1. The innervation of neurones in the submandibular
ganglion of neonatal and adult rats has been studied with intracellular recording, and light and electron microscopy. 2. Intracellular recordings from neurones in isolated ganglia from adult animals showed that about 75% of the
ganglion cells are innervated by a single preganglionic fibre. 3. However multiple steps in the post-synaptic potential (about five on average) were elicited in
ganglion cells from neonatal animals by graded stimulation of the preganglionic nerve. The same result was obtained when the preganglionic fibres were stimulated at their emergence from the brainstem, indicating that neonatal neurones are innervated by several different preganglionic nerve cells. 4. The number of preganglionic fibres innervating individual
ganglion cells gradually decreased during the first few weeks of life, and by about 5 weeks each
ganglion cell was generally contacted by a single preganglionic axon. 5. Synapses were made on short protuberances in the immediate vicinity of the neuronal cell bodies in both neonatal and adult ganglia as shown by staining presynaptic boutons with the
zinc-iodide osmium method, injection of
horseradish peroxidase into
ganglion cells, and electron microscopical examination. 6. Electron microscopical counts of synaptic profiles per
ganglion cell perimeter showed that the number of synaptic contacts made on
ganglion cells actually increased during the first few post-natal weeks, when the number of axons innervating each neurone was decreasing. 7. These results show that in the rat submandibular
ganglion there is a reorganization of neuronal connexions during the first few weeks of life which results in a transition from multiple to generally single innervation of
ganglion cells.