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PowerPoint 3

by: Brittany Woody

PowerPoint 3 PSB4504

Brittany Woody

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Inclues lecture notes and PowerPoint information for cellular mechanisms PowerPoint.
Developmental Psychobiology
Dr. Donald J. Stehouwer
Class Notes
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This 9 page Class Notes was uploaded by Brittany Woody on Sunday January 31, 2016. The Class Notes belongs to PSB4504 at University of Florida taught by Dr. Donald J. Stehouwer in Spring 2016. Since its upload, it has received 20 views.

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Date Created: 01/31/16
Wednesday, January 20, 2016 PowerPoint 3 Development of the CNS: Cellular Mechanisms - Axons must find their way to their targets, recognize and respond to midline of the body, recognize like-axons (form bundles), leave pathways, form topographical map, form synapses, match to target sizes - Sciatic nerve is a bundle of nerves in the lower back; as the bundle continues down the legs axons split off from the main trunk - Two classes of chemical factors: may be substrate-bound (membrane bound) or diffusible substances (free in extracellular fluid; form chemical gradients); chemicals can act as both neurotrophins and neurotropins • Neurotropins: guidance cues; lead axon to target • Neurotrophins: nourishing factors; growth and well-being of neuron - Physical, electrical, and temporal variables also play roles in axon guidance and formation of connection • Physical cue: nerves follow blood vessels • Temporal cue: order in which structures grow - Chemical cues are more directly gene products than other variables (easier to study) - Neuroblasts are apolar neurons - Neuron first forms many minor processes, one grows to become axon; no differentiation about destined axon to other processes - If an axon is cut, another minor process (dendrite) will grow to become an axon - Contact guidance: mechanical and substrate bound neurotropins; lend a directional component to the axon - Diffusible substances: AKA chemotaxis or chemotropism; attraction to certain chemicals - Guideposts: lead neurons - Timing: order in which neurons are born and axons grow - Galvanotaxis: electrical guidance 1 Wednesday, January 20, 2016 - Axons in early development do not have terminals; they have growth cones: critical for extension of axons; growth cone leads axon to its destination - Growth cone have filopodia (extentions of growth cone) and lamellopodia (valleys in growth cone); form “fingers” that stick to substrates to pull axon forward to grow - Paul Weiss (1920s) described growing axons; demonstrated axons cannot be pushed forward by cell body, must be pulled forward (by growth cone); he grew neurons in petri dishes and the axons grew along scratches in dish - Filopodia feel for something adhesive (a substrate) to grab to and pull growth cone forward; some substrates are more adhesive for certain neurons - Growth cone may split into two directions (common at the midline) and one will eventually retract to join the other; growth cone selects which direction to go in by gradients, substances secreted, stickiness of substrates, etc - Guidepost: diffusible or bound; highly localized, near target; no chemical gradient - Contact guidance: substrate adhesion - Chemoaffinity (attraction to chemical) and chemorepulsion (repulsion from chemical) - Sticky surfaces preferred are by axons - Growth cone collapses when it reaches its target (first step of neurogenesis) - Rita Levi-Montalcini was the first to hypothesize the existence of neurotropic and neurotrophic factors (1949); dubbed the neurotrophic factor she discovered “nerve growth factor” (NGF) - NGF acts on neural crest cells and central cholinergic cells. For 35 years, the only known neurotrophin/ neurotropin. NGF is now known to be only one of many diffusible tropic and trophic factors. Growth cones may respond either with positive chemotaxis (attraction) or negative chemotaxis (repulsion) - “hemmed in”: repulsion on two sides forcing the neuron to stay between them - NCAMs: attract other neurons, “Nerve Cell Adhesion Molecules”, homophilic (attracted to similar neurons) - Polysialic acid: contact repellent neuron; glycoprotein interferes with adhesion of NCAMs; allows neuron to continuing growing 2 Wednesday, January 20, 2016 - Netrins: diffusible and membrane bound; expressed at midline; can attract or repel on the same axon depending on receptor expressed by growth cone (Attract at DCC receptor; repellent at UNC receptor); from Sanskrit “to guide” - Semaphorins and slits: studied at midline; from Greek “sign-bearer” - Guidepost Theory: there is a series of discrete intermediate targets between neuron cell body and its target tissue; must encounter each target to stay on track; filopodium comes in contact with guidepost cell; guidepost cell also alters expression of genes in the neuron so that different receptors are expressed in preparation for the next guidepost cell - Pioneer neurons: first to traverse a pathway; their axons have a unique ability to “read” guidance cues; other neurons are follower neurons, do not have the ability to read guide cues; if pioneer neuron is destroyed, the others will not find their way to the target cell - Control of decussations at the midline: chemorepulsion: roof plate releases repulsion cues; floor plate releases attractive cues; axons are attracted to the midline - Singular cortex neurons are pioneer neurons for the corpus callosum - Glial wedges and sling cells “hem in” to prevent neurons from crossing the midline - Acallosal: do not have corpus callosum; cells do not cross over midline of the body; acallosal mice were given a glial sling and neurons were able to cross the midline - Slightly fewer than half of retinal neurons cross over midline in humans - Changes in gene expression for netrin and slit receptors mediate behavior of the growth cone at the midline; after a neuron crosses the midline, repulsive receptors are expressed so that it continues to move away from the midline - DCC: attractive receptor for netrin1; “Deleted in Colorectal Cancer”; a tumor- suppressing gene and its product - UNC5: repulsive receptor for netrin1 - Robo: silences DCC attractive receptors while axons are crossing midline; “roundabout” 3 Wednesday, January 20, 2016 - Topographical mapping: • Marcus Jacobson: neuroembryologist; tried to identify fate maps in developing nervous systems; describe topographical mapping and did early experiments; did work on frogs • Roger Sperry: psychologist; did work on mammals - If you rotate a frogs eye 180 degrees (severing optic nerve), the optic nerve will regenerate; with the regenerated eye, the frogs prey catching behavior is inverted; vision is rotated 180 degrees; same results with any degree of rotation - Chemoaffinity hypothesis: there is a matching of chemical signals between eye and optic tectum of frog (analogous to superior colliculus in mammals); thought that axons regrew in new rotated eye back to original myelin sheath so that eye sight would still the same, but the axons grew directly to tectum, inverting their eye sight • found that chemoaffinity works as repulsion; lower number of receptors, more ligands; repelled from the wrong target instead of being attracted to correct target - Frogs gain optic neurons in a circular fashion but tectal neurons are in a crescent shape, but can maintain topographical map • desyanpses: synapses are broken and form new synapses • synapses shift as more cells grow to maintain topographical map human topographical map for hearing shifts as we develop and our head shape • changes - In somatosensory cortex, there is a topographical map of our body; map changes throughout life; can change map within hours through training - Silent synapses: not normally active but present; can be activated if they are needed - RGC Specification: Jacobson rotated the eyes of tadpoles before retinal ganglion were established • behavioral responses were reversed if done after stage 31 in development • behavior was normal if done before stage 28 • if done at stage 30, dorsal-ventral responses were normal but anterior posterior behaviors were revered 4 Wednesday, January 20, 2016 • concluded that anterior-posterior were established between sage 28 and 30; each axis is developed independently and at slightly different time - If an eye in one stage was placed in an embryo of a different frog, it was the stage of development that the eye was in that determined the outcome (not the stage that the new frog was in); eye determines outcome even if placed somewhere else in the frog (did experiments by putting eyes in the frog’s tail) - Galvanotaxis/ Neurobiotaxis: electrical guidance; currents in the embryo can be directly measured to determine rostral-caudal direction - Neurotrophins: nourish neurons apoptosis: “programmed” cell death • • axon retraction: elimination of transient collaterals • synapse elimination: reduction in synaptic arborization - Apoptosis: cell shrinkage, nuclear fragmentation, chromatin condensation, chromosomal DNA fragmentation, involution of the cell • one regulator of apoptosis is concentration of Ca2+ in cell - Necrosis: cell swelling, chromatin digestion, disruption of cell membrane and organelle membranes, DNA hydrolysis, vacuolation of ER, organelle breakdown, lysis, inflammation due to release of cell contents - NGF in TrkA is lost in Alzheimer’s; “nerve growth factor” - TrkC: parasympathetic nervous system - Neurotrophins are synthesized then processed by protease cleavage to generate the mature, processed ligands. All the unprocessed neurotrophins bind with high affinity to the P75 receptor - P75 is death receptor; stimulation initiates apoptosis - One theory: Autophosphorylating: NGF phosphorylates TrkA receptors and a domino effect continues stimulation from growth cone to the cell body - Second theory: Retrograde effector: local signals emanating from Trk receptors in distal axons are themselves propagated to the cell bodies where they promote activation of cell body-associated Trk and consequently cell survival 5 Wednesday, January 20, 2016 - Effect of target size manipulation on neuronal population: can be regulated by trophic factors; more neurons than are needed are present initially; larger target sizes have larger neuronal populations - Axon retraction: selective loss of an axon collateral; requires that the neuron has multiple targets, each of which provides essential trophic factors; loss of any single target is insufficient to cause cell death; the remaining trophic factors from remaining targets is adequate to assure cell survival - Ipsilateral track in spinal cord is withdrawn; contralateral track maintains - Axon retraction is distinguished from withdrawal of processes of axons of growing neurons; axon retraction is restricted to withdrawal after functional synapses have been made; details poorly understood, but as with cell death, is thought to be mediated by neurotrophic interactions between the neuron and its target; electrical mechanisms may also contribute - Transient collaterals may serve functions other than information processing: • ipsilateral corticospinal tract • visuo-spinal projections - Synapse elimination: the rule when a neuron has multiple targets and is in competition with other neurons; the principal function of synapse elimination is refinement of pattern of innervation (fire together wire together); apoptosis results when a neuron is totally (or nearly so) deprived of its target; the principal function of apoptosis is the regulation of neuronal number - Neonatal muscle fibers have many neurons connected to them; they are eliminated so that each muscle fiber is only connected to one neuron; each post-ganglionic cell receives information from only one pre-ganglionic cell after excess connections are eliminated - Inhibitory synapses: increasing Cl- concentration would lower Cl- potential; when Cl- channels are opened, it would become excitatory instead of inhibitory; therefore GABA is excitatory in new cells (inhibitory in older cells) - Activity-dependent ACh receptor clustering: receptors cluster under receptor that received the most ACh - In synapse elimination: • Trophic factor: secreted by target; de-synapsis following axotomy (loss of axon) 6 Wednesday, January 20, 2016 • NO: nitric oxide: retrograde messenger; activates presynaptic 2nd messengers • Presynaptic Ca+ acts with trophic factor - Ca2+ / Caherin: adhesion - Ca2+/ Calmodulin: presynaptic differentiation into mature synapse in the presence of trophic factors - Synaptogenesis: “Ready, set, go” hypothesis 1. Guidance to target; trophic factors, diffusible and bound 2. Synaptic priming: once axon reaches target, secretion of priming factors by target cells and surrounding glia; growth cone collapse, adhesion; “ready” 3. Induction: synaptic specializations by other NCAMs (neural cell adhesion molecules); pre- and post- synaptic; “set” 4. Synapses begin rudimentary function; before maturity; “go” 5. Stabilization/ destabilization: maintaining or not maintaining synapse (based on activity) - Specification: “labeling” of cell’s identity; for example, position of RCGs in retina; Jacobson’s work - Recognition: ligand/receptor interactions; attractive or repulsive; for example, RGC to optic tectum • Miner’s skin rotations in amphibians: rotated skin on tadpoles; flipped dorsal and ventral skin; dorsal skin was white, ventral was green, opposite of normal frog; if you put acetic acid on a frogs skin, frogs are expected to wipe it off; after skin was rotated, the frogs still wiped on the correct side; once frogs grew more, the response switched to wiping the wrong side of the skin; Miner suggested cells become labeled by periphery; sensory neurons that used to innervate belly now innervate back skin and vice versa; periphery alters central connectivity - Induction: axon induces differentiation of target • barrel fields: whiskers of a rodent, cat, dog, etc are organized in a way specific to that species; cortex contains “barrels” for whiskers; if whiskers are removed (cut or immobilized), the respective barrels disappear; barrels depend on stimulation of whiskers; cortical organization is determined by sensory input 7 Wednesday, January 20, 2016 • taste cells: nerve ending grows and causes differentiation of cells; different axons induce different taste cells; sweet vs. salty; axons were severed between CN 7 and salt receptor, and CN 9 and sweet receptors; experiment crossed these axons so that 7 innervated sweet receptors and CN 9 innervated salty; CN 9 induced salt receptors and CN 7 induced sweet receptors; tissue determines type of receptor, not CN; switching cranial nerve connectivity did not change the type of taste bud that resulted • Herbst and Gandry corpuscles: found in beaks of birds; experimenters took beak skin and transplanted it into flank (back) of chick; beak skin was innervated by the spine, and corpuscles formed instead of peripheral cells; location of the tissue did not change the type of cells that resulted - Sensory layers in brain have large input layers and thin output layers; motor layers have thin input areas and large output layers - Garraghty experimented on ferrets because they are still immature when they are born; experiment eliminated some targets for lateral geniculate nucleus; ablated superior colliculus; lateral geniculate nucleus, superior colliculus, and inferior colliculus became smaller; retinal ganglion went to MGN instead of SC; MGN receives information from retina instead of inferior colliculus • MGN reacted to light instead of auditory information in new organization; animals could see with auditory cortex; visual resolution wasn’t as good as normal ferrets, but was still good; orientation columns formed in the auditory cortex; auditory cortex became organized in a way typical of visual cortex, with two dimensions instead of one, like a normal auditory cortex - Timing: first introduced by Paul Weiss as a mechanism to explain orderly innervation of target • innervation of muscles by spinal motoneurons as topographical in limb-innervating segments; if muscles are moved, the nerves will still innervate the first muscle available in orderly fashion • rostral to caudal motoneurons innervate proximal to distal muscles; mapping is maintained even after limb segment substitutions, additions, reversals, or transpositions; chicks with legs and wings switched flapped legs (like wings) and moved wings like legs; spinal cords were reversed in chicks and got the same result (movements were reversed) 8 Wednesday, January 20, 2016 • supernumerary limbs: extra limbs moved near the original limb; show “homotypic” (similar) responses if innervated by lumbar segments; if placed too far from original limb sight, do not “reprogram” thoracic segments 9


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