Changes in the distribution of the extrajunctional acetylcholine sensitivity along muscle fibers during development and following cordotomy in the rat.

Acetylcholine sensitivity along the entire length of muscle fibers was studied during postnatal development and following transection of the spinal cord in the rat. During postnatal development, the acetylcholine sensitivity in the soleus and extensor digitorum longus muscles decreased faster at the juxtajunctional region than near the tendons. Thus, the adult pattern of low acetylcholine sensitivity at the extrajunctional membrane was achieved through the uneven change of acetylcholine sensitivity during normal development. This uneven pattern of the sensitivity was found to appear in both muscles in older rats after cordotomy, and is in striking contrast to the uniform pattern in denervated muscles. The uneven appearance of the sensitivity could not be explained by changes in input resistance or resting membrane potential. In the soleus muscle whose nerve was implanted at an ectopic site, the lowest sensitivity also appeared at the ectopic juxtajunctional region after cordotomy. These results indicate that the motor nerve exerts regionally different effects along a fiber with respect to the appearance of acetylcholine receptors.