Accuracy of regeneration of vagal parasympathetic axons.

The degree of accuracy with which regenerating preganglionic parasympathetic fibers can restore their original connections was examined in the frog, Rana pipiens. The normal motor pool of the vagus nerve was determined by labeling the vagal cardiac branch fibers with horseradish peroxidase (HRP). Cardiac neurons form a relatively compact subgroup within the rostral half of the vagal motor column and represent less than 5% of the total vagal motor pool. Five to 14 weeks after crushing the vagus nerve, HRP labeling revealed that a more extensive population of vagal motor neurons has reinnervated the cardiac branch. The regenerated cardiac motor neuron pool is about twice as numerous as normal but is still centered in the rostral half of the vagal motor column. These experiments show that regeneration of the cardiac branch of the vagus nerve is neither a random process nor one which is completely accurate. Some degree of accuracy is maintained during regeneration following a crush lesion. Neurons nearer the original cardiac motor pool are more likely to reinnervate the cardiac branch than are vagal motor neurons located at some distance from the original cardiac pool.