Cell nucleus
Cell nuclei contain most of the cell's genetic material, organized as multiple long linear DNA molecules in a complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these chromosomes are the cell's nuclear genome and are structured in such a way to promote cell function. The nucleus maintains the integrity of genes and controls the activities of the cell by regulating gene expression—the nucleus is, therefore, the control center of the cell. The main structures making up the nucleus are the nuclear envelope, a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm, and the nuclear matrix (which includes the nuclear lamina), a network within the nucleus that adds mechanical support, much like the cytoskeleton, which supports the cell as a whole.
Trigeminal nerve
The mesencephalic nucleus is not a true nucleus; it is a sensory ganglion (like the trigeminal ganglion) embedded in the brainstem and the sole exception to the rule that sensory information passes through peripheral sensory ganglia before entering the central nervous system. It has been found in all vertebrates except lampreys and hagfishes. They are the only vertebrates without jaws and have specific cells in their brainstems. These "internal ganglion" cells were discovered in the late 19th century by medical student Sigmund Freud.