Penetration of systemically injected horseradish peroxidase into ganglia and nerves of the autonomic nervous system.

HRP given intravenously to rats and guinea-pigs passes within 5 minutes into the extracellular spaces of the superior cervical and coeliac ganglia, reaching the spaces between neurons and their associated satellite cells and the equivalent spaces between neuronal processes and satellite cells. The sympathetic nerve trunks have a blood-nerve barrier. The myenteric plexus does not contain blood vessels, but is permeable to tracer from the extracellular spaces of the adjacent muscle layers.