16.8:
Nervous Tissue: Myelin
Myelin is a protective, multilayered covering around axons, composed of lipids and proteins, that provides electrical insulation.
In the PNS, individual Schwann cells repeatedly coil around short segments of an axon. The inner layers of this wrapping form the myelin sheath, while the outer portion, the neurolemma, contains the Schwann cell's nucleus and cytoplasm.
Two adjacent myelinated regions, or internodes, are separated by exposed areas of the axon, known as nodes of Ranvier. Because myelin is insulating, a nerve impulse jumps along an axon from one node to the next, speeding up the propagation of nerve impulses.
In the CNS, oligodendrocytes send out flat, cellular processes that wrap around axons. Several oligodendrocytes contribute to the myelination of one axon, and one oligodendrocyte myelinates parts of many axons.
Myelinated axons in the CNS also have nodes of Ranvier and internodes but no neurolemma.
Unmyelinated PNS and CNS axons are thin and lack myelin sheaths. In the PNS, a single Schwann cell sometimes encloses many unmyelinated axons without coiling around them.
16.8:
Nervous Tissue: Myelin
The myelin sheath is a multilayered lipid and protein covering that insulates the axon of a neuron, enhancing the speed of nerve impulse conduction. Axons without this sheath are referred to as unmyelinated. Two types of neuroglia, Schwann cells in the peripheral nervous system (PNS) and oligodendrocytes in the central nervous system (CNS) are responsible for producing myelin sheaths.
Schwann cells begin to form myelin sheaths around axons during fetal development. They wrap around a small portion of a single axon's length, creating multiple layers of glial plasma membrane around the axon. The inner part of this structure, comprised of the Schwann cell membrane, is the myelin sheath. The outer layer, which includes the Schwann cell's cytoplasm and nucleus, is called the neurolemma.
This outer layer is only found around axons in the PNS and is crucial in aiding axon regeneration after injury by forming a tube that guides and stimulates growth.
On the other hand, an oligodendrocyte in the CNS can myelinate parts of several axons. Unlike PNS, neurolemma is absent in the CNS because the oligodendrocyte cell body and nucleus do not envelop the axon. This absence, along with an inhibitory influence exerted by the oligodendrocytes, is believed to limit the regrowth of axons in the CNS following injury.
Myelination continues from birth to maturity, significantly increasing the speed of nerve impulse conduction. Consequently, infants, whose myelination is still in progress, exhibit slower and less coordinated responses to stimuli than older children or adults.