Tremor pathophysiology

Tremor syndromes arise from diverse combinations of neuronal oscillation, resonance, and entrainment within interconnected cerebellar, thalamocortical, basal ganglia, and sensorimotor loops. This review synthesizes anatomical, physiological, pathological, clinical, and computational evidence to identify the most likely network mechanisms of common and uncommon tremor syndromes. Mechanical-reflex tremor reflects underdamped musculoskeletal resonance and reflex loop dynamics, whereas central neurogenic tremors arise from intrinsic membrane conductances, electrical coupling, and recurrent inhibitory and excitatory loops that support oscillation, frequency-selective resonance, and entrainment. Acute cerebellar lesions produce intention tremor through impaired dentate-mediated motor planning and interpositus-dependent predictive motor control, causing hypermetria and abnormal transcortical mechanical-reflex oscillation. Delayed tremor after a cerebellothalamic pathway injury reflects maladaptive plasticity, likely within the thalamocortical loop, and similar delayed maladaptive change plus abnormal basal ganglia oscillation produces Holmes tremor. Essential tremor probably originates from oscillatory cerebellar cortical circuits with thalamocortical amplification, shaped by heterogeneous genetic vulnerabilities. Parkinson tremor is produced by increased pallidal and subthalamic oscillation combined with thalamocortical resonance in response to a loss of dopaminergic modulation. Dystonic tremor arises from abnormal oscillation within interacting basal ganglia and cerebellar circuits. Cortical tremor stems from cortical hyperexcitability and reverberation. The properties of orthostatic tremor are most consistent with oscillation originating in the fastigium and associated cerebellar cortex with resonant amplification primarily in the pontobulbar reticular formation and secondarily in the thalamocortical loop. Neuropathic tremors appear to begin as disturbances of physiological mechanical-reflex and central neurogenic pathways but may evolve into disabling oscillation in the cerebellothalamocortical pathway. Tremor in sarcomeric myopathies probably emerges from unexplored central mechanisms of tremorogenesis. Future progress in tremor pathophysiology will require additional animal models and computational network models that can experimentally test proposed oscillatory mechanisms, distinguish causal oscillators from entrained amplifiers, and guide mechanism-based therapies.