Motor Neurons

Last update: September 15, 2022
By BrainMatters

There are two types of motor neurons: alpha and gamma.

Alpha motor neurons are the largest motor neurons. They originate on the anterior horn of the spinal cord and terminate in muscle tissue. They cause muscles to contract and thus allow limbs to move.

In addition to alpha motor neurons, there are also gamma motor neurons. These are located deeper in the muscle and ensure that the muscle remains in tension. Suppose an alpha motor neuron controls a contraction. The result is that the muscle shortens. If the muscle shortens a lot, the muscle spindles become flaccid and go "off air". They can then no longer pass on information to the brain about the length of the muscle. It doesn't get that far, however, because the gamma motor neurons are also activated. Gamma motor neurons cause a contraction of the inner muscle tissue surrounding the muscle spindle, so the muscle spindles still receive information.

The movement neurons are controlled in several ways. Not only from the brain do signals reach the cells, but also directly from the muscle or from the spinal cord. When a muscle is stretched, for example, a signal from the proprioceptors goes directly from the muscle to the dorsal horn of the spinal cord and through the dorsal horn back to the alpha cell. This happens, for example, when the doctor tests your patellar reflex. The tap with the hammer causes the muscle tissue to stretch very quickly. As a result, the muscle reflexively contracts. In this way, stability is maintained in the knee.

Motor neurons are also controlled from the brain, via the spinal cord. Sometimes this is done with the intervention of switch neurons. The control from the brain can be both stimulating and inhibitory. This interaction is important for smooth movements. For example, if the brain orders the biceps to flex the arm, an inhibitory signal must simultaneously be sent to the triceps so that it relaxes.

Author: Bart Aben (translated by Melanie Smekal)

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