Specific innervation of neurons in the paravertebral sympathetic ganglia of the chick.

I have explored the chick sympathetic system as a model for the study of specific synapse formation by examining the pattern of ganglion cell innervation by preganglionic axons in 2 different ganglia. Using intracellular recording and HRP labeling techniques, the innervation of the 12th and 15th cervical ganglia (C12 and C15) was examined. Sympathetic ganglion cells of the chick are innervated in a stereotyped manner by preganglionic axons arising from different levels of the spinal cord. While each ganglion is innervated by preganglionic axons arising from several spinal cord segments, individual ganglion cells are innervated by only some of the spinal segments that supply each ganglion as a whole. The subset of spinal segments is always contiguous, with 1 segment providing the dominant innervation to the cell. Spinal segments adjacent to the dominant segment provide synaptic inputs that diminish as a function of distance from the dominant segment. This pattern of ganglion cell innervation in the chick is similar to that of the mammal, where re-innervation studies have suggested that ganglion cell innervation is selective. The similarity in the innervation of avian and mammalian sympathetic neurons suggests that the rules that underlie the specificity of synapse formation in the sympathetic system of the 2 species are the same. Because of the accessibility of the chick embryo for experimental manipulations during development, it is now possible to study the cellular basis that underlies the specificity of synapse formation in this relatively simple nervous system.