Relation of animal size to convergence, divergence, and neuronal number in peripheral sympathetic pathways

The enormous range of animal size raises a fundamental problem: How do larger animals maintain adequate control of peripheral structures that are many times more massive and extensive than the homologous structures in smaller animals? To explore this question, we have determined neuronal number, the number of axons that innervate each neuron (convergence) and the number of neurons innervated by each axon (divergence), in a peripheral sympathetic pathway of several mammals (mouse, hamster, rat, guinea pig, and rabbit). The average adult weights of these species vary over approximately a 65-fold range. However, the number of superior cervical ganglion cells increases by only a factor of 4 between the smallest of these animals (mice; about 25 gm) and the largest (rabbits; about 1700 gm); the number of spinal preganglionic neurons that innervate the ganglion increases by only a factor of 2. Thus, the number of nerve cells in the sympathetic system does not increase in proportion to animal size. On the other hand, our results indicate that there are systematic differences across these species in the number of axons that innervate each ganglion cell and in the number of ganglion cells innervated by each axon. We suggest that modulation of convergence and divergence in sympathetic ganglia allows this part of the nervous system to effectively activate homologous peripheral targets over a wide range of animal size.