The Golgi Tendon Organ
Brittany Kearney and Meredith Wilkinson


Introduction
The Golgi tendon organ is an important neurological structure located within the muscular system. Its general function is to provide the central nervous system with information regarding tension in the muscles and tendons. Motor output can thus be modulated in order to protect the body from unnecessary damage. In addition, the tendon organ contributes to the production of regular patterns of intended movement, as seen in locomotion. These purposes render the Golgi tendon organ as an irreplaceable part of every muscle system.


Functional Anatomy
How is it structured?
Each Golgi tendon organ consists of a capsule of 3-25 extrafusal fibers located within the musculotendinous junction (embedded in the muscle). At one end it is connected in-series to multiple muscle fibers (ten to twenty-five per tendon organ), while the other end attaches in-parallel to the collagenous fibers of the tendon.


Figure 1:
z9k0080675710001.gif
http://jn.physiology.org/cgi/content/full/96/4/1789


There are two types of collagen fibers that make up the GTO: densely packed and loosely packed. The fibers that make up the outside of the capsule are densely packed and are organized in parallel, whereas the fibers within the capsule are loosely packed and convoluted (Fig. 1) (6).
How is it activated?
A Ib afferent nerve fiber enters the capsule bypassing the densely packed fibers and divides into branches that weave through the internal fascicle. When a tendon is stretched, some of the loosely packed collagen fibers will straighten. The Ib afferent endings get pinched between the straightening fibers, become depolarized, and open mechanically-gated ion channels. With an adequate stimulus, an action potential will result and propagate up the Ib afferent axon. As the signal reaches the dorsal root of the spinal cord it synapses with the Ib inhibitory interneuron.


Details of activation:
Upon activating the GTO there is a dynamic response in which the initial movement of the muscle generates a burst of afferent discharge. A static response closely follows the dynamic as the output reduces to a more constant firing rate. The initial discharge may be reduced by either self-adaptation or cross-adaptation of motor units. A motor unit will self-adapt if it has already been activated. Motor units can also undergo cross-adaptation in which the activation of one motor unit leads to the reduced dynamic discharge for a different motor unit firing after it (7).


Furthermore, multiple motor units can innervate a single tendon organ. Both types of motor units (type I slow and type II fast, fatiguing) initiate bursts of similar discharge frequencies in GTOs. When multiple units are activated for one GTO, the cumulative response is smaller than the sum of the individual responses. There is generally a greater contribution from motor units that are more likely to cross-adapt than from those that are less likely to do so. As any number of in-series fibers are stimulating afferent discharge, numerous in-parallel fibers are simultaneously producing an unloading effect on the structure. This unloading effect reduces the overall response of the motor units that are activated. Thus, after factoring in both types of fibers, the net response of afferent discharge is reduced (7).



Figure 2:
ch16f12-1.jpg
http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=neurosci&part=A1104



Function in the control of normal movement
The primary purpose of the GTO is to protect the muscles from the injurious effects of excessive force production. This is accomplished through autogenic inhibition. All afferent information of the GTO relates to the force of muscle contraction. When there is no load on the associated muscle, a tonic discharge is maintained. A light load will cause the GTO to fire more frequently. As the load increases, there is a subsequent rise in the force of muscle contraction. As a result, the inhibitory discharge rate of the GTO also increases. Through this mechanism, the GTO limits the muscle to a safe level of contraction.

The functioning of the GTO is also context-dependent. Therefore, its effects are not always inhibitory. There have been studies on cats in which the GTO actually activates the muscles during locomotion. The brain sends signals informing the spinal cord about the intent of walking and the Ib afferent signals the muscle to be activated. This is a reflex reversal in which autogenic inhibition is suppressed.

The dispersed placement of the GTOs within various walking muscles (i.e. gastroc, soleus, quadriceps, etc) coordinates stability during locomotion. As one muscle is being inhibited, others are disinhibited. Thus, during swing phase of one leg, the flexor muscles of the other leg are being inhibited to support the weight of the body. The combined influences of the Ia afferents of the muscle spindles and the Ib afferents of the Golgi tendon organs coordinate the competing levels of positive and negative force feedback. This allows for the coordination for both interjoint movements and the intermuscular movements within a single limb. These combined afferent contributions generate the rhythmic pattern of locomotion (5,8,9).

Central Drive
The functioning of the GTO can be modulated through descending input converging on the Ib interneuron. Most of the influence takes place either at the receptor site (the free nerve endings) or at the synaptic locations (Ib interneuron). Levels of sensitivity can be altered by central drive. Thus, if less muscle force is needed, excitatory input will synapse onto the Ib inhibitory interneuron to reduce muscle activation. Conversely, when more force is required, the descending input will operate to inhibit the Ib inhibitory interneuron, which will ultimately disinhibit muscle contraction.

The output pathway for the GTO
The general neuronal pathway responding to GTO activation is known as autogenic inhibition, or the inverse stretch reflex. An action potential propagates up the Ib afferent and into the dorsal root of the spinal cord. In the dorsal root, the Ib afferent synapses with a Ib inhibitory interneuron. At this point, the interneuron synapses with an alpha motor neuron which will travel back to the same muscle to inhibit further stretching.


Figure 3:
DCML.jpg http://www.rci.rutgers.edu/~uzwiak/AnatPhys/ChemicalSomaticSenses_files/image026.jpg


Some information from the GTO continues to the central nervous system via the DCML (Dorsal Column Medial Lemniscal Pathway).

DCML (figure 3): The Ib afferent travels to the dorsal root of the spinal cord, as mentioned above. Depending upon where the in the musculature the stretch has occurred, the information either ascends up the gracile tract or the cuneate tract. These tracts continue upward and synapse with another axon in the medulla. At this point, decussation occurs so that information from the left side of the body is projected to the right brain, and visa versa. The pathway continues to synapse at the VPL (ventral posterolateral nucleus) of the thalamus. Information eventually reaches the primary somatosensory cortex to be further processed.
Some proprioceptive information from the GTO will travel to the cerebellum via the spino cerebellar pathway. Information processed in the cerebellum aids in error detection so that movement can be as controlled and accurate as possible.


Abnormal Functioning
The most obvious result of a malfunctioning GTO would be muscle damage in response to excessive tension. If a muscle is stretched too much, the GTO will become active and through autogenic inhibition, that same muscle will be inhibited to prevent further stretching. Therefore, if the GTO is not functioning properly a muscle tear may result.
Parkinson’s disease is the most common disorder associated with an abnormally functioning GTO. Many patients with this condition show decreases in the effectiveness of autogenic inhibition. This is mostly due to an increased threshold of inhibitory responses which leads to a delayed conduction in the reflex connections. Some research has also shown that the delayed conduction may be attributed to a decreased diameter in the Ib afferent. When the GTO reflex pathway is not conducted properly, tonic and dynamic properties of the GTO function are disturbed. Therefore, rigidity and tremor may result (3).


Glossary of important terms:
Ib afferent- the sensory axon that innervates the Golgi Tendon Organ and carries a signal to the spinal cord.

Autogenic inhibition/ inverse stretch reflex- Reflex inhibition of a motor unit in response to excessive tension in the muscle fibers it supplies. This is the reflex mechanism associated with the GTO.

Contract-Relax Technique- a proprioceptive technique used to inhibit muscle spasm. The target muscle is engaged in an isometric contraction and then relaxed (1).

Musculotendonous junction- the place in which the muscle is joined by the tendon; where the GTO is located.

Parkinson’s disease- a degenerative disorder of the central nervous system that often impairs an individual’s motor functioning, speech, etc.



Further Readings

1. “Neurophysiology for Medicine”:
https://blackboard.gordon.edu/webapps/portal/frameset.jsp?tab_id=_2_1&url=%2fwebapps%2fblackboard%2fexecute%2flauncher%3ftype%3dCourse%26id%3d_3201_1%26url%3d

2. Youtube video:
http://www.youtube.com/watch?v=7T4NI_2qDEM

3. “Run the Planet”
http://www.runtheplanet.com/trainingracing/stretching/chap1-whathappens.asp

4. The Journal of Physiology:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2290032/


Quiz Questions:
Multiple Choice:
1. The general neuronal pathway responding to GTO activation is known as:
a. autogenic inhibition
b. stretch reflex
c. GTO reflex
d. Ib facilitation
e. None of the above
2. The sensory axon that innervates the Golgi Tendon Organ is the:
a. Ib interneuron
b. Alpha- motor neuron
c. Gamma-motor neuron
d. Ib afferent
e. None of the above
3. The most common disease associated with a dysfunctional GTO is:
a. Multiple sclerosis
b. Cancer
c. Arthritis
d. Huntington’s disease
e. None of the above
4. The initial discharge can be referred to as:
a. Tonic firing
b. Afferent firing
c. Static response
d. Dynamic response
e. None of the above
5. To what normal activity does the GTO contribute?
a. Throwing a ball
b. Walking
c. Reading a webpage
d. Sleeping
e. Chatting with friends
6. The channels that are opened to depolarize the Ib axon are:
a. Mechanically-gated
b. Voltage-gated
c. Ligand-gated
d. Transmitter-gated
e. None of the above

True/False:

7. The Golgi tendon organ is located within the extrafusal fibers of a muscle.
8. The inverse stretch reflex incorporates the GTO, a Ib afferent, a Ib interneuron, and a gamma motor neuron.
9. When the GTO is not working properly, muscle tremors and rigidity may result.
10. If more than one motor unit is activated at once, the cumulative response of the GTO is equal to the sum of its responses to individual units.


Answers
:
1. A
2. D
3. E
4. D
5. B
6. A
11. False
12. False
13. True
14. False

Short Answer:
1. Describe how central drive may affect the influence of the GTO.
2. Diagram the autogenic inhibition pathway.
3. Describe the effect of a dysfunctional GTO.


References
1. Archer P. Therapeutic Massage in Athletics. Baltimore: Lippincott Williams and Wilkins, 2007, p. 132-133.
2. Brodal P. The Central Nervous System: Structure and Function. Oxford: Oxford University Press, 2004. 150-152.
3.
Burne JA, Lippold O. Loss of Tendon Organ Inhibition in Parkinson’s Disease. Oxford Journals : 1996; 119: 1115-1121.
4. Gregory J E, Brockett C L, Morgan D L, Whitehead NP, Proske U. Effect of eccentric muscle contractions on Golgi tendon organ responses to passive and active tension in the cat. Journal of Physiology. 2002; 538(Pt 1): 209-218 [PubMed]
5. Grey M J, Nielsen J B, Mazzaro N, Sinkjaer T. Positive force feedback in human walking. Journal of Physiology. 2007; 581(Pt 1):99-105. [PubMed] 6. Horcholle-Bossavit G, Jami L, Petit J, Vejsada R, Zytnicki D. Unloading of tendon organ discharges by in-series motor units in cat peroneal muscles. Journal of Physiology. 1989; 408:185-198.
7.
Mileusnic M P, Loeb G E. Mathematical Models of Proprioceptors. II. Structure and Function of the Golgi Tendon Organ. Journal of Neurophysiology. 2006; 96:1789-1802. [PubMed]
8. Nichols T R. Receptor Mechanisms Underlying Heterogenic Reflexes Among the Triceps Surae Muscles of the Cat. Journal of Neurophysiology. 1999; 81:467-478. [PubMed]
9. Prochazka A, Gillard D, Bennett D J. Positive Force Feedback Control of Muscles. Journal of Neurophysiology. 1997; 77:3226-3236. [PubMed]