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The parasympathetic control of muscle tonus

The parasympathetic control of muscle tonus

Arch Neurol & Psychiatry 22(2): 265-281

A review is given of the facts which bear on Frank's hypothesis that tonic impulses are conveyed to the muscles antidromically over parasympathetic fibers in the dorsal roots. Antidromic vasodilation and the pseudomotor phenomenon are probably both brought about by the fine dorsal root fibers that accompany the blood vessels. Although the pseudomotor phenomenon and acetylcholine contrac-ture are inhibited by epinephrine, they are not inhibited by large doses of scopolamine and atropine. Hence the pharmacologic evidence is against the reactions falling definitely in the parasympathetic class. On the other hand, the favorable action of atropine and scopolamine on the Parkinsonian rigidities must not be forgotten. The importance of the dorsal roots for muscle tonus has been overstated. Contrary to the usual belief, deaffer-ented muscle does not remain permanently atonic, but recovers some of its tone and may for a time become hypertonic. Decerebrate rigidity, tonic crossed extensor reflexes and labyrinthine tonic reflexes can be obtained from deafferented muscle. This shows that whatever role the dorsal roots may play in tonus, the fibers of the ventral roots are sufficient in themselves to produce tonic contractions. It remains an open question whether or not there are any efferent fibers in the dorsal roots. There is no histologic evidence that dorsal root fibers have any endings on the muscle fibers in the limbs of mammals other than the endings in the muscle spindles.

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