Journal of Neurophysiology, Vol 59, Issue 5 1510-1523, Copyright © 1988 by APS
Effects of muscle shortening on the responses of cat tendon organs to unfused contractions
G. Horcholle-Bossavit, L. Jami, J. Petit, R. Vejsada and D. Zytnicki
Laboratoire de Neurophysiologie, College de France, Paris.
1. The discharges from individual Golgi tendon organs of peroneus tertius
and brevis muscles were recorded in anesthetized cats. Responses to unfused
isometric contractions of single motor units and combinations of motor
units were compared with responses to contractions eliciting muscle
shortening (i.e., shortening contractions). 2. In 75% of the examined
instances, the effect of muscle shortening during unfused contractions was
a slight decrease in tendon organ activation, in keeping with the reduction
of contractile tension recorded at the muscle tendon. In other instances
there was either no change in tendon organ response or, in less than 10% of
instances, a slight increase For two motor units eliciting similar
activation of a given tendon organ under isometric conditions, the effect
of shortening contraction was not necessarily the same. 3. The reductions
observed in tendon organ discharges upon muscle shortening were less than
proportional to the reductions of contractile tension and difficult to
correlate with the properties of motor units, as determined under isometric
conditions. The present observations suggest three main reasons for this
lack of relation. 4. The first reason depended on the properties of motor
units, in that the relation between length changes and tension changes was
not the same for all units. Two motor units developing similar isometric
tensions did not necessarily produce the same degree of muscle shortening.
Some units produced relatively significant shortening without much loss of
tension. 5. Second, the dynamic sensitivity of tendon organs is known to
exert a major influence on their responses to isometric unfused
contractions, accounting for 1:1 driving of discharge during tension
oscillations and high frequency bursts upon abrupt increase of tension.
Although less tension was produced and the rate of tension development was
slower in shortening contractions, similar manifestations of the dynamic
sensitivity of tendon organs were observed. In such cases, the responses of
tendon organs were the same whether or not the muscle shortened during
contraction. 6. Third, when several motor units were stimulated in
combination, the unloading influences of in-parallel units were facilitated
by muscle shortening so that unloading effects, which were hardly visible
under isometric conditions became evident during shortening contractions.
