The reorganization of synaptic connexions in the rat submandibular ganglion during post‐natal development.

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.