The postnatal development of the inferior oblique muscle of the cat. II. Effects of repetitive stimulation on isometric tension responses.

The changes with postnatal age in post-tetanic potentiation (PTP) and fatigue of the inferior oblique muscle have been studied in the cat. PTP of the twitch amplitude increased steadily with age up to 20 weeks after birth. Twitch contraction time (ct) and half relaxation time (hrt) was not significantly changed. The potentiation of the tetanic response also became more prominent with age. The young muscles (10 weeks and below) were slightly more susceptible to fatigue than older muscles, but recovery was rapid in muscles of all ages. Intensive stimulation induced post-tetanic depression of twitch responses in muscles older than 1 week, but none in the muscles of new-born cats. Ct and hrt were greatly prolonged in muscles above six weeks of age. In these muscles, but never in the very youngest, repetitive firing could be observed in response to post-tetanic single nerve stimulation. The results are compared with those obtained in similar experiments on hind-limb muscles by other workers. They further support the idea, raised in a previous paper, that slow eye muscle fibres develop quicker and reach maturity earlier than fast fibres.