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Why transcortical reflexes?

M Wiesendanger, D G Rüegg, G E Lucier

    The Canadian Journal of Neurological Sciences. Le Journal Canadien Des Sciences Neurologiques
    |August 1, 1975
    PubMed
    Summary

    The motor cortex uses transcortical reflexes to adjust muscle stiffness during voluntary movements, offering adaptive gain control crucial for skilled motor function and potentially impaired in Parkinsonian rigidity.

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    Area of Science:

    • Neuroscience
    • Motor Control
    • Systems Neuroscience

    Background:

    • Muscle stiffness is regulated by visco-elastic properties, segmental reflexes, and long-loop reflexes.
    • Long-loop reflexes are critical for compensating unpredictable load perturbations during voluntary movements and postural activity.

    Purpose of the Study:

    • To investigate the neural mechanisms underlying automatic compensation of load perturbations during voluntary movements.
    • To explore the role of the motor cortex in adjusting muscle stiffness via transcortical servo-loops.

    Main Methods:

    • Experiments in humans and monkeys involving voluntary movements and postural activity with controlled load perturbations.
    • Electrophysiological recordings in anesthetized and alert monkeys to track neural signals from stretch afferents to the motor cortex.

    Main Results:

    • Stretch afferent signals reach motor cortex neurons, potentially via area 3a, with latencies consistent with long-loop reflex responses.
    • Motor cortex cell discharges correlate in timing with electromyographic responses to load changes, supporting a causal relationship.
    • Transcortical servo-loops provide adaptive gain control at the cortical level, which is modifiable and under voluntary control.

    Conclusions:

    • The motor cortex utilizes a transcortical servo-loop to adjust motor output based on load conditions, enhancing skilled movement execution.
    • Adaptive gain control in the motor cortex is essential for preserving complex motor capabilities.
    • Dysfunction in this adaptive gain control may contribute to motor disorders like Parkinsonian rigidity.

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