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Characteristics of brain glial cells Mike Zuurman Med. Fysiologie Different brain cell types • Glial material was first described by Rudolf Virchow (1846). ”..substance that lies between the proper nervous parts, holds them together and gives the whole its form. ..its differences to other connective tissue has induced me to give it a new name, that of neuro glia.” • In 1891 the cellular structure of the brain was recognized: H.W.G von Waldeyer`s created the term neurons and C. Golgi discoverd and described astrocytes and suggested a nutritive role of neuroglia • In 1921 R.del Hortega distinguished 2 cells in brain which he named microglia and oligodendrocytes • C.L Schleich (1894) suggested active and dynamic roles for glial cells in the whole specturm of brain functions Glial cell markers Cell type Marker Oligodendrocytes Myelin basic protein (MBP) Galactocerebroside (GC) Microglia ED-1, OX-42, F40/80, MAC-1 Astrocytes Glial fibrillary acidic protein (GFAP), Glutamate astrocytic transporter (GLAST) Developmental origin of glial cells neuroectoderm Neuroectoderm Mesoderm Bone Marrow Neuroepithelial stem cells from developing ventricular zones ? Microglia Oligodendrocytes Astrocytes Neurons There are 10 times as much astrocytes in human brain as there are neurons 50% of the brain mass There are as much microglia or oligodendrocytes in human brain as neurons Glial cell lineages glial cells glial progenitor cells neuronal progenitor cells ? neurons stem cells neuronal ventricular zone oligodendrocytes O2A lineage O2A progenitor cell RAN-2 A2B5 + + growth factors and ECM GFAP + A2B5 + ? T1A lineage type 1 astrocyte T1A precursor RAN-2 + ? A2B5 ? type 2 astrocyte GC + GFAP A2B5 - radial glial cells GFAP + RAN-2 + A2B5 - ? neurons GFAP - A2B5 GFAP GC radial glial cells in the developing CNS survace of the developing cortex ventricular zone time Radial glia: multi-purpose cells for CNS development T1A lineage type 1 astrocyte T1A precursor ? radial glial cells ? neurons mitosis + Functions of oligodendrocytes in the brain • • synthesize, assemble and maintain myelin form myelin sheath which are wrapped around axons Oligodendrocytes and formation of myelin origin of microglial cells Invasion of the brain by monocytes and development of microglia ED 18 Invasion by monocytes birth PD3-4 PD 5 PD 22 Formation of the BBB Beginn of ramification End of ramification Functions of microglia in the brain resting (ramified) microglia fully activated (phagocytic) microglia intermediate states • • • Major immuncompetent cell of the brain MHCII expression and antigen presentation beneficial (production of neurotrophins) and detrimental (synaptic stripping, phagocytose) effects on neurons different types of astrocytes type 2 astrocytes GFAP + A2B5 - type 1 astrocytes GFAP + A2B5 + Functions of astrocytes in the brain • • • • • • • • • • formation of growth tracts to guide the migration of neurons during early devlopment production of trophic factors for neurons before they make connections with postsynaptic cells participate in the immune response of the brain scar tissue formation following neuronal loss storage of glycogen as an energy reserve in the brain uptake and release of neuroactive compounds buffering of the extracellular ion homeostasis (spatial buffering of K+ ions) participate in the formation of the blood brain barrier metabolic coupling between astrocytes and neurons neuron-glia and glia-neuron communication calcium: metabolism and its role in neuronal death Mechanism of excitotoxicity Mg++ Na+ Na+ Non-NMDA NMDA K+, Ca++ K+, Ca++ Ca++ Neuronal death Metabolic coupling between astrocytes and neurons from Tsacopoulos and Magistretti (1996) Termination of glutamate signaling and a subsequent activation of glycolysis in cases of energy failiure (ischemia) astrocytes are the main source for the high extracellular glutamate concentrations Glutamate and intracellular calcium signaling: the basis for glia-glia, neurone-glia and glia-neurone communication? • • • • glutamate is the major exitatory neurotransmitter in brain ionotropic receptors and metabotropic receptors, which are expressed by neurons and astrocytes stimulation of glutamate receptors may induce calcium signaling over stimulation with glutamate leads to neuronal death, glutamate induced neurotoxicity is the major damage in ischemia • • • • calcium is important intracellular messenger and has a vast array of different functions in the cell calcium signals can be distinguished in singel calcium spikes and calcium waves calcium waves can occur intracellular as well as intercellular and they can occur in nearly all cells the function of calcium waves are still unkown Features of astrocytes in the brain • • • • • Allmost all receptors found on neurones were expressed by astrocytes as well (neurotransmitters, neuropeptides, growth factors, cytokines…) they express a great variety of ion channels (voltage gated and transmitter gated) transport systems for ions, neurotransmitters……. formation of “networks” via gap junctions size of astrocytic networks is brain region dependent Calcium signaling in cultured astrocytes Calcium signals in astrocytes are (partly) stimulus specific, reproducible, complex in terms of strengh, velocity (10-30 um/s) and spacial aspects Calcium signals observed in various brain cell preparations stimulation of neurones causes calcium signals in astrocytes in culture, (1992) in vivo like systems (1996) Calcium waves spread through astrocytes in culture, (1990) in vivo like systems (1995) Questions? what is the neuron-astrocyte signal? how does the calcium signal spread through the astrocyte network? what is the astrocyte-neurone signal? what does it all mean, are there physiological effects, is this real communication between neurones and astrocytes? Astrocytic calcium signaling induces calcium signals in neurones (1994) Neuron-astrocyte signaling Hippocampal slices neurons in Schaffer collaterals astrocytic intracellular calcium in stratum radiatum low frequency no effect high frequency Calcium transients in astrocytes Transmitter: glutamate Astrocytic calcium waves are dependent on neuronal activity Porter and McCarthy, 1996; Pasti et al., 1997 Astrocyte-astrocyte signaling Astrocyte-astrocyte signaling ATP Cotrina et al., 1998, 2000 Synaptic neuronal activity controls the development of astrocytic networks (Rouach et al., 2000) astrocyte - neuron signaling Glutamate - Parpura et al., 1994 and several other papers Gap-junctions - Nedergaard, 1994 - gap-junctions between neurons and astrocytes in culture (Froes et al., 1999) - gap-junctions between neurons and astrocytes in locus ceruleus (Alvarez-Maubecin et al., 2000) Astrocytes may control synaptic plasticity Araque et al., - astrocytic glutamate modulates the magnitude of action potential-evoked transmitter release (1998a) - as well as the the probability of transmitter release in unstimulated synapses (1998b) - astrocytes mediate potentiation of inhibitory synaptic transmission in hippocampal slices (Kang et al., 1998) - astrocytic modulation of neuronal activity in the retina (Newman and Zahs, 1998) Astrocytes, integrators of synaptic activity? Two principles of signal transmission in brain: Wiring against Volume transmission (Zoli and Agnati, 1985) • Single “transmission channel” made by cellular structures and with a region of discontinuity not larger than a synaptic cleft • Diffusion of a cell source of chemical signals in the extracellular fluid for a distance larger than the synaptic cleft • Hardware for WT neurons and astrocytes, synapses and calcium signaling • NO, dopamine, adenosine neuropeptides are known to diffuse for more than 1mm in brain neuromodulators differ from neurotransmitters in that their effects are generally more global and longer lasting than the effects of the latter (F. Bloom, 1988) Hardware for VT extracellular fluid in the interstitial volume fraction (20% of the brain volume) • • Swelling of astrocytes may influence “volume wiring” in the brain open synapse closed synapse This has not been shown so far, it is a theory. But it is pretty clear that astrocytic volume changes occur in invivo like preparations and that the swelling and shrinkage of astrocytes is controlled by intracellular calcium signaling neuron : astrocyte ratio leech 10:1 rodents 1:1 human 1:10 Conclusions • • • • • • • Like neurons are glial cells derived from ventricular zone stem cells oligodendrocytes built up the myelin sheath around axons miroglia are the major immunocompetent cell of the CNS astrocytes are involved in CNS pattern formation astrocytes and neurons are metabolically coupled glutamate and intra- inter-cellular calcium signals between astrocytes and neurons are capable of modulating synaptic activity “….one now begins to feel more comfortable with the concept that the majority of cells in the CNS are no longer “passive partners” to neurons” A. Vernadakis (1996)