Selective Activation of Human Finger Muscles after Stroke or Amputation

Abstract

Individuated finger movements of the human hand require selective activation of particular sets of muscles. Such selective activation is controlled primarily by the motor cortex via the corticospinal tract. Is this selectivity therefore lost when lesions damage the corticospinal tract? Or when the motor cortex reorganizes after amputation? We studied finger movements in normal human subjects and in patients who had recovered substantially from pure motor hemiparesis caused by lacunar strokes, which damage the corticospinal tract without affecting other pathways. Even after substantial recovery from these strokes, individuation of finger movements remained reduced— both for flexion/extension and for adduction/abduction motion of the fingers. Stroke subjects regained the ability to move the instructed digit through a normal range, but unintentional motion of other digits was increased. This increase did not result from a change in the passive biomechanical coupling of the fingers. Rather, voluntary contractions of muscles that move the intended digit were accompanied by inappropriate contractions in muscles acting on additional digits. These observations suggest that the normal corticospinal system produces individuated finger movements not only by selectively activating certain muscles, but also by suppressing activation of other muscles during voluntary effort to move a given digit. In a separate experiment, reversible amputation of the hand was produced in normal subjects by ischemic nerve block at the wrist. Motor output to the intrinsic muscles and sensory input both become blocked under these conditions, effectively amputating the hand from the nervous system. But the long extrinsic muscles that flex and extend the digits remain normally innervated, and thus flexion forces still can be generated at the fingertips. During reversible amputation of the hand produced by ischemic nerve block, the ability of subjects to activate subdivisions of extrinsic muscles and to exert flexion force at individual fingertips continued to show essentially normal selectivity. Voluntary activation of the remaining muscles thus continues to be selective after amputation, in spite of both the loss of sensory input from the amputated hand, and reorganization within the primary motor cortex. During cortical reorganization after amputation, then, voluntary patterns of motor output intended for finger muscles may not be lost. We therefore examined activity in the stump muscles of above-elbow amputees, who have no remaining hand muscles. Different movements of the phantom hand were accompanied by different patterns of EMG in remaining proximal muscles, distinct from the EMG patterns associated with movement of the phantom elbow. We infer that voluntary motor output patterns that normally control finger movements after amputation may become diverted to remaining proximal muscles.

Bringing transcranial mapping into shape: Sulcus-aligned mapping captures motor somatotopy in human primary motor hand area

“…The quantification of discrete changes in M1 excitability is also relevant for clinical research, e.g. in patients with Stroke, Parkinson's disease or Focal Hand Dystonia who show an altered pattern of M1 HAND representation/function and dynamical muscular representation (Beck et al, 2009;Granert et al, 2011;Schieber et al, 2009).…”

The superposition principle applied to grasping an object producing moments outside anatomically-defined axes

“…Despite this, the central nervous system (CNS) tends to select certain families of solutions, or synergies, when grasping objects (Santello and Soechting, 2000;Rearick and Santello, 2002;Shim et al, 2003Shim et al, , 2005aJohnston et al, 2005Johnston et al, , 2010Shim and Park, 2007). A better understanding of such control mechanisms can lead to improved rehabilitation of neuromuscular dysfunction (Nowak, 2008;Alberts and Wolf, 2009;Schieber et al, 2009), and the development of more dextrous robotic limbs (Bae et al, 2005;Choi et al, 2008;Lee et al, 2009;Yang et al, 2009). …”

Tactile feedback plays a critical role in maximum finger force production

“…Previous studies have administered digital anesthesia as a primary modality to remove cutaneous feedback from the digits and investigated the role of cutaneous feedback on CNS control over hand and finger actions (Gandevia and McCloskey, 1977;Kilbreath et al, 1997;Reilly et al, 2008;Schieber et al, 2009). Studies on sub-maximal force production tasks such as grasping and pinching have shown that effective removal of cutaneous feedback leads to an increase in errors during motor performance (Nowak et al, 2001;Monzee et al, 2003).…”