The plan of all movement finds its origin in the brain. The major part of the brain involved in the initiation and control of voluntary movements is the primary motor cortex. Motor neurons in the brain are called upper motor neurons (UMN), while motor neurons in the brainstem and spinal cord are called lower motor neurons (LMN). UMNs cannot leave the CNS; they must therefore establish a synapse with LMNs whose axons can leave the CNS, allowing them to establish a synapse with muscles throughout the body. So, in a normal situation, messages from the UMNs are transferred to the LMNs, and from there, to specific muscles. The UMNs and LMNs are responsible for movements such as walking and chewing, respectively, as well as movements of the arms, legs, chest, and face. This allows healthy people to voluntarily move their muscles with ease. Amyotrophic lateral sclerosis is also called motor neuron disorder (MND), because it is characterized by the continued degeneration of upper and lower motor neurons. These motor neurons, as previously stated, are responsible for the body's voluntary muscles, and as the neurons degenerate or die, they are unable to send messages to the muscles. When muscles can no longer receive signals, they become unable to function and, in turn, weaken and waste away. Eventually, the brain loses its ability to initiate and control voluntary movements, resulting in paralysis. Since ALS only affects voluntary motor neurons, the senses (sight, smell, taste, hearing and tactile sensations) are spared. Involuntary motor neurons are also not affected (e.g. the heart and digestive system). Additionally, the individual is often cognitively unaffected....... middle of article......iclesMitchell, JD and GD Borasio. 2007. Amyotrophic lateral sclerosis. Seminar. 369:2031-2041. Roth-Kauffman, M. and J. Niebauer. 2012. Amyotrophic lateral sclerosis: diagnosis and appropriate management. Opinion of clinicians. 22(7): 15-21Rowland, LP and NA Shneider. 2001. Amyotrophic lateral sclerosis. New England Journal of Medicine. 344(22): 1688-1700. Wijesekera, LC and PN Leigh. 2009. Amyotrophic lateral sclerosis. Orphanet Journal of Rare Diseases. 4(3): doi:10.1186/1750-1172-4-3.WebpagesALS Association. About ALS. http://www.alsa.org/about-als/what-is-als.html. Updated 2010. Accessed March 13, 2014. National Institute of Neurological Disorders and Stroke. NINDS information page on amyotrophic lateral sclerosis (ALS). http://www.ninds.nih.gov/disorders/amyotrophiclateralsclerosis/ALS.htm. Updated February 4, 2014. Accessed March 13, 2013.