ECE/BIOM 537:
Biomedical Signal Processing
Colorado
State University
Student:
Minh Anh Nguyen
There are several definitions of electric
activities of cells which can be found on the internet and in the Biomedical
Signals and Images textbook. One
definition may provide a clearer concept than another. Chapter 8 of the textbook, Biomedical
Signals and Image Processing, provided a definition of Electric Activities of cells. According to the textbook definition, the
electric activities of the cells constitute a major physical property that
allows a wide spectrum of functionalities such as messaging, cellular
communications, timing of cellular activities and even regulation of
practically all biological systems. However, in my opinion, this definition is
too shallow, especially for some students or readers who do not have strong
backgrounds in cell biology and electrical characteristics. Cell biology and
their electric activities are two separate topics.
Cells comprise all living organisms or
tissues, and give life to all creatures and growing organisms. Cells are
typically surrounded by water or fluid. There are several different kinds of
cells, depending on the characteristics and size of the cells. Each type of
cell has its own individual function and purpose. For example, nerve cells
carry electrical impulses, and muscle cells, which include skeletal muscles,
cardiac muscles, and smooth muscles, provide contraction and allowing the body
to move. The Connecting Human Biology & Health Choices book
provided this definition of nerve and muscle cells: 1) Nerve tissue carries
messages from the brain to the rest of your body by sending electrical
impulses. The electrical impulses tell the muscles when to contract affecting
everything from the beating of your heart to the wiggling of your toes. 2)
Muscle tissue functions in movement because the muscle cells contract, shorten
and relax. Muscle cells know when to
contract or shorten because they receive electrical signals from nerve tissue. Cells need to take nutrients from the outside
and remove waste products from internal chemical reactions. Cells have cell membranes,
which hold the cell together and keep the cell separated from its environment.
The main functions of membranes are to control what can go into and out of the
cell, and prevent ions, proteins and other molecules from diffusing into areas
where they should not be. The cell membrane is not a solid structure, rather it
is made of a phospholipid bilayer, which has a polar (hydrophilic) head and two
nonpolar (hydrophobic) tails, and proteins floating in it. These proteins
attach to both the inside and outside surface of the cells. The phospholipid
bilayer allows small nonpolar molecules, for example, hydrocarbons, carbon
dioxide and oxygen to cross through easily. These phospholipids are typically
arranged tail to tail so the nonpolar areas form a hydrophobic region on the
inside of the cell membrane between the hydrophilic heads which form the outer
surfaces of the membrane. This
double-layered sheet has very unique properties: 1) heads are attracted to water because polar molecules
interact with other polar molecules and ions they are located on the outside of
the cell. 2) Tails repel water because
nonpolar molecules do not interact with polar molecules, and they are located
the middle of the membrane. This creates
two surfaces of molecules called the lipid layer. The tail to tail region is
surrounded by cholesterol or fatty acid, which helps to regulate the fluidity of the membrane and makes the
cells flexible. The membrane must be fluid to work properly. Blood and antibody
antigens are proteins which are normally found on the outside of the cell. The main purposes of these proteins are to
interact with any molecule outside the cells, transport molecules into and out
of the cells, and bind to other cells in wound healing and immune
response. Therefore, in animal and
human cells, the cell membrane is very important because it keeps unwanted
particles from entering the cell body and protects the organelles.
Cell membrane transport procedures control the formation
and dissipation of ion gradients. Ion gradients store energy in the form of an
electrochemical potential. This energy can be converted into other forms of
energy. There are several methods of
proteins transport through or across the cell membrane. The first method is
active transport. In this technique, proteins contained in the cell's lipid
bilayer are able to move molecules and ions into or out of the cells only when
they cross the bilayer. For example, if
proteins move glucose, then they will not move calcium (Ca) ions because the
membrane proteins are very specific. The
movement of molecules from a region of higher concentration to a region of
lower concentration without any energy required by cells is known as diffusion.
The movement of diffusion is due to collisions among the particles. If the areas have higher concentration of
particles, then there will be more collisions in those areas. These collisions create energy. Particles
with higher temperatures will move faster because they have more energy; as a
result, the overall energy of movement is proportional to the temperature. The
diffusion rate increases when temperature increases because the particles move
faster, the collisions among particles become more energetic, causing particles
to move from areas of higher concentration to lower concentration at a faster
rate. Particles with a large size and mass move slower, and may be influenced
by numerous collisions with many nearby smaller particles. Particles which have
a smaller size and mass will move faster. The movement of specific molecules
across cell membranes through protein channels is known as facilitated
diffusion. This movement of molecules also does not require any energy because
particles move from high to low concentration along a gradient; as a result,
the energy is generated by the concentration gradient. In the neurons, proteins have to work
against a concentration gradient, constantly pumping ions from areas of lower
to higher concentration or in and out to get the membrane of the neuron ready
to transmit electrical impulses.
In order for cell membrane to move molecules
in and out of the cells, the cell membrane must have electrical gradient across
it. The
electrical gradient represents the difference in electrical charge across the
membrane and opposes the chemical gradient. Since the membrane is very thin, it will generate a small
amount of voltage but enough to pump ions into and out of cells. This subject
is related to electric charge or current and voltage, which is the second
topic.
According
to the section 8.2.1 of chapter 8, Transmembrane Potential, a cell-layer
thickness distance from the cell membrane is approximately 1um thickness. All cell membrane features can be modelled as
an electrical circuit by using the concepts of capacitance, resistance, and a
battery as charge. Ion gradients of a
membrane are measured by electrodes attached to volt meters. Some of the measurement values of ion
concentrations are provided in the table 8.1 of chapter 8. The voltage equation that
describes the relationship between ion gradients and the membrane potential of
cell membranes is the Goldman-Hodgkin-Katz voltage equation, which is provided
in section 8.2.1. It
determines the relationship between equilibrium voltage, the membrane potential
and concentrations of sodium, potassium, and chlorine (Na, K, and Cl) ions. At
rest, muscle cells have a potential -90mV and nerve cells have potential -70mV. This Goldman equation also can
be used to describe the balance of a cell under the conditions that Na and Cl
current are equal to each other and are not equal to zero, which is explained
in the section 8.2.1 as well. This condition happens when more than one ion
channel is open in the membrane. For example, if many channels of Na open, the
PNa (membrane permeability of Na)
will be high. If only a few Na channels open, the PNa will be small.
If all Na channels are closed or if no Na channels exist in the membrane,
the PNa will be zero. If pk (membrane
permeability of K) is much larger than pNa
and pCl, Vm (the Goldman-Hodgkin-Katz
voltage) will be closer to the equilibrium potential for K (VK)
than it will be to the equilibrium potential for Na+ (VNa)
or Cl- (VCl).
The Vm will be exactly at VK when the pNa
= pCl = 0. Since K is bigger, it means the concentration
inside the cell membrane is greater than the concentration outside; therefore,
diffusion gradients are directed towards the outside making the cell negative.
The electric field will flow toward the negative or from inside to outside.
This condition is known as a concentration imbalance of ions and it causes a
resting potential.
When all channels are closed, ions are not
able to enter the cell; this is a hyperpolarization condition because the
action potential is increased. When all channels are open, ions are rushed into
the inside of the cell; this is a depolarization condition because the action
potential is decreased. When diffusion and electrical forces are competing
again each other, a steady state is occurred. This means that the cell membrane
is polarized. The action potential is known as a property of the cell and is
defined as a rapid change in the membrane potential to return to the resting
membrane potential. The action potential results from a chemical and electrical
stimulus. The action potential is important to the functioning of the brain
because it creates information in the nervous system. The action potential is described in detail
in section 8.3.4 of chapter 8.
Repolarization happens when the depolarization stage continues until the
maximum positive potential is reached after cell start, which is defined in
section 8.3.4.
The phospholipid bilayers are quite good
insulators, so there are no free ions in the membrane and no carriers to
transport charges. This is known as a conductance. The resistance value can be
calculated by inversing the value of conductance or one divided by conductance.
A resistor can be used to represent the leakage of current through the
membrane. In section 8.3.3, Resistance,
R, is defined as the reciprocal of the conductance and it has unit Ohm. There are many kinds of ion channels and
other pores penetrating the membrane and allowing additional current to flow.
Current usually flows from positive to negative. This current makes cells
behave in complex and interesting ways.
The inside and the outside of the cell are surrounded by various salts
in water, which are referred to as two conductors separated by an insulator,
the membrane. This makes it possible to have different amounts of electrical
charges inside and outside the cell, and it acts a parallel plate capacitor.
Capacitors formed by any conductor have surfaces and charges can accumulate on
those surfaces, and can influence any close-by conductors. A capacitor is a
device which stores electric charge and potential energy, and it has
capacitance property. The membrane capacitance is explained and an equation to
calculate the capacitance is provided in section 8.3.2. For a parallel plate capacitor, the
capacitance will have a bigger value if the areas of plates are larger and the
distance between two plates is smaller without breakdown. The capacitance value can be calculated by
dividing the area of the plates by the distance separation between the plates.
According to section 8.3.2, the charges are separated by the thickness of the
membrane, which is only a few molecular chains thick and is the order of 7.5
nm. The capacitance of a typical cell membrane is relatively high. This is due
to the fact that thickness of the cell membrane is very small while the area of
the membrane is relatively large. The capacitance value for muscle cells and
neurons is 1uF/cm2. This information is very useful because it can
be used to create a model that represents a cell membrane.
There
is a relationship between the electric potential, which is created across the
plates, and the amount of charge on the plates. The amount of energy stored in
a capacitor can be calculated by multiplying the capacitance with the potential
difference (potential voltage difference). The electric field can be related to
the potential difference, which is created across the parallel plate
capacitor. The capacitor’s main function
is to store electric charges or energy. The movement of electric charges around
a closed loop is known as electric current.
The electric current (I) has unit of Ampere meaning one Coulomb of charge passes any point in one
second of time. The electric current can
be estimated by dividing total charge by total time in seconds. Therefore, if
the total charge is large then the total amount of current will be large. The flow of current is due to electron flow,
which responds to the nature of the electric field or potential. This current
is a direct current (DC). If the electric field or potential is constant in
time, the flow of current will be constant in time and convention is only one
direction. One example of a medical device which contains a capacitor is a
defibrillator. This device gives a strong electric shock to the heart, which
helps restore normal contract rhythms in a heart when a patient has had a
cardiac arrest, a process which is described in the Defibrillation article
published on the American Heart Association website.
Cells and electric properties appear to be
two separate topics but some of the characteristics of them are same when compared
side by side, which can be useful to both biology and electrical students or
readers. The cell is very small and the main function unit in organisms, and it
is a building block of the body. Cells
usually bind together to form a tissue.
The outer boundary of a cell is a membrane which is composed of a
bilayer of phospholipids with a thickness 7.5 nm. A cell has four zones: the innermost or
central zone is negative because it has protein and amino acids, the inner zone
is positive because it has cations or potassium, the outer zone is positive
because it has ions or sodium, calcium and potassium, and the outermost zone is
negative because it has glycolipids. The
two negative zones or layers are the most important layers because these layers
are responsible for the electrical charges of the cell and change the cell
properties. The inside of a cell has higher potassium (K) and low sodium. The
outside of a cell has low potassium, high sodium and calcium. There are electrical charges inside the cell
that move in and out of the cell. Some of the electrical charges are bound to
be inside and can be found in either single or compound form. Since some ions
are able to move from the inside to outside or from the outside to inside, they
create a current or convection current.
When Ions flow they create an electric field, which opposes flow until
equilibrium is reached. This concept is
similar to P-N junction, which electrical engineering students studied in the
semiconductor topic, where ions move by diffusion and create a potential
difference that prevents further flow of charged ions. The electricity of the
cell is important because it is a signal which comes to the cell in a form of
electric charge and it is one way that cells communicate with others. A disease
will happen if this communication was to fail or not work properly. If the
disease occurred, then external electricity or electric devices will be used to
treat it or to correct electrical charges within the cell. However, there are also some differences
between electrical and biological systems. The differences between electrical
and biological circuits are: 1) the electric circuit uses electrons and needs
occasional replacement of components. An electric charge must exist in a dry
environment and moved without leakage otherwise shock will occur. Energy is
needed only when the circuit is working and getting a faster response. Electric circuits can work in long pathways.
2) The biological circuit uses atoms and ions. Components of the circuit are always
charging. The electric charge must exist in a wet environment and there needs
to be area charge differences and continuous leakage. Energy is needed all the
time and there is a slower response rate. Biological circuits only work in
short pathways or distances. These differences are important to biomedical
students because they need to understand them in order to design and use
bioelectricity correctly; especially, if there is a change in cell function.
References:
1. Biomedical Signal and Image
Processing, 2nd. ed. by K. Najarian and R.
Splinter, CRC Press, 2012.
3. Defibrillation:http://www.heart.org/HEARTORG/Conditions/Arrhythmia/PreventionTreatmentofArrhythmia/Defibrillation_UCM_305002_Article.jsp
4.
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