Cyclobenzaprine: Studies on its site of muscle relaxant action in the cat

Vibration (150 μ × 300 Hz) of the gastrocnemius-soleus muscle was used to elicit a reflex muscle contraction (tonic vibration reflex) which was facilitated or inhibited by electrical stimulation of various areas within the central nervous system. The results obtained support previous suggestions that the principal site of action of cyclobenzaprine as a skeletal muscle relaxant agent is upon neurons of the brain stem reticular formation. Vestibular nerve and medial reticular formation-elicited responses were more sensitive to the action of cyclobenzaprine than those induced through activation of Deiters' vestibular nucleus or its spinal projection through the lateral reticular formation. Least sensitive to the action of cyclobenzaprine were responses elicited through activation of these pathways at the level of the spinal cord. It may be noted that vestibular nerve afferents as well as vestibular nuclei project to the medial reticular formation including reticulospinal neurons within the nucleus pontis oralis and caudalis. Further, stereotaxic coordinates for electrical stimulation of the medial reticular formation impinged upon these pontine nuclei. This would suggest that in the present study the muscle relaxant action of cyclobenzaprine resulted from its suppression of these latter pontine reticulospinal pathways.