The density of reinnervation of adult rat superior cervical sympathetic ganglionic neurons is limited by the number of available postsynaptic sites.

The adult rat superior cervical ganglion has about 27,000 neurons and is innervated by about 9000 preganglionic axons which make a total of nearly 11 million synapses. Surgical removal of the upper part of the ganglion, reducing the number of neurons to about 20%, causes an overall reduction of the number of synapses to about 30%, but has no effect on the numbers of preganglionic axons. Thus, a 5-fold increase in the axon/neuron ratio causes an increase of only about 50% in the number of synapses per cell. Axotomy followed by regeneration of the preganglionic axons causes no further increase in the number of synapses per cell, even though the average number of synapses per axon is reduced to about one-quarter of the normal. This suggests that the ganglionic neurons can only accept a limited number of synapses, and that in the normal situation there is only possibility for a relatively minor increase before this limit is reached. This study is complementary to a previous one in which the numbers of preganglionic axons were surgically reduced and it was found that, when allowed to regenerate into an entire denervated ganglion, the remaining axons could not increase their numbers of synapses. Thus, in the normal rat superior cervical sympathetic ganglion the total number of synapses is such that while the preganglionic axons are probably expressing close to their full synaptogenic potential, the ganglionic neurons express only about two-thirds of their ability to receive synapses.