- Biololgy - most (all) rapid events have electrical component - great for studying
- 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.
- 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
- Detect electric field and changes fluorescence.
- Other potential metric dyes change position in membrane with potential, e.g. mericyanin.
- 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
BME 111 Cardiac