Myelination of the nerve fibers in the central nervous system is the job of the oligodendrocyte. True or False .. C) association neuron. D) glial cell A) the myelin sheath. B) large nerve fibers Schwann cells are functionally similar to ______. Schwann cells or neurolemmocytes are the principal glia of the peripheral nervous system 5 See also; 6 References; 7 External links Myelinating Schwann cells begin to form the myelin sheath in mammals during fetal A well- developed Schwann cell is shaped like a rolled-up sheet of paper, with layers of myelin. The sheath of Schwann is also called the ______. A) myelin sheath. B) axolemma A) Schwann cell. B) dendrite. C) axon C) efferent neuron. D) association neuron Schwann cells are functionally similar to ______.
The radial diffusion of low molecular weight dyes across the myelin sheath was not interrupted in myelinating Schwann cells from cxnull mice, indicating that other connexins participate in forming gap junctions in these cells. Owing to the unique geometry of myelinating Schwann cells, a gap junction-mediated radial pathway may be essential for rapid diffusion between the adaxonal and perinuclear cytoplasm, since this radial pathway is approximately one million times faster than the circumferential pathway.
Schwann cell - Wikipedia
In the PNS, Schwann cells make the myelin sheath, which is composed of two distinct domains, compact myelin and noncompact myelin, each containing distinct proteins. Compact myelin contains protein zero, peripheral myelin protein of 22 kD, and myelin basic protein. E-cadherin is localized to adherens junctions by immunoelectron microscopy Fannon et al. Although adherens junctions and gap junctions are typically found between neighboring cells, in the myelin sheath they join adjacent layers.
Chapter 11 Fundamentals of the Nervous System and Nervous Tissue (TB)
These radially arrayed junctions are thus unique examples of how reflexive junctions Herr and Heidger, ; Majack and Larsen, ; Rumessen et al. In addition to these molecular specializations, there has been a renewed interest in the electrophysiological functions of myelinating Schwann cells.
Although myelinating Schwann cells are typically characterized as providing relatively static axonal insulation, they also possess several kinds of ion channels Chiu, ; Sontheimer, ; Mi et al. Moreover, when their contact with axons is disrupted, they develop voltage-dependent sodium and potassium conductances Chiu,may become coupled by gap junctions Chandross et al.
The finding that Cx32 is localized to the incisures and paranodes, where putative gap junctions have been observed, led us to investigate whether incisures and paranodes have functional gap junctions. Such gap junctions could promote intracellular communication, since reflexive gap junctions could provide a radial pathway directly across the myelin sheath. True or False True During depolarization, the inside of the neuron's membrane becomes less negative.
True or False True Strong stimuli cause the amplitude of action potentials generated to increase. True or False False The oligodendrocytes can myelinate several axons. True or False True Enkephalins and endorphins are peptides that act like morphine.
- Myelin: an invention by vertebrates AND invertebrates
- Schwann cell
True or False True A synapse formed between the axon ending of one neuron and the cell body of another neuron is called an axosomatic synapse.
True or False True In myelinated axons the voltage-regulated sodium channels are concentrated at the nodes of Ranvier. True or False True Action potentials can be generated by virtually all cells of the body because all cells possess cell membranes.
True or False False Voltage is always measured between two points and may be called the potential between these two points. True or False True Neurons that are far away from the center of the neuron pool and that are not easily excited by an incoming stimulus are in the discharge zone. This in turn greatly reduces the radial leakage of transient currents flowing through the sheath during nerve impulses current flowing through a capacitor is proportional to the time derivative of voltage-change across it in regions between nodes the "internodes".
Although sodium channels are concentrated at the nodes at densities well above those of typical unmyelinated fibers, the mean density averaged over the length of the fiber is much less, resulting in a smaller ionic imbalance that must be restored at the expense of metabolic energy ionic pumps after an impulse passes. The smaller internodal current loss leaves more current available to raise distant nodes to threshold, which will thus happen more quickly, speeding impulse propagation.
Further, the reduced size of exposed nodal membrane, reduces the area of membrane into which this current must flow and increases the rate of change of voltage at the node technically, the time-constant for charging nodal capacitance is reducedallowing threshold to be reached faster, further speeding the impulses. Conduction speed also depends on axial resistance through the interior of the fiber.
Functional Gap Junctions in the Schwann Cell Myelin Sheath
The larger the diameter of this interior space, the lower the resistance, a principle that holds for unmyelinated as well as myelinated fibers. This explains the frequent observation of "giant" axons in invertebrates, especially prevalent in circuits involved in rapid escape reactions e. The thickness of the myelin sheath, however, varies with the interior diameter, typically maintaining a fairly constant ratio to it ca 0.
The result is that internode capacitance per unit area of axon decreases with fiber diameter, adding to the effects from decreased axial resistance and giving the conduction speed a first power dependency on inner or outer diameter over a substantial range. What is myelin and who else has it? The essential structural features that produce these properties are the restriction of leakage current to cross multiple membrane lamellae in the internode and the reduction of surface area of nodal membrane.
If we take these as the defining characteristics of "myelin" then myelin occurs in several taxa of phylogenetically distant invertebrates: There are several variants in myelin structure seen in invertebrates which still achieve the same functional results. Vertebrate myelin is spirally wrapped. That is, a continuous double lamella laid down by a Schwann cell or oligodendrocyte winds around the fiber starting against the axon and spiraling outward.
Compact myelin is the form most typical of mature vertebrate myelin, with both cytoplasmic and extracellular spaces eliminated.
In EM cross section, this gives rise to a regularly banded alternation of thick and thin lines referred to as the "major dense line" apposed ecytoplasmic membrane leaflets and the "intraperiod line" apposed extracellular leaflets. Vertebrate myelin has regions Schmidt-Lantermann incisures that retain cytoplasm over short segments and these form continuous spiral intracytoplasmic pathways from just outside of the axon to the outer layer of glial membrane. Spiral wrapping has the disadvantage of requiring specializations to prevent radial current leak following along the spiral path between lamellae.
Reports of myelin in invertebrates are scattered among several phylogenetically diverse groups as shown on the phyletic tree below. Not all of these reports have been confirmed in the electron microscope yet asterisks in the figure below and recent EM evidence has failed to confirm its presence in one of the polychaete groups indicated in the figure below bamboo worms - see Hartline and Kong Myelin of oligochaetes especially the earthworm is the best studied of invertebrate myelin at the electron microscope level.
It is spirally wrapped, at least in places, as in the vertebrate case Roots et al. It consists of 20 to layers, often, but not always, compact. The non-compact regions typically have thin layers of cytoplasm sanndwiched between glial cell membranes.
Being intracellular and narrow, however, their capability for compromising sheath insulation appears limited. While conduction speed of earthworm myelinated fibers is high compared to that for non-myelinated fibers of the same diameter, the advantage is only a few fold Gunther, All crustacean myelin so far described has proven to be concentrically arranged: Concentric wraps are electrically more efficient, requiring only that tight seals be made at the margins of the myelinating cells, the "seams," to prevent short circuiting of the insulation.
Thus myelin in the decapod shrimp is sometimes compact and sometimes only semicompact, that is, it excludes only the extracellular gap while retaining cytoplasm or vice versa. What is important for its electrical integrity is that the space between layers are sealed from each either by a continuous membranous barrier or by tightly joined appositions at the seams. Two somewhat different forms have been described for different shrimp taxa. In the more "advanced" Caridean shrimp including the prawnseach myelin layer includes a thin sheet of sandwiched cytoplasm and extends fully around the axon, meeting itself on the opposite side in a seam.