Amyotrophic Lateral Sclerosis (ALS) is a fatal neurodegenerative disease characterized by the rapid and progressive degeneration of motor neurons in the central and peripheral nervous system. ALS is considered a terminal disease with no current cure and represents the most common motor neuron disease.
ALS is classified into a group of diseases known as neurodegenerative disorders, which also include disorders such as Parkinson’s disease and Huntingdon’s Disease. The term “neurodegenerative” is a combination of “neuro” referring to the nerve cells or neurons and “degenerative” signifying the loss of function of a bodily structure. In regards to ALS, the matter of neurons is crucial in the understanding of this disease, because one of its hallmark characteristics is that it includes the involvement of both upper motor neurons (UMN) and lower motor neurons (LMN). The human body contains trillions of neurons with the brain alone containing over 100 billion of these nerve cells. There are both sensory and motor neurons within the body; however, regardless of type; these cells are primarily comprised of a cell body and two extending branches known as the axon and the dendrite ("The Life and Death of a Neuron," 2014 ). Axons are responsible for transmitting messages from a neuron while dendrites receive messages transmitted by other cells ("Neurons and Their Jobs," 2008). In the human body, motor neurons innervate muscle fibers of the body. A single motor neuron can innervate many muscle fibers and the combination of a single neuron with the muscle fibers it innervates is referred to as a motor unit. In the case of ALS, the motor neurons begin to deteriorate impairing their ability to send impulses to the muscle fibers. Thus, without these messages being transmitted, the neuron is no longer able to control its motor unit resulting in denervation. Lacking sufficient impulses from the motor neurons, the brain is unable to initiate and control voluntary muscle movement.
Due to the denervation of muscle fibers, weakness is one of the predominant features in the development of ALS. In the initial stages of the disease, this weakness is likely to be restricted to a focal area of the body such as the head, neck, or limb regions. Further, depending on where symptoms first begin to appear, individuals with ALS can be stratified into two different onset types based on the area(s) affected. These divisions are commonly regarded as either a spinal or bulbar-onset ALS, and these classifications help discern the pathway of disease progression in a given ALS patient. For persons who do develop ALS, manifestations of the disease are more likely to begin in the limb musculature as 75-80% of these cases can be classified as spinal-onset. The remaining individuals will initially experience difficulties involving speech and swallowing and be regarded as bulbar-onset patients. Notwithstanding onset type, the extensive degeneration of upper and lower motor neurons in persons with ALS results in impairments of speech, swallowing, movement, and respiration to develop in these persons. This paper and study are mainly concerned with furthering an understanding of how UMN and LMN damage uniquely affects the function of speech and swallowing throughout disease progression and explore the potential role of lingual strengthening in this unique patient population.
Genes associated with ALS
Over the past few decades, the number of genes believed to be associated with ALS pathology has increased and so too has understanding the functions of these genes and their possible links to ALS. Understanding these genes, including Superoxide Dismutase 1 (SOD1), alsin, senataxin, VAMP/synaptobrevin-associated protein, dynactin, angiogenin, ataxin 2, profilin, Fused in Sarcoma/Translocated in Sarcoma (FUS), the C9ORF72 hexanucleotide gene expansion and Trans-active Response (TAR) DNA-binding protein (TDP-43), to name a few, has been the topic of much research and has inspired great interest in determining the root causes of ALS as well as other neurodegenerative diseases, especially since many seem to share many underlying mechanisms. Currently, some of the widely accepted hypotheses regarding the contributions of these genes to ALS revolve around RNA and protein homeostasis as well as RNA processing being dysregulated, which subsequently can lead to neurodegenerative phenotypes.
A distinct feature of neurodegenerative diseases, including ALS, is the presence of hyperphosphorylated and ubiquitinated, misfolded cytoplasmic and nuclear protein aggregates that are spread across the nervous system. Post mortem brain and spinal cord tissue exhibit pathological cytoplasmic inclusion bodies that contain the 43 kDa DNA and RNA binding protein, TDP-43. TDP-43 is a nuclear protein expressed in several tissues including heart, lung, liver, spleen, kidney, muscle, and, at high levels in the brain. Evidence exists to support the notion that TDP-43 has multiple functions in transcription, mRNA splicing, microRNA biogenesis, RNA transport and translation, and stress granule formation and binds at least 6,000 RNAs. Furthermore, mice deficient for TDP-43 displayed embryonic lethality, demonstrating TDP-43 to be essential for both development and viability.