<unk>  (Greek νέυρο-, néuro-, "nerval" and Latin dēgenerāre, "to decline" or "to worsen") is a condition in which cells of the brain and spinal cord are lost. The brain and spinal cord are composed of neurons that do different functions such as controlling movements, processing sensory information, and making decisions. Aside from a small number of neural stem cells that are created daily, cells of the brain and spinal cord are not readily regenerated en mass, so excessive damage can be devastating. <unk> s result from deterioration of neurons which over time will lead to neurodegeneration and disabilities resulting from this. They are crudely divided into two groups according to phenotypic effects, although these are not mutually exclusive:
Following Stanley Prusiner, some hypothesize that infectious proteins called prions are pathogens that result in the degeneration of the central nervous system. Prions are proteins that suffer post-translational modification(s) that change their shape so that they can no longer perform their cellular functions.  Worse, they are hypothesized to trigger equivalent modifications in normal proteins, thus creating a cascade of damage that eventually results in significant neurodegeneration.  In humans, the result is Creutzfeldt-JaKob Disease (Mad Cow Disease).<unk> s can result from stroke, heat stress, head and spinal cord trauma (blunt or infectious pathology), and bleeding that occurs in the brain, the pressure from which eventually causes the death of one or more neurons.
Many times neuronal death begins long before the patient will ever experience any symptoms. It can be months or years before any effect is felt. Symptoms are noticed when lots of cells have died and certain parts of the brain have been weakened to the point that they can no longer function properly.
Regulation, or production of microglia by the immune system, in a process of neuroinflammation, is currently being rigorously studied for its role in<unk> s.
In recent studies it has been suggested that Monosodium Glutamate (MSG), a food additive, can result in<unk> s. MSG appears to cause lesions at specific locations in the brain of laboratory animals as well as concomitant proliferations of neuroendocrine dysfunction. Glutamic acid, where MSG is a source of, has not been identified as a neurotransmitter which transmits impulses. These neurotransmitters develop specific types of<unk> s.
Neurodegenerative disease
Basal ganglia disease
Central nervous system infection
Spinal disease
Spinal tumor
Neuromuscular disease
Autoimmune disease
Idiopathic inflammatory demyelinating diseases