title: Acute and chronic hypoxia: implications for cerebral function and exercise tolerance.
abstract: OBJECTIVE
  To outline how hypoxia profoundly affects neuronal functionality and thus compromise exercise-performance.
  
  
  METHODS
  Investigations using electroencephalography (EEG) and transcranial magnetic stimulation (TMS) detecting neuronal changes at rest and those studying fatiguing effects on whole-body exercise performance in acute (AH) and chronic hypoxia (CH) were evaluated.
  
  
  RESULTS
  At rest during very early hypoxia (<1-h), slowing of cerebral neuronal activity is evident despite no change in corticospinal excitability. As time in hypoxia progresses (3-h), increased corticospinal excitability becomes evident; however, changes in neuronal activity are unknown. Prolonged exposure (3-5 d) causes a respiratory alkalosis which modulates Na+channels, potentially explaining reduced neuronal excitability. Locomotor exercise in AH exacerbates the development of peripheral-fatigue; as the severity of hypoxia increases, mechanisms of peripheral-fatigue become less dominant and CNS hypoxia becomes the predominant factor. The greatest central-fatigue in AH occurs when SaO2is ≤75%, a level that coincides with increasing impairments in neuronal activity. CH does not improve the level of peripheral-fatigue observed in AH; however, it attenuates the development of central-fatigue paralleling increases in cerebral O2availability and corticospinal excitability.
  
  
  CONCLUSIONS
  The attenuated development of central-fatigue in CH might explain, the improvements in locomotor exercise-performance commonly observed after acclimatisation to high altitude.
authors.affiliation: Faculty of Health and Life Sciences, Northumbria University, Newcastle, UK.
authors.forename: Stuart
authors.initials: S
authors.lastname: Goodall
authors.affiliation: School of Sport and Service Management, University of Brighton, Eastbourne, UK.
authors.forename: Rosie
authors.initials: R
authors.lastname: Twomey
authors.affiliation: Department of Medicine, University of Utah, Salt Lake City, UT, USA.
authors.forename: Markus
authors.initials: M
authors.lastname: Amann
keywords: brain; exercise; hypoxia; muscle; oxygen