Motor neuron

Motoneurons are the nerve cells responsible for the formation and coordination of movements. According to the location of the motoneurons, a distinction is made between the “upper motoneurons”, which are located in the cerebral cortex, and the “lower motoneurons”, which are located in the spinal cord.

The lower motor neuron

The lower motoneuron is located in the so-called “anterior horn” of the spinal cord, an anatomical structure that is known for the fact that here the extensions of the upper motoneurons are connected to the lower motoneurons. These in turn have extensions that reach down to the muscle, where they trigger a contraction. Depending on their position in the spinal cord (cervical [in the neck], thoracic [in the chest], or lumbar [in the lumbar region]) the respective motoneuron is responsible for another muscle. For example, the arm muscles are served by motoneurons located in the neck, while the leg muscles are served by motoneurons located in the lumbar region. Thus, it is also possible to understand why paraplegia at the level of the cervical spine very often leads to a quadriplegia – a paralysis of arms and legs, while lower lying paraplegia usually “only” leads to paralysis of the legs.

The upper motor neuron

The “upper” motoneurons are located in the brain, more precisely in the cerebral cortex, in a relatively central part, the so-called “motocortex”. Due to their conspicuous size, they are also called “Bethze giant cells”. Their extensions (medically: axons) are sometimes up to one meter long and reach into the lower parts of the spinal cord. In the motocortex, movements are initiated and the commands for movement are sent out. To do this, the electrical signal runs from the upper motoneuron in the brain to the “lower motoneuron” in the spinal cord, to which it is then connected.

Hemiplegia

Brain damage with loss of the right hemisphere always leads to loss of the left hemisphere. This is because the motoneurons of the right hemisphere of the brain cross over to the opposite side at the level of the neck after leaving the skull, and paralysis is therefore also to be expected on the opposite side.