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Section II: Sensory Systems
1. Motor Units and Muscle Receptors
Part 4 of 4

James Knierim, Ph.D.
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Golgi Tendon Organ

The Golgi tendon organ is a specialized receptor that is located between the muscle and the tendon (Figure 1.7).  Unlike the muscle spindle, which is located in parallel with extrafusal fibers, the Golgi tendon organ is located in series with the muscle and signals information about the load or force being applied to the muscle.  A Golgi tendon organ is made up of a capsule containing numerous collagen fibers (Figure 1.11).  The organ is innervated by primary afferents called Group Ib fibers, which have specialized endings that weave in between the collagen fibers.  When force is applied to a muscle, the Golgi tendon organ is stretched, causing the collagen fibers to squeeze and distort the membranes of the primary afferent sensory endings.  As a result, the afferent is depolarized, and it fires action potentials to signal the amount of force.




Figure 1.11

Golgi tendon organ.

 

Figure 1.12 illustrates the difference in information conveyed by muscle spindles and Golgi tendon organs.  At the resting position, the Ia afferents of spindles in the triceps muscle fire at a steady rate to encode the present length of the muscle, and the Ib afferents of the Golgi tendon organs of the biceps muscle fire at a low rate.  When a light object (a balloon) is placed in the hand, there is little change in the firing rate of either afferent.  When the hand starts to rise, however, the triceps muscle is stretched, and the Ia afferent fibers increase their firing rate as a function of muscle length. The Ib fibers do not change appreciably, because the balloon does not add much load to the muscle.  What if a heavy object (a bowling ball) were placed in the hand instead?  Because a heavy load is now placed on the biceps, the Ib afferents fire vigorously.  Note that the Ia afferent is not affected, as the muscle length has not changed.  When the arm begins to rise, however, the Ia afferents fire, just as with the balloon.

Figure 1.12

Difference between muscle spindle and Golgi tendon organ.

In summary,

  1. Muscle spindles signal information about the length and velocity of a muscle
  2. Golgi tendon organs signal information about the load or force applied to a muscle

 

Test Your Knowledge

  • Question 1
  • A
  • B
  • C
  • D
  • E

Types of fibers contained within muscle spindles include...

A. Dynamic nuclear bag fibers

B. Dynamic nuclear chain fibers

C. Group Ib afferent fibers

D. Extrafusal fibers

E. Group IV afferent fibers

Types of fibers contained within muscle spindles include...

A. Dynamic nuclear bag fibers  This answer is CORRECT!

B. Dynamic nuclear chain fibers

C. Group Ib afferent fibers

D. Extrafusal fibers

E. Group IV afferent fibers

Types of fibers contained within muscle spindles include...

A. Dynamic nuclear bag fibers

B. Dynamic nuclear chain fibers This answer is INCORRECT. Nuclear chain fibers signal only static muscle length.

C. Group Ib afferent fibers

D. Extrafusal fibers

E. Group IV afferent fibers

Types of fibers contained within muscle spindles include...

A. Dynamic nuclear bag fibers

B. Dynamic nuclear chain fibers

C. Group Ib afferent fibers   This answer is INCORRECT. Group Ib afferents are associated with Golgi tendon organs.

D. Extrafusal fibers

E. Group IV afferent fibers

Types of fibers contained within muscle spindles include...

A. Dynamic nuclear bag fibers

B. Dynamic nuclear chain fibers

C. Group Ib afferent fibers

D. Extrafusal fibers  This answer is INCORRECT. Extrafusal fibers are outside the muscle spindle.

E. Group IV afferent fibers

Types of fibers contained within muscle spindles include...

A. Dynamic nuclear bag fibers

B. Dynamic nuclear chain fibers

C. Group Ib afferent fibers

D. Extrafusal fibers

E. Group IV afferent fibers  This answer is INCORRECT. Group IV afferent fibers are not part of the muscle spindle.

 

 

 

 

 

 

 

 

  • Question 2
  • A
  • B
  • C
  • D
  • E

Muscle force is controlled in part by...

A. Alpha-gamma coactivation

B. Intrafusal fibers

C. Rate code

D. Golgi tendon organs

E. Gamma motor neurons

Muscle force is controlled in part by...

A. Alpha-gamma coactivation  This answer is INCORRECT. Alpha-gamma coactivation ensures that muscle spindles maintain sensitivity to stretch over a wide range of muscle lengths.

B. Intrafusal fibers

C. Rate code

D. Golgi tendon organs

E. Gamma motor neurons

Muscle force is controlled in part by...

A. Alpha-gamma coactivation

B. Intrafusal fibers   This answer is INCORRECT. Intrafusal fibers do not contribute significantly to muscle force.

C. Rate code

D. Golgi tendon organs

E. Gamma motor neurons

Muscle force is controlled in part by...

A. Alpha-gamma coactivation

B. Intrafusal fibers

C. Rate code This answer is CORRECT!

D. Golgi tendon organs

E. Gamma motor neurons

Muscle force is controlled in part by...

A. Alpha-gamma coactivation

B. Intrafusal fibers

C. Rate code

D. Golgi tendon organs  This answer is INCORRECT. Golgi tendon organs signal information about muscle force, but do not control that force directly.

E. Gamma motor neurons

Muscle force is controlled in part by...

A. Alpha-gamma coactivation

B. Intrafusal fibers

C. Rate code

D. Golgi tendon organs

E. Gamma motor neurons   This answer is INCORRECT. Gamma motor neurons innervate intrafusal fibers, which do not contribute significantly to muscle force.
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