a scheme of differential treatment.

Plasticity resembling spike-timing dependent synaptic plasticity: the evidence in human cortex

Spike-timing dependent plasticity (STDP) has been studied extensively in a variety of animal models during the past decade but whether it can be studied at the systems level of the human cortex has been a matter of debate. Only recently newly developed non-invasive brain stimulation techniques such as transcranial magnetic stimulation (TMS) have made it possible to induce and assess timing dependent plasticity in conscious human subjects. This review will present a critical synopsis of these experiments, which suggest that several of the principal characteristics and molecular mechanisms of TMS-induced plasticity correspond to those of STDP as studied at a cellular level. TMS combined with a second phasic stimulation modality can induce bidirectional long-lasting changes in the excitability of the stimulated cortex, whose polarity depends on the order of the associated stimulus-evoked events within a critical time window of tens of milliseconds. Pharmacological evidence suggests an NMDA receptor mediated form of synaptic plasticity. Studies in human motor cortex demonstrated that motor learning significantly modulates TMS-induced timing dependent plasticity, and, conversely, may be modulated bidirectionally by prior TMS-induced plasticity, providing circumstantial evidence that long-term potentiation-like mechanisms may be involved in motor learning. In summary, convergent evidence is being accumulated for the contention that it is now possible to induce STDP-like changes in the intact human central nervous system by means of TMS to study and interfere with synaptic plasticity in neural circuits in the context of behavior such as learning and memory.

Cutaneous Inputs Can Activate the Ipsilateral Primary Motor Cortex During Bimanual Sensory-Driven Movements in Humans

Using transcranial magnetic stimulation (TMS), we examined whether sensory input from a finger affects activity of the ipsilateral primary motor cortex (M1) when human subjects hold a virtual object bimanually and whether this ipsilateral activation varies under different contexts. Subjects used both index fingers to hold two plates, which were subjected to unpredictable pulling loads from torque motors. Loads were delivered in a random sequence to either plate or concurrently to both, although the latter occurred most frequently. Finger forces vertical to the plates and surface electromyographs from the first dorsal interosseous muscles were recorded bilaterally during the task. TMS was sometimes applied over the finger area of the left M1 at variable times relative to load onset to examine cortical excitability. Strength of TMS was set around the active motor threshold of the right finger muscle while subjects waited for loading to the handheld plates. When one plate was singly loaded, the M1 contralateral to the loaded finger was activated, causing automatic force increases in the finger. In addition, the ipsilateral M1 was activated during such loading, associated with transient force increases in the contralateral nonloaded finger. Activations in the ipsilateral M1 were also observed during concurrent loading, when activations were stronger than those following single loading of the contralateral plate. Ipsilateral activations weakened when concurrent loading was less frequent. These results suggest interactions between bilateral sensorimotor cortices during bimanual coordinated movements, with strength varying by context.

Is the long-latency stretch reflex in human masseter transcortical?

A long-latency stretch reflex (LLSR) has been described in the human masseter muscle, but its pathway remains uncertain. To investigate this, the excitability of corticomotoneuronal (CM) cells projecting to masseter motoneurons during the LLSR was assessed with transcranial magnetic stimulation (TMS). A facilitated response to TMS would be evidence of a LLSR pathway that traverses the motor cortex. Surface electromyogram electrodes were placed over the left or right masseter, and subjects ( n=10) bit on bars with their incisor teeth at 10% of maximal electromyographic activity (EMG). Servo-controlled displacements were imposed on the lower jaw to evoke a short- and long-latency stretch reflex in masseter. TMS intensity was just suprathreshold for a response in contralateral masseter. Trials consisted of: (1) stretch alone, (2) TMS alone, and (3) TMS with a preceding conditioning stretch at varied conditioning-testing (C-T) intervals chosen to combine TMS with the short-latency stretch reflex (3 ms, 5 ms) and the LLSR (23-41 ms). Masseter EMG was rectified and averaged. With TMS alone, mean (+/- SE) MEP area above baseline was 56+/-9%. The area of masseter MEPs above baseline in the C-T trials was calculated from each EMG average following subtraction of the response to stretch alone. Conditioning muscle stretch had no significant effect on masseter MEPs evoked by TMS with any C-T interval (ANOVA; P=0.90). In addition, subjects were unable to modify the SLSR or LLSR by voluntary command. It is concluded that the long-latency stretch reflex in the masseter does not involve the motor cortex and is not influenced by "motor set".

The aging hand

Hand function decreases with age in both men and women, especially after the age of 65 years. A review is presented of anatomical and physiological changes in the aging hand. The age-related changes in prehension patterns (grip and pinch strength) and hand dexterity in the elderly population are considered. Deterioration in hand function in the elderly population is, to a large degree, secondary to age-related degenerative changes in the musculoskeletal, vascular, and nervous systems. Deterioration of hand function in elderly adults is a combination of local structural changes (joints, muscle, tendon, bone, nerve and receptors, blood supply, skin, and fingernails) and more distant changes in neural control. These age-related changes are often accompanied by underlying pathological conditions (osteoporosis, osteoarthritis, rheumatic arthritis, and Parkinson's disease) that are common in the elderly population. Assessment of hand function and prehension patterns is needed in order to determine specific treatment approaches.

Suppression of cortical myoclonus by levetiracetam

A 16-year-old boy suffered severely disabling posthypoxic myoclonus. Neurophysiological investigation showed cortical but not reticular reflex myoclonus. Add-on therapy with levetiracetam significantly improved the patient's clinical condition, suppressed cortical myoclonus-associated spikes, and enabled further neurorehabilitation.

Silent period evoked by transcranial magnetic stimulation in unilateral thalamic infarcts

We studied 9 patients with isolated, unilateral thalamic infarcts of different location and size imaged by MRI and (in one case) by CCT. The cortical silent period (c-SP) of thenar muscles evoked by transcranial magnetic stimulation was evaluated on both sides with low (c-SP1) and high (c-SP2) stimulus intensity. Additionally, the motor-evoked thenar potentials by transcranial magnetic stimulation (MEP) and the central motor conduction time (CMT), the silent period of thenar muscles evoked by electrical median nerve stimulation (p-SP), electrically evoked short- (HR) and long-latency reflexes (LLR) of the thenar and somatosensory evoked potentials of the median nerve were investigated bilaterally. The c-SP1 was prolonged contralateral to the thalamic infarcts in 7 patients and c-SP2 in all 9 patients. Prolongation of c-SP did not correlate with location and size of the infarct, clinical symptoms or with the other parameters, which are based on pyramidal motor system (MEP, CMT), thalamocortical circuits (SEP, LLR), or spinal excitability level (p-SP, HR). These results suggest the central pathways of the c-SP to be independent from those of the remaining parameters and indicate the c-SP as a sensitive parameter reflecting thalamocortical modulation of cortical inhibition.

Stiff man syndrome: neurophysiological findings in eight patients

The neurophysiological findings in eight patients with the stiff man syndrome (SMS), including four of six tested with autoantibodies against glutamic acid decarboxylase, are presented. Neurophysiological findings did not make it possible to discriminate between patients with and those without autoimmunity against GABAergic neurons. Investigation of mono- and polysynaptic reflexes revealed abnormal results in a variable number of SMS patients, the abnormalities largely corresponding to those seen in spastic paresis. A stereotyped motor response to electrical stimulation of peripheral nerves was recorded from the trunk muscles of all patients investigated. This response was termed spasmodic reflex myoclonus and consisted of a sequence of 1-3 synchronous myoclonic bursts, 60-70 ms after median nerve stimulation, followed by a tonic decrescendo activity over a number of seconds. The recruitment order of muscles along the neuraxis in spasmodic reflex myoclonus suggested that the latter was generated in the spinal cord and conveyed via propriospinal tracts. It is thought that spasmodic reflex myoclonus may serve not only as a diagnostic tool, but also as a key to understanding some aspects of the pathophysiology of both spasms and stiffness in SMS. It is speculated that stiffness is a fragment of spasms, both being generated by common neuronal mechanisms tentatively ascribed to interneurons in the spinal grey matter.