Post-natal reduction of neural unit size in the rabbit ciliary ganglion

We have studied the innervation of adult and neonatal ciliary ganglia in the rabbit to determine the average number of ganglion cells innervated by each preganglionic neurone at different stages of development. 1. The adult ciliary ganglion comprises about 380 ganglion cells which are innervated by about forty preganglionic neurones. 2. Ciliary ganglion cells in adult rabbits are on average innervated by 2.2 different axons; in contrast, neonatal ganglion cells are on average innervated by 4.6 different axons. The transition to the adult pattern of innervation occurs gradually during the first few post-natal weeks. 3. The numbers of ganglion cells and preganglionic neurones do not change appreciably after birth. Accordingly, the loss of some innervation to individual neurones during post-natal development indicates that each preganglionic axon innervates progressively fewer ciliary ganglion cells. 4. The number of synaptic boutons found in ganglia at birth, however, is less than the number of synaptic boutons found in adult ganglia. 5. We conclude that synaptic connexions in this ganglion age gradually rearranged in early post-natal life such that each preganglionic neurone focuses an increasing number of synaptic contacts on a progressively smaller subset of the ganglion cell population.