Amyotrophic lateral sclerosis presents as an upper motor neuron UMN, mixed motor neuron MMN, or a lower motor neuron picture when patients first see their doctor about symptoms. Upper motor neuron signs are thought to be most common in human adults over the age of 50, with a peak incidence of 55 to 65 years of age. The UMN deficits are more prevalent as one goes up the primate phylogenetic scale with incidents higher in chimps, who share 95% of the human genome with Homo sapiens. Motor neurons are greater in numbers 40,000, in humans as compared to the great apes which lack greater numbers. Upper motor neuron signs would be one of my experimental variables, lower motor neuron numbers the second variable, and mixed motor neurons the third variable.
Upper motor neuron signs include weakness in the bulbar muscles, shoulder muscles, and hand muscles. Loss or diminishment in control of motor plans and executions is the key feature present. UMNs are supraspinal in control and more specifically corticospinal in nature. Rubrospinal neurons are also upper motor neurons originating in the red motor nucleus of the brainstem. Some forms of motor neuron disease do originate in the spinal cord lower motor neurons but most originate in the cerebral cortex, hence, UMNs. Weakness in the limbs is the final common symptoms for ALS, followed by dysphagia, dysphonia, and respiratory paralysis in the end. The latter of the CNS are lower motor neuron signs. These motor neurons are controlled by the spinal cord and cranial nerve motor nuclei.
Presentations of ALS in a scale of upper and lower motor neuron signs are the following:
Multilimb upper motor neuron signs (Loss of control and execution of movements) Multilowerlimb lower motor neuron signs (Weakness, fasciculations, paralysis) Hand dysfunction