The responses to stretch have been studied in living, isolated Golgi tendon organs (GTOs) from tail muscles of cat. Experiments were performed in vitro and consisted of subjecting single GTOs to controlled ramp-and-hold stretch while recording the response from their sensory axons raised in oil. 2. The threshold force required for sustained afferent discharge was measured directly, and found to be between 8 and 170 dynes at 24 degrees C for nine GTOs tested. Beyond threshold, the discharge frequency is approximately proportional to applied static tension over a wide range. Sensitivy to tension varies among different GTOs and appears to be inversely correlated with mechanical stiffness. 3. With impulse activity blocked by tetrodotoxin, graded receptor potentials could be recorded whose amplitude varied in proportion to applied static tension. All GTOs examined showed in addition a dynamic response, which became larger with increasing velocity of ramp stretch. This dynamic sensitivity appears in the receptor potential and is then augmentd by an apparent accommodative process at the impulse initiating site. 4. Based on the above findings, possible mechanical models are discussed for the sensory transduction mechanism.
