Chronaxie varies for different nerve fibers. Which of the following types of nerves has minimum chronaxie?
Mixed nerves
Motor nerves
Myelinated nerve
Sensory nerves
Chronaxie is the minimum time for which a direct current equivalent to twice the intensity of the rheobase must be applied to trigger an action potential. Rheobase of an axon is the threshold current for an infinitely long-duration stimulus. The chronaxie is inversely propoional to the degree of fiber myelination, therefore chronaxie is minimum in myelinated nerves. Sensory fibers are less myelinated than motor fibers. Consequently, a sho duration electrical stimulus should trigger action potentials in motor fibers (A alpha) without triggering the sensory fibers (A delta, C). Chronaxie of C fiber (unmyelinated): 400 microsecond Chronaxie of A delta fiber (myelinated): 150 microsecond Chronaxie of A alpha fiber (myelinated): 40-100 microsecond Ref: Pediatric Anesthesia By Bruno Bissonnette, 2011, Page 758.