Final answer:
The speed of a nerve signal is fastest in the white matter, which consists of myelinated axons covered with myelin sheath. Gray matter contains the cell bodies of neurons and is responsible for receiving and processing information.
Step-by-step explanation:
The speed of a nerve signal is fastest in the white matter. White matter consists mainly of axons covered with myelin sheath, which allows for fast transmission of electrical signals. These myelinated axons make up the white matter, while cell bodies and interneurons make up the gray matter. Gray matter mainly contains the cell bodies of neurons and is responsible for receiving and processing information.