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Golgi tendon organ
A muscle is stretched passively and then contracts strongly against a fixed load. Which receptor fires in each situation, and how many synapses lie in each reflex arc?
- A. Both fire on stretch, and both arcs are monosynaptic
- B. Golgi tendon organ on stretch through a monosynaptic arc; muscle spindle on tension through a disynaptic arc
- correctC. Muscle spindle on stretch through a monosynaptic arc; Golgi tendon organ on tension through a disynaptic arc
- D. Both fire on tension, and both arcs are disynaptic
Explanation
Key fact
The spindle measures length; the tendon organ measures tension. One excites its own muscle, the other inhibits it.
Spindle on stretch, monosynaptic; tendon organ on tension, disynaptic. The extra synapse is the inhibitory interneurone.
Why
Muscle spindles lie in parallel with the fibres and sense length and its rate of change.
Their Ia afferent synapses directly on the alpha motor neurone - one synapse.
Golgi tendon organs lie in series in the tendon and sense tension.
Their Ib afferent synapses on an inhibitory interneurone first - two synapses.
The tendon organ therefore relaxes the muscle it monitors, protecting the tendon.
Two receptors, two arcs, opposite effects
Why not the others
- A. Both fire on stretch, and both arcs are monosynaptic — loses the distinction entirely. Only the spindle reflex is monosynaptic.
- B. Golgi tendon organ on stretch through a monosynaptic arc; muscle spindle on tension through a disynaptic arc — swaps the two receptors and both arcs.
- D. Both fire on tension, and both arcs are disynaptic — would leave nothing measuring muscle length, and no monosynaptic stretch reflex to elicit.
Remember
Parallel measures length and excites; in series measures tension and inhibits.
Go deeper
The knee jerk is the clinical monosynaptic spindle reflex, and it is the only reflex arc in the body with a single synapse.
Gamma motor neurones keep the spindle taut as the muscle shortens, so it can keep reporting length throughout a contraction.
The clasp-knife phenomenon in spasticity is attributed to the tendon organ reflex suddenly overriding the stretch reflex.
Ganong's Review of Medical Physiology, 27th ed. (McGraw Hill, 2025); Guyton and Hall Textbook of Medical Physiology, 15th ed. (Elsevier, 2026).