The responses elicited in individual tendon organs by the contraction of single motor units were studied in peroneus longus, peroneus brevis, tibialis anterior and soleus muscles. 2. No simple relation was found between the discharge frequency of a tendon organ and the tension produced in the muscle tendon by the contraction of individual motor units. 3. The sensitivity of a given tendon organ to contractile tension was not the same for each of the motor units which elicited its discharge. There was no correlation between the sensitivity of the receptor and the strength of the motor units. 4. Upon repetitive stimulation of a tendon-organ-activating motor unit at increasing rates, the frequency of the receptor sustained discharge reached a maximal value for rates of stimulation eliciting submaximal tetanic tension. Higher rates only produced an increase in the dynamic component of the tendon organ response. 5. These observations show that the contractile tension sensed by a tendon organ is not a simple fraction of the tension which appears at the muscle tendon. They might be accounted for as consequences of the fine structure of tendon organs and of variations in the number of muscle fibres contributed by different motor units to the bundle inserted on each receptor. The location of most tendon organs at musculo-aponeurotic junctions rather than in the tendon proper, could also be responsible for some of the observed discrepancies.
