Sensitivity of a cat muscle fiber to locally applied ACh was tested by moving the tip of an ACh pipette close to the muscle membrane at points separated by distances of about 0.5 mm. Upper trace in each record is the potential change, recorded by an intracellular pipette, to the pulse of ACh, ejected by the current pulse indicated in each lower trace. In an innervated fiber (B) only the visible end‐plate region is sensitive to ACh. In a 14‐day denervated fiber a constant pulse of ACh produced at each point of the membrane a potential change as shown in A. Time marker: 10 ms

Acetylcholine sensitivity of fibers from rat soleus muscle, tested by iontophoretic application of ACh. The muscle had been denervated for 7 days without stimulation, and then directly stimulated for 7 days. Δ, ○, ACh sensitivity of two different fibers at the locus of high sensitivity (presumably the former end plate). •, sensitivity of a number of fibers in the same muscle at a point midway between the end plate and tendon. ×, sensitivity at a comparable region of fibers in the opposite soleus muscle that had been only denervated for 14 days. Note that, at this extrajunctional site, stimulation of the denervated muscle has decreased the expected sensitivity by approximately 3 orders of magnitude

Effect of diffuse application of ACh to a partially denervated muscle fiber. Simultaneous intracellular recordings from a denervated (upper traces) and an innervated (lower traces) end plate in the same muscle fiber 15 days after section of both pelvic nerve branches. Distance between end plates, 14.5 mm. Resting potentials were 86 and 84 mV at the denervated and innervated end plates, respectively. During exposure to ACh: A, 10⁻⁸ M; B, 5 × 10⁻⁸ M; C, 10⁻⁷ M at 20‐min intervals. Voltage calibration same for both traces, B as in A; time calibration in C applies to A and B. Usual arrangement of nerve branches in frog's sartorius is illustrated at lower right. ×, sites where nerves were cut. Note that the membrane near the denervated end plate has become supersensitive, although the muscle fiber was presumably still activated by its remaining innervation. At the innervated region, sensitivity was low

Series of twitches of the soleus (SOL) and flexor digitorum longus (FDL) muscles of the cat to show the effect of nerve cross‐union. The upper control records show contractions of muscle whose nerves have been severed, then resutured (self‐reinnervation). Lower records show contractions of muscles whose nerves have been severed and cross‐sutured (cross‐reinnervation). The self‐innervated muscles have normal contraction speeds. Note the speeding of the slow‐twitch soleus and the slowing of the fast‐twitch FDL following cross‐reinnervation. TM, temperature of muscle; TP, time to peak of contraction.

Cross sections of soleus muscle of rats incubated for actomyosin‐ATPase activity. Darkly stained fibers are presumed to be of the fast‐twitch type, others of slow‐twitch type. A: normal, unoperated rat. B: ipsilateral soleus, 8 mo after denervation of its antagonists by transection of the common peroneal nerve. Note the increased proportion of fast‐twitch fiber types. C: 21 wk after excision of the soleus synergists, gastrocnemius and plantaris. Note decreased proportion of fast‐twitch fibers in the operated limb (right). There is also a decrease in the fast‐twitch fibers in the contralateral, unoperated limb (left)