Golgi tendon organ

Each Golgi tendon organ is a ≈0.5–1 mm capsule at the junction between muscle and tendon. Inside, the collagen strands are attached in series to a small group of ≈10–20 extrafusal muscle fibres. A group Ib afferent axon branches among the collagen strands. When the attached fibres contract, the strands straighten and squeeze the nerve endings, so the receptor signals tension, especially actively generated tension. It is very sensitive to the contraction of even a single motor unit.

Ib afferents excite inhibitory interneurons in the spinal cord that inhibit the α-motor neurons of the same muscle. This is autogenic inhibition, also called the inverse myotatic reflex. They also facilitate the antagonists. Classically this was seen as a protective reflex against excessive force. It is now regarded as continuous force feedback that helps regulate tension and share load between muscles. It is the basis of proprioceptive neuromuscular facilitation stretching.