Neurons – Biological Digital Circuits

1. Neurons – Biological Digital Circuits Alex Hodes EECS 713

2. 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

3. 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)

4. Noise Margin • Ability to tolerate noise • High transmitted=high received, low transmitted= low received • Noise Margin – difference between signal and decision threshold level

5. 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

6. 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)

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

8. 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

9. Nerve Transmission Line • V gated ion channels = conductance • Membrane of neuron = capacitance • Difference in ion amounts = voltages

10. Crosstalk • Crosstalk between adjacent transmission lines can occur • Signal induced by current and magnetic field affects • Proportional to distance between traces

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

12. 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?

13. 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

14. 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

15. 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/

16. 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://www.researchgate.net/publication/6523904_Transmission-line_model_for_myelinated_nerve_fiber • http://people.eecs.ku.edu/~callen/713/EECS713.htm