Chapter 12 - Section 12.3 - Supportive Cells (Neuroglia) - Before You Go On - Page 447: 9

Myelin is a fatty substance that wraps around the axons of neurons, providing insulation and enhancing the speed of electrical signal transmission. Myelin is produced by specialized cells called glial cells, which differ in the central nervous system (CNS) and the peripheral nervous system (PNS). The two primary types of glial cells responsible for myelin production are oligodendrocytes in the CNS and Schwann cells in the PNS. Here's how myelin production differs in the CNS and PNS: **1. Central Nervous System (CNS):** - **Oligodendrocytes:** In the CNS, myelin is produced by oligodendrocytes. Oligodendrocytes are capable of myelinating multiple axons at once. They send out processes that wrap around and insulate different segments of several axons. This is known as "multifocal" or "multiple-wrapping" myelination. - **White Matter Tracts:** In the CNS, myelinated axons are primarily found in white matter tracts. These tracts contain bundles of axons with myelin sheaths, and they facilitate communication between different regions of the brain and spinal cord. - **Role in Diseases:** Diseases affecting myelin in the CNS include multiple sclerosis (MS), where the immune system mistakenly attacks and damages oligodendrocytes, leading to demyelination and impaired nerve signaling. **2. Peripheral Nervous System (PNS):** - **Schwann Cells:** In the PNS, myelin is produced by Schwann cells. Each Schwann cell typically myelinates a single axon by wrapping itself around the axon multiple times. This is known as "unifocal" or "single-wrapping" myelination. - **Individual Myelination:** Unlike oligodendrocytes in the CNS, Schwann cells myelinate axons individually. Each Schwann cell is responsible for a single axon segment. This results in a "node of Ranvier" between adjacent Schwann cell segments along the same axon. - **Role in Regeneration:** Schwann cells also play a crucial role in nerve regeneration. After injury, they can guide and support axon regrowth to help restore nerve function in the PNS. - **Role in Diseases:** Diseases affecting myelin in the PNS include conditions like Guillain-Barré syndrome, where the immune system targets Schwann cells, leading to demyelination and muscle weakness. In summary, myelin production differs in the CNS and PNS primarily due to the distinct glial cell types involved (oligodendrocytes in the CNS and Schwann cells in the PNS) and the manner in which they myelinate axons. These differences have important implications for nerve function, regeneration, and the pathophysiology of diseases affecting myelin in these two parts of the nervous system.

Myelin is a fatty substance that wraps around the axons of neurons, providing insulation and enhancing the speed of electrical signal transmission. Myelin is produced by specialized cells called glial cells, which differ in the central nervous system (CNS) and the peripheral nervous system (PNS). The two primary types of glial cells responsible for myelin production are oligodendrocytes in the CNS and Schwann cells in the PNS. Here's how myelin production differs in the CNS and PNS: **1. Central Nervous System (CNS):** - **Oligodendrocytes:** In the CNS, myelin is produced by oligodendrocytes. Oligodendrocytes are capable of myelinating multiple axons at once. They send out processes that wrap around and insulate different segments of several axons. This is known as "multifocal" or "multiple-wrapping" myelination. - **White Matter Tracts:** In the CNS, myelinated axons are primarily found in white matter tracts. These tracts contain bundles of axons with myelin sheaths, and they facilitate communication between different regions of the brain and spinal cord. - **Role in Diseases:** Diseases affecting myelin in the CNS include multiple sclerosis (MS), where the immune system mistakenly attacks and damages oligodendrocytes, leading to demyelination and impaired nerve signaling. **2. Peripheral Nervous System (PNS):** - **Schwann Cells:** In the PNS, myelin is produced by Schwann cells. Each Schwann cell typically myelinates a single axon by wrapping itself around the axon multiple times. This is known as "unifocal" or "single-wrapping" myelination. - **Individual Myelination:** Unlike oligodendrocytes in the CNS, Schwann cells myelinate axons individually. Each Schwann cell is responsible for a single axon segment. This results in a "node of Ranvier" between adjacent Schwann cell segments along the same axon. - **Role in Regeneration:** Schwann cells also play a crucial role in nerve regeneration. After injury, they can guide and support axon regrowth to help restore nerve function in the PNS. - **Role in Diseases:** Diseases affecting myelin in the PNS include conditions like Guillain-Barré syndrome, where the immune system targets Schwann cells, leading to demyelination and muscle weakness. In summary, myelin production differs in the CNS and PNS primarily due to the distinct glial cell types involved (oligodendrocytes in the CNS and Schwann cells in the PNS) and the manner in which they myelinate axons. These differences have important implications for nerve function, regeneration, and the pathophysiology of diseases affecting myelin in these two parts of the nervous system.