I believe you are talking about the denevan neural parasites from the episode Operation--Annihilate!
You may be referring to the Star Trek: The Original Series and Star Trek: The Remastered Original Series episode "Operation: Annihilate!". They are referred to as "neural parasites" not because they infest human neural tissue, but because they are said to be parasitic in nature, and resemble human brain cells (however, the material they inject into a humanoid host may infest neural tissue). The Star Trek Star Fleet Medical Manual refers to them as "blastoceles". ProfessorTrek 20:32, July 16, 2011 (UTC)