Differentiation of electrical and contractile properties of slow and fast muscle fibres.

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Abstract
The development of the electrical and contractile properties of slow anterior latissimus dorsi (ALD) and fast posterior latissimus dorsi (PLD) muscles of the chick was studied. At the earliest ages studied (14-16 days of incubation), the membrane resistance was similar in both muscles. Subsequently the membrane resistance of PLD decreased and that of ALD increased. Electrical differentiation continued after hatching. The contractile speeds of ALD and PLD muscles were similar in embryos at 14-16 days of incubation and suddenly differentiated at 17-18 days. Unlike the electrical properties, contractile speeds changed little after hatching. The different electrical properties of the membrane of the 2 types of muscle fiber developed as a result of the particular type of excitation imposed by their nerves.