The origin of parkinsonian muscular rigidity is not clearly understood. The manifest clinical differences between it and the spasticity in disease of the pyramidal tract have led to many speculative theories which in the main have produced only controversial confusion.
The simplest and most fundamental consideration, that the rigidity is dependent on the integrity of the proprioceptive arc arising in the muscle, has not been established definitely. Walshe1 showed that intramuscular injections of procaine hydrochloride, so graded as to paralyze the afferent nerve fibers in the muscle and to leave the activity of the motor nerve fibers unimpaired, abolishes or greatly diminishes the rigidity of paralysis agitans. Voluntary movements carried out by the muscles thus rendered flaccid are of greater amplitude and speed and are less readily fatigued than similar movements carried out when the muscles concerned are still rigid. It seems, therefore, that the integrity of the proprioceptive
POLLOCK LJ, DAVIS L. MUSCLE TONE IN PARKINSONIAN STATES. Arch NeurPsych. 1930;23(2):303–319. doi:10.1001/archneurpsyc.1930.02220080087006
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