Motor neurons are the nerve cells that are responsible for the creation and coordination of movements. According to the localization of the motor neurons, a distinction is made between the “upper motor neurons”, which are located in the cerebral cortex, and the “lower motor neurons”, which are located in the spinal cord.
The lower motor neuron
The lower motor neuron is located in the so-called "Front horn“Of the spinal cord, an anatomical structure that is known for the fact that the processes of the upper motor neurons are interconnected here with the lower motor neurons. These in turn have appendages that down to the muscle range, and then trigger a contraction in this.
Depending on the position in the spinal cord (cervical [in the neck area], thoracic [in the chest area], or lumbar [in the lumbar area]), this is respective motor neuron for another muscle responsible.
For example, the arm muscles are served by motor neurons located in the neck area, while the leg muscles are served by motor neurons located in the lumbar area.
So you can understand why one Paraplegia at the level of Cervical spine very often to one quadriplegia - leads to paralysis of the arms and legs, while deeper-lying cross-sections usually "only" lead to paralysis of the legs.
The upper motor neuron
The "upper" motor neurons are located in the brain, more precisely in the Cerebral cortex, in a relatively central part, the so-called "Motocortex". Due to their eye-catching size, they are also "Betzsche giant cells" called. Their processes (medically: axons) are sometimes up to a meter long and extend into the lower parts of the spinal cord.
Be in the motocortex Initiated movements and sent the orders to move. For this purpose, the electrical signal runs from the upper motor neuron in the brain to the "lower motor neuron" in the Spinal cordto which it is then interconnected.
A Brain damage failure of the right hemisphere always leads to one Loss of the left side of the body. This is because the Motor neurons the right hemisphere after emerging from the skull at the level of the neck cross on the opposite side, and the paralysis can therefore also be expected on the opposite side.