Responses of tendon organs in a lizard.

1. In the lizard Tiliqua the tendons of the caudo-femoralis muscle are supplied by a nerve which runs separately from the muscle nerve. 2. Recordings of afferent discharges in the tendon nerve revealed the presence in the tendon of stretch-sensitive mechanoreceptors which responded to both passive changes in limb position and to muscle contraction. 3. A preparation of the tendon and its nerve were dissected free of surrounding tissue and studied in isolation while recording the activity of single functional units. The minimum tension in the tendon necessary for a maintained response from a receptor lay in the range 5-35 g (mean 16 g) and the firing rates at these tensions were in the range 5-14 impulses/sec (mean 9 impulses/sec). 4. Receptors showed a steep increase in firing rate with increase in tension up to about 120 g. The firing rate 30 sec after the onset of a tension change did not exceed 40 impulses/sec. 5. During the tension change the receptor responded with a burst of impulses whose frequency depended on the velocity of stretch. With large, rapidly rising tension steps peak firing rates of up to 300 impulses/sec were observed. 6. Tension and length changes recorded during rapid tendon-stretches were very similar, with little sag in tension at the new length. The response of all units however continued to fall throughout the stretch. Some of the possible causes of this adaptation have been discussed.