FIBRE DIAMETER INCREASE IN NERVES TO “SLOW‐RED” AND “FAST‐WHITE” CAT MUSCLES DURING POSTNATAL DEVELOPMENT

The nerve fibres supplying mammalian “slow-red” and “fast-white” muscles are known to differ in calibre, those to “slow-red” muscles being the smaller (Hay 1901, Eccles & Sherrington 1930, Boyd & Dauey 1966). There is also a difference in duration of the after-hyperpolarization between motoncurons supplying “slow-red” and “fast-white” mammalian muscles (Eccles, Eccles & Lundberg 1958). The latter authors also suggested that, within a muscle, the fast-contracting fibres should be supplied by fast-conducting nerve fibres. In expcrimenls on motor units of cat lumbrical muscle, Bessou, Emonef-Denand & Laporte (1963) found a linear relation between the two parameters. Such a relation also exists in rat-tail muscles ( S f e g 1964). If the same relation were to exist in newborn kittens, one might expect the nerve fibres to future “slow-red” and “fast-white” muscles to be of fairly equal diameter a t that age, since the contraction times of the respective types of muscle are fairly similar (Denny-Brown 1929, Buller, Eccles & EccZes 1960). However, with respect to another physiological fcature-the posttetanic potentiation (PTP) of twitch tension-the soleus and gastrocnemius muscles differ in the newborn kitten (Nystrom 1968a), the soleus having a high PTP and the gastrocnemius mainly lacking this phenomenon. The PTP phenomenon has been ascribed by some authors to events taking place in the nerve or its terminals (see Hughes 1958). Consequently, it is considered of interest to know: ( A ) are there any differences between the maturity of soleus and gaslrocncmius nerve fibres in newborn kittens, maturity being measured as nerve fibre diameter (cf. Skoglund & Romero 1965) and conduction velocity (cf. Skoglund 1960a) ? Skoglund (1960b) showed that the absence of PTP in the monosynaptic path of newborn kittens is due to hyperpolarization of the immature afferent terminals to block during tetanization. If, in analogy therewith, the gastrocnemius nerve terminals of newborn kit-

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