The relationship of these glial cells to the structure of the CNS is seen in Figure One of the two types of glial cells found in the PNS is the satellite cell. Satellite cells surround the cell bodies of neurons in the PNS.
They provide support, performing similar functions in the periphery as astrocytes do in the CNS—except, of course, for establishing the BBB. The second type of glial cell is the Schwann cell , which insulate axons with myelin in the periphery. Schwann cells are different than oligodendrocytes in that a Schwann cell wraps around a portion of only one axon segment and no others. Oligodendrocytes have processes that reach out to multiple axon segments, whereas the entire Schwann cell surrounds just one axon segment.
The nucleus and cytoplasm of the Schwann cell are on the edge of the myelin sheath. The relationship of these two types of glial cells to ganglia and nerves in the PNS is seen in Figure Whereas the manner in which either cell is associated with the axon segment, or segments, that it insulates is different, the means of myelinating an axon segment is mostly the same in the two situations.
Myelin is a lipid-rich sheath that surrounds the axon and by doing so creates a myelin sheath that facilitates the transmission of electrical signals along the axon. The lipids are essentially the phospholipids of the glial cell membrane. Myelin, however, is more than just the membrane of the glial cell.
It also includes important proteins that are integral to that membrane. Some of the proteins help to hold the layers of the glial cell membrane closely together. The glial cell is wrapped around the axon several times with little to no cytoplasm between the glial cell layers.
For oligodendrocytes, the rest of the cell is separate from the myelin sheath as a cell process extends back toward the cell body. A few other processes provide the same insulation for other axon segments in the area. For Schwann cells, the outermost layer of the cell membrane contains cytoplasm and the nucleus of the cell as a bulge on one side of the myelin sheath.
During development, the glial cell is loosely or incompletely wrapped around the axon Figure The edges of this loose enclosure extend toward each other, and one end tucks under the other. The inner edge wraps around the axon, creating several layers, and the other edge closes around the outside so that the axon is completely enclosed. The axon contains microtubules and neurofilaments that are bounded by a plasma membrane known as the axolemma.
Outside the plasma membrane of the axon is the myelin sheath, which is composed of the tightly wrapped plasma membrane of a Schwann cell. What aspects of the cells in this image react with the stain to make them a deep, dark, black color, such as the multiple layers that are the myelin sheath?
Myelin sheaths can extend for one or two millimeters, depending on the diameter of the axon. Axon diameters can be as small as 1 to 20 micrometers. If the myelin sheath were drawn to scale, the neuron would have to be immense—possibly covering an entire wall of the room in which you are sitting. Multiple sclerosis MS is one such disease.
It is an example of an autoimmune disease. The antibodies produced by lymphocytes a type of white blood cell mark myelin as something that should not be in the body.
This causes inflammation and the destruction of the myelin in the central nervous system. As the insulation around the axons is destroyed by the disease, scarring occurs.
This is where the name of the disease comes from; sclerosis means hardening of tissue, as occurs in a scar. Multiple scars are found in the white matter of the brain and spinal cord. This medullary sheath is interrupted at intervals by the nodes of Ranvier. Nerve cells are functionally connected to each other at a junction known as a synapse , where the terminal branches of an axon and the dendrites of another neuron lie in close proximity to each other but normally without direct contact.
Information is transmitted across the gap by chemical secretions called neurotransmitters. It causes activation in the post-synaptic cell. Template:WikiDoc Sources. Category : Tissues. Cookies help us deliver our services. By using our services, you agree to our use of cookies. Other parts of the nervous system control involuntary processes.
These include keeping a regular heartbeat, releasing hormones like adrenaline, opening the pupil in response to light, and regulating the digestive system. When a neuron sends a message to another neuron, it sends an electrical signal down the length of its axon. At the end of the axon, the electrical signal changes to a chemical signal. The axon then releases the chemical signal with chemical messengers called neurotransmitters pronounced noor-oh-TRANS-mit-erz into the synapse pronounced SIN-aps —the space between the end of an axon and the tip of a dendrite from another neuron.
The neurotransmitters move the signal through the synapse to the neighboring dendrite, which converts the chemical signal back into an electrical signal. Ependymal cells line each ventricle , one of four central cavities that are remnants of the hollow center of the neural tube formed during the embryonic development of the brain. The choroid plexus is a specialized structure in the ventricles where ependymal cells come in contact with blood vessels and filter and absorb components of the blood to produce cerebrospinal fluid.
These glial cells appear similar to epithelial cells, making a single layer of cells with little intracellular space and tight connections between adjacent cells.
They also have cilia on their apical surface to help move the CSF through the ventricular space. The relationship of these glial cells to the structure of the CNS is seen in Figure 4. Figure 5. One of the two types of glial cells found in the PNS is the satellite cell.
Satellite cells are found in sensory and autonomic ganglia, where they surround the cell bodies of neurons. This accounts for the name, based on their appearance under the microscope. They provide support, performing similar functions in the periphery as astrocytes do in the CNS—except, of course, for establishing the BBB. The second type of glial cell is the Schwann cell , which insulate axons with myelin in the periphery.
Schwann cells are different than oligodendrocytes, in that a Schwann cell wraps around a portion of only one axon segment and no others. Oligodendrocytes have processes that reach out to multiple axon segments, whereas the entire Schwann cell surrounds just one axon segment.
The nucleus and cytoplasm of the Schwann cell are on the edge of the myelin sheath. The relationship of these two types of glial cells to ganglia and nerves in the PNS is seen in Figure 5.
Whereas the manner in which either cell is associated with the axon segment, or segments, that it insulates is different, the means of myelinating an axon segment is mostly the same in the two situations. Myelin is a lipid-rich sheath that surrounds the axon and by doing so creates a myelin sheath that facilitates the transmission of electrical signals along the axon. The lipids are essentially the phospholipids of the glial cell membrane.
Myelin, however, is more than just the membrane of the glial cell. It also includes important proteins that are integral to that membrane. Some of the proteins help to hold the layers of the glial cell membrane closely together.
The glial cell is wrapped around the axon several times with little to no cytoplasm between the glial cell layers.
For oligodendrocytes, the rest of the cell is separate from the myelin sheath as a cell process extends back toward the cell body. A few other processes provide the same insulation for other axon segments in the area. For Schwann cells, the outermost layer of the cell membrane contains cytoplasm and the nucleus of the cell as a bulge on one side of the myelin sheath.
During development, the glial cell is loosely or incompletely wrapped around the axon Figure 6a. The edges of this loose enclosure extend toward each other, and one end tucks under the other. The inner edge wraps around the axon, creating several layers, and the other edge closes around the outside so that the axon is completely enclosed. Figure 6. The Process of Myelination. Myelinating glia wrap several layers of cell membrane around the cell membrane of an axon segment.
A single Schwann cell insulates a segment of a peripheral nerve, whereas in the CNS, an oligodendrocyte may provide insulation for a few separate axon segments. Myelin sheaths can extend for one or two millimeters, depending on the diameter of the axon. Axon diameters can be as small as 1 to 20 micrometers. Figure 1, Figure 4, and Figure 5 show the myelin sheath surrounding an axon segment, but are not to scale. If the myelin sheath were drawn to scale, the neuron would have to be immense—possibly covering an entire wall of the room in which you are sitting.
Several diseases can result from the demyelination of axons. The causes of these diseases are not the same; some have genetic causes, some are caused by pathogens, and others are the result of autoimmune disorders. Though the causes are varied, the results are largely similar. The myelin insulation of axons is compromised, making electrical signaling slower. Multiple sclerosis MS is one such disease.
It is an example of an autoimmune disease.
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