Lecture 13 Target Selection 03 18 2015 Chapter 6 Nerve bundles might be going right past a particular target but some axons defasiculate from bundle and go towards that target Once they go through target they need to know what to innervate and what not to go into need mechanisms to stay where they are supposed to be Barrier of some kind secreted signals or difference in extracellular matrix to keep axons within their target Next need to stay within specific region of their target molecular coordinate lays down topography of target tissue and matches to particular axon to keep in the right A P or D V area Then particular subset of neurons or muscle fibers found in particular layer So they need to synapse onto their particular layers Within the layers specificity of connecting to certain subset of cells Need to synapse on particular subset of target cells in particular layer Axons don t nee dto be depolarizing back to neural cellbody nucleus axon will still find its way if cut off through local cues After local cues once target is reached forming a synapse Support that tells it that its synapsing the right cell and therefore it won t die off or regress back Sema repellant stops things from responding see video Defasciculation axon or set of axons exiting from a larger bundle of axons SEE SLIDE Different things can be used together or individually to cause defasciculation Long range attraction can cause axons to enter target Neurotrophins cause sympathetic axons to innervate ear or mouse ear is secreting neurotrophins which recruits axons to enter that target tissue If you knockout NT 3 then tey won t innervate If you knockout NT 3 but then inject NT 3 you can rescue the phenotype and they will innervate correct target This is a positive signals see slide for neurotrophins they tell the axons to come innervate the tissue Can rescue phenotypes oftentimes by using a different neurotrophin so they aren t entirely specific See slide for receptors Growing along FGF pathway grows kind of fast Optic tectum no FGF cue anymore so it hits the brakes and stalls Once axons hit their target their growth slows shown in A Bottom panels retinal axons finding way to optic tectum then they start branching off Lack of FGF at tectum slows down their growth Experimentally express FGF where it should be low Axons don t stop and they fly out past their target site Exit eye go through optic chiasm where slit shh is guiding them then Ephrin B decides if contralateral or ipsilateral Once contralateral start growing along path of FGF inhibited by sema3A Lack of FGF says it s reached its target LOSS of a signal can also help guide to target Why does axon start branching when it doesn t have its positive signal anymore Once it reaches target no more FGF telling it to grow growth cone starts to collapse a little and there is disorganization of actin and everything So growth cone pauses and branch comes off then Growth cone stops growing then it starts to grow again and where it stopped is where branch comes off Inhbitory signals OR loss of positive signals at target collapses growth cone which induces the branching How to keep withing target site and away from nearby tissues Find target defasciculate find general tissue and need signals to make borders Keep handplate area where bones are formed from axons Brown part is semaphoring staining Don t want neurons to go past their to screw up innervation differentiation when knockout right they go where they shouldn t grow too early This is one example of having a boundary for ex keep motor neurons from innervating skin but let sensory neurons in Nasal and temporal regions of eye know how to connect it to the brain All laid down by topographical maps Position of retina in eye is laid down matching to particular region in optic tectum Ears cochlear cells don t have so much spatial cue but they have high sensitivity to frequencies which each map to particular region in tectum Langley s identification of the superior cervical ganglion map Topographical map of SCG Regeneration showed that nerves can re find their target neuron targets Spinal cord on left with sympathetic ganglia on right part of peripheral nervous system Axons from SC iinnverate within sympathetic ganglia that go into target tissue By activating nerve bundles exiting thorsassic spinal cord particular response Activate T1 electrical stimulation very specific response takes place of dilation of pupil Activate T4 response of constriction of blood vessels in ear Rostral to caudal axis map of what things synapse upon to give rise to behavior of animal Activating different nerves bundles showed there is topographical map Experimentally it was very clear Peripheral nervous sstem regenerates pretty well Cut sympathetic nerve branch so T1 and T4 can t reach target in ganglia axons still going off to eye and ear though Axons from spinal cord regenerated and found proper targets so they found their way to proper nerves that innervate the eye and showed that beyond development there is some chemical talk between axon and target to make sure they go to the right place and stay there Not even a spatial property Can move ganglia around and axon still finds its way to proper target 6 7 8 map onto T5 Took T5 and put where SCG normall is they proper axons found their way al the way up and innervated proper target Some map laid down along rostral caudal axis that once you lay it down it isn t changed There s some matching between them chemical cues are laid down Amphibians very resilient to surgery Rotated dorsal ventral axis so eye was backwards If fly was dorsal ventral part is activated Dorsal innervates ventral portion of optic tectum When you flip the eye around chemical specificity still matches it to correct target so fly goes in wrong direction Chemoaffinity hypothesis laying down topographical map in latitudinal and longitudinal axes would guide axons to innervate proper targets Took decades and decades to identify molecules to explain this hypothesis What finally identified these molecules was using in vitro assays Stripe assay axons grow along stipe that contains molecules they like and WON T grow in stripe of inhibitory molecule Made stripes of anterior and posterior membranes Outside of the ey and inside of the eye How do they map along these regions Temporal towards outside these axons prefer anterior region Nasal regions don t have specificity of A P Heated to a point where proteins degraded so properties