THE RATIO OF PREGANGLIONIC TO POSTGANGLIONIC NEURONS AND RELATED ISSUES IN THE AUTONOMIC NERVOUS-SYSTEM

The motor outflow of the autonomic nervous system (ANS) is differentia ted into two major divisions, parasympathetic (PSNS) and sympathetic ( SNS). Both are organized hierarchically into pre- and postganglionic l evels, but classically the two divisions have been assumed to differ i n their ratios of pre- to postganglionic neurons. The PSNS has been ch aracterized as having lower ('one-to-few') ratios, whereas the SNS has been described as possessing higher ('one-to-many') ratios. These pat terns have been assumed to measure differing divergences of the outflo ws. In this review, a ratio of pre- to postganglionic neurons is calle d a ratio index, and the idea that the PSNS and SNS have characteristi cally different ratio indexes and divergences is called the ratio rule . The putative differences in the ratio indexes of the two divisions - as well as Fulton's influential proposal that they form one of the ba ses of contrasting functional capacities of the PSNS and SNS - have be en widely accepted for nearly three quarters of a century. A survey of the original observations yielding the concept of the ratio rule as w ell as the more recent estimates of pre- and postganglionic numbers, h owever, challenges both the generality and the adequacy of the ratio r ule and indexes. The originally formulated differences between the PSN S and SNS represent an overgeneralization since they were based on obs ervations of only two ganglia, the ciliary ganglion in the PSNs and th e superior cervical ganglion in the SNS. Furthermore, these original e stimates were based on limited samples and were subject to a number of counting artifacts. A survey of the literature suggests that ratio in dexes vary much more within each ANS division than they do between the two divisions. When ganglia other than the ciliary and superior cervi cal are examined, the two divisions of the ANs have broad, largely ove rlapping ranges of ratio indexes. Additionally, other PSNS-SNS pairs c an be found in which the relative sizes of their respective indexes ar e completely contrary to the ratio rule. For a given ganglion, there a re substantial differences in the ratio index between species, between individuals of the same species, and between stages of development in the same species. Furthermore, both divisions of the ANS have wide an d largely overlapping ranges of physiological effects varying from spe cific to diffuse, from local to widespread. Finally, the ratio index m easure ignores the degree of convergence found in different ganglia, a nd it is insensitive to the fact that many ganglia have multiple funct ionally distinct motor neuron pools, each with separate inputs varying in their degrees of divergence and/or convergence. Thus ratio indexes do not differentiate the PSNS from the SNS, and conclusions based on such putative distinctions are questionable.