Neuronal Measurement (Lectures 20-22)

What did Richard Caton do?

showed electrical activity in animal brain

What did Hans Berger do?

- invent EEG

- showed electrical activity in a human brain

EEG Waves

Delta:

-0.5Hz-4Hz

- stage 3 NREM sleep

Theta:

- 4-7Hz

- learning/memory

Alpha

- 8-12Hz

- inhibition/fatigue

Beta

-12-30Hz

- attention

Gamma

-30-80Hz

- cognitive activity/sensory

Cortical Geometry

- pyramidal neurons, cell bodies look like pyramids

- source of EEG signal

Microelectrodes

- 50μm

- local field potentials (postsynaptic potentials)

Macroelectrodes

-1.27mm

- deep brain stimulation

Electrocorticography (EcoG)

- 2.3mm

- surgical mapping

Electroencephalography (EEG)

-7-10mm

- brain surface

Stereo-EEG (sEEG)

- 1mm

- precisely locates and record electrical activity for diagnostic and treatement purposes

Volume conduction

propagation of ionic current through brain from source to detection due to passive diffusion

Primary Current

- PSP in dendrites of pyramidal neurons

Secondary Current

- volume-conducted current (passive ion movement due to electric field propagation)

Is resistivity higher in transverse or longitudinal spine?

Tramsverse because cell membrane blocks transmission

Longitudinal is more conductive

Relationship between bone and brain with respect to conductivity

Bone (skull) is 1000x less conductive than the brain

Oxidation Reaction

- atom loses an electron

- dominant from electrode to electrolyte

- stimulating body through electrodes

Reduction Reaction

- atom gains an electron

- dominant from electrolyte to electrode

- recording measurements

Half-Cell Potential

- potential difference between eletrode/electrolyte interface (at intersection point and when current becomes 0)

- want low half cell potential due to less resistance/less energy. Especially for brain signals since brain signals are so small, you want lower thermal resistance

Equilibrium Half-Cell Potential

No net current flow

Polarization

- accumulation of charge on the surface of an electrode

Overpotential

difference between observed H.C.P. (flow current) and equilibrium half cell potential (no flow current)

Ohmic Overpotential

voltage drop across electrolyte due to resistivity

Concentration Overpotential

difference in H.C.P due to movement of ions at interface

Activation Overpotential

difference in activation energy compared to equilibrium condition

Polarizable

- buildup of + and - charge at interface

- perfect = no current flow = capacitor

- good for stimulation

Non-polarizable

-current flows with little resistance

- good for small signals/recording

Best biopotential electrode and why

Ag-Ag-Cl

- nonpolarizable (good for small signals from body)

- nontoxic

- low half cell potential

What happens to impedance at high frequencies and low frequencies?

High frequency (apporaching infinity), w approaches infinity, impedance is 0 ( acts like a wire/short circuit)

Low frequency (approaching 0), w approaches 0, impedance approaches infinity (open circuit)

What is the major contributor to skin impedance?

Stratum Corneum

Motion Artifacts

- movement of an electrode wrt an electrolyte

-effects polarizable electrodes mainly at low frequencies

Electrode vs Electrolyte

electrode = slightly negative

electrolyte = slightly positive

Why do we want a low half-cell potential?

Less resistance/energy required for current to flow