Distribution of the innervating neurons of pupillary dilator and tarsal muscles in the superior cervical ganglion.

In an experimental study using adult cats, we analyzed the distribution of the sympathetic postganglionic neurons in the superior cervical ganglion (SCG) which innervate the pupillary dilator and tarsal muscles to determine whether there are direct fiber projections from the spinal cord to the iridic and tarsal muscles. This study consisted of three experiments. In Experiments 1 and 2, respectively, horseradish peroxidase (HRP) was injected unilaterally into the iridic stroma or superior tarsal muscles. The SCG and spinal cord were then removed for microscopic analysis. In Experiment 3, the ganglion was examined microscopically after the injection of bisbezimide into the iris and of HRP into the ipsilateral superior tarsal muscle. Experiment 1 revealed more numerous HRP-labeled neurons in the rostral part than in any other region of the ipsilateral superior cervical ganglion. In Experiment 2, HRP-labeled neurons were evenly distributed over the whole ipsilateral SCG. No HRP-labeled neurons were detected in the spinal cord in either of these experiments. In Experiment 3, no doubly-labeled neurons were discernible in the ganglion. These results clearly indicate the localization of internal carotid nerve cells in a specific region of the SCG, and provide evidence for the absence of postganglionic neurons that innervate both the dilator muscle of the pupil and the tarsal muscles.