Histochemically, the anterior tibial muscle of the albino rat presents a mosaic pattern of fibres with different enzymatic activity. From the standpoint of esterase and oxidative enzymes, such as succinic dehydrogenase, three different types of fibres have been distinguished: type A fibres, which are the classical white muscle fibres, and B and C fibres, which aretwokinds of red fibres (Stein and Padykula, 1962). The A fibres are large with low enzymatic activity and the C fibres small with high enzymatic activity. The B fibres form a more heterogeneous group with various intermediate sizes and intensities of activity.It has been shown that muscle contractions induced by shock stimulation to the sciatic nerve produce striking changes in the phosphorylase activity and glycogen content, which vary in the different types of fibres (Kugelberg and Edstr6m, 1968a, b Kugelberg, 1968). METHODSThe investigations were carried out on 20 adult albino rats weighing 400 g, anaesthetized with sodium pentobarbital. The body temperature was maintained between 37 and 39°C.The animal was placed in the prone position on a steel plate with the vertebral column fixed at two points. The left hind limb was fixed with steel pins drilled through the tibia near the knee joint and through the calcaneus and clamped so that the leg was held rigidly with the thigh and lower leg at right angles. The foot was fixed in a holder. The left hind limb was denervated by transecting the nerves to all the muscles with the exception of the anterior tibial muscle and the extensor digitorum longus and extensor hallucis longus. The latter muscles were not denervated because it was found that this procedure was apt to impair the circulation to the anterior tibial. The tendon of the anterior tibial muscle was dissected free, cut and attached with a thin steel wire to a Grass Ft 03 force-displacement transducer for recording from single units, and to an Ft 10 for recording whole muscle contractions. The transducer was fastened to a rack and pinion device which permitted easy adjustment of muscle length and tension. The transducer was connected to a BUDD model P-350 portable digital strain indicator and its output displayed on a Tektronix 502A oscilloscope and photographed. Isometric contractions of the muscle were recorded with the initial tension of the muscle adjusted to 12 g, which is the lower range of optimal tension. The L4 and L5 ventral roots were exposed by laminectomy and transected proximally near the spinal cord. The skin around the incision over the vertebral column was drawn up to form a pool for mineral oil, which bathed the roots. The temperature of the fluid was maintained at 37 to 38°C. The filaments of the L4 or L, motor roots were dissected under a microscope until it was ensured that it contained only one motor fibre to the muscle. The criterion was an all-or-nothing response in the muscle to finely graded stimuli from sub-threshold up to supramaximal current strength.In some experiments the sciatic nerve was cut and stimulated with sup...

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