Neuromuscular adaptations probably occur at all levels of the motor pathway, following changes in muscular activity. Adaptations have been mostly investigated in muscle fibers after heavy-resistance and endurance training. In strength training the rapid improvement is due to poorly understood neural factors, with muscle fiber hypertrophy occurring rather later; fiber hyperplasia is probably unimportant. In contrast to other mammals, muscle fibers in humans do not readily change from "slow" to "fast" and vice versa, although there may be a period of plasticity in infancy especially responsive to weightbearing. A proportion of the changes in the fibers, following training, is probably brought about by stretch-activated mechanisms, in which second messengers express immediate early genes and the latter, in turn, promote the transcription of "muscle" genes; other cellular adaptations occur at the level of mRNA translation. In the spinal cord, adaptations following hemiparesis include a loss of functioning motor units. Impaired motor drive occurs not only in organic brain lesions but also after periods of disuse. In persons with amputations, the peripheral nerve stump undergoes degenerative changes; somatosensory cortical evoked responses, following stimulation of the stump, are diminished. It is possible that both sensory and motor cortical representations are increased for neighboring regions of the body.