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Neocortex: Consciousness Cerebellum Grey matter (chips) White matter (the wiring: the brain mainly talks to itself) Neocortex: consciousness Cerebellum: unconscious control of posture & movement brains 1. Golgi-stained section of cerebral cortex 2. One of Ramon y Cajal’s faithful drawings showing nerve cell diversity in the brain cajal Neuropil: perhaps 1 km2 of plasma membrane - a molecular reaction substrate for 1024 voltage- and ligand-gated ion channels. light to Glia: 3 further cell types 1. Astrocytes: trophic interface with blood, maintain blood brain barrier, buffer excitotoxic neurotransmitters, support synapses astros Oligodendrocytes: myelin insulation oligos Production persists into adulthood: radiation myelopathy 3. Microglia: resident macrophages of the CNS. Similarities and differences with Langerhans cells, the professional antigen-presenting cells of the skin. 3% of all cells, normally renewed very slowly by division and immigration. Normal Neurosyphilis microglia Most adult neurons are already produced by birth Peak synaptic density by 3 months EMBRYONIC POSTNATAL week: 0 6 12 18 24 30 36 Month: 0 6 12 18 24 30 36 Year: 4 8 12 16 20 24 Cell birth Migration 2* Neurite outgrowth Synaptogenesis Myelination 1* Synapse elimination Modified from various sources inc: Andersen SL Neurosci & Biobehav Rev 2003 Rakic P Nat Rev Neurosci 2002 Bourgeois Acta Pediatr Suppl 422 1997 timeline 1 Synaptogenesis 100% * Rat RTH D BI E A Density of synapses in T PUBERTY primary visual cortex H at different times post- 0% conception. 100% (logarithmic scale) RTH Cat BI D E A T PUBERTY H The density values equivalent 0% to 100% vary between species 100% but in Man the peak value is Macaque 6 3 RTH 350 x10 synapses per mm BI D E PUBERTY A T The peak rate of synapse H formation is at birth in the 0% macaque: extrapolating to 100% the entire cortex, this Man RTH BI amounts to around 800,000 D E synapses formed per sec. A PUBERTY T H 0% (Modified from Bourgeois, JP, 10 days 100 days 2.5 years 25 years Acta Pediatr Suppl. 422: 1997) synapse density Plasticity in the adult cerebral cortex: phantom limbs. Referred phantom hands These phenomena could be explained if cortical afferents (blue) sprout new connections (red to occupy the vacated sensory field, but the timescale is too short. There may be more widespread (but latent) cortical connectivity than we are accustomed to believe. Ramachandran Myelination continues through the 2nd decade in some regions There is evidence that axons cannot myelinate until they form synapses, so the schedule may reflect gradual acquisition of hard-wired status. myelin schedule * 2. The hippocampus: site of transfer of declarative memories from short term into long term storage in the neocortex. Dentate gyrus: 1 of 2 significant sites of neuro genesis in the adult brain Hippocampus Dentate gyrus hippocampus Neurogenesis in the adult rat dentate gyrus 2 million granule neurons in the adult rat About 10,000 new granule neurons produced per day, ie, 0.5% of total. New granule cells differentiate as proper neurons but only survive for about 1m. May compete for survival or have a specific transient role. Rate of division enhanced by environment enrichment, activity & learning. Reduced by stress & steroids and no doubt affected by hormones during puberty. Sources include: Cameron & McKay, J Comp Neur 2001 Gould E, et al, Nature Neuroscience 2001 Shors, TJ et al, Nature 2001 Monje M & Palmer T, Curr Op Neurol 2003 Modified from Cameron & McKay J Comp Neur 2001 rat dentate The mitotic dance in the early neuroepithelium: exponential growth Cell numbers in Man: cerebral cortex 15 billion cerebellum 70 billion other 5 billion In the macaque, all cortical neurons are produced in 2 months between E40 & E100, during which the average rate of production is 3000.sec-1 Cell cycle time ... 7hrs Early spinal cord neurogenesis 1 Later development: linear growth by unequal divisions producing 1 post-mitotic neuroblast (dark blue) and 1 renewed stem cell (light blue) 2 ventricular 1 germinal zone ventricle neurogenesis 2 Vascular niches in the hilus of the dentate gyrus support adult neurogenesis Therapeutic cranial X-irradiation halts the production of neurons, but this is an inflammatory effect. Il-6 made by microglia switches the fate of the dividing stem cells from neurons to glia. NORMAL STRESSED new astrocyte new granule neuron microglia neuroblasts glioblasts stem cell Sources include: Alvarez & Lim Neuron 2004 Kemperman & Neumann Science 2003 niches Childhood development of the CNS At 9 months, the computer rolls off the production line apparently complete. We have a fair understanding of the mechanics of assembly, but very little idea of what happens while the software is installed during the ensuing 24 years. Stages in cognitive development have been defined,but we have almost no mechanistic understanding of what they entail. Science can respond to social priorities. Much of what we know about the production line arose from shock at the consequences for the developing brain of foetal irradiation. We now live in an age worried by problems with software installation in the young. Is there a single cause for their sometimes unwelcome behaviour? Something in the water? Big Macs? EMF? I seriously doubt the latter, but we know so little about what goes on during installation that anything is possible. homily More than 90% of cortical neurons connect to other parts of the cortex: less than 10% connect with othe parts of the brain and the spinal cord I remote excitatory connections II local excitatory collaterals III Pyramidal local inhibitory connections cells IV 75% V Interneurons VI 25% 90% intracortical connections WM 10% connections to other parts of the CNS cell types The adult complement of neurons is in place by birth Peak synaptic density by 3 months EMBRYONIC POSTNATAL week: 0 6 12 18 24 30 36 Month: 0 6 12 18 24 30 36 Year: 4 8 12 16 20 24 Cell birth Migration * Neurite outgrowth Synaptogenesis Myelination * Synapse elimination Modified from various sources inc: Andersen SL Neurosci & Biobehav Rev 2003 Rakic P Nat Rev Neurosci 2002 Bourgeois Acta Pediatr Suppl 422 1997 timeline 1. Pre-GD3 Oligodendrocyte PDGFR development DM20 CNP 2. OPC/O2-A - local embryonic origin PDGFR DM20 CNP - migration and peak + GD-3 proliferation perinatally 3. Premyelinating oligodendrocyte DM20 - susceptible to CNP MBP perinatal stress and GalC irradiation in childhood and in adult 4.Myelinating oligodendrocyte DM20 + PLP CNP MBP oligo 1.
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