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What is the role of Schwann cells in peripheral nerves?
What is the role of Schwann cells in peripheral nerves?
What is a characteristic feature of myelinated nerve fibers in the peripheral nervous system (PNS)?
What is a characteristic feature of myelinated nerve fibers in the peripheral nervous system (PNS)?
In what way do oligodendrocytes in the CNS differ from Schwann cells in the PNS?
In what way do oligodendrocytes in the CNS differ from Schwann cells in the PNS?
Why might the myelin sheath partly be removed by standard histologic procedures?
Why might the myelin sheath partly be removed by standard histologic procedures?
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What is the appearance of the myelin sheath under high-magnification TEM?
What is the appearance of the myelin sheath under high-magnification TEM?
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How does the formation of myelin around axons differ for Schwann cells compared to oligodendrocytes?
How does the formation of myelin around axons differ for Schwann cells compared to oligodendrocytes?
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What happens to axons of large diameter growing in the PNS?
What happens to axons of large diameter growing in the PNS?
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What term describes the area where the plasma membrane of a Schwann cell fuses and extends itself around an axon?
What term describes the area where the plasma membrane of a Schwann cell fuses and extends itself around an axon?
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What is the functional significance of the myelin sheath in nerve cells?
What is the functional significance of the myelin sheath in nerve cells?
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What is the primary function of the major dense lines in the sheath?
What is the primary function of the major dense lines in the sheath?
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Which ultrastructural feature is associated with myelin clefts?
Which ultrastructural feature is associated with myelin clefts?
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What characterizes the myelin sheath at nodes of Ranvier?
What characterizes the myelin sheath at nodes of Ranvier?
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How does saltatory conduction contribute to nerve impulse movement?
How does saltatory conduction contribute to nerve impulse movement?
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What factor determines the length of an internodal segment in Schwann cells?
What factor determines the length of an internodal segment in Schwann cells?
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What distinguishes the intraperiod lines seen in light-staining layers?
What distinguishes the intraperiod lines seen in light-staining layers?
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Why does the myelin sheath partially cover axons at nodes of Ranvier?
Why does the myelin sheath partially cover axons at nodes of Ranvier?
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Where are second neurons of the parasympathetic series typically found?
Where are second neurons of the parasympathetic series typically found?
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What distinguishes parasympathetic ganglia from sympathetic ganglia?
What distinguishes parasympathetic ganglia from sympathetic ganglia?
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What allows some replacement of neurons throughout life in certain regions of the CNS?
What allows some replacement of neurons throughout life in certain regions of the CNS?
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Which cells are involved in the process of axonal regeneration in peripheral nerves?
Which cells are involved in the process of axonal regeneration in peripheral nerves?
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What process involves the formation and remodeling of synaptic connections throughout life?
What process involves the formation and remodeling of synaptic connections throughout life?
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During embryonic development, what happens to neurons that fail to establish correct synapses?
During embryonic development, what happens to neurons that fail to establish correct synapses?
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Why is regeneration and restoration of function within the CNS after major injury very difficult?
Why is regeneration and restoration of function within the CNS after major injury very difficult?
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What gives peripheral nerves a better capacity for axonal regeneration compared to CNS tissue?
What gives peripheral nerves a better capacity for axonal regeneration compared to CNS tissue?
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What family of proteins controls neural plasticity and reformation of processes in adult mammals after an injury?
What family of proteins controls neural plasticity and reformation of processes in adult mammals after an injury?
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Where are neuronal stem cells found in the adult central nervous system (CNS)?
Where are neuronal stem cells found in the adult central nervous system (CNS)?
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What type of cells can neuronal stem cells in the adult CNS supply?
What type of cells can neuronal stem cells in the adult CNS supply?
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In which structure do growing astrocytes at injured sites interfere with successful axonal regeneration?
In which structure do growing astrocytes at injured sites interfere with successful axonal regeneration?
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What could potentially allow tissue regeneration and functional recovery within the CNS components?
What could potentially allow tissue regeneration and functional recovery within the CNS components?
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Which cells have a much greater potential for regeneration and return of function in histologically simpler peripheral nerves after injury?
Which cells have a much greater potential for regeneration and return of function in histologically simpler peripheral nerves after injury?
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Why can fully differentiated, interconnected CNS neurons not temporarily disengage connections to replace lost cells?
Why can fully differentiated, interconnected CNS neurons not temporarily disengage connections to replace lost cells?
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What is controlled by several growth factors produced by both neurons and glial cells after an injury in adult mammals?
What is controlled by several growth factors produced by both neurons and glial cells after an injury in adult mammals?
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What characterizes the process of chromatolysis in neurons?
What characterizes the process of chromatolysis in neurons?
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How are distal portions of axons affected when isolated from their source of new proteins and organelles?
How are distal portions of axons affected when isolated from their source of new proteins and organelles?
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What is the role of blood-derived macrophages in the process of axon regeneration?
What is the role of blood-derived macrophages in the process of axon regeneration?
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Which cells align to serve as guides for the regeneration of axons?
Which cells align to serve as guides for the regeneration of axons?
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What happens to the Nissl substance in neurons during the process of chromatolysis?
What happens to the Nissl substance in neurons during the process of chromatolysis?
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In the process of axon regeneration, what occurs to the proximal segment close to the wound?
In the process of axon regeneration, what occurs to the proximal segment close to the wound?
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What do surrounding Schwann cells do when axons are isolated from their source?
What do surrounding Schwann cells do when axons are isolated from their source?
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Study Notes
Peripheral Nerves
- Peripheral nerves are bundles of nerve fibers (axons) individually surrounded by Schwann cells and connective tissue.
- Axons are sheathed by Schwann cells or neurolemmocytes, which may or may not form myelin around the axons depending on their diameter.
- Myelinated nerve fibers are formed when axons of large diameter grow in the PNS, and they are engulfed by a series of differentiating neurolemmocytes.
Myelin Sheath
- The myelin sheath is composed mainly of lipid bilayers and membrane proteins.
- A Schwann cell forms myelin around only a portion of one axon.
- The myelin sheath appears as a thick electron-dense axonal covering in which the concentric membrane layers may be visible.
- The prominent electron-dense layers visible ultrastructurally in the sheath are the major dense lines, which represent the fused, protein-rich cytoplasmic surfaces of the Schwann cell membrane.
- The myelin sheath serves to insulate axons and maintain a constant ionic microenvironment most suitable for action potentials.
Nodes of Ranvier
- Between adjacent Schwann cells on an axon, the myelin sheath shows small nodes of Ranvier (or nodal gaps), where the axon is only partially covered by interdigitating Schwann cell processes.
- At these nodes, the axolemma is exposed to ions in the interstitial fluid and has a much higher concentration of voltage-gated Na+ channels.
- The nodes of Ranvier are responsible for the rapid movement of nerve impulses, producing saltatory conduction.
Sympathetic and Parasympathetic Ganglia
- Sympathetic second neurons are located in small ganglia along the vertebral column.
- Second neurons of the parasympathetic series are found in very small ganglia always located near or within the effector organs.
- Parasympathetic ganglia may lack distinct capsules altogether, perikarya and associated satellite cells simply forming a loosely organized plexus within the surrounding connective tissue.
Neural Plasticity and Regeneration
- The nervous system exhibits neuronal differentiation and formation of new synapses even in adults.
- Neural plasticity and reformation of processes are controlled by several growth factors produced by both neurons and glial cells in a family of proteins called neurotrophins.
- Neuronal stem cells are present in the adult CNS, located in part among the cells of the ependyma, which can supply new neurons, astrocytes, and oligodendrocytes.
- In the peripheral nerves, injured axons have a much greater potential for regeneration and return of function.
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Description
Test your knowledge on peripheral nerves and nerve fibers, including the role of Schwann cells and myelination. Learn about the structure and function of nerves in the peripheral nervous system.
