Quantification of spinal reflexes in neurological disorders

Proprioceptive reflexes play an important role in the control of movement and posture. In this study the involvement of disturbed modulation of proprioceptive reflexes in Parkinson´s disease (PD) was investigated. PD is a neurodegenerative disorder characterized by tremor, bradykinesia, rigidity, and impaired postural reflexes. With the gradual loss of dopamine producing brain cells the smooth initiation of movements and fine control of muscles vanish and subsequently the symptoms develop. In this study it is hypothesized that also the modulation of proprioceptive reflexes is involved in the development of the symptoms. The modulation of proprioceptive reflexes at the shoulder joint is assessed during postural control task. Force disturbances were applied via a handle while the subjects were instructed to ´minimize the deviations´ of the handle. Several settings for the external damping, imposed by the (haptic) manipulator, are applied. It is found that the intrinsic muscle stiffness is significantly larger in patients compared to healthy control subjects. In patients the position feedback gain does increase with external damping but not as steeply as in controls, indication that the range of reflex gain modulation is confined with PD.