Gamma Motoneuron - Gamma Motor Neuron Development

Gamma Motor Neuron 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 (Shh) are an important part of the development process which 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 (5-ht 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 (a reflex arc involving only a sensory and motor neuron), 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 unecessary 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.