Two types of synaptic selectivity and their interrelation during sprouting in the guinea-pig superior cervical ganglion.

1. The synaptic connexions of the guinea-pig superior cervical ganglion were examined after collateral sprouting provoked by cutting the preganglionic nerve supply from spinal segments T3-T7. The selective properties of the connexions made by the remaining segments C8, T1 and T2 were explored both with respect to the segmental origin of the preganglionic axons (Njå & Purves, 1977 a) and with respect to their conduction velocity (Wigston, 1983). 2. After sprouting, the ventral root T1 elicited strong sympathetic effects on both the eye and the ear, while the normal response is largely confined to the region of the eye. This effect on the expressed selectivity was confirmed by intracellular recording in vitro, which showed that virtually all the ganglion cells were now innervated by T1. 3. However, neurones that received dominant innervation from T1 were more frequently innervated by sprouted axons from C8 than were neurones whose dominant innervation derived from T2. This indicates that basic mechanisms promoting segmental selectivity were still functioning. 4. The selective innervation of ganglion cells with respect to the conduction velocity of the preganglionic axons was well maintained after sprouting. 5. These results show that during sprouting from a restricted set of preganglionic axons the synaptic partners are still matched according to both types of selectivity. This suggests that the recognition mechanisms which guide neural development are preserved in adult life, but that the differences in the available sets of preganglionic axons lead to different competitive interactions and different resulting patterns of innervation.