PubMed · 4169147
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J W Lance. 1968-01-13. Physiological tremor.. https://doi.org/10.1016/s0140-6736(68)90103-7
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OBJECTIVES: To study the role of corticomotor neuronal pathways in primary orthostatic tremor. METHODS: Transcranial magnetic stimuli at an intensity 10% above the resting motor threshold were delivered over the leg motor cortex in two patients with primary orthostatic tremor while standing still. Electromyographic responses in both tibialis anterior muscles were recorded after 20 stimuli given randomly at intervals of 120 to 180 seconds. Differences between predicted and actual times of occurrence of tremor bursts after the stimuli were used to calculate a resetting index, with a value of 0 representing no resetting and a value of 1 representing complete resetting. RESULTS: Transcranial magnetic stimulation evoked EMG responses in both tibialis anterior muscles, followed by transient suppression of tremor before reappearance of rhythmic EMG activity. Analysis of the timing of tremor bursts from EMG recordings before and after the magnetic stimuli disclosed that the phase of orthostatic tremor could be reset by brain stimulation (mean resetting indices 0.93 and 0.82). CONCLUSION: The results suggest that a central oscillator, involving the motor cortex, has a crucial role in either the generation or modulation of orthostatic tremor.
This article reviews some recent applications of time and frequency domain cross-correlation techniques to human motor unit recording. These techniques may be used to examine the pre-synaptic mechanisms involved in control of motoneuron activity during on-going motor tasks in man without the need for imposed and artificial perturbations of the system. In this review we examine, through several examples, areas in which insights have been gained into the basic neurophysiological processes that bring about motoneuron firing in man and illustrate how these processes are affected by central nervous system pathology. We will demonstrate that synchronization and coherence may be revealed between human motor unit discharges and give examples that support the hypothesis that these phenomena are generated by activity in a focused common corticospinal input to spinal motoneurons. Disruption of central motor pathways due to diseases of the nervous system leads to pathophysiological alterations in the activity of these pre-synaptic motoneuron inputs that can be revealed by cross-correlation analysis of motor unit discharges. The significance of these studies and outstanding questions in this field are discussed.
In this paper, the following experimental methods for studies of the motor system in freely moving human subjects will be considered: (i) eliciting the H-reflex and understanding its use as a test response, (ii) methods to measure reciprocal inhibition between antagonist muscles, (iii) methods to measure presynaptic inhibition of Ia-afferent terminals in the spinal cord, (iv) certain aspects of the interpretation of peri-stimulus time histograms (PSTH) of single motor unit discharge, and finally, (v) stimulation of the motor cortex and the measurement of response parameters that may reflect task dependent changes. Two closely related ideas bearing directly on these methods will be emphasized--the influence of the background level of motor activity on input output properties of the neural pathway investigated and the operating point on the input-output curves at which the experimental variable is measured. Finally, in the discussion a simple model that is easily understandable in geometric terms is presented to help predict and interpret the outcome of these sorts of experiments.