The postnatal development of the inferior oblique muscle of the cat. I. Isometric twitch and tetanic properties.

The postnatal development of the inferior oblique muscle in the cat has been studied with physiological and histochemical techniques. This paper describes the changes with age in isometric twitch and tetanic response characteristics. The twitch amplitude increased and the twitch contraction time (ct) and half-relaxation time (hrt) decreased almost linearly from birth to adulthood. The relation between the strength of nerve stimulation and twitch ct and hrt changed during development with a threshold slow response appearing at 10 weeks. Twitch responses in cats 6 weeks of age or older were of longer duration than in younger cats, in spite of the longer ct and hrt in young cats. Fusion frequency of the tetanic response reached a constant level in muscles 6 weeks or older. The maximum rate of tension rise remained the same from birth to 6 weeks of age and later increased markedly up to 20 weeks of age. The contracture induced by succinylcholine was the same in muscles of all ages. These data were related to previous findings on the postnatal development of fast and slow muscles and motor units in the hindlimb of the cat. A differentiation of the development of fast and slow eye muscle fibers is suggested. Slow fibres seemed to have completed their maturation at about ten weeks of age, while the development of fast fibre properties continued, probably up to the adult stage.