Neurons – Biological Digital Circuits Alex Hodes EECS 713

Neuron Cell body –performs basic living function Axon – 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 received Noise Margin – difference between signal and decision threshold level

Threshold Potential Noise Margin Threshold potential – if reached signal is fired (HI) Around -55mV Varies 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) http://www.ploscompbiol.org/article/info%3Adoi%2F10.1371%2Fjournal.pcbi.1000850#pcbi.1000850-Kempter1

Noise Margin Different Neuron Central Nervous System Noise margin – 23mV Peripheral Nervous System Noise margin – 45mV Cardiac Cells Noise margin – 18mV

Neural Transmission Lines Axons – a signal propagates down an axon to reach other neurons Characteristic impedance of line Axons can be modeled with circuitry Ions – Potassium, Sodium channels describe conductance Signal travels unidirectional Speed of signals important for functions Propagation 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 affects Proportional to distance between traces

Crosstalk Reduction Increasing distance between transmission lines Providing continuous ground plane Using grounded guard traces Shielding

Crosstalk in Axons Transfer 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 Axon Myelin sheath (dielectric layer) Insulates axons, nourishes axon layer etc. Increases signal speed – ‘focuses electrical pulses’ Decreases membrane capacitance Increases electrical resistance

Demyelination of Axon Density of current reduced Signals propagate slower Difficulty sending signals Failure to transmit high frequency signals Important in fine motor skills Timing effects Crosstalk Diseases 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/

References http://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.pdf http://www.stanford.edu/group/dlab/papers/Blumhagen%20Nature%202011.pdf http://jn.physiology.org/content/90/2/924.full.pdf+html http://www.sciencedirect.com/science/article/pii/S0306452201001671 http://ir.library.tohoku.ac.jp/re/bitstream/10097/48000/1/10.1109-16.210207.pdf http://people.eecs.ku.edu/~callen/713/EECS713.htm