• Biololgy - most (all) rapid events have electrical component - great for studying signaling
  • Extracellular, historical -
    • wide area
    • small signals shunted by conductive volume of salty fluid (cranium, abdomen, etc. )
    • e.g. eeg, ecg, emg.  Global mechanism, very useful.  
    • Can record single action potentials, with careful discetion even small c-fibres.
  • Intracellular
    • record AP with good fidelity, see absolute potential of AP, epsp, ipsp. 
    • Excellent amplitude and temporal resolution, but can only look at very small number of cells at once. -
    • Invasive, loss of R.M.P. leak conductance, or cell attached patch clamp intracellular soluble factor diluted out of cell. e.g. Ca currents don't last long. 
    • Even with this resolution, it took chemical techniques to show that axon hillock does not exists in neurons.  Was a nice concept!
  • Nervous system more than single synapse. e.g. 1:1 N.M.J.
    • Most cells have many inputs, integrated over cell surface and over time. 
      • Look at integration of synaptic inputs over cell surface. 
      • Axons branch.  Divergence easy but convergence may underlay processing. 
    • Our interest in sensory encoding. 
    • Mechanosensors converge, integration of inputs from individual end organs. 
      • E.g. muscle spindle convergence of ~6 sensory endings, with no AP. 
      • Where is summation going on? 
      • Is there an analogue of the axon hillock. 
      • Are there pacemaker sites and if so where are they located. 
    • PC where is action potential initiated?
  • Optical techniques allow localization of electrical activity over surface of cell and over time. -
    • dyes not mechnically invasive like microelectrodes -
    • see ensemble activity say on surface of cortex - along axons - sample over larger population of cells.  
    • Test more cells, better statisitics, look at greater set of different cell types, e.g. cultured DRGs, look at small, medium & large  cells test with drugs. 
    • Great for screening, drug co. would like this!
  • Use fluorophores and epifloresecnce or ratio imaging. e.g. 8-di-ANEPPS
  • Weakly fluorescent in water amd high lipid / water partitian coeff.  Lipophilic too!
  • Detect electric field and changes fluorescence.
  • Other potential metric dyes change position in membrane with potential, e.g. mericyanin.   Slow.
  • fast < 1 ms. Spectra of di-8-ANEPPS in lipid.  Potentometric changes in F
  •  

• Optical detectors
  • Camera - slow temporal resolution at standard vidoe rates.
  • CCD - specialized
    • better temporal resolution but poor dynamic range and poor efficiency.  
    • Wells not full area of chip,
    • interline charge transfers produces switching noise slows down sampling time. 
    • Charge wells can only hold so much charge, which limits dynamic range. 
    • Bad with high illumination where changes in fluorescence is small.  
    • Single photodiodes good except for no spacial resolution. 
  • Solution - photo diode coupled with fiber optic WU array
    • good area sampling with little space betweens fibres.  
    • Couple fibres via short length to photodiodes. 
    • High quantum efficiency.  
    • Large dynamic range, no charge wells.
    • Avoid switching transients by having separate amplifier for each photodiode
  • How a photodiode works
    • photo -> hole pairs migrate to anode - gives current
    • I-V diode use voltage clamp so no capacitance giving faster response.
    • Good noise and dynamic range
  • The circuit - charging circuit over come lamp on transients (like patch amp!)
  • 2nd stage amp, electronically switch eveything since have hundreds of channels.   Can not make good switch bank with hundreds of contacts.  Switch gain, time constant
  • Filter sections.  Butterworth for best amplitude response.  Can not have hundred bank variable resisitors.  Solution - comutative filter system.   Switched mode - chop to capacitor with 50% charging time is twice as long. 

turtle cortex traces

Potential changes over cortex

Individual traces

Snail

BME 111  Cardiac