Responses of isolated Golgi tendon organs of the cat to muscle contraction and electrical stimulation.

1. Responses of Golgi tendon organs isolated from cat tail muscles to contraction of muscle fibers inserting directly into the receptor (GTO-muscle fibres) as well as to pulses of electrical current applied extracellularly through the sensory axon were studied. 2. Analysis of the responses to GTO-muscle fibre contraction indicated that the active force developed by each muscle fibre constituted an equally potent input to the receptor in proportion to the developed force. 3. The sensitivity of a Golgi tendon organ remained almost constant with changes in muscle length up to a length where maximum active tension was developed (lo). Beyond lo, the sensitivity tended to decrease. 4. The absolute force threshold (passive + active) at 1o for initiating an impulse in the afferent nerve was estimated for five preparations to be 4.5-14 mg. It was also demonstrated that the contraction of a single GTO-muscle fibre may initiate impulse discharge from the receptor. 5. A constant depolarizing current applied extracellularly to a GTO through its axon initiated a train of impulses, probably originating from a site near or within the receptor capsule. Analysis of responses to constant currents of various intensities suggested that a single impulse initiation site was involved. 6. During combined stimulation, responses of a tendon organ to GTO-muscle fibre contraction simply added to the response initiated by a constant current pulse, suggesting that the impulse initiation sites activated by each mode of stimulation were identical, or situated very close to each other in the nerve terminal.