Biomedical Instrumentation

Electrocardiogram and Electroencephalogram

The Heart
 Blood (poor with oxygen) flows from the
body to the right atrium and then to the
right ventricle. The right ventricle pump the
blood to the lung.
 Blood (rich with oxygen) flows from the
lung into the left atrium and then to the left
ventricle. The left ventricle pump the blood
to the rest of the body.
 Diastole: is the resting or filling phase (atria
chamber) of the heart cycle.
 Systole: is the contractile or pumping phase
(ventricle chamber) of the heart cycle.
 The electrical events is intrinsic to the heart
itself.
 The walls of atria and ventricles and
interventricular septum can be considered
the major action current sources
responsible for the production external field
potentials recorded from the heart.
2

1
 The Heart
Electrical system
 Distribution of specialized conductive
tissues in the atria and ventricles, showing
the impulse-forming and conduction
system of the heart. The rhythmic cardiac
impulse originates in pacemaking cells in
the sinoatrial (SA) node, located at the
junction of the superior vena cava and the
right atrium. Note the three specialized
pathways (anterior, middle, and posterior
internodal tracts) between the SA and
atrioventricular (AV) nodes. Bachmann's
bundle (interatrial tract) comes off the
anterior internodal tract leading to the
left atrium. The impulse passes from the
SA node in an organized manner through
specialized conducting tracts in the atria
to activate first the right and then the left
atrium.

The Heart
Electrical system
 Passage of the impulse is delayed at the AV
node before it continues into the bundle of
His, the right bundle branch, the common
left bundle branch, the anterior and
posterior divisions of the left bundle branch,
and the Purkinje network. The right bundle
branch runs along the right side of the
interventricular septum to the apex of the
right ventricle before it gives off significant
branches. The left common bundle crosses to
the left side of the septum and splits into the
anterior division (which is thin and long and
goes under the aortic valve in the outflow
tract to the anterolateral papillary muscle)
and the posterior division (which is wide and
short and goes to the posterior papillary
4 muscle lying in the inflow tract).

2
 The Heart
Electrical system
 Different action potential waveforms for each
specialized cells found in the hearth.

The Heart
Electrical system

3
 The Heart
Electrical system
 Automaticity: cells ability
to spontaneously
depolariza, reach
treshold and propagate an
AP
 Found only in pacemaker
cells
 Depolarization depends
on Ca influx
 Slow depolarization and
repolarization
 Excitability recovery
time is longer

The Heart
Electrical system
 Involves non-pacemaker
cells (all cells of the heart
except pacemaker cells)
 Depolarization depends on
Na influx
 Rapid depolarization
 AP duration depends on Ca
influx at plateau stage.
 Has a stable resting state

4
 The Heart
Electrical system
 Cardiac electrical activity is result of the movement of ions (charged particles such as
sodium, potassium and calcium) across to cell membrane,
 In the resting state cardiac muscle cells are polarized, which means an electrical
difference exists between the negatively charged inside and the positively charged
outside of the cell membrane,
 As soon as an electrical impulses is initiated, cell membrane permeability changes and
sodium Na+ move rapidly into the cell while potassium K+ exits the cell,
 This ionic exchange begins depolarization (electrical activation of the cell) converting
the internal charge of the cell to a positive one,
 The repolarization is return of the cell to its resting state occurs as the cell returns to
its baseline. This corresponds to relaxation of myocardial muscle,
 After the rapid influx of sodium into the cell during depolarization the permeability of
cell membrane to calcium is changed calcium enters the cell and is released from
intracellular calcium stores
 The increase in calcium, which occurs during plateau phase of repolarization is much
slower than that of sodium and continuous for a longer period.
9

The Heart
Electrical system

10

5
 The Heart
Electrical system
 Recording depolarization and repolarization waves

 Zero potential difference - isoelectric point Resting stage - Polarized

11

Electrocardiogram

 P wave – depolarization of atria

 QRS complex – depolarization of ventricular muscle
 T wave – repolarization of ventricular muscle
 *repolarization of atria lies under QRS
 The U-wave believed to be due repolarization of ventricular papillary muscles.
 P-R interval is caused by delay in the AV node
 S-T segment is related to the average duration of the plateau regions of the individual
ventricular cells.
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6
 Electrocardiogram
 The electric potentials generated by the heart appear throughout the body and on its
surface.
 The electrical signals of the Cardiac Conduction System can be picked up with sensors
on the chest.
 These signals result in an ECG, or electrocardiogram (in Germany EKG).
 Different pairs of electrodes at different locations generally yield different voltages
because of the spatial dependence of the electric field of the heart.
 This graph is frequently used to detect normal heart function.
 The familiar ECG is a reading of the different electrical signals that go off.
 The chart to the right explains the electrical significance of the different spikes in
potential.

13

Electrocardiogram
 The electric dipole, consists of two equal and opposite charges, separated by some (usually
small) distance
 The potential differences arising in the heart (cardiac dipoles) can be represented by
electrical vectors
 Amplitude and direction
 All basic vector operations can be applied to the cardiac vectors
 Each depolarizing myocardial cell is in fact a dipole and thus can be represented by a
vector = elementary vector
 The sum of all elementary vectors will create an instantaneous vector the potential
differences generated by the heart change from moment to moment during the cardiac
cycle
 Once a single cell is stimulated the depolarization will propagate in every direction: a
propagating wave of depolarization will be created
 Each of these moments can be described by an instantaneous vector (with a different size
and orientation)
 All these vectors can be brought to a single common point: electrical center of the heart
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7
 Electrocardiogram
 By recording the magnitude and direction of the electrical forces that are generated by
the heart by means of a continuous series of vectors that form curving lines around a
central point one can record the vectorcardiography

 The projection of this curve as function of time on an axis corresponding to a lead is

actually the ECG in that particular lead

• cos ∙

θ → voltage induced in a lead

→ Cardiac vector
→ Lead vector

15

Electrocardiogram

Positive isoelectric
deflection Negative
deflection

When wave of depolarization moves towards a

positive electrode, the deflection is positive.

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8
 Electrocardiogram
 If an electrode is placed so that wave of
depolarization spreads toward the
recording electrode, the ECG records a
positive (upward) deflection.
 If wave of depolarization spreads away
from recording electrode, a negative
(downward) deflection occurs.

17

Electrocardiogram
 Electrodes for recording the potential changes of the heart are placed on the body
surface in a standard way

 Potential changes recorded by specifically connected electrodes is called a lead

 Each lead will be assigned with an axis and each of the axes will have an orientation:
by convention the sense of the axis is toward the positive electrode

 The projection of the cardiac vectors as function of time on the axis corresponding to
a lead is actually the ECG trace in that particular lead

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9
 Electrocardiogram
1. Two different points on the body (bipolar leads)
2. One point on the body and a virtual reference point with zero electrical potential,
located in the center of the heart (unipolar leads)
 The standard EKG has 12 leads:
1. 3 Standard Limb Leads
2. 3 Augmented Limb Leads
3. 6 Precordial Leads
 The axis of a particular lead represents the viewpoint from which it looks at the
heart. Limb Leads Precordial Leads

Bipolar I, II, III -

(standard limb leads)
Unipolar aVR, aVL, aVF V1-V6
(augmented limb leads)
19

Electrocardiogram  6 limb leads – define electrical

activity in frontal plane

 6 precordial leads – define

electrical activity in transverse
plane

 The 3 augmented leads compare

one limb electrode to the average
of the other two. (aVR, aVL,
aVF).

 Leads are made of a combination

of electrodes that form imaginary
lines in the body along which the
electrical signals are measured.

20

10
 Electrocardiogram

 Electrical axis of the heart is normally between -30 and 90 degrees.

21

Electrocardiogram

22

11
 Electrocardiogram

23

Electrocardiogram
 Three additional leads are used for frontal plane measurements.
 These are the measurements at the specific electrodes, with respect to a reference
electrode
 One commonly used reference electrode is the Wilson Central Terminal, obtained
through a resistive network, combining limb electrodes
 The new set of leads obtained by combining the standard limb electrodes to the
Wilson terminal form the augmented leads
 These leads provide additional vector views of cardiac depolarization in the frontal
plane. Unlike leads I, II, III, the augmented leads utilize WCT, a central negative
terminal. This virtual "electrode" is calculated by the EKG computer to measure
vectors originating roughly at the center of the heart.
 Note that the voltage at Wilson’s terminal is zero

24

12
 Electrocardiogram
 R should be very high
(~5 Ω) or buffers
(voltage followers) are
used between each
electrode so that the
loading of any particular
lead will be minimal.

25

Electrocardiogram
 Example: Show that the voltage in lead aVR is %50 greater than that in lead VR at the
same instant.
 Considering the connections for aVR and VR, we can draw the equivalent circuits. The
voltages between each limb and ground are , , . When no current is drawn by
the voltage measurement circuit, the negative terminal for aVR (the modified Wilson’s
central terminal) is at a voltage of with respect to ground, which can be
determined as follows:

2

2 2

2
2 2

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13
 Electrocardiogram
 To find the Wilson’s central terminal voltage we simplify the circuit by taking the
Thevenin equivalent circuit of the two right hand branches:

2

3 ⁄2 2 3
≡

2
2
3 3

2 3
VR
3 2

27

Electrocardiogram

28

14
 Electrocardiogram
 Additional set of six leads, placed on the chest, also known as the precordial leads.
These too are unipolar, that is they measure the potential with respect to WCT.
 The main reason for recording all 12 leads is that it enhances pattern recognition. This
combination of leads gives the clinician an opportunity to compare the projections of
the resultant vectors in two orthogonal planes and at different angles.

29

Electrocardiogram
 Cardiac Rhythms - Normal & Abnormal
 Normal
 Heart rate is about 70 beats per minute (bpm)
 Bradycardia: slower that normal (during sleep)
 Tachycardia: higher than normal (during exercise, emotional episodes, fever, fright)
 Abnormal
 Idioventricular heart rate is about 30 - 45 bpm (ventricles and atria beat
independly)
 Disease can alter the conducting pathways (e.g., rheumatic heart disease and viral
infections)
 Infarction (loss of blood supply and muscle death) can alter the heart muscle
conducting pattern

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15
 Electrocardiogram
 Atroventricular (AV) Block First-degree block

• First degree:
– AV node is diseased; P-R
interval is prolonged
• Second degree:
– Greater damage to the
AV node; some pulses
are not conducted (2:1,
3:1, etc.)
Complete block
• Third degree:
– Complete block; cells in
AV node are dead; atria
and ventricles beat
independently.

31

Electrocardiogram
 Premature Ventricular Contraction (PVC)

Normal ECG followed by an ectopic beat.

An irritable focus, or ectopic pacemaker, within the ventricle or specialized

conduction system may discharge, producing an extra beat, or extrasystole, that
interrupts the normal rhythm.

32 This extrasystole is also referred to as a premature ventricular contraction (PVC).

16
 Electrocardiogram
 Tachycardia

• Paroxysmal tachycardia.
An ectopic focus may
repetitively discharge
at a rapid regular rate
for minutes, hours, or
even days.
• Atrial flutter. The atria
begin a very rapid,
perfectly regular
"flapping" movement,
beating at rates of 200
to 300 beats/min;
rapid P waves.

33

Electrocardiogram
• Atrial fibrillation
 Fibrillation – Atrial and Ventricular
– Feeble,
uncoordinated
twitching
– Low-amplitude,
irregular ECG
– Blood pumping is
continued
• Ventricular fibrillation
– Disorganized
conduction & ECG
– Ventricles twitch
– No blood is
pumped

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17
 Electrocardiogram
 Cardiac Ischemia

• Control: action potentials

& ECG waveforms from
normal dog myocardium
• Challenge: a coronary
artery is occluded; cells
become ischemic; lose
K+ and gain Na+
• Early ischemia: the ST
segment of the ECG is
elevated
• Late ischemia: in addition
to ST segment elevation,
the TQ segment is
depressed
35

Electrocardiogram
 ECG machines can run at 50 or 25 mm/sec.
 Major grid lines are 5 mm apart, at standard 25 mm/s, 5 mm corresponds to .20
seconds.
 Minor lines are 1 mm apart, at standard 25 mm/s, 1 mm corresponds to .04 seconds.
 Voltage is measured on vertical axis.
 Standard calibration is 0.1 mV per mm of deflection.
 When myocardial muscle is completely polarized or depolarized, the ECG will not
record any electrical potential but rather a flat line, isoelectric line.
• ECG Signals have two components:
• ECG Waveform
• 0.05 Hz to 150 Hz bandwidth per Medical Standards
• Average R Wave Amplitude is 1.8mV
• Some waveforms can be as big as 10 mV p-p.
• T wave alterans are only a few microvolts in amplitude
• Pacing Artefact
36 • Medical Standards require 2mV and 200μs detection
• Average pulse is 1 mV and 500μs but can be much smaller

18
 Electrocardiogram

37

Electrocardiogram
Functional Blocks
 Sensing electrodes
Right leg
 Lead fail detect electrode

 Amplifier protection
Driven
circuit Sensing
electrodes
Lead-fail
detect
right leg
circuit
ADC Memory

 Lead selector
 Auto calibration Amplifier
protection Lead
selector
Preamplifier Isolation
circuit
Driver
amplifier
Recorder-
printer
circuit
 Preamplifier
 Baseline restoration Auto Baseline Isolated
power
calibration restoration supply
 Driven right leg circuit
Parallel circuits for simultaneous recordings from different leads
 Isolation circuit
 ADC & Memory system Microcomputer

 Driver amplifier Operator

display
Control
 Recorder-printer program
Keyboard
 Microcomputer ECG analysis
38 program
 Control software

19
 Electrocardiogram
Frequent Problems
 Frequency distortion
 High-frequency loss rounds the sharp edges of the QRS complex.
 Low-frequency loss can distort the baseline (no longer horizontal) or cause
monophasic waveforms to appear biphasic.
 Saturation/cutoff distortion
 Combination of input amplitude & offset voltage drives amplifier into saturation
 Positive case: clips off the top of the R wave
 Negative case: clips off the Q, S, P and T waves
 Ground loops
 Patients are connected to multiple pieces of equipment; each has a ground (power
line or common room ground wire)
 If more than one instrument has a ground electrode connected to the patient, a
ground loop exists. Power line ground can be different for each item of
equipment, sending current through the patient and introducing common-mode
noise.
 Open lead wires
39
 Can be detected by impedance monitoring.

Electrocardiogram
Artefacts

• Unwanted voltage transients

– Patient movement Effect of a voltage transient on an
ECG recorded on an
– Electrical stimulation signals, electrocardiograph in which the
like defibrillation transient causes the amplifier to
• Amplifier saturates saturate, and a finite period of time
• First-order recovery to baseline is required for the charge to bleed
– Recovery time set by low- off enough to bring the ECG back
frequency corner of the into the amplifier’s active region of
bandpass amplifier operation. This is followed by a
first-order recovery of the system.
40

20
 Electrocardiogram
Artefacts

• Upper figure: coupling of 60 Hz power line noise

– Electric-field coupling between power grid, instrument, patient, and wiring.
• Lower figure: coupling of electromyographic (EMG) noise
– Example of tensing chest muscles while ECG is being recorded.
41

Electrocardiogram
Power-Line Coupling Power line 220 V

 Small parasitic capacitors connect the C2 C1 C3

power line to the RA and LA leads,
and the grounded instrument case Z1 Id1
A
 Small ac displacement currents and Z2
Id2
are generated B Electro-
 The body impedance is about 500 W cardiograph
and can be neglected G

 If and are approximately equal: ZG Id1+ Id2

A mechanism of electric-field pickup of an
electrocardiograph resulting from the power
6 20 Ω 120
line. Coupling capacitance between the hot side
 Remedies of the power line and lead wires causes current
 Shield electrodes & connect to to flow through skin-electrode impedances on its
electrocardiograph (grounding tree) way to ground.
to reduce id
42  Reduce or match the electrode skin
impedances (minimize Z1 - Z2 )

21
 Electrocardiogram Cb Power line 220 V

Power-Line Coupling idb

• Power line is coupled into the body

• Small ac displacement current is
Electrocardiograph
generated, which produces a Z1
common mode voltage cm A
cm
Zin

0.2 50 Ω 10 B
Z2
• At the amplifier inputs: Zin

cm G
10 20 Ω⁄5 Ω 40
ZG idb
• Remedies:
– Reduce or match the electrode Current flows from the power line through the
skin impedances (minimize Z1 - body and ground impedance, thus creating a
Z2 ) common-mode voltage everywhere on the body.
Zin is not only resistive but, as a result of RF
– Increase Zin
43 bypass capacitors at the amplifier input, has a
reactive component as well.

Electrocardiogram
Magnetic Field Coupling
 Sources
 Power lines
 Transformers and
ballasts in fluorescent
lights
 Remedies
Magnetic-field pickup by the elctrocardiograph (a) Lead  Shielding
wires make a closed loop (shaded area) when patient and  Route leads away
electrocardiograph are considered in the circuit. The change from potential
in magnetic field passing through this area induces a current sources
in the loop.  Reduce the effective
(b) This effect can be minimized by twisting the lead wires area of the single-
together and keeping them close to the body in order to turn coil (twist the
subtend a much smaller area. lead wires)
44

22
 Electrocardiogram
Other Noise Sources
 Electromagnetic radiation
 Patient leads become antennas, especially if detached.
 Sources
 Radio
 Television
 Radar
 Research equipment
 Electrosurgical devices
 Arching fluorescent lights (needing replacement)
 Remedy
 Employ capacitors shunting the inputs to ground (eg., 200 pF).
 Do not lower the input impedance of the amplifier.

45

Electrocardiogram
Amplifier Protection
 Electrostatic discharge
 High voltages due to
electrosurgical
equipment
 Leads shorted to high
voltage by hospital
personnel
600mV 2-20V 50-90V  Voltage limiting devices
on each input lead are
used to protect the
equipment

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23
 Electrocardiogram
i
Driven Right Leg Circuit
d

 Patient is not grounded - v3

 Common mode voltage + Ra

is sensed by two
averaging resistors (Ra)
vcm
 Resistor output is - Ra
inverted, amplified, and v4 Rf
fed-back to the right leg +
vcm Common
 Negative feedback drives -mode Ro
the common mode to a RL gain is Auxiliary -
low value unity op amp +
RRL
 Body displacement
current flows to the
inverting OpAmp Minimizes common- mode interference. The circuit
 Provides safety: if the derives common-mode voltage from a pair of
OpAmp saturates, an averaging resistors connected to v3 and v4 in the
alarm sounds; Ro limits instrumentation amp. The right leg is not grounded
current out of the
47 but is connected to output of the auxiliary op amp.
feedback OpAmp.

Voltage and frequency ranges of some common biopotential signals; dc potentials

include intracellular voltages as well as voltages measured from several points on the
body. EOG is the electrooculogram, EEG is the elctroencephalogram, ECG is the
electrocardiogram, EMG is the electromyogram, and AAP is the axon action potential.
48

24
 Electrocardiogram
Preamplifiers
The first stage of the amplifier circuit
 Must be a low-noise device
 Its output is amplified many times, so any noise injected here also gets amplified
many times!
 Should be dc coupled to the electrodes
 Include no series capacitors in the input leads (input bias currents build charge on
series input capacitors).
 To preserve low frequency content of the input signals.
 Use relatively low gain for the preamplifier
 Input bias currents can build charge on polarizable electrodes, creating a dc offset
in the input signals.
 Use a high-input impedance OpAmp to reduce these charging effects.
 High gain will saturate the output of the preamplifier.
 Employ capacitive coupling for later stages of the amplifier circuit to avoid saturation
49 effects.

Electrocardiogram
Preamplifiers
for bias
compen-
sation

 Gain: 800 Low

pass
 DC stage: G=25
(input signals can
be 300 mV)
 AC coupled band-pass
stage: G=32
 With μA 776
OpAmps
 CMRR: 86 dB at
100 Hz
 Noise: 40 mV p-p
 Frequency response
 .05 to 150 Hz
Common- Coupling
 Flat over 4 - 40 Hz mode capacitor
adjustme (high pass)
50
nt

25
 Electrocardiogram
Cardiotachometers

Timing diagram for beat-to-beat

cardiotachometer

Block diagram of a beat-to-beat

51 instantaneous cardiotachometer

Electrocardiogram
Cardiac Monitor
Patient Electrodes Preamplifier Isolation Amplifier

Analog to
digital
Communication Display converter
RAM
port screen

Bus
Microcomputer
CPU

Program Chart Storage Alarm

Keyboard
PROM recorder medium indicator

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26
 Electrocardiogram
Lead-Failure Alarm



through
the
patient

Block diagram of a system used with cardiac monitors to detect increased electrode
impedance, lead wire failure, or electrode fall-off. When the electrode begins to fall off, the
impedance increases and the voltage at 50 Hz rises towards the threshold. When the
threshold is crossed, the alarm sounds. The back-to-back Zener diodes limit the voltage at
the current source output and protect the patient and other electronics from high voltage
53values.

Electrocardiogram
Biotelemetry

Block diagram of a single-channel radiotelemetry system

54

27
 Superior

Diencephalon

The Brain
Cerebrum

- Cerebrum Anterior
Posterior
- Conscious functions
- Brainstem (medulla, pons, midbrain,
Midbrain
diencephalon)
- Connecting link between the cerebral
cortex, spinal cord, and cerebellum
- An integrative center for several visceral Pons
functions (e.g. control of blood pressure Ventral
and ventilation) Cerebellum
Medulla oblongata
- Integration center for various motor
reflexes Caudal
Inferior
- Thalamus: integration center for all of the
general and special sensory systems,
gateway to cerebrum
- Hypothalamus: integrates the function of
the autonomic nerves system.
- Cerebellum (balance and voluntary-somatic
muscle system control)
55

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28
 The Brain
 The cerebrum, showing the four
lobes (frontal, parietal, temporal, and
occipital), the lateral and longitudinal
fissures, and the central sulcus.
 The cortex receives sensory
information from skin, eyes, ears, and other receptors. This information is compared
with previous experience and produces movements in response to these stimuli.
 The outer layer (1.5 – 4.0 mm) of the cerebrum is called cerebral cortex and consist
of a dense collection of nerve cells that appear grey in colour (gray matter).
 The deeper layer consists of axons (or white matter) and collection of cell body.

57

Peripheral nerve Cerebral hemisphere

1

The Brain
Lateral ventricle

Fourth ventricle
2 Spinal cord Thalamus
1. Ascending (sensory) nerve tracts: Third ventricle
3
 From peripheral nerve to CNS
Ascending spinothalamic tract
 Path:
1. Peripheral nerve Thalamocortical radiations

2. Secondary nerve located in the spinal cord (or in the brain stem)
3. Nerve in the brain (thalamus)
 Axon of the secondary neuron crosses to the other side of the cord.
2. Descending (motor) nerve tracts:
 From Cerebrum or cerebellum to motor neurons in the ventral horn of the
spinal cord.
 Control skeletal musculature
 Axon of the secondary neuron crosses to the other side of the cord.
 The two way communication link between the spinal cord and the brain.
Information is transmitted to the brain by means of a frequency-modulated train of
nerve impulses.
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29
 The Brain
 Electrical activity in either ascending or descending nerve fiber tracts may be
represented to a first approximation by an action current dipole oriented in the
direction of propogation
 Recording field potentials non-invasively from the relatively small volume of active
nerve tracts, invariably requires the use cumulative signal averaging techniques.
 The field potentials associated with long nerve tracts depends to a large extent:
 Whether the tract is straight or bent
 The resistance (geometry and specific
conductivity) of the surrounding volume
conductor media
 SER: somatosensory evoked response
 AER: auditory evoked response
 VER: visual evoked response

59

Excitatory
synaptic
input

The Brain 
 Two type of cells in the cortex
Lines of current flow
 Pyramidal cell
 Nonpyramidal cell
 small cell body Apical dendritic tree
Cell body (soma)
 Dendrites spring in all direction
 Axons most of the times don’t leave the cortex
 Unipolar field potentials recorded within the
cortical layers have shown that the cortical surface + Basilar dendrites
Axon
potential is largely due to the net effect of local
postsynaptic potentials of cortical cells.

60

30
 The Brain  Post-synaptic potentials:

1) When an Action Potential reaches

the axon terminal the neuron
releases a neurotransmitter.
2) The neurotransmitter binds to the
receptor.
3) The postsynaptic neuron gets
depolarized (Na+ inward currents –
excitatory - EPSP) or hyperpolarized
(Cl- inward currents – inhibitory -
IPSP).
4) EPSP and IPSP summate temporally
and spatially.
5) If the postsynaptic neuron reaches a
given depolarization threshold, an
action potential is generated.

61

The Brain
 Pyramidal neurons are spatially
aligned and perpendicular to
the cortical surface.

 Thus, EEG represents mainly

the postsynaptic potentials of
pyramidal neurons close to the
recording electrode.

 The electrical activity from

deeper generators gets
dispersed and attenuated by
volume conduction effects.

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31
63

The Brain
 Conducted action potentials in axons contribute little to surface cortical records,
because they usually occur asynchronously in time and at different spatial directions.

 Pyramid cells of the cerebral cortex are oriented vertically, with their long apical
dendrites running parallel to one another. So, the surface records obtained signal
principally the net effect of local postsynaptic potentials of cortical cells.

 Nonpyramidal cells in the neocortex are unlikely to contribute substantially to surface

records because their dendritic trees are radially arranged around their cells, so the
current sum to zero when viewed by electrode at a distance.

 When the sum of dendritic activity is negative relative to the cell, the cell is
depolarized and quite excitable. When it is positive, the cell is hyperpolarized and less
excitable.

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 Electroencephalogram (EEG)
 EEG is a superposition of the volume-conductor fields produced by a variety of active
neuronal current generators. The three type of electrodes to make the measurements are
scalp, cortical, and depth.
 Wave group of the normal cortex
 Alpha wave - 8 to 13 Hz, 20-200 V,
 Recorded mainly at the occipital region
 disappear when subject is sleep, change when subject change focus
 Beta wave (I and II) - 14 to 30Hz,
 During mental activity f=50Hz, beta I disappear during brain activity while beta II
intensified.
 Recorded mainly at the parietal and frontal regions
 Theta wave - 4 to 7 Hz, appear during emotional stress such as disappointment and
frustration
 Recorded at the parietal and temporal regions
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Electroencephalogram (EEG)
 Delta wave
 Below 3.5 Hz, occur in deep sleep, occur
independent of activity
 Occur solely within the cortex,
independent of activities in lower regions of
the brain.
 Synchronization is the underline process that
bring a group of neurons into unified action.
Synaptic interconnection and extracellular
field interaction cause Synchronization.
 Although various regions of the cortex capable
of exhibiting rhythmic activity they require
trigger inputs to excite rhythmicity. The
reticular activation system (RAS) provide
this pacemaker function.
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 Electroencephalogram (EEG)

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Electroencephalogram (EEG)

(a) Different types of normal EEG waves. (b) Replacement of alpha rhythm by an
asynchronous discharge when patient opens eyes. (c) Representative abnormal EEG
68waveforms in different types of epilepsy.

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