Tuesday, February 4, 2014 CC-BY-NC
Synapse formation

Maintainer: admin

1Synapse formation

  • Target selection : axon recognizes target. independent of activity
  • Address selection : involves major remodeling of the initial coarse distribution of synaptic connection. generally dependent of activity
    • synaptic modifications because there are more synapses than needed

2Visual system

  • retinal ganglion cells project axons to :

    • lateral geniculate nucleus (LGN) : main pre-cortical terminus for input to the visual ortex
    • superior colliculus
    • pretectum
  • LGN

    • each layer receives input from one eye
  • visual cortex

    • ocular dominance columns in the cortex
    • each column receives input from one eye
    • first described by Hubel and Wiesel with electrophysiologicla recordings
      • closing one eye of juvenile cat leads to almost all V1 (one of the columns) responding to only the open eye.
      • if eye not opened during the critial period, the closed eye never regains space.
      • injectoin of tritiated proline in one eye labels different ocular columns
        • caveate: proline might spread out of the axon
  • Larry Katz: examined the formation of ocular dominance using anterograde tracer

    • put in LGN
    • see columns in cortex
    • see segregation occuring before eye activity
    • therefore ocular domiance column doesn't require activity
  • However, still some indication of experience affecting ocular dominance

    • similar blood vessel pattern on retina and visual cortex
    • three-eyed frogs: axons from transplanted eyes project to LGN
    • strabismus : two eyes cannot align, lead to incurable visual impairment in one eye
  • Hebbs rule; neurons assisting in firing the post-synaptic cell should be strenghtened

    • sorting of LGN requires spontaneous activity in retinal ganglion cells
    • proposed mechanism: synchronous firing leads to a larger depolarization of the target cell which leads to increased amount of neurotrophic factor released by the cell
  • visual system:

    • ocular dominance : activity not required but experience during the critical period helps reenforce the connections
    • lateral genicular nucleus: sponataneous activity but not experience is required for sorting eye-specific projections in LGN

3Neuromuscular Junction (NMJ)

  • each muscle is innervated by one single neuron
    • at birth, multiple neurons innervate one muscle
    • later one single axonal innervation for one muscle
  • elimination of synapses at NMK
    • due to competition based on asynchronouse firing
      • at birth, activity is temporally correlated due to prevalent Gap junctional coupling.
      • later, correlated activity between neurons start to disappear
      • imbalance activity leads to elimination of synapses
      • proposed mechanism: at site of receptor activation, a local signal protects the active synaptic site while a destabilization signal affects the surrounding
    • Ach receptor on muscle don't go away as winning axon take over
    • winning neurons: the ones with less connections already (hence stronger transmission at their each connection)