Biomedical Engineering Electrophysiology

Biomedical Engineering Electrophysiology Dr. rer. nat. Andreas Neubauer Sources of biological potentials and how to record them 1. How are signals transmitted along nerves? Transmit velocity Direction Intensity Frequency 2. How can measurements be standardized? Electrode position Data visualization Andreas Neubauer I Slide 2 I 18.11.2014 1

The nervous system Andreas Neubauer I Slide 3 I 18.11.2014 Organization of the nervous system brain, nerves and muscles are the major components of the nervous system sensory/afferent nerves deliver information to the brain information is passed along motor/efferent nerves by the brain the nervous system is highly parallel synapses allow reflex loops via the spinal column can be suppressed by the brain Synapses Andreas Neubauer I Slide 4 I 18.11.2014 2

Neurons basic concept of nerves dendrites can be considered as the means of information input axons are the channels for output information cell bodies may be considered to be located in the brain/spinal cord axons supply muscles or carry information to the brain http://www.biotele.com/research.htm Andreas Neubauer I Slide 5 I 18.11.2014 Neural communication I electrical signals in the body are constant in amplitude and vary in frequency pain intensity is regulated by the frequency of the signals normal frequency 1 (pulse per second) relation of frequency and intensity is approx. logarithmic log Andreas Neubauer I Slide 6 I 18.11.2014 3

Neural communication II Example: Dynamic range of the ear: min 10 /1 120 The eye is sensitive to a similarly wide range of intensities Assume a linear relationship: Maximum transmission frequency: 100 pps min. sensory input would correspond to 10 impractical! with a logarithmic scale a dynamic range of 10 /1 is compressed to 25/1 recognition of different amplitudes is much worse Andreas Neubauer I Slide 7 I 18.11.2014 Why is smooth movement possible? increasing contraction is achieved be an increase in frequency not all muscle fibers twitch simultaneously Andreas Neubauer I Slide 8 I 18.11.2014 4

The Nernst equation consider a reservoir with de-ionized water add a volume with saline solution () enclosed by a semipermeabel (for ) membrane diffusion will go on until equilibrium is established diffusion gradient electrostatic force Nernst equation:! log & ' " # $ % )* & log ' & ( " +, - # & ( valid at room temperature.: Gas constant; /: Temperature; 0: Faraday constant; 1 2 : Valence transmembrane potential with respect to the outside of the membrane Andreas Neubauer I Slide 9 I 18.11.2014 Transmembrane potential 4 ions can hardly diffuse through the membrane when the cell is in resting state generation of a nerve action potential leads to 4 influx normally negative when the nerve is in resting state Ion Intracellular concentration (56) Extracellular concentration (56) Nernst potential inside wrt outside (57) 8 4 400 20 :75 4 50 450 55 40 550 :66 Andreas Neubauer I Slide 10 I 18.11.2014 5

Membranes and nerve conduction electrical impulses can travel along the nerve with a velocity of 50 -/ high/low intracellular potassium/sodium concentration is established by the membrane polarization i.e. resting potential = 4 >? 4 stimulation leads to an efflux/influx of potassium/sodium change in transmembrane potential avalanche effect DEPOLARIZATION! Andreas Neubauer I Slide 11 I 18.11.2014 Transmission of Nerve Action Potentials (NAPs) I impulse of depolarization which travels along a nerve muscle fibers can also transmit action potentials (MAPs) ionic currents will flow from depolarized to polarized parts source of bioelectric signals! myelinated fibers transmit APs 10 times faster than non-myelinated fibers Andreas Neubauer I Slide 12 I 18.11.2014 6

Transmission of NAPs II speed of transmission depends on: Membrane capacitance Myelin Axon resistance assume a cylindrical membrane with diameter @ and length A:. BC DEF; G: resistivity HΩmK LMDL; L: dielectric constant of neural membrane RS BV WDE. GL DWF ; time constant of the membrane HK DE F X typical values: Membrane capacitance: 1 YZ [UF,@10]-,A 10 --,^1 Ω- 3 10 ` ]F,.1.3 10 * Ω, time constant 0.4. [c22%d 2.14 Ω TU Andreas Neubauer I Slide 13 I 18.11.2014 Muscle Action Potentials (MAPs) Smooth muscle intestines and blood vessels intrinsically active Striated muscle skeletal muscle voluntarily active Andreas Neubauer I Slide 14 I 18.11.2014 7

Volume conductor effects I electrical potential: Φ + Df gh i d assumptions: Potential at infinity equal zero Tissue is homogeneous j k g cylindrical nerve fiber: l B d Dd Fj Bh Dd j Bm nop Dq contribution made to the potential field at rs,u,v j s s Φ s,u,v g x u:u x v:v x y F Bh n p D pp z F 4{ zf 4 zf y F s Andreas Neubauer I Slide 15 I 18.11.2014 Volume conductor effects II connection of k U to the transmembrane potential k U s k } k c HT p T pop ~ Bop HT p4op T p ~ : x~ o F T Bop B op F FT Andreas Neubauer I Slide 16 I 18.11.2014 8

Detection and analysis of ECG/EKG Andreas Neubauer I Slide 17 I 18.11.2014 ECG/EKG characteristics electrical events can be recorded from the body surface complex relation to the source lighthouse analogy recording is only possible when potentials are changing record of the changing activity of the heart Andreas Neubauer I Slide 18 I 18.11.2014 9

Electrocardiographic planes standardization of recorded signals is needed Andreas Neubauer I Slide 19 I 18.11.2014 The frontal plane ECG/EKG lead configurations electrical activity of the heart can be described as movement of an electrical dipole cardiac vector is the line joining the charges of the dipole Einthoven s triangle: triangle between RA, LA and LL lead configurations: Lead I: RA : to LA Lead II: RA : to LL Lead III: LA : to LL plotting the measured signal in the three leads at any time of the cardiac cycle on Einthoven s triangle leads to the cardiac vector body build and age influence the cardiac vector Andreas Neubauer I Slide 20 I 18.11.2014 10

The transverse plane ECG/EKG recorded unipolarly wrt an indifferent electrode (LA + RA + LL) usually with six electrodes in a line round the chest Andreas Neubauer I Slide 21 I 18.11.2014 The sagittal plane ECG/EKG also recorded with an indifferent electrode catheter with electrode is placed down the oesophagus rarely used in practice http://www.cardiocommand.com/research_cathinsert.html Andreas Neubauer I Slide 22 I 18.11.2014 11

Electrodes and amplifiers good skin preparation leads to an electrode impedance 10 Ω amplifier input impedance of 1 Ω is adequate electrodes do not have the same impedances common-mode voltage is produced 80 common-mode rejection with 10 Ω difference impedance between electrodes requires a common-mode input impedance of 100 Ω normally the majority of EMG spectra lies above the ECG spectra apply bandpass filter Andreas Neubauer I Slide 23 I 18.11.2014 Detection of EEG signals Andreas Neubauer I Slide 24 I 18.11.2014 12

Sources of the EEG signal electroencephalographic signals were first recorded in 1929 (ECG/EKG in 1895) electroencephalograph means graph of electrical changes from the enkephalos (Greek for brain) sources of the EEG signals are the neuronal potentials of the brain attenuation by bone, muscle and skin electrocorticography (ECoG) records signals directly from the cortex EEG signals are between 10 and 300 ]V Ag-AgCl discs are best to record an EEG time consuming skullcaps are much more convenient in use Andreas Neubauer I Slide 25 I 18.11.2014 EEG equipment and settings differential amplifiers are used for signal amplification min. eight channels at the recorder assume 16 differential amplifiers 32 input connections plus one earth connection standard EEG settings: Chart speed 30 --/ Gain setting: 100 ]V/ƒ- Time constant: 0.3 (corresponds to a :3 point of 0.53 1/) Filters: High frequency response is a -3 at 75 1/ electrode impedance 10 Ω Andreas Neubauer I Slide 26 I 18.11.2014 13

Normal EEG signals a quiet environment is required only one person should be in the room with the patient wide-awake normal persons produce an unsynchronized highfrequency EEG rhythmic activity at 8 13 1/ is produced if a normal person closes the eyes Andreas Neubauer I Slide 27 I 18.11.2014 Artifacts electrode artifacts electrode impedances interference movement of the cables perspiring of the patient potential difference of several - between the back and front of the eyes ECG may be seen if recording electrodes are spaced a long way dental fillings may produce artifacts http://www.psychologie.uzh.ch/fachrichtungen/plasti/lab or.html http://bipolaraspiemom.wordpress.com/20 11/07/15/are-you-still-awake-our-eegstory/ Andreas Neubauer I Slide 28 I 18.11.2014 14

Detection of EMG signals Andreas Neubauer I Slide 29 I 18.11.2014 Sources of electromyographic (EMG) signals record signals of nerves and muscles needle and surface electrodes can be used examine shape and sound of the signal with needle electrodes overall activity of the muscle is recorded with surface electrodes functional unit of a muscle is one motor unit Andreas Neubauer I Slide 30 I 18.11.2014 15

EMG equipment recording of an EMG is possible between two surface electrodes reducing the distance below 4 mm leads to a significant signal drop surface electrodes will always record signal from multiple muscles needle electrodes are more accurate but uncomfortable fine wire electrodes are excellent for long term EMG recording surface electrodes record less high-frequency content than needle electrodes signals up to 2 - are typical Andreas Neubauer I Slide 31 I 18.11.2014 EMG settings Standard settings for the pre-amplifier Amplification 100 Input impedance 10 Ω Noise with input shorted Common-mode rejection ratio 2 ]V : 80 Bandwidth (:3 points) 10 1/ 10` 1/ equipment testing short circuit the inputs of the amplifier and set maximum gain only noise should be visible check leads and plugs Andreas Neubauer I Slide 32 I 18.11.2014 16

Normal EMG signals voluntary EMG pattern is recorded with a needle electrode several points must be observed normal EMG sounds like gunfire normal APs last few milliseconds and contain two/three deflections myopathic muscles produce smaller APs with more deflections Andreas Neubauer I Slide 33 I 18.11.2014 Neural stimulation III Andreas Neubauer I Slide 34 I 18.11.2014 17

Nerve conduction velocity measurement measurement of time between stimulus and response conduction time (average of myelinated fibers: 50 -/) myelination is not complete at birth nerve conduction increases over the first years of life Andreas Neubauer I Slide 35 I 18.11.2014 Motor nerve conduction velocity measure latency of proximal/distal stimulation calculate velocity from the values of obtained latencys Andreas Neubauer I Slide 36 I 18.11.2014 18