Fast and slow motor units in ageing.

The differences in onset and degree of old age changes in different muscles are explained by the differentiation and different reactivity of fast and slow motor units with respect to physiological, structural and biochemical characteristics. The main changes in the motor units in old age are described. The general basic change is a progressive random disturbance of neuromuscular contact ascribed to a decrease of the trophic function of the neuron. The main motor disturbances in old age, i.e; slowness, decrease of muscle strength and lack of fine coordination are explained in terms of physiological changes in senescent motor units. The reactions of senescent motor units differ from one unit to another as shown, e.g. in denervation, reinnervation and regeneration (transplantation) studies. The trend to a shift from a heterogeneous to a more uniform muscle fibre pattern and the defficiencies in recovery of the original muscle fibre pattern in reinnervation and regeneration of senescent muscle is demonstrated and explained by a decrease of the differentiating capacity of different motor units. The changes in the heterogeneous fibre pattern of skeletal and the homogeneous fibre pattern of the papillary heart muscle in old age are contrasted.