On the branching of motoneurons

Innervation ratios were estimated for motor units supplying superficial lum brical muscles of the cat after assigning units to one of three types, FF/F I, FR, and S, and estimating their axonal conduction velocity, based on pub lished data. Similarly, unit tensions were converted to muscle fiber number using published values for specific tension and fiber cross-sectional area . A relation was established between axonal area of cross-section and numbe r of fibers innervated which was matched reasonably well by a model of a br anching motoneuron with preterminal diameter of 4.28 mum and a daughter-to- parent ratio of 1.48 at each level of branching. It is proposed that the ma in features of the model are likely to apply to all motoneurons. (C) 2001 J ohn Wiley & Sons, Inc.