Alpha and Gamma Neurons: How do they work with Mechanoreceptors?

This post explains the relationship between muscles, mechanoreceptors, and alpha and gamma neurons. Previously I explained Muscle Spindles and Golgi Tendon Organs, two different types of mechanoreceptors. Muscle Spindles affect muscle length and tone, while Golgi Tendon Organs sense changes in tension. Both of these receptors are influenced by Alpha and Gamma neurons.

Alpha motor neurons innervate extrafusal fibers, the contracting fibers that supply the muscle with its power. Golgi Tendon Organs are found in extrafusal fibers, and affect the force createdwhen a muscle is activated. Gamma motor neurons innervate intrafusal fibers, which contract slightly. Muscle Spindles are found in the intrafusal fibers and affect movement. The more muscle spindle cells in a muscle the more refined the movement.

According to James Knieran, Ph.D., “The function of intrafusal fiber contraction is not to provide force to the muscle. Gamma activation of the intrafusal fiber is necessary to keep the muscle spindle taut, and therefore sensitive to stretch, over a wide range of muscle lengths. If a resting muscle is stretched, the muscle spindle becomes stretched in parallel. A subsequent contraction of the muscle, however, removes the pull on the spindle, and it becomes slack. If the muscle were to be stretched again, the muscle spindle would not be able to signal this stretch. Thus, the spindle is rendered temporarily insensitive to stretch after the muscle has contracted.”

Activation of gamma motor neurons prevents this temporary insensitivity by causing a weak contraction of the intrafusal fibers, in parallel with the contraction of the muscle. This contraction keeps the spindle taut at all times and maintains its sensitivity to changes in the length of the muscle. When the Central Nervous System instructs a muscle to contract, it not only sends the appropriate signals to the alpha motor neurons, it also instructs gamma motor neurons to contract the intrafusal fibers appropriately. This coordinated process is referred to as alpha-gamma coactivation”.

Knierim, James, Ph.D., Chapter 1: Motor Units and Muscle Receptors. Department of Neuroscience, The Johns Hopkins University, ©1997-Present. Webmaster email nba.webmaster@uth.tmc.edu