Biomedical Engineering - Home: UMM … Engineering Electrophysiology Dr. rer. nat. Andreas Neubauer...

10/25/2016

1

Biomedical EngineeringElectrophysiology

Dr. rer. nat. Andreas Neubauer

Andreas Neubauer I Slide 2 I 18.11.2014

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

10/25/2016

2


The nervous system


Organization of the nervous system

• brain, nerves and muscles are the major components of the nervous system

Brown, Medical Physics

• 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

• synapses allow reflex loops via the spinal column⇒ can be suppressed by the brain

10/25/2016

3


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


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��


10/25/2016

4


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


Why is smooth movement possible?

• increasing contraction is achieved be an increase in frequency

• not all muscle fibers twitch simultaneously


10/25/2016

5


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+,

&'&(�-�

.: Gas constant; /: Temperature; 0: Faraday constant; 12: Valence

⇒ transmembrane potential with respect to the outside of the membrane

valid at roomtemperature


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 wrtoutside (57)

84 400 20 :75

��4 50 450 �55

�� 40 550 :66


10/25/2016

6


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


• stimulation leads to an efflux/influx of potassium/sodium⇒ change in transmembrane potential⇒ avalanche effect

⇒ DEPOLARIZATION!

=4>?4


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

10/25/2016

7


Transmission of NAPs II

• speed of transmission depends on:• Membrane

capacitance • Myelin • Axon resistance Brown, Medical Physics

• assume a cylindrical membrane with diameter @ and length A:

⇒ . �BC

DEF; G: resistivity HΩmK

⇒ � LMDL; L: dielectric constant of neural membrane RS

TU

⇒ �. BV �WDE

DEF GL

�DWF

X; time constant of the membrane H�K

• typical values: Membrane capacitance: 1YZ

[UF , @ 10]-, A

10--, ^ � 1Ω-⇒ � � 3 �10�`]F, . � 1.3 �10*Ω,time constant � 0.4�⇒ .[c22%d � 2.14Ω


Muscle Action Potentials (MAPs)

Smooth muscle Striated muscle

intestines and blood vessels skeletal muscle

intrinsically active voluntarily active


10/25/2016

8


Volume conductor effects I• electrical potential: Φ

+

�Dfghid��

• assumptions:

• Potential at infinity equal zero

• Tissue is homogeneous

⇒ � j k gB

�DdF�j

l

d

Bh

�Dd

• cylindrical nerve fiber:

⇒ � j Bmnop

�Dq

• contribution made to the potential field at r�s‘, u‘, v‘

⇒ j s– s‘x� u : u‘ x � v : v‘ x

yF

⇒ Φ s, u, v gBhn p

�D p�pz F4{zF4|zFyF

�s



Volume conductor effects II

• connection of kU to the transmembrane potential

⇒ kU s k} –kc HT p � T p�op ~

Bop:

HT p4op � T p ~

Bop

x~

B

oFT

opF

FT

Brown, Medical Physics Brown, Medical Physics


10/25/2016

9


Detection and analysis of ECG/EKG


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

10/25/2016

10


Electrocardiographic planes

• standardization of recorded signals is needed



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

10/25/2016

11


The transverse plane ECG/EKG


• recorded unipolarly wrt an indifferent electrode (LA + RA + LL)• usually with six electrodes in a line round the chest


The sagittal plane ECG/EKG

http://www.cardiocommand.com/research_cathinsert.html

• also recorded with an indifferent electrode

• catheter with electrode is placed down the oesophagus

• rarely used in practice

10/25/2016

12


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


Detection of EEG signals

10/25/2016

13


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



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/�-

• Time constant: 0.3� (corresponds to a :3��point of 0.531/�)

• Filters: High frequency response is a -3��at 751/�

• electrode impedance � 10�Ω

10/25/2016

14


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 high-frequency EEG

• rhythmic activity at8– 131/� is produced if a „normal“ person closes the eyes


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/Labor.html

http://bipolaraspiemom.wordpress.com/2011/07/15/are-you-still-awake-our-eeg-story/

10/25/2016

15


Detection of EMG signals


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


10/25/2016

16


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



EMG settings

Standard settings for the pre-amplifier

Amplification 100

Input impedance 10�Ω

Noise with input shorted 2]V� : �

Common-mode rejection ratio 80��

Bandwidth (:3��points) 101/�–10`1/�

• equipment testing⇒ short circuit the inputs of the amplifier and set maximum gain

⇒ only noise should be visible⇒ check leads and plugs


10/25/2016

17


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




Neural stimulation III

10/25/2016

18


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



Motor nerve conduction velocity

• measure latency of proximal/distal stimulation⇒ calculate velocity from the values of obtained latencys

Date post:	01-Jul-2018
Category:	Documents
Upload:	phungliem
View:	215 times
Download:	0 times

Biomedical Engineering - Home: UMM … Engineering Electrophysiology Dr. rer. nat. Andreas Neubauer...

Documents