What happens to pyruvate molecules formed in glycolysis in the absence of oxygen?

Absence of oxygen means anaerobic respiration ; which has 2 different pathways.

First Pathway ; Alcoholic fermentation which takes place in yeast.

Pyruvate is decarboxylated into ethanal releasing co2 as waste product.
Ethanal is toxic , so its reduced by help of NADH to ethanol in presence of alcohol dehydrogenase enzyme , and Free NAD is released to continue glycolysis.

2) The second pathway is Lactic fermentation which takes place in mammalian muscles.

Pyruvate accepts hydrogen and is converted to lactate by the help of enzyme lactate dehydrogenase , again free NAD is released to continue glycolysis.

Hope I made it clear enough :)

It is called the medulla oblongata or people usually call it medulla. it is the lower half of the brain stem continuous with the spinal cord.