EU501-BIOMEDICAL SIGNAL
MEASUREMENT
IDA MARIA BINTI MOHD YUSOFF
TOPICS
1. THE ORIGIN OF BIOPOTENTIAL
2. BIOPOTENTIAL ELECTRODES
3. BIOPOTENTIAL AMPLIFIERS
4. CARDIOVASCULAR MEASUREMENT
5. RESPIRATORY MEASUREMENT
Course Assessment
Test - 1 (20%)
Quiz - 4 (10%)
Practical Work - 6 (50%)
Practical Test - 1 (15%)
End of Chapter - 2 (5%)
Essay Question - 1 (5%)
CHAPTER 1:
THE ORIGIN OF BIOPOTENTIAL
Define sources of bioelectric potentials
Explain resting and action potentials
resting potentials
action potentials
waveform of action potential
Determine propagation of action potential
Determine bioelectric potentials
ECG
EEG
EMG
ENG
ERG
EOG
Construct signals of bioelectric potentials
Cell Membrane Potentials
Cell Membrane
• very thin (7-15 nm) lipid-protein complex
◦ transmembrane ion channels (pores) allow flow of ions
across the membrane
◦ like a leaky capacitor: a thin dielectric material acts as a
charge separator
impermeable to intracellular protein and other organic anions
selectively permeable to sodium (Na+) potassium(K+)and
chlorine(Cl-)ions
ion concentration difference across membrane creates a
diffusion gradient
ions flow, creating an electric field that opposes flow, until an
equilibriumis established
◦ similar to p-n junction, ions flow by diffusion and create a
potential difference which inhibits further flow of charged
ions
Extracellular
Domain
Intracellular
domain
Structure of Cell Membrane
Electrical Activity of Excitable Cells
Biopotentials are produced as a result of electrochemical
activity of excitable cells: i.e., nervous, muscular (cardiac and
smooth) and glandular cells
Electrical states of excitable cells
resting state
action state
Factors influencing the flow of ions across the cell membrane
1. Diffusion gradients
2. Inwardly directed electric field (inside negative, outside
positive)
3. Membrane structure (availability of pores; K+, Na+ and
permeability of membrane to different ions)
4. Active transport of ions across membrane against
established electrochemical gradients
Resting Membrane Potential
● Higher concentration of potassium (K+) inside than outside the cell and a higher concentration of chloride (Cl-) and sodium (Na+) outside the cell than inside.
● The negatively charged protein molecules (A-) inside the neuron cannot cross the membrane
● Use sodium/potassium pump to expel 3 sodium ions from the cell, each time 2 potassium ions are moved inside the cell
● The cell membrane is fairly permeable to potassium ions, so potassium ions diffuse out of the cell, leaving behind them a negative chargeat the interior side of the cell membrane
● Resting membrane potential of a membrane is about –70 mV (mV=millivolt )
● At rest, potassium ions (K+) can cross through the membrane easily. ● Chloride ions (Cl-)and sodium ions (Na+) have a more difficult time crossing.
Resting Potentials
Nerve and muscle cells are encased in a semi-permeable
membrane that permits selected substances to pass through while
others are kept out.
Body fluids surrounding cells are conductive solutions containing
charged atoms known as ions.
In their resting state, membranes of excitable cells readily permit
the entry of K+ and Cl~ ions, but effectively block the entry of
Na+ ions (the permeability for K+ is 50-100 times that for Na+).
Various ions seek to establish a balance between the inside and
the outside of a cell according to charge and concentration
The inability of Na+ to penetrate a cell membrane results in the
following:
Na+ concentration inside the cell is far less than that outside.
The outside of the cell is more positive than the inside of the cell.
To balance the charge, additional K+ ions enter the cell,
causing higher K+ concentration inside the cell than outside.
Charge balance cannot be reached due to differences in
membrane permeability for the various ions.
A state of equilibrium is established with a potential difference,
with the inside of the cell being negative with respect to the
outside
A cell in its resting state is said to be polarized. Most cells maintain
a resting potential of the order of - 70 to -90 mV until some
disturbance or stimulus upsets the equilibrium
Electrically these cells exhibit a resting membrane potential
[Nernst (1) and Goldman/Hogkin and Katz equation (2)]
When appropriately stimulated, they generate an action
potential (flow of ions across the cell membrane and
generation of a propagating wave of depolarization along
the membrane)
Model-generated transmembrane potential vm and membrane
ionic conductance changes for sodium (gNa) and potassium
(gK) during the action potential.
These waveforms are obtained by solving the differential
equations developed by Hodgkin and Huxley for the giant axon
of the squid at a bathing medium temperature of 18:5 C. ENa
and EK are the Nernst equilibrium potentials for Na and K across
the membrane.
Action Potential
The action potential (AP) is the electrical signal that
accompanies the mechanical contraction of a single cell when
stimulated by an electrical current (neural or external)
It is caused by the flow of sodium (Na+), potassium (K+),chloride
(Cl~), and other ions across the cell membrane.
The action potential is the basic component of all bioelectrical
signals.
the sodium–potassium pump
actively transports Na+out of cell and K+into cell in the ratio
3Na+: 2K+
associated pump current iNaK is a net outward current that
tends to increase the negativity of the intracellular potential
energy for the pump is provided by a common source of
cellular energy, adenosine triphosphate(ATP) produced by
mitochondria in the cell
Na-K pump
Vm
Action potential
● as a "spike" or an "impulse" for the action
potential
● When it is stimulated, protein channels in the
membrane open, and let sodium into the cell,
making the membrane potential more
positive.
● Action potential is an explosion of electrical
activity that is created by a depolarizing
current
● A stimulus causes the resting potential to
move toward 0 mV
● When the depolarization reaches about -55
mV a it will fire an action potential.
● This is the threshold
RESTING MEMBRANE POTENTIAL
♦ Na+ channels and
K+ channels are
closed.
♦ The outside of the plasma membrane is positively charged compared to the inside.
DEPOLARIZATION
♣ Na+ channels open.
♣ K+ channels begin to open.
♣ Depolarization results because the inward movement of Na+ makes the inside of the membrane more positive
REPOLARIZATION
♠ Na+ channels close and additional
♠ K+ channels open.
♠ Na+ movement into the cell stops and K+
movement out of the cell increases, causing repolarization
Membrane Current
membran current im
im
t
time / ms
distance / mm
Action Potential = ALL x NOTHING
Action Potential
Action Potential = opening of sodium and potassium channels
Action Potential
K+ -channels
Na+ -channels
Vm
excitable cell
time
resting potential
Waveform of Action Potential
Transmembrane potential (v) and membrane
ionic conductance changes for sodium (gNa) and
potassium (gK) during the action potential
◦ Absolute refractory period
membrane cannot respond to any stimulus
no matter how intense
Relative refractory period
action potential can be elicited by an intense
superthreshold stimulus
◦ Set upper limit action potential frequency EX: for nerve axon with absolute refractory period of 1 ms
max action potential frequency is 1000 impulses/s
but, typical neuron firing rate is ~30 Hz
Electrical Recording from a Nerve
Fiber
Recording of cell activity can be made via a
penetrating micropipet
•Recording of action potential of an
invertebrate nerve axon
•cell membrane potential vs. time
•resting potential
•action potential
ACTION POTENTIAL
Propagation of Action Potential
An action potential propagates along a muscle fiber or an unmyelinated nerve fiber as follows
Once initiated by a stimulus, the action potential propagates along the whole length of a fiber without decrease in amplitude by progressive depolarization of the membrane
Current flows from a depolarized region through the intra-cellular fluid to adjacent inactive regions, thereby depolarizing them
Current also flows through the extra-cellular fluids, through the depolarized membrane, and back into the intra-cellular space, completing the local circuit
The energy to maintain conduction is supplied by the fiber itself
Myelinated nerve fibers are covered by an insulating sheath of myelin
The sheath is interrupted every few millimeters by spaces known as the nodes of Ranvier,where the fiber is exposed to the interstitial fluid
Sites of excitation and changes of membrane permeability exist only at the nodes, and current flows by jumping from one node to the next in a process known as saltatory' conduction
How Electrical Activity Allows Neurons to
Communicate in The Action Potential?
http://brainu.org/files/movies/action_potential_cartoon.swf
(a) Charge distribution in the vicinity of the active region of an
unmyelinated fiber conducting an impulse. (b) Local circuit current flow
in the myelinated nerve fiber.
Resting vs. Active State
•Resting State
Steady electrical potential of difference between
internal and external environments
Typically between -70 to -90mV, relative to the
external medium
•Active State
Electrical response to adequate stimulation
Consists of “all-or-none” action potential after
the cell threshold potential has been reached
ECG
Electrocardiograph
ElectroCardioGram
Change of electric potential
heart muscle activation
atrium depolarization
3 diff. recording schemes:
Einthoven, Goldberger, Wilson
Frequency = 1-2 Hz !
Heart
Eindhoven’s triangle
36
Heart
EEG
The EEG (popularly known as brain waves)
represents the electrical activity of the brain
• Delta (): 0.5 - 4 Hz;
• Theta (): 4 - 8 Hz;
• Alpha (): 8 - 13 Hz; and
• Beta (): > 13 Hz.
ElectroEncefaloGram
Waves:
•Delta: < 4 Hz ... sleeping, in awakeness pathological
•Theta: 4.5 -8 Hz ... drowsiness in children, pathological in aduls
(hyperventilation, hypnosis, ...)
•Alfa: 8.5 -12 Hz ... relaxation physical / mental
•Beta: 12 - 30 Hz ... wakefulness, active concentration
•Gama: 30–80 Hz …higher mental activity including perception and
consciousness
Brain
EMG
Skeletal muscle is organized functionally
on the basis of the motor unit which
consists of a single motor nerve fiber and
the bundle of muscle fibers to which it is
attached
Electromyograph
Measures electrical activity of the muscles
Electromyogram (EMG)
Skeletal muscle is organized
functionally on the basis of the
single motor unit (SMU).
SMU is the smallest unit that can be
activated by a volitional effort
where all muscle fibers are activated
synchronously.
SMU may contain 10 to 2000 muscle
fibers, depending on the location
of the muscle.
Factors for muscle varying strength
1. Number of muscle fibers
contracting within a muscle
2. Tension developed by each
contracting fiber
Muscle Fiber (Cell)
http://www.blackwellpublishing.com/matthews/myosin.html
Figure 4.10 Diagram of a single motor unit (SMU), which consists of a single motoneuron and
the group of skeletal muscle fibers that it innervates. Length transducers [muscle spindles, Figure
4.6(a)] in the muscle activate sensory nerve fibers whose cell bodies are located in the dorsal root
ganglion. These bipolar neurons send axonal projections to the spinal cord that divide into a
descending and an ascending branch. The descending branch enters into a simple reflex arc with
the motor neuron, while the ascending branch conveys information regarding current muscle
length to higher centers in the CNS via ascending nerve fiber tracts in the spinal cord and brain
stem. These ascending pathways are discussed in Section 4.8.
Field potential of the active fibers of an SMU
1- triphasic form
2- duration 3-15 msec
3- discharge rate varies from 6 to 30 per second
4- Amplitude range from 20 to 2000 V
Surface electrode record field potential of surface muscles and over a
wide area.
Monopolar and bipolar insertion-type needle electrode can be used to
record SMU field potentials at different locations.
The shape of SMU potential is considerably modified by disease such
as partial denervation.
Electromyogram (EMG)
Figure 4.11 Motor unit
action potentials from
normal dorsal interosseus
muscle during progressively
more powerful contractions.
In the interference pattern (c
), individual units can no
longer be clearly
distinguished. (d)
Interference pattern during
very strong muscular
contraction. Time scale is 10
ms per dot.
ENG
The ENG is an electrical signal observed as a
stimulus and the associated nerve action
potential propagate over the length of a nerve.
It may be used to measure the velocity of
propagation (or conduction velocity) of a
stimulus or action potential in a nerve
ENGs may be recorded using concentric needle
electrodes or silver -silver-chloride electrodes
(Ag - AgCl) at the surface of the body.
Electroneurogram (ENG)
Recording the field potential of an excited nerve.
Neural field potential is generated by
- Sensory component
- Motor component
Parameters for diagnosing peripheral nerve disorder
- Conduction velocity
- Latency
- Characteristic of field potentials evoked in muscle supplied by the
stimulated nerve (temporal dispersion)
Amplitude of field potentials of nerve fibers < extracellular potentials
from muscle fibers.
Figure 4.7 Measurement of neural conduction velocity via measurement of
latency of evoked electrical response in muscle. The nerve was stimulated at
two different sites a known distance D apart.
Reference
Velocity = u =
2 ms
V°(t)S2
S2
S1
S1
Muscle
+ - + -
D
R
L2
L1- L2
L1
t DV°(t)
V°(t)
1 m
V
Conduction Velocity of a Nerve
Field Potential of Sensory Nerves
Extracellular field response from the sensory nerves of the median or
ulnar nerves
Figure 4.8 Sensory nerve action potentials evoked from median nerve of a healthy subject at elbow and
wrist after stimulation of index finger with ring electrodes. The potential at the wrist is triphasic and of much
larger magnitude than the delayed potential recorded at the elbow. Considering the median nerve to be of the
same size and shape at the elbow as at the wrist, we find that the difference in magnitude and waveshape of
the potentials is due to the size of the volume conductor at each location and the radial distance of the
measurement point from the neural source.
To excite the large, rapidly conducting
sensory nerve fibers but not small pain
fibers or surrounding muscle, apply brief,
intense stimulus ( square pulse with
amplitude 100-V and duration 100-300
sec). To prevent artifact signal from
muscle movement position the limb in a
comfortable posture.
Reflexly Evoked Field Potentials
Some times when a peripheral nerve is stimulated, a two evoked
potentials are recorded in the muscle the nerve supplies. The time
difference between the two potentials determined by the distance
between the stimulus and the muscle.
Stimulated nerve: posterior tibial nerve
Muscle: gastrocnemius
Figure 4.9 The H reflex The four traces show potentials evoked by stimulation of the
medial popliteal nerve with pulses of increasing magnitude (the stimulus artifact increases
with stimulus magnitude). The later potential or H wave is a low-threshold response,
maximally evoked by a stimulus too weak to evoke the muscular response (M wave). As the
M wave increases in magnitude, the H wave diminishes.
Reflexly Evoked Field Potentials
Low intensity stimulus stimulate only the
large sensory fibers that conduct toward the
CNS. No M wave
Medium intensity stimulus stimulate
smaller motor fibers in addition to
the large sensory fibers. Motor
fibers produce a direct muscle
response the M wave.
With strong stimuli, the excited motor
fibers are in their refractory period so
only the M wave is produced.
Electroretinogram (ERG)
A transparent contact lens contains one electrode and the reference electrode can be
placed on the right temple.
ERG is a recording of the temporal sequence of changes in potential in
the retina when stimulated with a brief flash of light.
Glaucoma
High pressure
Aqueous humor
Electroretinogram (ERG)Ag/AgCl electrode impeded in a special contact lens.
Source of Retinal PotentialThere are more photoreceptors than ganglion cells so there is a convergence pattern.
Many photoreceptors terminate into one bipolar cell and many bipolar cells terminate into one
ganglion cell. The convergence rate is greater at peripheral parts of the retina than at the fovea.
Rod (10 million) is for vision in dim light and cone (3 million) is for color vision in brighter light.
Electroretinogram (ERG)The a-wave, sometimes called the "late receptor potential," reflects the general physiological
health of the photoreceptors in the outer retina. In contrast, the b-wave reflects the health of
the inner layers of the retina, including the ON bipolar cells and the Muller cells (Miller and
Dowling, 1970). Two other waveforms that are sometimes recorded in the clinic are the c-wave
originating in the pigment epithelium (Marmor and Hock, 1982) and the d-wave indicating
activity of the OFF bipolar cells (see Figure 3).
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http://webvision.med.utah.edu/ClinicalERG.html
Electro-Oculogram (EOG)
EOG is the recording of the corneal-retinal potential to determine the
eye movement.
By placing two electrodes to the left and the right of the eye or above
and below the eye one can measure the potential between the two
electrode to determine the horizontal or vertical movement of the eye.
The potential is zero when the gaze is straight ahead.
Applications1- Sleep and dream research,
2- Evaluating reading ability and visual fatigue.
Bionic Eyes