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The postnatal development of some twitch and fatigue properties of single motor units in the ankle muscles of the kitten.

Contractions of single motor units in the gastrocnemius, soleus and pretibial flexor muscles were elicited by intracellular stimulation of the innervating motoneurones. Kittens being 1, 2, 6 and 10 weeks of age were used. The aim was to establish the pattern of postnatal differentiation of the adult motor unit types (Burke et al. 1973, 1974, Hammarberg and Kellerth 1975 b). In the developing type S units of the soleus muscle the mean value for contraction time showed a transient decrease during the early postnatal period, but the twitch half-relaxation time (HRT), the duration of motoneurone post-spike afterhyperpolarization (AHP) and the susceptibility to fatigue remained virtually unchanged during the age period studied. In the units of the developing fast twitch muscles the contraction time, HRT and susceptibility to fatigue were significantly different from those of the soleus units already at 1 week of age. At 6-10 weeks of age the contractile characteristics typical of the adult type FF and FR units were attained. The AHP duration gradually decreased up to 10 weeks of age, and it was then considerably shorter than both at 1 week of age and in the adult stage.