Abstract
Membrane constants and distribution of acetylcholine (ACh) receptors were determined for multiply innervated fibers of the anterior latissimus dorsi (ALD) and singly innervated fibers of the posterior latissimus dorsi (PLD) muscles of 3-6 month old chickens. The values of the various membrane constants were: length constant, 1.78 mm (mean) in ALD, 0.68 mm in PLD; time constant, 35 msec in ALD, 3.7 msec in PLD; transverse membrane resistance, 4388 ohm cm(2) in ALD, 561 ohm cm(2) in PLD; and membrane capacitance, 8.2 microF/cm(2) in ALD, 7.0 microF/cm(2) in PLD. Peaks of ACh sensitivity occurred at intervals of ca. 740 micro on ALD fibers with a low sensitivity remaining between peaks. Only one peak of ACh sensitivity was detected on PLD fibers. The maximum ACh sensitivity found was 5 +/- 4 mv/ncoul for fibers of the ALD and 77 +/- 60 mv/ncoul for fibers of the PLD. The distance over which this sensitivity fell to 0.1 was ca. 225 micro in the ALD and 140 micro in the PLD. The membranes of these two muscle fiber types differ widely regarding some electrical properties and the disposition of ACh-sensitive receptor sites.