Apportionment of the terminals from single preganglionic axons to target neurones in the rabbit ciliary ganglion.

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Abstract
The apportionment of terminals from single preganglionic axons to target neurons was studied in the ciliary ganglion of adult rabbits. Both electrical recording and intra-axonal injection of horseradish peroxidase (HRP) showed that each preganglionic axon innervates only a small fraction of the ganglion cell population (about 10-20 of the .apprx. 400 ganglion cells). Neurons whose cell bodies were enveloped by HRP-labeled boutons from a single axon were often surrounded by other neurons which received no contacts from the labeled fiber EM examination of labeled presynaptic terminals on individual ganglion cells confirmed that the boutons of single axons were sharply confined to particular target cells. Individual target neurons (or portions of them) are probably the unit of innervation during the development of these synaptic connections. Comparison of the amplitudes of synaptic responses in singly and multiply innervated ganglion cells indicated that, on average, an individual axon made a weaker synaptic connection with a multiply innervated neuron than with a neuron that received only 1 input. Neurons innervated by several different axons tended to have fewer synapses on their somata than neurons innervated by only 1 or 2 preganglionic axons. Individual post-synaptic profiles were often contacted exclusively by labeled terminals when examined in the EM. Since many of these neurons are multiply innervated, this observation suggests some regional separation of the several inputs contacting the same cell. For several reasons this inference must be regarded as tentative. The several axons that initially innervate ganglion cells without dendrites evidently compete during early life until only a single input remains. On ganglion cells with dendrites, the number of inputs that persists is proportional to dendritic complexity. The diminished competition between axons innervating neurons with dendrites may result from some degree of terminal segregation on dendritic arborizations.

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