Mkgrady8 Neuro 1 Glia

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glial cells: number of them compared with number of neurons, size compared with neurons, what do they look like compared with neurons, can they divide compared with neurons, what do they do

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many more (3-10x) glia than neurons in the brain; smaller than neurons but make up half of the bulk; also possess numerous processes but do not propagate action potentials; mature glia retain ability to divide so an important clinical consequence is that most NS tumors are glial in origin such as malignant astrocytomas; perform many support functions including myelin sheath formation, structural support, aid in repair and regeneration, maintaining environment around synapses, may act to insulate neurons from eath other and become phagocytic after injury; as there is no connective tissue within the CNS (aside from the thin coatings of blood vessels) the areas around neurons, consisting of neuronal cell processes and glial cells, supply a supportive function and this area is called the NEUROPIL

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what are the names of the 3 main types of glial cells in the CNA

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astrocytes (many radiating processes), oligodendrocytes (fewer radiating processes), and microglial cells (a lot smaller); the other type of glial cell is ependymal cells

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astrocytes: what do they look like, what are the 2 types, what intermediate filament protein do they have, how are they linked together

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small, star shaped glial cells with many processes; there are 2 named types= protoplasmic (gray matter) and fibrous (white matter); both types possess a particular form of intermediate filament protein= glial fibrillary acidic protein (GFAP); antibody staining for GFAP is considered to be diagnostic for this cell type; most astrocytes are linked together via gap junctions to form a functional network

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astrocytic end feet

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are terminal expansions of astrocytic processes that delineate boundaries with blood vessels and pia mater but also cover parts of neurons; end feet support the blood brain barrier and are also closely associated with axons at synapses; tight junctions can keep even small molecules from crossing its barrier; form a tough layer underneath the pia mater to form the 'glial limitans'; cover various parts of neurons, including synapses and Nodes of Ranvier, to provide both structural and metabolic support; DO NOT form part of the blood brain barrier but may influence its stability and maintenance

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oligodendrocytes: form compared to astrocytes, 2 types, main function

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smaller cell bodies than astrocytes, fewer and thinner processes, small dark nuclei; 2 types= interfascicular and satellite; main function= wrapping axons with myelin in the CNS only

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oligodendrocytes: the 2 main proteins involved, how does it do it, what does this form

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oligodendrocytes may wrap segments of numerous axons tightly with multiple layers of membrane; in the CNS 2 main proteins are involved in the process= myelin basic protein (MBP) and proteolipid protein (PLP); MBP function remains unclear; PLP extracellular domains bind to similar domains of abutting membranes helping to compact the myelin layers; IMPORTANT CONCEPT= oligodendrocytes srap only small portions of each axon it is associated with forming 'internodes'; multiple oligo's are needed to fully myelinate an axon; the spaces between internodes are called NODES OF RANVIER

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why is there myelination?

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INCREASED CONDUCTION VELOCITY AS MEMBRANE DEPOLARIZATION BECOMES SALTATORY; 1. myelin lipids insulate the axolemma (plasma membrane) from outward leakage of ions helping to maintain the strength of the depolarizing current; 2. voltage gated sodium channels (whose activities are a slow component of A.P. propagation) are found only at the nodes of ranvier; 3. together and along with increased diameters of high velocity axons, the speed of propagation is greatly increased over unmyelinated axons

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summary of oligodendrocyte structure and function

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they are smallish cells with processes that wrap around the axons; 2 types= interfascicular and satellite (the function of satellite oligo's is unclear); myelination is extremely important for increasing nerve signal conduction speed; loss of myelin results in severe neurological deficits (examples include multiple sclerosis (immune mediated), vitamin B12 deficiency (metabolic), and virus mediated loss)

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microglia: form compared to oligodendrocytes, part of what system, derived from what, what do they do

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even smaller than oligodendrocyte cell bodies; can't distinguish from other glial cell types without the use of a special marker; part of the mononuclear phagocyte system; derived originally from the yolk sac; usually quiescent but can be activated by injury or disease; when activated microglial cells= proliferate, secrete cytokines, act as APCs, increase phagocytic activity

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ependymal cells: where are they, specialized type does what, morphology, communicate via what, what do they do

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line ventricles and the central canal of the spinal cord; specialized ependyma (choroid plexus) make cerebrospinal fluid (CSF); other specialized type are the tanycytes; they are ciliated and cuboidal to low columnar in morphology; they communicate via gap junctions; they line the ventricles of the CNS and their cilia beat in a coordinated way to aid the flow of cerebrospinal fluid (CSF)

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tanycytes: where are they found, where do their processes go, what do they do

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found mostly in the 3rd ventricle; have processes that extend into the hypothalamus; may function to provide feedback as to the composition of the CSF; probably won't test us on these cells

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choroid plexus cells: where are they found, what do they do

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exist as a neuroepithelial layer over a vascular core; located in the ventricular system; are specialized to secrete CSF

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schwann cells: what are they, what are they derived from, what do they do, compare them to oligodendrocytes

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the PNS equivalent of oligodendrocytes; derived from neural crest cells; provide myelin sheaths for axons of the PNS; use MYELIN PROTEIN ZERO instead of PLP; in contrast with oligodendrocytes each schwann cell invests only one internode of one axon in the PNS; therefore each axon will be wrapped by many Schwann cells; in addition they also wrap a single layer of membrane around unmyelinated axons (this is something that oligodendrocytes so not do)

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the single wrap layer of membrane around unmyelinated axons: what does that look like, can you visualize them, what do these unmyelinated nerves do; ***THIS KIND OF SLIDE MAKES FOR GOOD EXAM MATERIAL

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unmyelinated nerves 'sit' in an indented pocket of schwann cell but there is no wrapping of glial cell membrane; unmyelinated nerves are not routinely visible with light microscopy as their diameter is below the resolution of LM; unmyelinated nerves transmit action potentials more slowly than do myelinated axons

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satellite cells: what are they, where are they, what are they derived from, what do they do

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satellite cells are modified schwann cells that are found in peripheral ganglia; they are also derived from neural crest cells; they either surround (in sensory ganglia like DRGs) or incompletely surround (in sympathetic ganglia) the nerve somas; in DRGs satellite cells effectively inhibit any axons from synapsing on the DRG soma while in sympathetic ganglia some synapses are observed; they may also provide both structural and metabolic support for the neurons

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clinical issues: PNS demyelinating conditions

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guillan-barre disease (immune mediated)= usually not fatal, pts often recover, often follows an infection; X linked charcot marie tooth disease (inherited)= connexin 32 mutated, myelin at nodes of ranvier affected; vit B12 deficiency (metabolic)

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glia summary: astroytes

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star shaped glial cells with many processes; astrocytes possess intermediate filaments composed of glial fibrilary acidic protein (GFAP) which is used as a diagnostic marker for these cells; multiple functions of astrocytes include support of neurons, isolation of neurons, regulation of the tight junctions that exist between brain endothelial cells (which forms the basis of the blood brain barrier)

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glia summary: oligodendrocytes

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glial cells with few processes, provide myelin in CNS; one oligo provides myelin segments to multiple axons

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glia summary: ependyma

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line ventricles; specialized ependyma (choroid plexus) make cerebrospinal fluid (CSF); tanycytes may provide feedback to the hypothalamus regarding CSF make up

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glia summary: microglia

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not true nervous system derivatives; are resident phagocytes derived from hematopoietic system; function in immune surveillance

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glia summary: schwann cells

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myelinate peripheral axons, also invest unmyelinated axons

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glia summary: satellite cells

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associated with peripheral ganglia, surround the somas