Neuroglia
Neuroglia or glia are the connective tissue of the central nervous system, consisting of several different types of cells associated with various types of neurons that are specialized for a variety of supporting functions. They provide structural and metabolic support for neurons , provide myelination, buffers extracellular pH and remove damaged processes. Macroglia and microglia are glial cells derived from the mesoderm that function as macrophages (scavengers) in the central nervous system and form part of the reticulo-endothelial system, microglia remove neuronal debris after damage through phagocytosis.
Neuroglia support the function of neurons. In part, they regulate the concentration of ions (atoms with an electrical charge, such as potassium) in the extracellular spaces of the brain and spinal cord, and they may play a role in the blood-brain barrier. In addition, they are involved in several other functions.Unlike most neurons, neuroglia are capable of cell division after both. In fact, they they are often the structural element in slow-growing tumors of the brain. Further, neuroglia do not form synapses. Although they do participate in slow, electrical phenomena, they are not capable of transmitting impulses.
In carrying out their role, the neurons depend on a population of cells called neuroglia. In the CNS, these include macroglia, microglia, and ependymal glia. In the PNS, they include satellite and Schwann cells. The macroglia include the astrocytes and oligodendrocytes. Microglia are the smallest of the neuroglia. They actively proliferate in response to injury within the CNS. During the resting stage, they have small, dark, elongated nuclei and scant cytoplasm. When activated (as in scavenging for foreign bodies or injured tissue), microglia swell up with ingested material. In this condition they are known as "gitter cells."
Schwann Cells are found in the peripheral nervous system and are counterparts of the neuroglial cells of the central nervous system which are astrocytes.Types of glial cells include Schwann cells, named after Theodor Schwann, a german neurophysiologist who showed that animals tissue is composed of cells. The neurons of the peripheral nervous system lie embedded in a sheath of Schwann cells. Two of it's functions are well understood: its role in the formation of myelin and its role during nerve regeneration.
Oligodendrocytes are a glial cell similar to an astrocyte but with fewer protuberances, concerned with the production of myelin in the central nervous system. The oligodendrocytes are responsible for covering axons in myelin. Protoplasmic astrocytes are an important interface between neurons and blood vessels and
myelin sheath. Astrocytes are star-shaped glial cells of the central nervous system. Ependymal cells are thin membranes of glial cells that line the ventricles of the brain and the central canal of the spinal cord. Ependymal glia are the cells that line the cavity of the brain and spinal cord.
Astrocytes are the most common type of glia found in the CNS. They are of two kinds, protoplasmic and fibrous. Protoplasmic astrocytes are found primarily in gray matter (unmyelinated areas, consisting largely of the cell bodies and dendrites of neurons) and are characterized by numerous branching processes. Fibrous astrocytes are found primarily in white matter (bundles of myelinated axons) and exhibit long, thin, relatively unbranched processes. Both types are larger than oligodendrocytes.
Some processes of astrocytes terminate in expansions called "end feet" or attachments. They are applied to the surfaces of blood vessels, to dendrites, and to the pia mater, the innermost covering of the brain and spinal cord. Capillary end feet may be involved in transporting substances between blood vessels and neurons. Dendritic attachments are applied to the parts of dendrites not covered by synapses and may be involved in metabolic support of the neuronal membrane. The pial attachments of the asttrocytes form a functionally protective membrane with the pia mater called the pia-glial membrane.
Oligodendrocytes are smaller than astrocytes and have fewer processes. Satellite are found close to the neuronal cell membranes. Interfascicular are known to form myelin around the axon of neurons of the CNS. The counterpart of the oligodendrocyte in the peripheral nervous system (PNS) is the Schwann cell; the two differ in that the Schwann cell myelinates one fiber at a time, whereas the oligo can myelinate several fibers simultaneously. Occasional oligodendrocytes with perivascular attachements are seen adjacent to capillaries.
The neuroglia of the PNS are the satellite cells surrounding the neuronal cell bodies of the spinal ganglia and the Schwann cells enveloping the axons of the PNS. Satellite cells appear to serve the same nutritive and supportive functions as the macroglia of the CNS. Schwann cells play an important role in nerve regeneration.
The neuron is capable of repair and regeneration of itself with glia as the principle constituent. The neuron and its various parts constitute a highly plastic entity, capable of a good deal of growth and reshaping, especially at the ends of the dendritic tree. But the dendritic spines are thought to the be the most plastic elements of the entire neuron - arising out of the membrane, growing, changing shape and size and probably disappearing, all in response to shiftng patterns of information coming onto the neuron.
A reverse transport mechanism (i.e. one moving material from distal to proximal) has been shown to move material at about half the rate of the proximal-distal mechanism. Thus, there is a form of "conveyor belt" that moves material to and from the cell nucleus. The role that such a system could play in the maintenance of distant portions of the neuron is readily apparent.
Such a two-way conveyer-belt system can also explain how the cell nucleus responds to damage to the distal parts of the axon (cell chromatolysis): it apparently produces increasing amounts of material. In this way, substances returning to the nucleus may provide a biochemical feedback that controls nuclear production of materials needed in the periphery.
Finally, the trophic substances postulated as being secreted from the terminals of nerves in order to affect postsynaptic membranes may be complex molecules synthesized under the genetic control of the nucleus.
While the fast transport rate of 400mm/day does not seem "fast" in ordinary terms, it is fast compared to the previously known rate of movement of materials in axons when a nerve fiber has been cut and the axon regenerates. The rate of growth of the distal axon is 1 to 10 mm/day, when it is regenerating.
The changes seen in a mammalian motor cell after transection of the axon and the subsequent regeneration are shown in the cycle. Note that the portion of the axon distal to the cut degenerates when it is cut off from the nucleus. The cell nucleus then undergoes a process of chromatolysis, during which the axon regenerates. The large volume of material that must be synthesized is indicated by the relatively large volume of the axon relative to that of the cell body.
In mammals, birds, and reptiles, regeneration of axons occurs in only the PNS. No functional regeneration in the CNS of these animals occurs after damage. The reasons are not clear, since regeneration can occur in the CNS of amphibia.
Neuroglia support the function of neurons. In part, they regulate the concentration of ions (atoms with an electrical charge, such as potassium) in the extracellular spaces of the brain and spinal cord, and they may play a role in the blood-brain barrier. In addition, they are involved in several other functions.Unlike most neurons, neuroglia are capable of cell division after both. In fact, they they are often the structural element in slow-growing tumors of the brain. Further, neuroglia do not form synapses. Although they do participate in slow, electrical phenomena, they are not capable of transmitting impulses.
In carrying out their role, the neurons depend on a population of cells called neuroglia. In the CNS, these include macroglia, microglia, and ependymal glia. In the PNS, they include satellite and Schwann cells. The macroglia include the astrocytes and oligodendrocytes. Microglia are the smallest of the neuroglia. They actively proliferate in response to injury within the CNS. During the resting stage, they have small, dark, elongated nuclei and scant cytoplasm. When activated (as in scavenging for foreign bodies or injured tissue), microglia swell up with ingested material. In this condition they are known as "gitter cells."
Schwann Cells are found in the peripheral nervous system and are counterparts of the neuroglial cells of the central nervous system which are astrocytes.Types of glial cells include Schwann cells, named after Theodor Schwann, a german neurophysiologist who showed that animals tissue is composed of cells. The neurons of the peripheral nervous system lie embedded in a sheath of Schwann cells. Two of it's functions are well understood: its role in the formation of myelin and its role during nerve regeneration.
Oligodendrocytes are a glial cell similar to an astrocyte but with fewer protuberances, concerned with the production of myelin in the central nervous system. The oligodendrocytes are responsible for covering axons in myelin. Protoplasmic astrocytes are an important interface between neurons and blood vessels and
myelin sheath. Astrocytes are star-shaped glial cells of the central nervous system. Ependymal cells are thin membranes of glial cells that line the ventricles of the brain and the central canal of the spinal cord. Ependymal glia are the cells that line the cavity of the brain and spinal cord.
Astrocytes are the most common type of glia found in the CNS. They are of two kinds, protoplasmic and fibrous. Protoplasmic astrocytes are found primarily in gray matter (unmyelinated areas, consisting largely of the cell bodies and dendrites of neurons) and are characterized by numerous branching processes. Fibrous astrocytes are found primarily in white matter (bundles of myelinated axons) and exhibit long, thin, relatively unbranched processes. Both types are larger than oligodendrocytes.
Some processes of astrocytes terminate in expansions called "end feet" or attachments. They are applied to the surfaces of blood vessels, to dendrites, and to the pia mater, the innermost covering of the brain and spinal cord. Capillary end feet may be involved in transporting substances between blood vessels and neurons. Dendritic attachments are applied to the parts of dendrites not covered by synapses and may be involved in metabolic support of the neuronal membrane. The pial attachments of the asttrocytes form a functionally protective membrane with the pia mater called the pia-glial membrane.
Oligodendrocytes are smaller than astrocytes and have fewer processes. Satellite are found close to the neuronal cell membranes. Interfascicular are known to form myelin around the axon of neurons of the CNS. The counterpart of the oligodendrocyte in the peripheral nervous system (PNS) is the Schwann cell; the two differ in that the Schwann cell myelinates one fiber at a time, whereas the oligo can myelinate several fibers simultaneously. Occasional oligodendrocytes with perivascular attachements are seen adjacent to capillaries.
The neuroglia of the PNS are the satellite cells surrounding the neuronal cell bodies of the spinal ganglia and the Schwann cells enveloping the axons of the PNS. Satellite cells appear to serve the same nutritive and supportive functions as the macroglia of the CNS. Schwann cells play an important role in nerve regeneration.
The neuron is capable of repair and regeneration of itself with glia as the principle constituent. The neuron and its various parts constitute a highly plastic entity, capable of a good deal of growth and reshaping, especially at the ends of the dendritic tree. But the dendritic spines are thought to the be the most plastic elements of the entire neuron - arising out of the membrane, growing, changing shape and size and probably disappearing, all in response to shiftng patterns of information coming onto the neuron.
A reverse transport mechanism (i.e. one moving material from distal to proximal) has been shown to move material at about half the rate of the proximal-distal mechanism. Thus, there is a form of "conveyor belt" that moves material to and from the cell nucleus. The role that such a system could play in the maintenance of distant portions of the neuron is readily apparent.
Such a two-way conveyer-belt system can also explain how the cell nucleus responds to damage to the distal parts of the axon (cell chromatolysis): it apparently produces increasing amounts of material. In this way, substances returning to the nucleus may provide a biochemical feedback that controls nuclear production of materials needed in the periphery.
Finally, the trophic substances postulated as being secreted from the terminals of nerves in order to affect postsynaptic membranes may be complex molecules synthesized under the genetic control of the nucleus.
While the fast transport rate of 400mm/day does not seem "fast" in ordinary terms, it is fast compared to the previously known rate of movement of materials in axons when a nerve fiber has been cut and the axon regenerates. The rate of growth of the distal axon is 1 to 10 mm/day, when it is regenerating.
The changes seen in a mammalian motor cell after transection of the axon and the subsequent regeneration are shown in the cycle. Note that the portion of the axon distal to the cut degenerates when it is cut off from the nucleus. The cell nucleus then undergoes a process of chromatolysis, during which the axon regenerates. The large volume of material that must be synthesized is indicated by the relatively large volume of the axon relative to that of the cell body.
In mammals, birds, and reptiles, regeneration of axons occurs in only the PNS. No functional regeneration in the CNS of these animals occurs after damage. The reasons are not clear, since regeneration can occur in the CNS of amphibia.