Role of the stretch reflex in oxotremorine tremor.
Although the basic mechanism for parkinsonian tremor is known to be central, the case for the oscillator(s) of physiological tremor and shivering is still a matter of dispute. In this case, an important role has been proposed for muscle afferents. Oxotremorine tremor has in common with shivering its frequency range (10-28 Hz) and the co-contraction of flexors and extensors. On the other hand, in contradistinction, it can be blocked by atropine, as is the case with parkinsonian tremor. Thus it was of interest to analyze the role of muscle afferents in the production and maintenance of oxotremorine tremor in the acutely decorticated cat. This was studied with two experimental approaches. In one, either a front or hind limb of a cat was completely deafferented by dorsal rhizotomy and the tremor activity recorded electromyographycally in a pair of antagonistic muscles. In the other, the nerve and muscle electrical activity and force produced by the tremoring muscle (gastrocnemius) were recorded simultaneously. Tremor activity was induced by oxotremorine injection (200-750 micrograms/kg i.p.) given at different postoperative periods. The drug induced a regular tremor in the chronically deafferented animals (3 weeks to 5 months) but not in the acutely deafferented limb (3 hrs after dorsal rhizotomy). The tremor observed in the former group was very regular and had the same frequency range (10-28 Hz) for both normal and deafferentd sides. Section of all the muscle afferents of a given pool (agonist and its main synergist) did not have any effect on the regularity and frequency of the oscillation (tremor) of that pool. The role of other muscle and/or cutaneous afferents, although perhaps of some importance, remains unclear. A pure spinal mechanism to account for the regular oscillations (tremor) is proposed.