Evidence for strong synaptic coupling between single tactile afferents from the sole of the foot and motoneurons supplying leg muscles. J Neurophysiol 94: 3795-3804, 2005. First published August 3, 2005 doi:10.1152/jn.00359.2005. It has been known for some time that populations of cutaneous and muscle afferents can provide short-latency facilitation of motoneuron pools. Recently, it has been shown that the input from individual low-threshold mechanoreceptors in the glabrous skin of the hand can modulate ongoing activity in muscles acting on the fingers via spinally mediated pathways. We have extended this work to examine whether such strong synaptic coupling exists between tactile afferents in the sole of the foot and motoneurons supplying muscles that act about the ankle. We recorded from 53 low-threshold mechanoreceptors in the glabrous skin of the foot via microelectrodes inserted percutaneously into the tibial nerve of awake human subjects. Reflex modulation of ongoing whole muscle electromyography (EMG) was observed for each of the four classes of low-threshold cutaneous mechanoreceptors (17 of 21 rapidly adapting type I; 2 of 4 rapidly adapting type II; 7 of 18 slowly adapting type I; and 4 of 10 slowly adapting type II). Reflex modulation of the firing probability in single motor units (5 of 11) was also observed. These results indicate that strong synaptic coupling between tactile afferents and spinal motoneurons is not a specialization of the hand and emphasizes the potential importance of cutaneous inputs from the sole of the foot in the control of gait and posture.

This study demonstrates that Wii-based Movement Therapy is an effective upper limb rehabilitation poststroke with high patient compliance. It is as effective as modified Constraint-induced Movement Therapy for improving more affected upper limb movement and increased independence in activities of daily living.

Cutaneous sensation deteriorates with age. It is not known if this change is consistent over the entire hand or if sensation is affected by changes in skin mechanics. Cutaneous perceptual thresholds were tested at eight sites in the glabrous skin and two in the hairy skin of both hands in 70 subjects (20-88 years), five male and five female per decade, using calibrated von Frey filaments, two-point discrimination, and texture discrimination. Venous occlusion at the wrist (40±10 mmHg) and moisturizer were used to alter skin mechanics. Cutaneous thresholds increased significantly with age (p<0.001); von Frey thresholds were 0.04 g [0.02-0.07] (median and interquartile range) in the 20s and 0.16 g [0.04-0.4] in the 80s, with differences between hands for older females (p00.044) but not males. The pattern of changes in cutaneous sensation varied according to the site tested with smaller changes on the fingers compared to the palm. Two-point discrimination deteriorated with age (p00.046), but with no interaction between sex, handedness, or changes in skin mechanics. There were no significant differences for texture discrimination. Changes in skin mechanics improved cutaneous thresholds in the oldest males after moisturizing (p00.001) but not otherwise. These results emphasize the complex pattern of age-related deterioration in cutaneous sensation with differences between sexes, the hands, sites on the hand, and the mode of testing. As the index fingertip is not a sensitive indicator of sensory decline, the minimum assessment of age-related changes in cutaneous sensation should include both hands, and sites on the palm.

An intensive 2-week protocol resulted in significant and clinically relevant improvements in functional motor ability post-stroke. These gains translated to improvement in activities of daily living.

Electrical stimulation of digital nerves elicits short‐latency excitatory and inhibitory spinal reflex responses in ongoing EMG in muscles acting on the fingers and thumb. Similar responses are elicited by stimulating a population of muscle spindles but not when a single muscle spindle is activated. The current study investigated whether short‐latency EMG responses could be evoked from the discharge of a single cutaneous afferent. Thirty‐three tactile afferents were recorded via tungsten microelectrodes in the median nerve of awake humans. Spike‐triggered averaging revealed EMG events time‐locked to the afferent discharge. The afferents were activated by an external probe and the EMG was elicited by a weak voluntary contraction. Eleven cutaneous afferents (33 %) showed a short‐latency response in the ongoing EMG. Overt increases or decreases in EMG were observed for seven afferents (onset latency 20.0‐41.1 11hms1h). For four slowly adapting (SA) type II afferents, EMG showed a periodicity that was correlated to the afferent interspike interval (r= 0.99). The EMG associated with two rapidly adapting (FA) type I afferents (29 %) showed a short‐latency excitation while five showed neither excitation nor inhibition. Seven SA II afferents (39 %) showed excitation and 11 no response; and none of the six SA I afferents showed any response. We conclude that, unlike muscle spindle afferents, the input from a single cutaneous afferent is strong enough to drive, via interneurones, motoneurones supplying muscles acting on the digits. The potent short‐latency response we found supports the important role of cutaneous mechanoreceptors in fine motor control of the human hand.

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.

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Two simple unambiguous and objective tests of gross (BBT) and fine (grooved pegboard test) manual dexterity discriminated 3 groups of motor function ability for a heterogeneous group of patients after stroke.