Gamma Motoneuron - Abnormal Gamma Motor Neuron Activity

Abnormal Gamma Motor Neuron Activity

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. Opposite to this, 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.

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. This can sometimes cause a gamma-bias (constant discharge of some gamma motor neurons) to be greater or less than usual. In the case for patients with excess gamma bias, the sensory endings within muscle spindles are discharging too frequently causing there to be more muscle activity than appropriate. Furthermore, this hyperactivity in the gamma spindle loop can cause spasticity.

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 can be adversely affected. Fine motor skills such as movements with the fingers and eyes are affected most since any lack of tautness within the muscle spindle hinders its ability to detect the amount of stretch through the sensory endings. This means that the muscle will not be able to precisely move accordingly. Lesions controlling descending pathways in lower motor neurons to the upper limbs, can cause a loss in patient's ability to have fine movement control.

In clinical settings, it is possible to test if someone has an abnormally low or high gamma gain simply by moving the patients arm. Gamma gain is the process where acceleration, velocity, and length of muscle changes are scaled up equally, enabling more accurate movements to take place in the appropriate situation. 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.

Oscilloscopes can be used to measure action potentials of an axon from a motor neuron in order to assess general muscle activity. Though it cannot distinguish alpha motor neurons from gamma motor neurons, it is useful in understanding whether one has abnormal motor neuron activity. With low rates of activity of the descending pathway, fewer and smaller motor neurons are activated leading to a small amount of muscle force. This will appear on the oscilloscope as lower peaks on the y-axis.