The Golgi Tendon Organ Lies in Series Between Muscle and Tendon and Detects Muscle Tension

Each Golgi tendon organ is a slender capsule, at the junction between muscle and tendon, in series with 15 to 20 extrafusal skeletal muscle fibers (Figure 8-5). The capsule of each tendon organ contains a complement of braided collagen fascicles, among which the branches of a single sensory neuron are intertwined.

Because the Golgi tendon organ is in series with a group of extrafusal fibers and the tendon, when the extrafusal fibers shorten during contraction, tension is applied to the tendon organ. This causes the braided collagen fibrils of the organ to tighten and squeeze the endings of the sensory neuron. Action potentials are therefore generated and sent to the CNS along the sensory neuron at a frequency proportional to the tension developed by the muscle. In contrast, the muscle spindle is arranged in parallel with the extrafusal muscle fibers, and when they contract, the spindle reduces its action potential frequency.

When action potentials from spindle sensory neurons reach the CNS, as mentioned earlier, they Inonosynaptically produce EPSPs in the α motor neurons returning to the same muscle. Action potentials along sensory neurons from Golgi tendon organs have the opposite effect: they activate inhibitory interneurons, polysynaptica]∣y producing IPSPs on α motor neurons to the same muscle. This leads to a reduced extrafusal muscle fiber contraction.

The two skeletal muscle receptors discussed in this chapter provide the CNS with vital information about muscle length (the spindle) and muscle tension (the Golgi tendon organ), and through their reflex circuitry, they help to keep these parameters within optimal ranges. The information these receptors provide, along with information from receptors of skin and joints, is essential if the CNS is to coordinate posture and locomotion.