Golgi tendon organ · Lobeworks/17

The Golgi tendon organ is a sensory receptor that measures a muscle's force, a capsule where the muscle fibres join the tendon, whose group Ib ending is squeezed between collagen strands each time the muscle pulls, so its firing tracks the tension the muscle produces.

Golgi tendon organ. The Golgi tendon organ is a sensory receptor that measures a muscle's force, a capsule where the muscle fibres join the tendon, whose group Ib ending is squeezed between collagen strands each time the muscle pulls, so its firing tracks the tension the muscle produces.

It sits in series with the muscle, where the muscle spindle sits in parallel, and that geometry decides what each one hears. Stretch the relaxed muscle and the spindle fires while the tendon organ barely does; let the muscle contract and the spindle is unloaded while the tendon organ fires harder. Its answer is mostly tonic, a rate that follows the force held, as when a weight is kept still in the hand, with a dynamic part as well: it answers more strongly while the force is rising or falling, and current accounts stress that it signals changes in force more than its steady level.

It can relax the muscle it measures. In the spinal cord Ib fibres inhibit the motor neurons of their own muscle through an interneuron (autogenic inhibition), one of the brakes that shape how force is graded.

Length and tension are two different messages. The brain needs both to tell an arm that stopped because it reached its place from one that something stopped.

Fast wiring. Ib fibres are thick and myelinated, close in speed to the spindle's Ia.

The spindle measures how long the muscle is; the tendon organ measures how hard it pulls.

One lies beside the fibres, the other at their end.

Questions: Muscle spindle or Golgi tendon organ: which reports length and which force? The spindle reports length and the tendon organ force, and the reason is where each one sits. The spindle lies in parallel with the muscle fibres, so it is stretched when the muscle is stretched and unloaded when the muscle contracts; the tendon organ lies in series, between muscle and tendon, so it is squeezed whenever the muscle pulls. Stretch a relaxed muscle and the spindle fires; make it contract and the tendon organ does.