The motor system is a tag team of nerve cells (neurons) that carries messages from the area that controls movement in the brain to the muscle (Figure 1 below). The first part of the motor system carries the signal from the brain to the lower part of the brain (brainstem) and the spinal cord. It is referred to as the upper motor neuron (UMN) or corticospinal tract. The upper motor neuron contacts a second motor neuron referred to as the lower motor neuron (LMN) or anterior horn cell. The LMN then carries the signal to the muscle. The LMNs in the brainstem control the muscles used for speech and swallowing. Involvement of this area is what is referred to as bulbar involvement. The highest part of the spinal cord is known as the cervical cord and motor neurons in this area send messages to the arm muscles and diaphragm (one of the muscles that are important for breathing). The middle part of the spinal cord is called the thoracic cord and neurons here innervate (control) the muscles of the trunk and the muscles of the chest important for breathing. The lowest part of the spinal cord is the lumbar spinal cord and the motor nerves at this level innervate the leg muscles.

Motor neuron diseases (MND) damage the motor system. They can affect either the upper motor neuron (UMN), the lower motor neuron (LMN), or both. These diseases are named for the part of the motor system they involve. In ALS both the UMN and the LMN are damaged.

UMN:

• Primary Lateral Sclerosis (PLS)
  • Variants include Bulbar Palsy
• Familial Spastic Paraparesis

LMN:

• Spinal Muscular Atrophy (SMA)
• Progressive Muscular Atrophy
• Monomelic amyotrophy (one extremity with slow progression)
• Brachial amyotrophic diplegia (progressive weakness of both arms with no bulbar or respiratory involvement)
• Motor Neuropathy with or without conduction block*
• Kennedy's Disease (an hereditary disease of the androgen receptor)

UMN and LMN:

• Amyotrophic Lateral Sclerosis (ALS)
  • Variants include Bulbar Palsy

* Motor neuropathy with conduction block is an important disease as it is treatable. It is an autoimmune disorder characterized by "conduction block" on Nerve Conduction studies, predominantly lower motor neuron clinical picture, the presence of anti GM1 antibodies (a blood test), and elevated spinal fluid protein. It responds to intravenous gamma globulin.