Motor neuron disease (MND): â€¢ A systemic (i.e. bilateral) neurological disease of unknown cause affecting the MOTOR SYSTEM only (UMN or LMN or both). â€¢ Rarely presents before 40 years. â€¢ A degenerative disease of a gradual onset and progressive course. â€¢ 5 clinical types (NO ataxia or sensory signs/symptoms): 1. 1ry lateral sclerosis (UMN affection in spinal cord). 2. Pseudo-bulbar palsy (UMN affection in brain stem or cerebral hemisphere) = bulbar symptoms (e.g. dysarthria) + quadriplegia + signs of UMNL in ULs and LLs + exaggerated palatal and pharyngeal reflexes + appearance of jaw reflex if the lesion is above pons + NO tongue wasting or fasciculations. 3. Progressive muscular atrophy (LMN affection in AHCs of lower C or L region in spinal cord) = weakness with signs of LMNL e.g. wasting, hypotonia and fasciculations + affects distal ms before proximal ms + THE BEST PROGNOSIS. 4. PROGRESSIVE BULBAR PALSY (TRUE BULBAR PALSY) (LMN affection in the cranial nerve motor nuclei) = bulbar symptoms + NO quadriplegia + lost palatal and pharyngeal reflexes + absent jaw reflex + tongue is wasted and shows fasciculations + THE WORST PROGNOSIS. 5. Amyotrophic lateral sclerosis (ALS) (THE MOST COMMON TYPE = 50% of patients) (combined UMN & LMN affection) = UMNL signs (e.g. m tone) + LMNL signs (e.g. absent reflexes) in ULs (TONIC ATROPHY) & UMNL signs in LLs + in familial cases the gene responsible lies on chromosome 21 and codes for superoxide dismutase.