What is contained in white matter in the brain?

White matter in the brain is primarily composed of myelinated nerve fibers that facilitate communication between neurons. The myelin sheath, which wraps around these nerve fibers, serves to increase the speed of electrical impulses as they travel along the axons, allowing for efficient communication between different areas of the brain and between the brain and spinal cord. This is crucial for the integration of information and coordination of functions across various regions of the nervous system.

The presence of myelinated fibers distinguishes white matter from gray matter, which contains the cell bodies of neurons, their dendrites, and unmyelinated axons. The efficiency provided by the myelin makes white matter essential for the overall functioning of the central nervous system, as it enables rapid signal transmission necessary for both conscious and unconscious processes.

By contrast, the other choices highlight components that are either associated with the central nervous system's gray matter, such as nerve cells and dendrites, or do not pertain directly to the structural composition of white matter, like pleasure circuits and voluntary muscle structures.