Proprioceptive Mechanisms and the Human Hand

The hand is complex and used in many functions, including eating and communication. To control the hand accurately the brain needs information about the position, velocities and forces around each joint, which is provided by proprioception. Despite being studied for over a century, there is much to learn about this enigmatic sense. The first part of this chapter summarises the key historical debates and the evidence that shaped the current view of proprioception. The main part then highlights recent evidence that has profoundly changed the understanding of proprioception. One recent development is the discovery that the firing rates of muscles spindles depend upon the contraction history of the muscle and that it is possible for muscle spindles to become insensitive to movement of the joint, remaining quiet during small joint movements. This alters the perceived position of joints. Other experiments show that illusions of joint movement can be induced by stretching the skin, providing evidence that slowly adapting cutaneous receptors contribute to movement sense. However, at least at finger joints, it seems that rapidly adapting cutaneous receptors interfere with the detection of the direction of movement. Recent evidence reveals illusions of altered joint position and movement with voluntary efforts during paralysis and anaesthesia. Thus command signals generated by the brain provide direct information about joint position and movement. Other experiments using anaesthesia have shown key roles of muscle receptors in generating the body maps stored by the brain. Together this recent work shows that the textbook view of proprioception needs revision.