AJP - Cell Physiology, Vol 253, Issue 4 C599-C606, Copyright © 1987 by American Physiological Society
Mechanics of tonus fibers of frog muscle
E. Bozler
Department of Physiology, Ohio State University, Columbus 43210.
Contractions with two phases of relaxation are induced by brief strong
stimulation in some frog muscles. The first phase with rapid relaxation is
produced by the twitch fibers; the second phase, which is very slow and is
only present after strong stimulation, represents the relaxation of the
tonus fibers. At moderate loads, half time of isotonic relaxation of these
fibers is as long as 30 min at 2 degrees C, but the rate varies with the
load and depends on the condition of the frogs. With regard to the rate of
relaxation, the tonus fibers resemble molluscan catch muscles. In tonus
fibers, rapid isotonic and isometric relaxation can be induced by a small
extension; shortening opposes this effect. These responses are like the
length responses previously found in various types of striated muscle. They
go in the same direction as the well-known metabolic effects of length
changes (Fenn effect). After a large extension by an increase in load there
is no active shortening when the load is returned to the previous value.
This and other observations show that the slowness of relaxation is not due
to sustained activity, but is determined by the strength of the contractile
bonds formed during contraction. Because activity during relaxation is very
low, it is unlikely that length responses are caused by a modification of
the cross-bridge cycle. It is suggested that length changes act through a
mechanism that is separate from that initiating contraction, but alters the
speed of relaxation by making the cross bridges weaker or stronger.
