Tremor

INDEX

Tremor is a rhythmic oscillation of a body part that occurs physiologically (e.g., fear, cold), or as the commonest movement disorder.  Clinically, it is postural or goal directed in almost all cases, with the notable exception of pure parkinsonian resting tremor. Tremor affects one or several regions of the body about a joint axis, and is usually produced by alternating or synchronous contractions of reciprocally innervated agonistic and antagonistic muscles.

The oscillation of tremor is generated by rhythmical discharges in a neuronal network that is maintained by feedback and feed-forward loops.  Under specific circumstances, a negative feedback loop will tend to oscillate (Figure). The cerebello-thalamo-cortical pathway is involved in virtually all pathologic tremors.

Overall, a high stability in tremor frequency, as seen in essential tremor, may be explained by a single oscillator with a specific frequency, or by multiple oscillators that are tightly coupled. By contrast, low stability in tremor frequency, as observed in Parkinson’s disease, may point towards several uncoupled oscillators with different frequencies2.

 

Tremor generation by an oscillating circuit.

An oscillation may arise whenever there is a delay in a negative feedback loop or an increase in gain in the control signal. A reference signal provides input about the target or goal state; a comparator compares the sensed information and the reference signal; gain transforms comparison into a control signal (i) that brings the sensed position closer to the reference signal, thus negating the error; the plant converts control signals into real output1.

From: Albanese A, Sorbo F Del. Dystonia and Tremor: The Clinical Syndromes with Isolated Tremor. Tremor Other Hyperkinet Mov (N Y). 2016;6:319.

 

 

References

1. Albanese A, Sorbo F Del. Dystonia and Tremor: The Clinical Syndromes with Isolated Tremor. Tremor Other Hyperkinet Mov (N Y). 2016;6:319. 

2. di Biase L, Brittain J-SS, Shah SA, et al. Tremor stability index: A new tool for differential diagnosis in tremor syndromes. Brain. 2017;140(7):1977-1986.