Spatial distribution of motor unit fibres in fast- and slow-twitch rat muscles with special reference to age

The spatial arrangement and morphometrical properties of the muscle fibres within single motor units (motor unit fibres) were studied in fast-twitch units of the tibialis anterior (TA) and in slow-twitch units of the soleus, using a computer-assisted model. The motor unit fibres were identified by the glycogen-depletion technique and the position of each fibre was defined by (x, y)-coordinates. The distance between each fibre and the nearest motor unit fibre (nearest-neighbour distance), and the distance between each fibre and each of the other fibres in the unit (interfibre distance), was calculated and plotted. Comparisons were made between young adult (3-6 months) and old (20-25 months) rats. In old animals, the motor units of TA and the soleus were larger (P less than 0.05 and P less than 0.01), contained an increased number of muscle fibres (P less than 0.01) and covered a larger portion of the muscle cross-section (P less than 0.01 and P less than 0.1). These changes indicate the presence of an age-related denervation-reinnervation process in both types of muscles. In the young adult group, the fast-twitch motor unit fibres of TA were non-randomly arranged (P less than 0.05-0.01) whereas the fibre arrangement within the slow-twitch motor units of the soleus was not significantly different from random. In old animals, the fibre arrangement was non-random in both fast- and slow-twitch motor units. In TA, the distribution of nearest-neighbour distances showed an increased (P less than 0.05) proportion of short distances in old age, whereas the distribution of interfibre distances was unchanged. In the soleus, the distribution of interfibre distances showed an age-related displacement to the left at short distances (P less than 0.05) and to the right at long distances (P less than 0.01), but the distribution of nearest-neighbour distances was not significantly altered. It is concluded that motor unit fibres are non-randomly arranged in the fast-twitch motor units studied and that a non-random rearrangement of motor unit fibres takes place in both fast- and slow-twitch units during the ageing process. This age-related rearrangement is secondary to a denervation-reinnervation process and it appears as if different types of reinnervation predominate in fast- and slow-twitch units.