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1. The ability of preganglionic axons to re-establish their normal pattern of synaptic connexions with superior cervical ganglion cells has been studied after section of the cervical sympathetic trunk.

2. In vivo stimulation of the last cervical (C8) and the first seven thoracic ventral roots (T1—T7) 3-4 months after section of the trunk produced end-organ responses similar to those observed in normal animals.

3. The pattern of innervation of individual neurones, determined by intracellular recording of synaptic potentials 4-9 months after cutting the sympathetic trunk, was also similar to that observed in normal neurones. Both normal and re-innervated ganglion cells were contacted by pre-ganglionic axons arising from C8 to T7, and each neurone was usually innervated by a contiguous subset of these segments.

4. Re-innervated neurones, as normal cells, were typically dominated by the innervation from a particular spinal cord segment, with the adjacent segments contributing a synaptic influence that decreased as a function of distance from the dominant segment. This was true whether the amplitude of the post-synaptic potential, or the estimated number of contributing axons, was used as the criterion of segmental dominance.

5. Re-innervated neurones, however, showed some abnormalities. The average number of ventral roots contributing innervation to each neurone was reduced from 4·1 to 3·0, and discontinuities in the sequence of innervating segments were more frequent than in normal neurones. Moreover, fewer preganglionic axons contacted each neurone after regeneration.

6. A further difference between normal and re-innervated neurones during the period covered by these experiments was that axons from the more caudal spinal cord segments were less successful in re-establishing contacts with ganglion cells than those from the rostral segments. The more caudal the position of the preganglionic neurones, the more pronounced was this relative deficiency.

7. Although anomalies of ganglion cell innervation were apparent, the basis for the restoration of normal functional effects appears to be the re-establishment of a pattern of innervation of individual neurones similar to that observed in normal ganglia.