Final answer:
In the central nervous system, myelinated axons that run together in bundles are known as tracts. These are insulated by myelin sheaths formed by oligodendrocytes, which facilitates efficient nerve impulse transmission. The term changes to 'nerve' when these bundles of axons are located in the peripheral nervous system.
Step-by-step explanation:
The myelinated axons that run together in bundles in the CNS are referred to as tracts. Myelinated neurons in the CNS are associated with oligodendrocytes, which are a type of glial cell responsible for forming the insulating myelin sheath. This sheath enhances the efficiency of electrochemical nerve impulse transmission. In contrast, in the peripheral nervous system (PNS), similar bundles of axons are known as nerves. These nerves are composed not only of nervous tissue but also contain connective tissues and blood vessels. The terminology depends on the location of the axons; the same bundle is called a 'nerve' in the PNS and a 'tract' in the CNS, such as the optic nerve becoming the optic tract after it enters the cranium at the point known as the optic chiasm. White matter in the CNS is mainly composed of such myelinated axons, which give it a white appearance, distinguishing it from the gray matter, which contains neuron cell bodies.