Final answer:
The myelin sheath is a lipid-rich insulating layer that surrounds axons, enhancing the speed of nerve impulse transmission by allowing signals to jump from node to node.
Step-by-step explanation:
The lipid (fat) covering the axon that speeds up the rate of impulse transmission is called the myelin sheath. This lipid-rich layer acts as insulation around the axon of a neuron, much like the plastic coating around an electrical wire. The myelin sheath is produced by special types of cells: oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS).
It allows nerve impulses to travel more rapidly down the axon by facilitating the transmission of electrical signals. The presence of nodes of Ranvier, which are gaps in the myelin sheath, enables the nerve impulse to 'jump' from node to node, which speeds up the impulse considerably, a process known as saltatory conduction.