The influence of innervation on differentiating tonic and twitch muscle fibres of the chicken.

1. The anterior (ALD) and posterior (PLD) latissimus dorsi muscles of adult chickens were denervated by section of their motor nerves. Four weeks later the contractile and membrane properties of these muscles were studied in vitro at room temperature.2. Although the time course of the PLD muscle twitch was slightly prolonged, the qualitative difference in contractile characteristics of the slow ALD and the fast PLD muscles was maintained after denervation.3. The difference in the passive membrane characteristics of the ALD and PLD muscle fibres was not lost after denervation, although the membrane resistance (R(m)) and space constant (lambda) of the denervated muscles fell. The membrane resistance, space (lambda) and time (tau(m)) constants of the ALD muscle remained significantly greater than for the PLD muscle fibres. The absolute values of tau(m) in both muscles increased, implying that in the case of the ALD the membrane capacitance (C(m)) was increased above normal after denervation. This is discussed in terms of the ultrastructural changes in this muscle after denervation.4. The ALD muscle was cut into small pieces and replaced in the bed of the PLD muscle, which in turn was minced and placed into the bed of the ALD muscle. These muscles regenerated and became reinnervated by the PLD and ALD nerves respectively. They aligned themselves in the muscle bed and adopted the former shape of the muscle that they replaced. The passive cable properties of the regenerated ALD muscle fibres innervated by the PLD nerve resembled the control PLD fibres and the regenerated PLD fibres reinnervated by the ALD nerve resembled those of the control ALD. Regenerated ALD and PLD reinnervated by their own nerves had contractile and membrane properties similar to those of control muscles.5. The results show that the fundamental differences, between slow and fast muscles once established, persist even when they are deprived of their innervation. The properties of developing muscle fibres however are determined by the motor nerves even in the adult animal.