Microsystems for Neuroscience and Medicine. Lecture 9

Transcription

1 1 Microsystems for Neuroscience and Medicine Lecture 9

2 2 Neural Microsystems Neurons - Structure and behaviour Measuring neural activity Interfacing with neurons Medical applications - DBS, Retinal Implants etc.

3 3 Neuron Dendrite Soma/Cell Body Axon Terminal Node of Ranvier Nucleus Axon Myelination

4 4 Neuron

5 5 Chemical Synapses Synaptic Vesicle Neurotransmitter Voltage-Gated Ca2+ Channel Re-uptake Pump Axon Terminal Post-Synaptic Density Receptor Synaptic Cleft Dendrite

6 6 Activating a Neuron Neuron membrane potential defined by the ionic concentration difference across it Resting potential is typically 70mV, defined by concentrations of K+, Na+ & Cl ions E m = RT F ln PK [K + ] out + P Na [Na + ] out + P Cl [Cl ] in P K [K + ] in + P Na [Na + ] in + P Cl [Cl ] out PX is membrane permeability of ion type X

7 7 Activating a Neuron Receptors receiving synaptic signals cause either excitation or inhibition of neuron Excitation is a positive change in membrane voltage while inhibition is negative Receptors can control the opening and closing of ion channels to achieve this At some threshold the neuron is activated

8 8 Action Potentials

9 9 Action Potentials Membrane Potential (mv) Threshold Time

10 10 Action Potentials Membrane Potential (mv) Na + channels open, Na + enters cell Threshold Excitation Time

11 11 Action Potentials Membrane Potential (mv) K + channels open, K + begins to leave cell Na + channels open, Na + enters cell Threshold Excitation Na + channels close Time

12 12 Action Potentials Membrane Potential (mv) K + channels open, K + begins to leave cell Na + channels open, Na + enters cell Threshold Excitation Na + channels close K + leaves cell Time

13 13 Action Potentials Membrane Potential (mv) K + channels open, K + begins to leave cell Na + channels open, Na + enters cell Threshold Excitation Na + channels close K + leaves cell K + channels close Time

14 14 Action Potentials Membrane Potential (mv) K + channels open, K + begins to leave cell Na + channels open, Na + enters cell Threshold Excitation Na + channels close K + leaves cell K + channels close Excess K + outside cell diffuses away Time

15 15 Real Action Potential

16 16 Real Action Potential

17 17 Action Potential Transport Action potentials travel down the axon in the same way as they are generated Voltage gated Na + channels open, then close and K + channels repolarise the membrane The myelin sheath speeds up propagation by allowing APs to jump from node to node

18 18 Neurons Firing

19 19 Recording Neuron Activity Voltage clamp fixes membrane voltage First use with squid giant axon V in Control Amplifier Diff. Amp. G=Large G(V in -V m) Internal Sensing Electrode G=1 Diff. Amp. V m Discovery of ion channels involved in action potential Current Electrode I m Axoplasm External Potential Sensing Electrode I R m s G=1 Diff. Amp. V=I mrs Current Measurement

20 20 Current Clamp V m (Vp) sensed with voltage follower V in R R + R Summing amplifier stage R + R V in V in +Vp R out Feedback for constant voltage (Vin) across R Therefore current V p + Voltage Follower V p I Current connection Cell is I = Vin/Rout Solution is earthed I

21 21 Patch Clamp Connection to a patch of cell membrane This may have one or more ion channels Suction used to make a high resistance seal Study ion channel current flow

22 22 Patch Clamp Patch clamp on a bacterial spheroplast Whole cell recording involves suction to break cell membrane Direct connection to cytoplasm - destructive

23 23 Patch Clamp Amplifier V2 Change Vin to activate ion channels Measure Vout to observe ion current What are the voltages? + V1 R f V out Diff. Amp. + V in Bath Electrode Cell

24 24 Planar Patch Clamp Micromachined aperture on a planar surface Cells sucked down over hole to make patch clamp Could be used to make arrays and integrate with microfluidic systems

25 25 Planar Patch Clamp Fabrication

26 26 Planar Patch Clamp Fabrication

27 27 Extracellular Measurements Electrodes in proximity to neurons can measure voltage changes due to ion current Low voltages (<1 mv) so it can require sensitive, low noise amplifiers In-vivo the electrodes may be very fine platinum wires insulated up to the tip Microelectrodes can measure single neurons

28 Micro-Electrode 28 Arrays (MEA) MEAs are designed to allow recording and stimulation of neurons in-vitro This can be on both sectioned slices of tissue and dissociated cells Passive systems with over 60 electrodes are common, higher densities are possible Integrated electronics for larger arrays

29 29 Equivalent Circuit Bath Solution Cell Passivation Layer Glass Substrate Metal Electrode Electrical Connection

30 30 Equivalent Circuit REF Bath Solution C FM V M R FM Cell R J C JM V J R JM C R b C C JE FE sh V Output Amplifier pad Passivation Layer Glass Substrate Metal Electrode Electrical Connection R + V out R

31 31 Equivalent Circuit REF C FM V M R FM R J C JM V J R JM C R b C C JE FE sh V Output Amplifier pad V out + V pad V J = C JE C E + C sh A JE A E R R

32 32 MEAs for Tissue Slices Electrodes can stimulate and measure Stimulate AP and measure propagation Similar for dissociated cells in a network

33 33 Issues with MEAs Good contact between cells and electrodes is important - perforated chips? Sectioned tissue may require 3D spiked electrodes to get through damaged cells Integration with silicon electronics allows large arrays but also problems with imaging Growing controlled networks of neurons?

34 Control of Neural Growth on 34 Silicon Research at UoE Fabrication at SMC Patterning of neurons and glial cells Parylene patterns incubated in serum

35 35 In-Vivo Neural Electrodes Micromachined silicon electrodes Known as the Utah Array Used successfully in animal trials 2D electrode array

36 36 In-Vivo Neural Electrodes Alternative to Utah array technology University of Michigan, multi-electrode needles Silicon based with onboard signal processing Stack in 2-3D arrays

37 37 In-Vivo Neural Electrodes Alternative to Utah array technology University of Michigan, multi-electrode needles Silicon based with onboard signal processing Stack in 2-3D arrays

38 38 Deep Brain Stimulation DBS is used in the treatment of Parkinson s Disease Electrical stimulation reduces symptoms of tremor in patients May be applied to other similar diseases

39 39 Visual Prostheses

40 Light Sensitive 40 Neurons PROGRESS a Optical fibre Blue light Cannula b ChR2 K + Channelrhodopsin-2: a light gated ion channel Cranioplastic cement Skull Cortex Specific cells types can be modified to express ChR2 Illumination c Electrical stimulation Na + Depth electrode (1.27 mm diameter) Targeted neuron type Modulation Modulation Intended effect Side effect Currently for research only May be an answer to problems with DBS Optical stimulation Implanted optical fibre (0.2 mm diameter) ChR2 or NpHR Adjacent non-targeted neuron Targeted neuron type expressing ChR2 or NpHR Modulation Unaffected Intended effect No side effect Adjacent non-targeted neuron Figure 2 In vivo optical neuromodulation in animal models of neuropsychiatric disease. a A cannula is implanted into the head of the experimental animal to guide an optical fibre to the targeted brain region. b The optical fibre is coupled to a strong light source (here a 488 nm laser diode 16 ) to bring blue or yellow light into the brain. c Genetic targeting of channelrhodopsin (ChR2) or halorhodopsin (NpHR) into defined classes of disease-model-relevant neurons may allow cell-specific neuro-

41 41 Summary I Neurons are electrically excitable cells capable of communication and computation Methods for interfacing with neurons: Voltage and Current clamp Patch Clamp External Microelectrodes

42 42 Summary II Microsystems have been developed to measure and actuate neurons This can be done in-vitro or in-vivo New technologies may allow this to be done with light.