The Journal of Neurophysiology Vol. 81 No. 6 June 1999, pp. 2647-2661
Copyright ©1999 by the American Physiological Society
Sag During Unfused Tetanic Contractions in Rat Triceps Surae
Motor Units
J. S.
Carp,
P. A.
Herchenroder,
X. Y.
Chen, and
J. R.
Wolpaw
Wadsworth Center, New York State Department of Health and State
University of New York at Albany, Albany, New York 12201
Carp, J. S.,
P. A. Herchenroder,
X. Y. Chen, and
J. R. Wolpaw.
Sag During Unfused Tetanic Contractions in Rat Triceps Surae
Motor Units. J. Neurophysiol. 81: 2647-2661, 1999.
Sag during unfused tetanic contractions
in rat triceps surae motor units. Contractile properties and
conduction velocity were studied in 202 single motor units of intact
rat triceps surae muscles activated by intra-axonal (or intra-myelin)
current injection in L5 or L6 ventral root to
assess the factors that determine the expression of sag (i.e., decline
in force after initial increase during unfused tetanic stimulation).
Sag was consistently detected in motor units with unpotentiated twitch
contraction times <20 ms. However, the range of frequencies at which
sag was expressed varied among motor units such that there was no
single interstimulus interval (ISI), with or without adjusting for
twitch contraction time, at which sag could be detected reliably.
Further analysis indicated that using the absence of sag as a criterion
for identifying slow-twitch motor units requires testing with tetani at
several different ISIs. In motor units with sag, the shape of the force profile varied with tetanic frequency and contractile properties. Simple sag force profiles (single maximum reached late in the tetanus
followed by monotonic decay) tended to occur at shorter ISIs and were
observed more frequently in fatigue-resistant motor units with long
half-relaxation times and small twitch amplitudes. Complex sag profiles
reached an initial maximum early in the tetanus, tended to occur at
longer ISIs, and were more common in fatigue-sensitive motor units with
long half-relaxation times and large twitch amplitudes. The differences
in frequency dependence and force maximum location suggested that these
phenomena represented discrete entities. Successive stimuli elicited
near-linear increments in force during tetani in motor units that never
exhibited sag. In motor units with at least one tetanus displaying sag,
tetanic stimulation elicited large initial force increments followed by
rapidly decreasing force increments. That the latter force envelope
pattern occurred in these units even in tetani without sag suggested
that the factors responsible for sag were expressed in the absence of
overt sag. The time-to-peak force (TTP) of the individual contractions
during a tetanus decreased in tetani with sag. Differences in the
pattern of TTP change during a tetanus were consistent with the
differences in force maximum location between tetani exhibiting simple
and complex sag. Tetani from motor units that never exhibited sag did
not display a net decrease in TTP during successive contractions. These
data were consistent with the initial force decrement of sag resulting
from a transient reduction in the duration of the contractile state.
