Motoneurones "learn" and "forget" physical activity.

In spite of our knowledge of activity related adaptations in supraspinal neurones and skeletal muscles, very little is known concerning adaptations in alpha-motoneurones to alterations in chronic activity levels. Recent evidence shows that the biophysical properties of alpha-motoneurones are plastic and adapt to both increases and decreases in chronic activation. The nature of the adaptations--in resting membrane potential, spike threshold, afterhyper-polarization amplitude,and rate of depolarization during spike generation--point to involvement of density, type, location, and/or metabolic modulation of ion conductance channels in the motoneuronal membrane. These changes will have significant effects on how motoneurones respond when activated during the generation of movements, and on the effort required to sustain activation during prolonged exercise. Since the adaptations most likely involve structural changes in the motoneurones and changes in protein synthesis, and change the output response of the cells to input, they are considered to be learning responses. Future research directions for examining this issue are outlined.