The review discusses thesteps of vitamin B12 metabolism and its role in 
maintaining of neurological functions. The term "vitamin B12 (cobalamin)" refers 
to several substances (cobalamins) of a very similar structure. Cobalamin enters 
the body with animal products. On the peripherу cobalamin circulates only in 
binding with proteins transcobalamin I and II (complex cobalamin-transcobalamin 
II is designated as "holotranscobalamin"). Holotranscobalamin is absorbed by 
different cells, whereas transcobalamin I-binded vitamin B12 - only by liver and 
kidneys. Two forms of cobalamin were identified as coenzymes of cellular 
reactions which are methylcobalamin (in cytoplasm) and hydroxyadenosylcobalamin 
(in mitochondria). The main causes of cobalamin deficiency are related to 
inadequate intake of animal products, autoimmune gastritis, pancreatic 
insufficiency, terminal ileum disease, syndrome of intestinal bacterial 
overgrowth. Relative deficiency may be seen in excessive binding of vitamin B12 
to transcobalamin I. Cobalamin deficiency most significantly affects functions 
of blood, nervous system and inflammatory response. Anemia occurs in 13-15% of 
cases; macrocytosis is an early sign. The average size of neutrophils and 
monocytes is the most sensitive marker of megaloblastic hematopoiesis. The 
demands in vitamin B12 are particularly high in nervous tissue. Hypovitaminosis 
is accompanied by pathological lesions both in white and gray brain matter. 
Several types of neurological manifestations are described: subacute combined 
degeneration of spinal cord (funicular myelinosis), sensomotor polyneuropathy, 
optic nerve neuropathy, cognitive disorders. The whole range of neuropsychiatric 
disorders with vitamin B12 deficiency has not been studied well enough. Due to 
certain diagnostic difficulties they are often regarded as "cryptogenic", 
"reactive", "vascular» origin. Normal or decreased total plasma cobalamin level 
could not a reliable marker of vitamin deficiency. In difficult cases the 
content of holotranscobalamin, methylmalonic acid / homocysteine, and folate in 
the blood serum should be investigated besides carefully analysis of clinical 
manifestations.