Rachel Heckman

This subpage was created during my Training Workshop.

Gamma Motor Neurons

Gamma motor neurons are a type of lower motor neurons used in muscle contraction, and represents about 30% of fibers going to the muscle. They originate from from the bulboreticular facilitatory region of brainstem with a diameter of only 5um. The presence of myelination allows a conduction velocity of 4-24 meters per second, significantly faster than nonmyelinated axons. However, the conduction velocity is slower than the alpha motor neurons. Unlike the alpha neurons, gamma motor neurons do not directly adjust the lengthening or shortening of muscles. However, their role is important in keeping muscle spindles taut, thereby allowing firing of alpha neurons to continue to discharge. These neurons also play a role in adjusting the sensitivity of muscle spindles.

General Background of Muscles

Muscle Spindles

Muscle spindles are the receptors located within muscles that allow communication to the spinal cord and brain with information of where the body is in space (proprioception) and how fast body limbs are moving with relation to space (velocity) They are mechanoreceptors in that they respond to stretch and are able to signal changes in muscle length. The sensitivity of detecting changes in muscle length are adjusted by fusimotor neurons- gamma and beta motor neurons. Muscle spindles can be made up of three different types of intrafusal muscle fibers: dynamic nuclear bag fibers (bag1 fibers), static nuclear bag fibers (bag2 fibers), nuclear chain fibers, and the axons of sensory neurons.

Types of Lower Motor Neurons

Muscle spindles consist of both sensory neurons and motor neurons in order to provide proprioception and make the appropriate movements via firing of motor neurons. There are three types: alpha motor neurons, gamma motor neurons, and beta motor neurons. Alpha motor neurons, the most abundant type, are used in the actual force for muscle contraction and therefore innervate extrafusal muscle cells. Gamma motor neurons on the other hand, innervate only intrafusal (within the muscle spindle) muscle cells while beta motor neurons which are present in very low amounts innervate both intrafusal and extrafusal muscle cells and have a conduction velocity greater than both other types of lower motor neurons. Alpha motor neurons are highly abundant and larger in size than gamma motor neurons. Little is currently known about beta motor neurons.

Alpha Gamma Co-Activation

When the central nervous system sends out signals to alpha neurons to fire, signals are also sent to gamma motor neurons to do the same. This process is called alpha gamma co-activation, which is a process that maintains tautness of the muscle spindles. It is important to note that nuclei of spindle muscle cells are located in the middle of these spindles and intrafusal muscle fibers do not have myofibrils. Furthermore, the lack of contractile elements leads to the equatorial region of fibers to not contract. Without gamma motor neurons, muscle spindles would be very lose and the muscle contracts more and more. This does not allow for muscle spindles to detect an precise amount of stretch since it is so limp. However, with alpha gamma co-activation where both alpha neurons and gamma neurons are present, fibers with the muscle spindles are pulled parallel to the extrafusal contraction causing the muscle movement. The firing of gamma motor neurons in synch with alpha motor neurons pulls muscle spindles from polar ends of the fibers as this is where gamma motor neurons innervate the muscle. The parallel pulling keeps muscle spindles taut and readily able to detect more minute changes in stretch.

Fusimotor System

The fusimotor system is comprised of muscle spindles along with fusimotor neurons - beta and gamma motor neurons. Because beta motor neurons innervate extrfusal as well as intrafusal muscle fibers, they are more specifically named skeletofusimotor neurons. Gamma motor neurons are the efferent part of the fusimotor system, whereas muscle spindles are the afferent part as they send signals towards the spinal cord and brain.

Types of Gamma Motor Neurons

Static Gamma Motor Neurons

Static gamma motor neurons innervate static bag muscle cells and chain muscle cells

Dynamic Gamma Motor Neurons

Dynamic gamma motor neurons innervate dynamic bag muscle cells

Spasms

Spasms can be caused by a disparity between how much alpha and gamma motor neurons are firing ie. too much gain of one or the other. The inbalance causes an inaccurate reading from muscle receptors in the muscle spindle. Therefore, the sensory neurons feeding back to the brain and spinal cord are misleading. For example, if a patient has over active gamma motor neurons, there will be a resistance to passive movement causing stiffness, also called spasticity. This is often found in individuals with damage to higher centers affecting the descending pathways.

Muscle Tone

Although muscles can be in a relaxed state, muscles have a general resting level of tension. This is termed muscle tone and is maintained by the motor neurons innervating the muscle. Its purpose is to maintain posture and assist in quicker movements, since if muscles were completely loose, then more neuronal firing would need to take place. The amount of tension in the muscles depend on the resting level discharge of alpha motor neurons. It is mainly the Ia spindle afferents that aid in this process. Not only are the alpha motor neurons involved in muscle tone, gamma motor neurons are also involved through the gamma efferent system's action on intrafusal muscle fibers. These intrafusal muscle fibers controls the resting level of the Ia afferent pathway, which in turn creates a steady level of alpha neuron activity.

Hypotonia

Hypotonia can be due to damage to alpha neurons or Ia afferents carrying sensory information to the alpha neurons. This creates a decrease in muscle tone.

Hypertonia

Hypertonia is caused by damage to descending pathways that terminate in the spinal cord. It increases muscle tone by increasing the total responsiveness of alpha motor neurons from its Ia sensory input.

Clinically Testing

If it is more difficult to bend a patients arm at the elbow back and forth, then he/she has higher gamma gain while someone who's arm moves very easily will have lower gamma gain.

Fine Motor Movement

Gamma motor neurons assist in keeping the muscle spindle taut, thus adjusting sensitivity. Therefore, if proper gamma motor neuronal firing does not occur, muscle movement particularly those of fine motor movements can be adversely affected.

Lesions

A lesion controlling descending pathways in lower motor neurons to the upper limbs, can cause a loss in patient's ability to have fine movement control.

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Citations

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References

Najem A., Al-Falahe, Vallbo, A. (1988) Role of the human fusimotor system in a motor adaptaion task. Journal of Physiology. 401(77): 77-79.

D, Barker. (1948.) The Innervation of the Muscle-Spindle. Journal of Microscopial Sciences. 3: 143-185.

^ Russell NJ (1980) Axonal conduction velocity changes following muscle tenotomy or deafferentation during development in the rat. J Physiol 298:347-360.