Organization and properties of spinal motoneurones and motor units.

Publisher Summary This chapter describes a brief survey of some known facts and current problems concerning motoneurones and motor units of the cat's hindlimb. A survey is given of the experimental results concerning the organization and properties of motoneurons and motor units of cat hindlimb. Particular attention is devoted to properties, which are of importance for the rate- and recruitment-gradation of muscle force. Two kinds of motoneurone-muscle matching are described and analyzed: (i) the “speed-match’’ between the isometric speed of muscle fibres and intrinsic rhythmic properties (frequency range, late adaptation) of their motoneurones, (ii) the “recruitment-match’’ between contractile properties of muscle units (force, speed, endurance) and excitability-related intrinsic properties of their motoneurones. The most easily recruited motoneurones of a pool are generally known to be equipped with relatively weak, slow, and fatigue-resistant muscle units. Experiments with long-term muscle activation suggest that this recruitment-match is partly produced by the fact that great amounts of daily activity tend to make muscle fibres weaker, slower, and more fatigue resistant.

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