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

Preganglionic axons arising from different levels of the mammalian spinal cord make preferential connexions with different classes of superior cervical ganglion cells (Langley, 1892, 1900; Njå & Purves, 1977a). For example, preganglionic axons from the first thoracic segment (T1) make relatively strong connexions with ganglion cells activating end-organs of the eye; conversely, axons arising from T4 selectively innervate ganglion cells projecting to the ear. In the present work we have asked whether this selectivity reflects the function of the pre- and post-synaptic cells, and aspect of their respective positions, or some other criterion. 1. 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, however, tend to be activated by different spinal segments. Thus the segmental innervation of ganglion cells is correlated with the position rather than the function of post-ganglionic targets. 2. The role of target position in ganglion cell innervation was examined directly by recording from neurones sending axons to different destinations. Superior cervical ganglion cells running dorso-medially in a spinal nerve receive, on average, innervation from more caudal segments than cells projecting ventro-laterally. 3. These selective connexions do not depend on intraganglionic cell position: neurones located at different points along the major axes of the superior cervical ganglion receive, on average, the same segmental innervation. In accord with this observation, retrogradely labelled neurones innervating a particular target such as the eye or ear are widely and randomly distributed within a large portion of the ganglion. Thus the importance of post-ganglionic target position in ganglion cell innervation is not simply a reflexion of ganglionic topography. 4. We conclude that one purpose of the selective connexions 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.