Cobalt and horseradish peroxidase tracer studies in the stellate ganglion of octopus.

The connectivity of the stellate ganglion of Octopus was investigated using cobalt and horseradish peroxidase (HRP) tracers applied to severed preganglionic and postganglionic nerves. Centripetal cells, defined as cells in the ganglion which send their axons back toward the brain, were discovered by both methods. Application of HRP to the pallial (preganglionic) nerve labeled the preganglionic fibers which enter the ganglion and spread widely within the neuropil. Application of either tracer to a single stellar (postganglionic) nerve labeled motor neuron cell bodies spread over 1/4 to 1/3 of the ganglion. Since there are 25-40 such stellar nerves per ganglion, it may be inferred that there is only a limited topographic relationship between the location of motor neuron cell bodies in the stellate ganglion and the segment of mantle muscle which they innervate. The axon and many side branches of these motor neurons were also labeled. The smooth, diffuse form of HRP labeling produced in pre- and portganglionic axotomized neurons in Oct0pus is similar in appearance to that produced in damaged mammalian neurons.