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Neurological Effects of Exposure to Galactic Cosmic Rays.     Gregory A. Nelson, Department of Radiation Medicine, Loma Linda University,  11175 Campus St. CSP A1010, Loma Linda, California 92354. 

   This presentation constitutes a review of the observed effects of radiation on the central nervous system (CNS) with emphasis on high LET radiation (galactic cosmic rays and accelerated particles) and low/medium dose ranges.  First, the general pathological responses of the brain and spinal cord to ionizing radiation are considered.   Following this introduction, selected radiation responses for neurons, glia and vasculature are discussed in terms of cell population changes, functional output and alterations in tissue architecture.  Examples of functional changes include blood-brain barrier breakdown and alterations in electrophysiological parameters.  Various high-LET radiation effects are compared and contrasted with corresponding low-LET effects with emphasis on results from low dose in vivo experiments. The two most sensitive CNS responses to radiation are neurogenesis and behavioral changes.  These responses are interrelated and have been subjected to numerous investigations with charged particles in rodent models.  Inhibition of neurogenesis, in particular, may determine dose limits for cognitive impairment.  Predictions from network modeling studies will be presented to illustrate the importance of redundancy and high connectivity to network failure modes.  Finally, the experimental results and observations on low LET radiation-induced neurological and cognitive impairment in humans are synthesized and discussed in terms of potential risk limits for space missions.