Role of the human fusimotor system in a motor adaptation task.

Abstract

SUMMARY1. Single-unit activity was recorded with the microneurographic technique from the radial nerve of attending human subjects. During active finger movements, impulses in spindle afferents from the extensor digitorum muscle were analysed along with joint movements, size of imposed load and EMG activity of the receptor-bearing muscle.2. In a simple motor adaptation task the subjects were requested to perform ramp-and-hold movements of prescribed amplit-ades and velocities at a single metacarpo-phalangeal joint. A test run consisted of a series of movement cycles when the flexor muscle was continuously loaded with a constant torque, immediately followed by cycles when this load was abruptly decreased during the flexion movement, producing a fast stretch of the receptor-bearing muscle. The subjects' task was to strive for movements of constant velocity and particularly to minimize the effect of the disturbance. In order to allow prediction on the basis of immediately preceding cycles, the disturbance was always injected at the same angular position in a number of successive cycles.3. Motor adaptation was manifested as a successive decrease of the perturbation amplitude, usually associated with the development of a continuous and growing EMG activity in the parent muscle and a growing reflex response of long latency (60 ms). Short-latency reflexes were not seen.4. The main mechanism accounting for the improved performance was a cocontraction of the agonist-antagonist muscle pair during voluntary movements, producing an increased muscular stiffness. The reflex did not contribute to the motor adaptation because it was not fast enough to curtail the perturbation.5. The development and the growth of the reflex were not due to a growing fusimotor drive during adaptation, because spindle discharge actually decreased when the reflex increased. The size of spindle response was related to the amplitude of perturbation rather than to the amplitude of the reflex. These findings suggest that reflex modifications were due to central excitability changes which paralleled the muscle contraction.6. Spindle firing rate during active movements was generally higher in disturbed cycles compared to undisturbed cycles, indicating a higher fusimotor drive. Since muscle contraction was present mainly in the former, this finding may simply represent a case of fusimotor activation along with skeletomotor activation. No indication of an independence between the two was found.

Is human muscle spindle afference dependent on perceived size of error in visual tracking?

“…The present findings are consonant with several previous microneurography recordings from spindle afferents in the upper and lower limb in that it failed to demonstrate gamma independence while, in other studies, minute indications of a faint gamma dissociation were occasionally found in a small minority of the spindle afferents recorded (Burg et al 1973(Burg et al , 1974Burke et al 1980a, b;Hagbarth 1981;Vallbo and Hulliger 1981;Gandevia and Burke 1985;Al-Falahe and Vallbo 1988;Aniss et al 1988Aniss et al , 1990aVallbo and Al-Falahe 1990;Gandevia et al 1994;Nielsen et al 1994;Wessberg and Vallbo 1995;Kakuda et al 1996). As a summary of microneurography studies on the issue of gamma independence, it seems fair to conclude that clear evidence of major and consistent fusimotor resetting has not been identified in man except when subjects increase or decrease the amount of extrafusal contraction in the parent muscle.…”

“…In human subjects, on the other hand, only minute, often dubious, changes of spindle firing rates have been found in a small minority of afferents, except when the parent muscle contraction is adjusted (Burg et al 1973(Burg et al , 1974Burke et al 1980a, b;Hagbarth 1981;Vallbo and Hulliger 1981;Gandevia and Burke 1985;Al-Falahe and Vallbo 1988;Aniss et al 1988Aniss et al , 1990aVallbo and AlFalahe 1990;Gandevia et al 1994;Nielsen et al 1994;Wessberg and Vallbo 1995;Kakuda et al 1996). In contrast, one single study based on recordings from presumed gamma-efferents suggests that substantial fusimotor adjustments occur with various manoeuvres (Ribot et al 1986).…”

Reflex responsiveness of a human hand muscle when controlling isometric force and joint position

“…The magnitude of the reflex response has been shown to increase in proportion to the level of preexisting tonic muscle activation (Gottlieb and Agarwal, 1979;Al-Falahe and Vallbo, 1988), which is often greater for novel tasks involving unstable loads due to the coactivation of agonist and antagonist muscles (De Serres and Milner, 1991;Osu et al, 2002). In an attempt to minimize the effects of coactivation on tonic activity of the first dorsal interosseus muscle, all subjects practiced the MVC, force, and position tasks during a familiarization session 1-3 days prior to the experiment.…”

Human muscle spindle response in a motor learning task.

“…The finger splint which immobilized the interphalangeal joints of the test finger was connected by a hinged bar of low mass to a computer-controlled actuator which has, in part, been described elsewhere (Hulliger, Nordh & Vallbo, 1982;Al-Falahe & Vallbo, 1988 (Edin & Vallbo, 1987 (Matthews, 1972).…”