Neurodegenerative diseases are morphologically featured by progressive cell loss in specific vulnerable neuronal populations of the central nervous system, often associated with cytoskeletal protein aggregates forming intracytoplasmic and/or intranuclear inclusions in neurons and/or glial cells. The major basic processes inducing neurodegeneration are considered multifactorial ones caused by genetic, environmental, and endogenous factors. Multiple neurochemical mechanisms (neurotransmitters, regulatory peptides, neurotrophic growth factors, and proteins of the signaling transducer systems) maintain the integrity of nerve cell circuits, facilitate the responses to environmental demands, and promote the recovery of a function after injury.