39 Questions
What is the role of Schwann cells in peripheral nerves?
Encapsulating nerve fibers
What is a characteristic feature of myelinated nerve fibers in the peripheral nervous system (PNS)?
The presence of multiple layers of Schwann cell membrane
In what way do oligodendrocytes in the CNS differ from Schwann cells in the PNS?
Oligodendrocytes produce myelin around multiple axons
Why might the myelin sheath partly be removed by standard histologic procedures?
Due to its lipid composition
What is the appearance of the myelin sheath under high-magnification TEM?
Thick electron-dense covering with concentric membrane layers
How does the formation of myelin around axons differ for Schwann cells compared to oligodendrocytes?
Schwann cells form myelin around one axon segment only
What happens to axons of large diameter growing in the PNS?
They become engulfed by differentiating neurolemmocytes
What term describes the area where the plasma membrane of a Schwann cell fuses and extends itself around an axon?
'Mesaxon'
What is the functional significance of the myelin sheath in nerve cells?
Maintains a constant ionic micro-environment suitable for action potentials
What is the primary function of the major dense lines in the sheath?
Allow transient movement of cytoplasm for membrane maintenance
Which ultrastructural feature is associated with myelin clefts?
Disappearance of major dense lines
What characterizes the myelin sheath at nodes of Ranvier?
Exposure to ions in the interstitial fluid
How does saltatory conduction contribute to nerve impulse movement?
Facilitates rapid movement from node to node
What factor determines the length of an internodal segment in Schwann cells?
Axonal diameter
What distinguishes the intraperiod lines seen in light-staining layers?
Apposed outer bilayers of Schwann cell membrane
Why does the myelin sheath partially cover axons at nodes of Ranvier?
To expose axolemma to ions for action potential renewal
Where are second neurons of the parasympathetic series typically found?
Near or within effector organs
What distinguishes parasympathetic ganglia from sympathetic ganglia?
Lack of distinct capsules
What allows some replacement of neurons throughout life in certain regions of the CNS?
Presence of neural stem and progenitor cells
Which cells are involved in the process of axonal regeneration in peripheral nerves?
Schwann cells and macrophages
What process involves the formation and remodeling of synaptic connections throughout life?
Neural plasticity
During embryonic development, what happens to neurons that fail to establish correct synapses?
They are eliminated by apoptosis
Why is regeneration and restoration of function within the CNS after major injury very difficult?
Complexity and distances of neuronal and glial interconnections
What gives peripheral nerves a better capacity for axonal regeneration compared to CNS tissue?
Simpler organization
What family of proteins controls neural plasticity and reformation of processes in adult mammals after an injury?
Neurotrophins
Where are neuronal stem cells found in the adult central nervous system (CNS)?
In the ependyma
What type of cells can neuronal stem cells in the adult CNS supply?
Astrocytes
In which structure do growing astrocytes at injured sites interfere with successful axonal regeneration?
Spinal cord tracts
What could potentially allow tissue regeneration and functional recovery within the CNS components?
Neural stem cells
Which cells have a much greater potential for regeneration and return of function in histologically simpler peripheral nerves after injury?
Damaged axons
Why can fully differentiated, interconnected CNS neurons not temporarily disengage connections to replace lost cells?
Inability to divide
What is controlled by several growth factors produced by both neurons and glial cells after an injury in adult mammals?
Neural plasticity
What characterizes the process of chromatolysis in neurons?
Swelling of the cell body
How are distal portions of axons affected when isolated from their source of new proteins and organelles?
They degenerate
What is the role of blood-derived macrophages in the process of axon regeneration?
Removal of cellular debris and myelin
Which cells align to serve as guides for the regeneration of axons?
Schwann cells
What happens to the Nissl substance in neurons during the process of chromatolysis?
Initially diminished
In the process of axon regeneration, what occurs to the proximal segment close to the wound?
It degenerates for a short distance
What do surrounding Schwann cells do when axons are isolated from their source?
Dedifferentiate and shed myelin sheaths
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.
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.
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