What do “first motor neuron” and “second motor neuron” mean?

“Motor neurons” (also known as “motoneurons”) are nerve cells in the brain and spinal cord that are responsible for controlling the voluntary muscles of our body. A basic distinction is made between motor neurons located in the motor cortex (“first motor neuron”) and those located in the spinal cord (or brainstem) (“second motor neuron”).

This distinction is important because in ALS, both groups of nerve cells—the first and second motor neurons—are affected, and damage to them is associated with different symptoms. Damage to the first motor neuron (1st MN) typically causes uncontrolled muscle tension (muscle stiffness), which is referred to as spasticity. Degeneration of the second motor neuron (2nd MN), on the other hand, is associated with muscle wasting (myopathy), muscle weakness (paresis), and muscle twitching (fasciculations).