Motor unit contractions initiating impulses in a tendon organ in the cat.

1. Responses, identified as coming from Golgi tendon organs, were recorded in afferent nerve fibres supplying the cat soleus muscle. 2. Receptor discharge was recorded during stimulation of single, selected motor units. The degree of summation of responses to combined stimulation of pairs of motor units was compared with the effect of stimulating each motor unit separately. Whenever individual response frequencies differed by a large amount there was little summation on combined stimulation. 3. The response of tendon organ to the second of a pair of closely spaced tetanic contractions of a motor unit showed adaptation of the discharge. Adaptation could also be induced by a conditioning contraction from another motor unit. The amount of 'cross-adaptation' could be correlated with the difference in individual response frequencies of the two motor units and with the degree of summation on combined stimulation. 4. A mechanical model is proposed to account for these observations. The muscle fibre from each motor unit is envisaged to pull on a collagen strand which supports one of the receptor terminals. The non-linear summation of responses on combined stimulation and the phenomenon of cross-adaptation are accounted for by mechanical cross-links between collagen strands such that one muscle fibre pulls on more than one receptor terminal.