Neuronal and muscle unit properties at different rostro-caudal levels of cat's motoneurone pool.

The rostrocaudal organization of motoneurones of the cat's m.peroneus longus (PerL) was investigated with respect to motoneuronal size (soma diameter) as well as muscle unit contractile properties (twitch speed, fatigue resistance, maximum force). Sizes of cell bodies were measured after retrograde labelling with horseradish peroxidase. Other properties were studied by aid of electrical stimulation of rostro-caudally distinct subfilaments of ventral roots. A weak but significant tendency was found for units to become somewhat slower and more fatigue-resistant at caudal than at more rostral levels within the pool. Neurones of different sizes and unit properties were, however, very widely intermingled at all levels of the pool. Rostro-caudally distinct root filaments gave rise to differentially located electromyographic signals in the muscle. The experimental results led to the following main conclusions: a type-specific intraspinal innervation of PerL motoneurones is unlikely to be arranged predominantly on topographical principles; and the intraspinal site of PerL motoneurones is related to the intramuscular site of their muscle fibres.