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Unformatted text preview: Lecture 5 30/04/2007 19:04:00 Plasticity-brain changes throughout life (development and experience) Rapid changes during early development (first several years) Number/size of neurons, number of glia, synaptic changes CNS starts to form at two weeks (gestation) Neural plate neural tube CNS At birth, brain <1lb; at 1 yr, 2.2lb; adult=3.1lbs Five steps of neuron development 1. Proliferation (neurogenesis)-Production of neurons and glia from cells lining ventricles o Some of these cells become stem cells and continue dividing o Others begin migrating to permanent home o Human adult brain neurogenesis-olfactory receptor neurons and hippocampus (hpc) 2. Migration-movement of neurons and glia from ventricles to final brain destination o guided by radia glia cell (guide wires) and chemicals trails immunoglobulins and chemokines low levels=impaired migration, smaller brains, smaller axons, mental retardation 3. Differentiation-once in place, grow axons, dendrites o acquire shape appropriate to brain region (e.g., cerebellar versus motor neuron) o molecular factors from neighboring neurons can direct cells to differentiate 4. Myelination-glia produce myelin on axons; occurs over decades 5. Synaptogenesis-formation of synapses; dendrites/axons grow out to make synaptic connections o throughout life but more so when young o prefrontal cortex (PFC) important for responding to stimuli that are no longer visually present PFC develops lots of synapses between 7-12 months of age Babies <9 months will not seek out hidden objects (dont yet have object permanence) How do neurons know specifically where to go? (in slides) Axons must travel great distances across the form the correct connections Sperrys (1954) research with newts indicated that axons follow a chemical trail to reach their appropriate target Growing axons reach their target area by following a gradient of chemicals in which they are attracted by some chemicals and repelled by others Retinal axons match up with neurons in the tectum by following two gradients The protein TOPdv is concentrated in dorsal retinal and the ventral tectum Axons rich in TOPdv attach to tectal neurons that are also rich in that chemical Similarly, a protein directs axons from the posterior retinal to anterior portion of the tectum Sperry=cut optic nerve and invert eye=optic nerve axons grow back to original targets Evidence for chemical guide During development large numbers of neurons die (apoptosis-programmed cell death) Axons that fail to make synapses die...
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- Spring '07