Gamma motor neuron

A gamma motor neuron, also called gamma motoneuron, or fusimotor neuron, is a type of lower motor neuron that takes part in the process of muscle contraction, and represents about 30% of fibers going to the muscle. Like alpha motor neurons, their cell bodies are located in the anterior grey column of the spinal cord. They receive input from the reticular formation of the pons in the brainstem. Their axons are smaller than those of the alpha motor neurons, with a diameter of only 5 μm. Unlike the alpha motor 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 the continued firing of alpha neurons, leading to muscle contraction. These neurons also play a role in adjusting the sensitivity of muscle spindles.
The presence of myelination in gamma motor neurons allows a conduction velocity of 4 to 24 meters per second, significantly faster than with non-myelinated axons but slower than in alpha motor neurons.

General background of muscles

Muscle spindles

are the sensory receptors located within muscles that allow communication to the spinal cord and brain with information of where the body is in space and how fast body limbs are moving with relation to space. 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 muscle fibers: dynamic nuclear bag fibers, static nuclear bag fibers, and nuclear chain fibers.

Types of lower motor neurons

Muscle spindles are innervated by 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 of lower motor neurons involved in muscle contraction: 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 fibers. Gamma motor neurons, on the other hand, innervate only intrafusal muscle fibers, whereas beta motor neurons, which are present in very low amounts, innervate both intrafusal and extrafusal muscle cells. Beta motor neurons have a conduction velocity greater than that of both other types of lower motor neurons, but there is little currently known about beta motor neurons. Alpha motor neurons are highly abundant and larger in size than gamma 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 maintains the tautness of muscle spindles and is called alpha gamma co-activation. The nuclei of spindle muscle cells are located in the middle of these spindles, but unlike extrafusal muscle fibers, the myofibril contractile apparatus of spindle fibers are located only at both ends of spindle. Efferent stimulation of the spindle by gamma motor neurons contracts the myofibrils, tautening the central region of spindle—which maintains the muscle spindle's sensitivity to muscle's length change.
Without gamma motor neurons, muscle spindles would be very loose as the muscle contracts more. This does not allow for muscle spindles to detect a precise amount of stretch since it is so limp. However, with alpha gamma co-activation and both alpha and gamma neurons firing, muscle fibers within the muscle spindles are pulled parallel to the extrafusal contraction causing the muscle movement. The firing of gamma motor neurons in sync with alpha motor neurons pulls muscle spindles from polar ends of the fibers as this is where gamma motor neurons innervate the muscle. The spindle is innervated by type Ia sensory fiber that go on to synapse with alpha motor neurons, completing the gamma-loop. The parallel pulling keeps muscle spindles taut and readily able to detect minute changes in stretch.

Fusimotor system

The central nervous system controls muscle spindle sensitivity via the fusimotor system that consists of muscle spindles along with gamma motor neurons also called fusimotor neurons. Beta motor neurons innervate extrafusal as well as intrafusal muscle fibers, and 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 relaying information from muscles toward the spinal cord and brain.

Gamma bias

Gamma bias is gamma motor neurons' consistent level of activity. Smaller neurons require a smaller amount of excitatory input to reach its threshold compared to larger neurons. Therefore, gamma motor neurons are more likely to fire than the larger alpha motor neurons. This creates a situation with relatively few alpha motor neurons firing but some gamma motor neurons constantly firing in conditions where muscle stretch or force is not occurring. The sensitivity of sensory endings of the muscle spindle are based on the level of gamma bias

Types

Static

Static gamma motor neurons innervate static nuclear bag fibers, a type of nuclear bag fiber and nuclear chain fibers. Both of these fiber types are part of the intrafusal muscle spindle fibers, where the static gamma motor neurons innervate onto. Nuclear chain fibers' nuclei are organized in longitudinal columns, which is where it gets its name from, whereas the nuclear bag fibers' nuclei are clumped in the midsection of the muscle spindle. There is approximately a 2:1 ratio of nuclear chain fibers to nuclear bag fibers. The static gamma motor neurons increase their firing, in response to an increase in magnitude of change in length and controls the static sensitivity of the stretch reflex. For this reason, this type of gamma motor neuron is mostly used in the maintenance of postures and slower movements such as lifting a box, rather than activities requiring rapid changes due to rapid change in muscle length.

Dynamic

Dynamic gamma motor neurons innervate the dynamic nuclear bag fibers, another type of nuclear bag fiber smaller than the static nuclear bag fibers. This type of gamma motor neuron can enhance the sensitivities of Ia sensory neurons. It is done so because the dynamic nuclear bag fibers, which are innervated by the dynamic gamma motor neurons, receive Ia sensory innervation. Furthermore, the firing of dynamic gamma motor neurons removes the slack in dynamic nuclear bags, bringing Ia fibers closer to the firing threshold. Dynamic gamma motor neurons alter muscle spindle sensitivity and increases its discharge in response to velocity, the rate of change, of muscle length rather than simply the magnitude as it is with static gamma motor neurons. Therefore, this type of gamma motor neuron can be used for activities requiring quick changes in muscle length to adjust such as balancing on a rail.

Effects of nuclear chain fibers

The effect of nuclear chain fibers on primary endings is to drive the discharge up to a frequency of around 60 Hz in a linear fashion, above which the discharge can become irregular. The activities of bag₂ fibers show an initial sharp peak in discharge, which gets less as the receptor adapts. Bag₂ fibers also reduce the dynamic sensitivity of the Ia afferent and sometimes also reduce the length sensitivity. Activation of bag₁ fibers has the effect of increasing both the length sensitivity and the dynamic sensitivity of the primary ending.
It is believed that the secondary sensory endings serve to measure length and muscle contractions of nuclear chain fibers at the pole via the static γ-motoneurons both excite the ending and increase its length sensitivity. Bag₁ and bag₂ fibers receive very little innervation from secondary endings, and activation of these fibers has a minimal effect on the discharge of the secondary ending.

Development

Gamma motor neurons develop similarly to alpha motor neurons at the beginning. They originate in the basal plate, which is the ventral portion of the neural tube in the developing embryo. Sonic hedgehog genes are an important part of the development process that is secreted by the notochord creating gradients of concentrations. After the hedgehog genes, various other molecular markers and transcription factors play a role in differentiating motor neurons into the specific gamma motor neurons.
Gamma motor neurons, like all cells, express specific genetic markers at birth. Muscle spindle derived GDNF neurotrophic factors must also be present for postnatal survival. Wnt7A is a secreted signaling molecule selectively in gamma motor neurons by embryonic day 17.5 of mice. This is the earliest molecule present in gamma motor neurons that differentiates them from alpha motor neurons, illustrating the divergence of these two types of lower motor neurons.
In addition, serotonin receptor 1d has been concluded to be a novel marker for gamma motor neurons enabling researchers to distinguish between the various types of lower motor neurons. Mice lacking this serotonin receptor 1d, displayed lower monosynaptic reflex, which may be caused by a reduced response to sensory stimulation in motor neurons. In addition, knockout mice without this serotonin receptor exhibited more coordination on a balance beam task, suggesting that less activation of motor neurons by Ia afferents during movement could reduce the unnecessary excess of muscle output.
Another distinguishing molecular marker of gamma motor neurons is transcription factor Err3. It is expressed at high levels in gamma motor neurons, but very little in alpha motor neurons. On the other hand, neuronal DNA binding protein NeuN, are present in significantly greater quantities in alpha motor neurons. Osteopontin, a protein also expressed in bones, hence the "osteo-" prefix, is a marker for alpha motor neurons. This in turn can provide scientists a way of eliminating gamma motor neurons if alpha motor neurons are of interest. One study in particular made this conclusion based on the fact that osteopontin was present in larger cell bodies, indicating the alpha motor neurons as they have larger cell bodies than gamma motor neurons.