On the purpose of selective innervation of guinea‐pig superior cervical ganglion cells.

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Abstract
Preganglionic axons arising from different levels of the mammalian spinal cord make preferential connections with different classes of superior cervical ganglion cells (Langley, 1892, 1900; Nja and Purves, 1977). Preganglionic axons from the first thoracic segment (T1) make relatively strong connections with ganglion cells activating end-organs of the eye; conversely, axons arising from T4 selectively innervate ganglion cells projecting to the ear. Whether this selectivity reflects the function of the pre- and post-synaptic cells, an aspect of their respective positions or some other criterion was investigated in the guinea pig. End-organs with different functions at the same locus (the eye) respond to stimulation of the same ventral roots; end-organs of a single modality (hairs or blood vessels) at different positions tend to be activated by different spinal segments. The segmental innervation of ganglion cells is correlated with the position rather than the function of post-ganglionic targets. The role of target position in ganglion cell innervation was examined directly by recording from neurons sending axons to different destinations. Superior cervical ganglion cells running dorsomedially in a spinal nerve receive, on the average, innervation from more caudal segments than cells projecting ventrolaterally. These selective connections do not depend on intraganglionic cell position: neurons located at different points along the major axes of the superior cervical ganglion receive, on the average, the same segmental innervation. In accordance with this observation, retrogradely labeled neurons innervating a particular target such as the eye or ear are widely and randomly distributed within a large portion of the ganglion. The importance of post-ganglionic target position in ganglion cell innervation is not simply a reflection of ganglionic topography. One purpose of the selective connections in the superior cervical ganglion is to bring together preganglionic axons arising from different levels of the spinal cord and ganglion cells whose axons innervate particular regions of the superior cervical territory.