Download Chapter 3 Bioelectromagnetism Bioelectromagnetism is sometimes

Chapter 3
Bioelectromagnetism
Bioelectromagnetism
is sometimes equated with bioelectricity. It is
refers to the electrical, magnetic or electromagnetic fields produced by
living cells, tissues or organisms. Examples include the cell membrane
potential and the electric currents that flow in nerves and muscles, as a
result of action potentials. Biological cells use bioelectricity to store
metabolic energy, to do work or trigger internal changes, and to signal
one another. Bioelectromagnetism is the electric current produced by
action potentials along with the magnetic fields they generate through
the phenomenon of electromagnetism.
Bioelectromagnetism is studied primarily through the techniques of
electrophysiology. In the late eighteenth century, the Italian physician
and physicist Luigi Galvani first recorded the phenomenon while
dissecting a frog at a table where he had been conducting experiments
with static electricity. Galvani coined the term animal electricity to
describe the phenomenon, while contemporaries labeled it galvanism.
Galvani and contemporaries regarded muscle activation as resulting
from an electrical fluid or substance in the nerves.
Bioelectromagnetism is an aspect of all living things, including all plants
and animals. Some animals have acute bioelectric sensors, and others,
such as migratory birds, are believed to navigate in part by orienting
with respect to the Earth's magnetic field. Also, sharks are more
sensitive to local interaction in electromagnetic fields than most
humans. Other animals, such as the electric eel, are able to generate
large electric fields outside their bodies.
In the life sciences, biomedical engineering uses concepts of circuit
theory, molecular biology, pharmacology, and bioelectricity.
Bioelectromagnetism is associated with biorhythms and chronobiology.
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Biofeedback is used in physiology and psychology to monitor rhythmic
cycles of physical, mental, and emotional characteristics and as a
technique for teaching the control of bioelectric functions.
Bioelectromagnetism involves the interaction of ions. Their are multiple
categories of Bioelectromagnetism such as brainwaves, myoelectricity
(e.g., heart-muscle phenomena), and other related subdivisions of the
same general bioelectromagnetic phenomena. One such phenomenon
is a brainwave, which neurophysiology studies, where
bioelectromagnetic fluctuations of voltage between parts of the
cerebral cortex are detectable with an electroencephalograph. This is
primarily studied in the brain by way of electroencephalograms.
3.1 Bioelectricity of Cell Membranes
Bioelectricity deals with cell membrane transport processes that
control the formation and dissipation of ion gradients. Ion gradients
store energy in form of an electrochemical potential. This energy can be
converted into other forms of energy. The electrochemical potential is
available to organisms for biosynthesis (photosynthesis and
respiration), transport of metabolites (absorption and secretion),
mechanical work (bacterial flagella rotor, swimming, crawling), and
signaling processes (action potentials). Action potentials are a form of
information used by electrically excitable membranes to control the
activity of cells (calcium signaling, muscle contractility) and to support
or suppress communication between cells (release of chemical signaling
molecules; hormones, neurotransmitters).
3.2 Cell Membrane
Cell membrane is a thin membrane that bound all living cells. It
composed primarily of phospholipids and proteins and are typically
described as phospholipids bi-layer as shown in figure 3.1.
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Figure 3.1 The Cell membrane
The spheres represent the phosphate, which is polar and water soluble
(Hydrophilic). The twin extensions represent the fatty acid components
which are not water soluble (hydrophobic).
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Transport of Ions and Molecules through the Cell Membrane
The membrane consist mainly of two parts:
1) Lipids
Lipid bilayer : It is not miscible with either the extracellular
fluid or the intracellular fluid.
 It constitutes a barrier for the movement of most
water molecules and water-soluble substances.
The type of diffusion here is Simple Diffusion.
2) Protein
Channel Protein
-
- It have watery spaces all the way through
the molecules and allow free movement of
certain ions or molecules.
- Highly selective in the types of molecules
or ions that are allowed to cross the
membrane.
- The type of diffusion here is Simple Diffusion.
Carrier Protein
- It bind with substance that are to be
transported, and conformational changes
in the protein molecules
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- It then move the substance trough the
interstices of the molecule to the other
side of the membrane.
- The type of diffusion here is Facilitated Diffusion.
Figure 3.2 Transportation through cell membrane
Membrane permeability and the rate of diffusion
The different factors that affect cell membrane permeability are:
1) Thickness of the membrane, the greater the thickness, the
less the rate of diffusion.
2) Number of protein channels through which the substance
can pass, the rate of diffusion is directly related to the
number of channels per unit area.
3) The temperature, the greater the temperature, the greater
is the thermal motion of molecules and ions in a solution. So
the diffusion increases directly in proportion to
temperature.
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4) The molecular weight of the diffusing substance, the velocity
of thermal motion of dissolved substance is inversely
proportional to the square root of its molecular weight.
5) Area of the membrane, the permeability is directly
proportional to the area of the membrane.
Where dn/dt is the rate of diffusion, A is the area, d is the
thickness and (C1 - C2) is the difference in concentration.
Where D is the diffusion constant.
d
Where  is the permeability of specific ion.
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While if we write
Thus dn/dt here is the rate of diffusion per unit area.
dn/dt
Simple Diffusion
Facilitated
C%
Figure 3.3 The relation between diffusion rate and concentration
3.3 The Origin of Membrane Potentials
3.3.1 The Rest Potential
It is important to understand the origin of membrane potentials.
Biological membranes are electrical insulators due to their phospholipid
bilayer structure and are impermeable to ions, unless specific ion
channels are temporarily open. In real cells, several different ion types,
each with its own gradient contribute to this charge separation. Any ion
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that forms an ion gradient across a membrane, and that is permeable
contributes to the actual membrane potential. At rest, most cells have a
potential around -40 to -90mV indicating that they are dominated by K
or Cl permeability (see table).
Ion
Intracellular Extracellular
Na+
10 mM
+
K
140 mM
Cl5 mM
Nernst Equation
100 mM
4 mM
125mM
Ratio
In : Out
Nernst
Potential
1 : 10
35 : 1
1 :25
+ 60 mV
 90 mV
 70 mV
V
If we have two liquids differ in
concentration where C1 ≠ C2 Thus
there will be potential (Vc ) due to this
difference where:
C1
C2
If C2 = C1
Thus log 1 = 0
C1 ≠ C2
Figure 3.4 Potential due to difference in concentration
Therefore Vc = zero
For living cell C2 is concentration outside and C1 is concentration inside
This means also it is for only one type of ions
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Thus according to Nernst the total voltage = 90 + 60 – 70 =  100 mV
However, the measured value is different this is due to Nernst
considers that the permeability to all ions is equal and that is not true.
Goldman’s Equation
He solves the mistake of Nernst by considering the permeability of ions
is relative permeability.
Notice that for Cl- ions we use up the inside concentration and down
the outside concentration this is due to the Cl is –ve so the field and
potential is opposite to the other ions.
Electrical equivalent circuit of membrane during rest state
The concentration act as batteries connected in series to a resistor its
magnitude is inversely related to the conductance of the membrane to
the ion.
Resistance of
membrane to Na+
g
Na =
1
VNa+
Resistance of
+60 mV
+
membrane to K
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VK +
g = 100
k
Resistance of
-90 mV
membrane to Cl-
g
Cl =
VCl60 to 150
-70 mV
RMP
-70 mV
Figure 3.4 The equivalent circuit of potential membrane
The large resistance represents the low permeability of the membrane
to Na+. While the small resistance represents the high permeability of
the membrane to K+ and Cl-. RMP results from connecting the 3
batteries resistance combination in parallel.
The membrane act as capacitor charges by RMP – 70 mV battery and
resistance R of membrane.
Outside
RMP
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Rm
Inside
Figure 3.5 The equivalent circuit of cell membrane
3.3.2 The Sodium-Potasium Pump
The process of moving sodium and potassium ions across the cell
membrane is an active transport process involving the hydrolysis of
Adenosine triphosphate (ATP) to provide the necessary energy. It
involves an enzyme refered to as:
Where ADP is Adenosine diphosphate. This process is responsible for
maintaining the large excess of Na+ outside the cell and large excess of
K+ ions on the inside. Figure 3.6 (a - f) shows step by step how this
process happen.
Step 1
K+
K+
Three Na+
ions come
close to the
gate from
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inside and
two K+ ions
Na+
Na+
Na+
Figure 3.6-a Step 1
Step 2
K+
K+
The Na+ ions
enter the
gate and the
ATP come
close
Na+
Na+
Na+
P
K+
K+
+
Na
+
Na
Na
+
P
K+
K+
+
Na
+
Na
Na
+
P
Figure 3.6-b Step 2
K+
Step 3
K+
The ATP stuck
to the gate
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Na+
Na+
Na+
P
K+
K+
+
Na
+
Na
Na
+
P
K+
K+
P
+
Na
+
Na
Na
+
K+
K+
+
Na
+
Na
Na
+
Figure 3.6-c Step 3
Step 4
K+
K+
One Phosphate
remain stuck to
the gate and
the ADP leave.
The gate close
from inside and
open from
outside.
Na+
Na+
Na+
P
K+
K+
+
Na
+
Na
Na
+
P
K+
K+
+
Na
+
Na
Na
+
P
K+
K+
+
Na
+
Na
Na
+
Figure 3.6-d Step 4
Na+
Na+
Na+
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Step 5
K+
K+
The Na+ ions
leave the gate
and go to the
outside and
two K+ ions
enters the
gate
Figure 3.6-e Step 5
Na+
Step 6
Na+
Na+
K+
K+
P
The Phosphate
leave and the
gate open from
inside
Then potassium
ions enter the
inside of cell
K+
K+
+
Na
+
Na
Na
+
Figure 3.6-f Step 6
and close from
outside.
Stimulation
We can classify stimulus into four types:
1Electrical
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2Mechanical
3Thermal
4Chemical
Any stimulus have intensity (height) and duration.
Intensity
Duration
The Threshold value is the value in which below it no action
potential happens.
• If the initial depolarization is equal or more than it:
1- Membrane permeability to Na+ increases.
2- Positive feedback process happens.
3- Reversal of action potential from -70mV  +20 mV.
3.3.3 The Action Potential
Cell membranes have stable potentials (resting potentials) that depend
on the gradients of permeable ions and in excitable cells can be induced
to form self-propagating, dynamic action potentials. An action potential
can be induced when the membrane potential changes electrotonically
reaching a threshold needed to trigger an action potential as shown in
Figure (3.7). The process involves several steps (for example for nerve
cell):
1- A stimulus is received by the dendrites of a nerve cell. This causes
the Na+ channels to open. If the opening is sufficient to drive the
interior potential from -70mV up to -55mV.
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2- Having reached the action threshold, more Na+ channels
(sometimes called voltage-gated channels) open. The Na+ influx
drives the interior of the cell membrain up to +30mV. The process
to this point is called depolarization.
3- The Na+ channels close and the K+ channels open. Since the K+
channels are much slower to open, the depolarization has time to
be completed. Having both Na+ and K+ channels open at same
time would drive the system toward neutrality and prevent the
creation of the action potential.
2
+ 30mV
3
0
4
Depolarization
Repolarization
55mV
Gate
Thershold
Stimulus
6
70mV
Rest Potential
1
90mV
Hyperpolarizatio
5n
Figure 3.7 The action potential curve
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4- With the K+ channels open, the membrane begins to repolarize
back toward to rest potential.
5- The repolarization typically overshoots the rest potential to about
-90mV. This is called hyperpolatization. The hyperpolarization
prevents the neuron from receiving another stimulus during this
time, or at least raises the threshold foe any new stimulus.
6- After hyperpolarization, the Na+/K+ pump eventually brings the
membrane back to its resting state of -70mV.
3.3 Membrane Currents
Measuring membrane currents instead of potentials has been the way
of understanding mechanisms underlying action potentials. Membrane
currents are the result of opening ion selective channels which causes
ions to flow across cell membranes. This flow is spontaneous because
all ion types are distributed unevenly between cellular and extracellular
compartments. In general, cell contain high loads of K+, but low Na+
and Ca++ ions, while extracellular fluids contain high Na+ and Ca++
ions, but low K+ concentrations. Accordingly, ion gradients ranging
from 10 to 10,000 fold depending on the ion species exist. When
channels are activated, ions will always start diffusing through the
pores in either direction, although more ions will flow from the high to
the low concentration (down hill). These ion diffusion is an important
part of bioelectricity maintaining resting potentials and generating
action potentials. It is also used to couple the transport of secondary
solutes that can be up concentrated inside or outside according to
metabolic needs. Finally, ATP hydrolyzing pumps reverse the flow of
ions regenerating the gradients dissipated by the activity of channels
and secondary transporters. Summarily, chemical energy is used to
maintain the formation and use of membrane potentials and ion
gradients. While almost all transporters somehow involve the flux of
ions across membranes, ion channels are unique in their fast kinetics
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facilitating the flow of up to 10 million ions per second. Pumps work at
a roughly 10,000 fold slower rate. The methods to study membrane
currents are voltage clamp (two electrodes) and patch clamp (one
electrode) techniques. The latter allows the measurement of currents
through single channel units, while the former is used to measure
macroscopic currents, which are the result of the simultaneous activity
of hundreds to thousands of channels. The noise recorded in the early
days of electrophysiology indicated the presence of unit conductances
which later have been correlated with the presence of ion channels, the
physical units of electrical conductivity in cell membranes.
3.3.1 Electrocardiography
Electrocardiography is a quick, simple, painless procedure in which the
heart's electrical impulses are amplified and recorded on a piece of
paper. This record, the electrocardiogram (ECG), provides information
about the part of the heart that triggers each heartbeat (the
pacemaker), the nerve conduction pathways of the heart, and the rate
and rhythm of the heart. Usually, an ECG is obtained if a heart disorder
is suspected. It is also obtained as part of a routine physical
examination for most middle-aged and older people, even if they have
no evidence of a heart disorder. It can be used as a basis of comparison
with later ECGs if a heart disorder develops.
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Figure 3.8 The electrocardiogram
An electrocardiogram (ECG) represents the electrical current moving
through the heart during a heartbeat. The current's movement is
divided into parts, and each part is given an alphabetic designation in
the ECG. Each heartbeat begins with an impulse from the heart's
pacemaker (sinus or sinoatrial node). This impulse activates the upper
chambers of the heart (atria). The P wave represents activation of the
atria. Next, the electrical current flows down to the lower chambers of
the heart (ventricles). The QRS complex represents activation of the
ventricles. The electrical current then spreads back over the ventricles
in the opposite direction. This activity is called the recovery wave,
which is represented by the T wave. Many kinds of abnormalities can
often be seen on an ECG. They include a previous heart attack
(myocardial infarction), an abnormal heart rhythm (arrhythmia), an
inadequate supply of blood and oxygen to the heart (ischemia), and
excessive thickening (hypertrophy) of the heart's muscular walls.
Certain abnormalities on ECG can also suggest bulges (aneurysms) that
develop in weak areas of the heart's walls. Aneurysms may result from
a heart attack. If the rhythm is abnormal (too fast, too slow, or
irregular), the ECG may also indicate where in the heart the abnormal
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rhythm starts. Such information helps doctors begin to determine the
cause. To obtain an ECG, an examiner places electrodes (small round
sensors that stick to the skin) on the person's arms, legs, and chest. These
electrodes measure the magnitude and direction of electrical currents in
the heart during each heartbeat. The electrodes are connected by wires to
a machine, which produces a record (tracing) for each electrode. Each
tracing shows the electrical activity of the heart from different angles.
The tracings constitute the ECG. ECG takes about 3 minutes, is painless,
and has no risks.
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