POSTNATAL DEVELOPMENT OF MOTOR NERVE TERMINALS IN “SLOW‐RED” AND “FAST‐WHITE” CAT MUSCLES

Differences hetwwn motor nerve terminals in “slow-red” and “fastwhite” niamnialian muscles with respect lo size were noted by CoPrs (1955). As far as the structure of the terminals in the two types of muscle is concerned, Tschieriew (1879) and H a y (1901) found no differences, whereas COCO & Dislefano (1903) reported a difference. However, as pointed out by Tiegs (1953), the pictures of the latter authors are unconyincing. Kriiger (1960), too, reported differences between terminals in “slow-red” and “fast-white” muscles of some mammalian species, classifying those in the former muscles as “terminaisons en grappe” (Tschier iew 1879) and those in the latter as “terniinaisons en plaque”. The “en grappe” type is generally regarded to be confined to the slow muscle fibres of cold-blooded animals (Gi in fher 1949, Gray 1957, Lannergren & Smith 19661, but it is found in some bird muscles (Kriiger 1950, 1952) and in mammalian extraocular muscles as well (Re t z ius 1892, Hess 1961). The “en grappe” type of ending has also been considered as an immature form of future “en plaque” endings (see Kriiger 1952). A differentiation in structure of motor endings during development was described by Boeke (1911) in the mole, mouse, and man and also by Tello (1922) and Iwanaga (1925) in man. Kruger (1960) reported “en grappe” endings to be present in adult cat soleus muscle. According to the classical descriptions, this type of ending is supplied by unmyelinated or thin, only slightly myelinated nerve fibres. Consequently, in order to ascertain whether true “en grappe” endings exist in “slow-red” or “fast-white” muscles in the cat, the diameter of the nerve fibres was measured close to the terminals. In addition to various other differences between “slow-red” and “fast-white” mammalian muscles, a difference has been reported in the magnitude (Bul ler & Lewis 1965) and mechanism ( S f a n d a e r t 1964) of post-tetanic twitch potentiation (PTP) as seen in cat soleus and gastrocnemius muscles. Furthermore, the capacity for twitch potentiation