Lesions and Muscle Tone - Control of Posture and Movement

LESIONS AND MUSCLE TONE

If there is a lesion in the lower motor neuron, the muscle it supplies atrophies (becomes smaller). There is loss of muscle tone, resulting in flaccidity. No re-flexes can be elicited because the muscle cannot be stimulated.

If there is a lesion in the upper motor neuron, the presentation is different because the lower motor neuron is intact. Also, the presentation will depend on which upper motor neuron is affected.

In a normal person, some descending tracts inhibit stretch reflexes and others stimulate; however, the in-hibitory effect is more prominent. If the corticospinal tract (has stimulatory effect) alone is injured, the muscle tone is diminished (hypotonic) and there is muscle weakness (paresis) rather than complete loss of movement.

If the extrapyramidal tracts are injured, the in-hibitory effect on the lower motor neuron is removed and the muscle tone is increased (hypertonic/spastic) and the reflexes are exaggerated. There is little muscle atrophy.

If the cerebellum or its projections are injured, there is incoordination of movement.