Loss of large-diameter nerve sensory input changes perceived posture.

Our previous studies (Inui et al. in J Physiol 589:5775-5784, 2011, Exp Brain Res 218:487-494, 2012) showed that a fully flexed or extended finger, wrist, and elbow became perceived as an extended or flexed 'phantom' hand and arm as ischemic anesthesia progressed. Here, we examined what happened if the wrist was fixed in full extension while the elbow was in full flexion before and during the anesthesia, and vice versa. Ten healthy participants demonstrated the perceived postures of their right wrist and elbow during an ischemic block of the right upper arm with the left hand and arm. If the actual wrist was fully extended while the actual elbow was fully flexed, then the perceived position of the wrist moved toward flexion and that of the elbow moved toward extension. Conversely, if the actual wrist was fully flexed while the actual elbow was fully extended, then the wrist was perceived to extend and the elbow was perceived to flex. Following the loss of the afferent signal coming from the main muscles acting at the two joints, the two perceived postures moved toward the opposite direction independently. The changes in the perceived postures are a shift in the body schema depending on the balance of the proprioceptive inputs that determine limb posture.