Sympathetic prions.

Transmissible spongiform encephalopathies are a group of invariably fatal neurodegenerative diseases. The infectious agent is termed prion and is thought to be composed of a modified protein (PrP Sc or Pr PRES ), a protease-resistant conformer of the normal host-encoded membrane glycoprotein, PrP C. Bovine spongiform encephalopathy, scrapie of sheep, and Creutzfeldt-Jakob disease are among the most notable transmissible spongiform encephalopathies. Prions are most efficiently propagated trough intracerebral inoculation, yet the entry point of the infectious agent is often through peripheral sites like the gastrointestinal tract. The process by which prions invade the brain is termed neuroinvasion. We and others have speculated that, depending on the amount of infectious agent injected, the injection site, and the strain of prions employed, neuroinvasion can occur either directly via peripheral nerves or first through the lymphoreticular system and then via peripheral nerves.