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

16. References