A number of facts have led physiologists to look on tonus in skeletal muscle as a function wholly distinct from other types of contraction. Many believe that it calls into play a different mechanism from that which produces the simple twitch evoked by artificial stimulation.
Decerebrate rigidity, a state in which the extensor muscles of the limbs remain for a long time in sustained contraction with extraordinary freedom from fatigue, has been taken as the most perfect example of tonus, and has for several reasons been held to exemplify those properties wherein tonus differs from ordinary contraction. In the first place the economy of metabolism and the lack of fatigue and of heat production are so marked, as compared with contraction of the same strength artificially induced, as to suggest the employment of a different mechanism. In the second place, the property of plasticity, as noted by Sherrington, whereby a
ALEXANDER FORBES. TONUS IN SKELETAL MUSCLE IN RELATION TO SYMPATHETIC INNERVATION. Arch NeurPsych. 1929;22(2):247–264. doi:10.1001/archneurpsyc.1929.02220020063003