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

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Abstract
Postnatal development of the inferior oblique muscle in the cat was studied with physiological and histochemical techniques. Changes with age in isometric twitch and tetanic response characteristics were described. Twitch amplitude increased and twitch contraction time (ct) and half-relaxation time (hrt) decreased almost linearly from birth to adulthood. The relation between strength of nerve stimulation and twitch ct and hrt changed during development with a threshold slow response appearing at 10 wk. Twitch responses in cats 6 wk 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 wk or older. The maximum rate of tension rise remained the same from birth to 6 wk of age and later increased markedly up to 20 wk of age. The contracture induced by succinylcholine was the same in muscles of all ages. 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 development of fast and slow eye muscle fibers was suggested. Slow fibers seemed to have completed their maturation at about 10 wk of age, while development of fast fiber properties continued, probably up to the adult stage.

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