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Golgi Tendon Organ - The 'Tension-Reader' of Muscles

The Golgi Tendon Organ (GTO) is a proprioceptor present in the muscle tendon. Remember, tendons connect muscle to bone.

 
Golgi-tendon
 

Ten to fifteen muscle fibres are connected to one GTO.

Essentially, the GTO reads tension in the muscle.

What do we mean by tension? It is a complex concept, but simply put, muscle tension is regarded as force exerted by muscle on an object. It is the state of contraction of the muscle fibres that thus exert a force on an object. The GTO is mainly an inhibitory organ i.e., it gives negative feedback to the muscles. This probably evolved as a protective mechanism to prevent damage from excessive load.

If the tension in the muscles increases, the GTO is stimulated and causes muscle relaxation.
Decreased tension decreases the stimulation of the GTO, leading to reflex contraction.

At different parts of the tendon there are some areas with increased tension and some with decreased tension (reflecting the tension in the muscle itself). The summation of the above mentioned effects helps to spread the load on the muscle, to distribute it equally and prevent damage by excessive load. When the load on the muscle increases beyond a certain point, it causes an instantaneous relaxation of the muscle which is known as “the lengthening reaction”.

The afferent nerve pathway is through fast Ib nerves, which connect with the dorsal horn of the spinal cord. They ultimately synapse on anterior horn cells which are inhibitory neurons- hence the negative feedback as was mentioned before.

Through spinocerebellar tracts they also inform cerebellum of changes in muscle tension. There are other connections to the cerebrum as well, which is why we have conscious awareness of changes in muscle tension - for example, imagine holding a cup in mid-air without changing the position of your arm: we’re aware of the tension building in our muscles and are forced to bring the cup down when we cannot manage the load. This brings to our attention the fact we are aware of muscle tension but not muscle length. 

The protective mechanism of the GTO is debatable because we have a conscious level of awareness of how much load our muscles are taking, and so we are able to voluntarily adjust ourselves to decrease that load. 

If you read the article on the muscle spindle, you’ll now be able to understand that these two proprioceptors are essential to how the musculoskeletal system reacts to internal and external changes. There is still a lot to learn about the physics of our bodies, and the only way to find out is delving deeper into the biomechanics and microsystems of organs of locomotion!

Author: Shruthi Sivakumar

Sources and citations

1.Guyton and Hall Textbook of Medical Physiology- 13th edition
2.Human Physiology- From Cells to System, 8th edition, Lauralee Sherwood