Neurons – Biological Digital Circuits

Alex HodesEECS 713

NeuronCell body –performs

basic living functionAxon – where signal

is transmitted Axon hillock – where

transmission decision is made (action potential)

Dendrites – receive signals and transmit them to cell body

Action Potential (Neuronal Signal)Occurs at axon hillock Sodium, potassium and chlorine concentration differences

create resting potential across neuron (-70mV) Threshold potential (-55mV) – when reached, signal is sent All or none response (digital signal – high or low)

Noise Margin Ability to tolerate noise

High transmitted=high received, low transmitted=

low receivedNoise Margin – difference

between signal and decision threshold level

Threshold Potential Noise MarginThreshold potential – if reached signal is fired

(HI)Around -55mVVaries

Neuron type Frequency of signal Ion gates (Na, K, etc.)

Min and max threshold (in vivo) values define a Noise Margin

Threshold voltages for different voltage gated sodium channels• Main determinant of threshold

potential

Central Nervous System Exhibits• 1.1, 1.2, 1.3, 1.6 • Threshold range ~ (-75, -52)

Peripheral Nervous System Exhibits• 1.7, 1.8, 1.9 • Threshold range~ (-90, -45)

Cardiac Cells Exhibit• 1.4, 1.5• Threshold range~ (-87, -105)

Noise Margin Different NeuronCentral Nervous System

Noise margin – 23mV

Peripheral Nervous SystemNoise margin – 45mV

Cardiac CellsNoise margin – 18mV

Neural Transmission LinesAxons – a signal propagates down an axon to

reach other neuronsCharacteristic impedance of line

Axons can be modeled with circuitryIons – Potassium, Sodium channels describe

conductanceSignal travels unidirectional

Speed of signals important for functionsPropagation delay in axons

Nerve Transmission Line

• V gated ion channels = conductance• Membrane of neuron = capacitance• Difference in ion amounts = voltages

Crosstalk Crosstalk between adjacent transmission

lines can occur Signal induced by current and magnetic field

affectsProportional to distance between traces

2HD1

Kcrosstalk

Crosstalk Reduction Increasing distance between

transmission linesProviding continuous ground planeUsing grounded guard tracesShielding

Crosstalk in AxonsTransfer of ions across axon membrane

transmits signal Ions may escape axon

Signal does not propagate down axon May affect other axons – alter nearby signals Membrane potential will be affected

How to reduce?

Myelination of AxonMyelin sheath (dielectric layer)

Insulates axons, nourishes axon layer etc.Increases signal speed – ‘focuses electrical

pulses’Decreases membrane capacitanceIncreases electrical resistance

Demyelination of Axon Density of current reducedSignals propagate slower

Difficulty sending signalsFailure to transmit high frequency signals

Important in fine motor skillsTiming effects

CrosstalkDiseases such as multiple sclerosis attack

myelin sheath

Why Important?Electrical characteristics of neuronal

networks allows for modeling of biological systems with circuitry

SpiNNaker project – simulation of neural networks in real time

http://apt.cs.man.ac.uk/projects/SpiNNaker/project/

Referenceshttp://

stan.cropsci.uiuc.edu/people/LSY_teaching/Fall2008/bioengin2008/Top/Lit/Peasgood2003.pdf

http://www.researchgate.net/publication/6523904_Transmission-line_model_for_myelinated_nerve_fiber

http://www.pnas.org/content/89/20/9662.full.pdfhttp://

www.stanford.edu/group/dlab/papers/Blumhagen%20Nature%202011.pdf

http://jn.physiology.org/content/90/2/924.full.pdf+htmlhttp://

www.sciencedirect.com/science/article/pii/S0306452201001671http://

ir.library.tohoku.ac.jp/re/bitstream/10097/48000/1/10.1109-16.210207.pdf

http://www.researchgate.net/publication/6523904_Transmission-line_model_for_myelinated_nerve_fiber

http://people.eecs.ku.edu/~callen/713/EECS713.htm