vagus nerve graphicThis article looks at resting parasympathetic activity in patients with functional movement disorders (tremor, cervical dystonia) versus control population and finds that it is diminished.  They interpret this as an increased stress vulnerability, but I think there might be wider ramifications.  Does the decreased vagal tone lead to more movement dysfunction?  Are they somehow interconnected? If you improve the vagal tone does the movement disorder lessen?

Parkinsonism Relat Disord. 2016 Jun 16.
Impaired resting vagal tone in patients with functional movement disorders.
Maurer CW1, Liu VD2, LaFaver K3, Ameli R4, Wu T5, Toledo R6, Epstein SA7, Hallett M8.
Author information
Abstract
INTRODUCTION:

The autonomic nervous system plays an integral role in the maintenance of homeostasis during times of stress. The functioning of the autonomic nervous system in patients with functional movement disorders (FMD) is of particular interest given the hypothesis that converted psychological stress plays a critical role in FMD disease pathogenesis. We sought to investigate autonomic nervous system activity in FMD patients by examining heart rate variability (HRV), a quantitative marker of autonomic function.
METHODS:

35 clinically definite FMD patients and 38 age- and sex-matched healthy controls were hospitalized overnight for continuous electrocardiogram recording. Standard time and frequency domain measures of HRV were calculated in the awake and asleep stages. All participants underwent a thorough neuropsychological battery, including the Hamilton Anxiety and Depression scales and the Beck Depression Inventory.
RESULTS:

Compared to controls, patients with FMD exhibited decreased root mean square of successive differences between adjacent NN intervals (RMSSD) (p = 0.02), a marker of parasympathetic activity, as well as increased mean heart rate (p = 0.03). These measures did not correlate with the depression and anxiety scores included in our assessment as potential covariates.
CONCLUSION:

In this exploratory study, patients with FMD showed evidence of impaired resting state vagal tone, as demonstrated by reduced RMSSD. This decreased vagal tone may reflect increased stress vulnerability in patients with FMD.