Nerve fiber

Most neurons have two types of nerve fibers extending from the cell body: dendrites and a single axon.

Dendrites: Although a neuron may have many dendrites, it has only one axon. Dendrites are usually short, highly branched processes. Together with the cell body membrane, dendrites form the main receptive surface of the neuron, receiving signals from other neurons.

Axon: The axon usually arises from the axon hillock and conducts nerve impulses away from the cell body. The cytoplasm of the axon contains numerous mitochondria, microtubules, and neurofibrils.

Although the axon begins as a single fiber, it may give off collateral branches. Near its termination, the axon divides into multiple branches, each ending in a presynaptic terminal that contacts the receptive surface of another neuron or an effector cell.

In addition to conducting nerve impulses, the axon transports substances synthesized in the neuron’s cell body. This process, known as axonal transport, carries organelles (such as vesicles and mitochondria), ions, nutrients, and neurotransmitter molecules from the cell body to the axon terminals.

Myelin and Schwann cells

Large axons in peripheral nerves are commonly enclosed by Schwann cells. These cells wrap tightly around the axon, forming multiple layers of cell membrane with little or no cytoplasm between them. These membranes are rich in lipid-protein (lipoprotein) material called myelin, which forms the myelin sheath.

The portions of Schwann cells that contain cytoplasm and nuclei remain outside the myelin sheath and form the neurilemma (neurilemmal sheath).

Smaller axons may also be enclosed by Schwann cells, but these cells do not wrap around the axon. As a result, such axons lack a myelin sheath.

Myelinated vs. Unmyelinated Nerve Fibers

• Myelinated (medullated) nerve fibers possess a myelin sheath.

• Unmyelinated nerve fibers lack a myelin sheath.

Bundles of myelinated fibers appear white and form the white matter of the brain and spinal cord. In the central nervous system, myelin is produced by oligodendrocytes, not Schwann cells. Additionally, central nervous system axons lack neurilemmal sheaths.

Unmyelinated nerve tissue appears gray. Therefore, gray matter in the brain and spinal cord contains a high concentration of unmyelinated fibers and neuron cell bodies.