Pathology of the peripheral nervous system - overview and differential diagnoses

Pathobiology

The peripheral nervous system (PNS) connects peripheral sensory and motor end organs to the central nervous system and provides the electrophysiological base for conscious and unconscious reactions to the outer environment. As much as the molecular and subcellular equipment of peripheral nerve cell components resemble those of the CNS, the type and composition of their vasculature, immediate (myelin) and peripheral (mesenchymal) sheaths and supporting cells differ.

Hence, the CNS-PNS compound axon, immediately after emergence from the CNS is enwrapped by Schwann cells that in a chain continue down to the terminal muscle junction or sensory end organ. The interface between central oligodendrocyte and peripheral Schwann cell is called transition node (of Ranvier) and marks the origin of the nerve root. Under pathological conditions the CNS-PNS border shifts into one or the other direction.

The peripheral nerve, in a strict sense, arises from the nerve root after the periradicular sheath (meningeal origin) transits into epi- and perineurium and gains lymphatic drainage. Moreover, the PNS contains nerve cell conglomerates (ganglia) for distal autonomic reflexes and for feeding sensory information into the CNS, via centripetal axons of the dorsal root. These nerve cells are supported by peripheral glial cells (satellite cells) but they are not protected by a considerable blood-tissue barrier in contrast to the brain and (to a lesser content) the peripheral nerve trunk. The same holds true for the distal nerve endings. [...]