The size-principle: a deterministic output emerges from a set of probabilistic connections

ABSTRACT To understand how even a single muscle is controlled, we must understand how a large ensemble of neurones can generate a complex function through the collective action of its members. The orderly recruitment of motoneurones (MNs) and motor units according to their sizes or tensions (Henneman, 1957; Henneman, Somjen & Carpenter, 1965a,b) is the product of collective action, illustrating how hundreds of MNs operate as a functional entity to control a muscle. Fig. 1 reproduces the recruitment of motor units recorded from a rabbit’s diaphragm during a series of progressively stronger inspirations (G. Yasargil, unpublished). With each contraction larger spikes were recorded in orderly sequence, indicating the discharge of larger motor units. Nearly invariant recruitment orders have been recorded from the MNs or muscles of more than 20 species (Henneman & Mendell, 1981).

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