AJP - Cell Physiology, Vol 236, Issue 1 35-C40, Copyright © 1979 by American Physiological Society
Properties of motor units of the frog iliofibularis muscle
A. R. Luff and U. Proske
The tension developed by single motor units of the iliofibularis muscle of
the frog Litoria aurea was recorded in response to single-shock and
repetitive stimulation of motor axons. The majority of units in each
muscle, 13 on the average, were of the twitch type; an additional 4 units
were slow or tonic. It appeared that slow units comprised a single
homogeneous population, but two types of twitch units could be recognized:
small fatigue-resistant units with long twitch times to peak (20--40 ms)
and larger, fatigable units with briefer times to peak (16--27 ms).
Evidence from a comparison of unit tetanic tensions indicated the presence
of polyneuronal innervation of both slow and twitch muscle fibers. The
relatively low incidence of polyneuronal innervation of twitch fibers in
iliofibularis, when compared with a muscle like sartorius (9), was
attributed to the difference in lengths of muscle fibers in the two
muscles. It was argued that slow muscle fibers probably receive a
multiterminal as well as polyneuronal innervation, with the terminals of
any one axon lying widely spaced along the muscle fiber.
