Approach to motor unit diseases and the floppy infant syndrome

The anatomy of the motor unit, also known as the lower motor neuron system or peripheral nervous system, has four major components: (1) a motor neuron in the brainstem or ventral horn of the spinal cord; (2) an axon, which is bundled with other axons to form a peripheral nerve; (3) a neuromuscular junction; and (4) a group of myofibers that are ultimately innervated by the motor neuron. Disorders affecting these motor units may be classified anatomically into these four categories and also classified into inherited versus acquired diseases and into those that present with acute or chronic manifestations.