Download Electricity Wthin The Body

Medical physics/Al-Kindy college of medicine
Dr. Ekhlass Jawad
Electricity within the Body
All functions in the body, voluntary & in voluntary electricity are involved. So all
movements are related to electrical movement.
There are 2 aspects of electricity & magnetism in medicine:
1 – Electrical & magnetic effects generated inside the body.
2 –using of electricity & magnetism to measure some of body functions. such as
ECG,EEG,EMG,ERG,EOG
The electricity generated inside the body serves for the control & operation of nerves,
muscles, & other organs.
The nervous system plays an important rule in every part of body the brain receives
internal & external signal & usually makes proper response. The information is transmitted as
electrical signals along various nerves. This efficient communication system can handle many
millions of pieces of information at one time with great speed.
Nervous System & the Neuron
Nervous system can be divided into two parts.
1 – Central Nervous sys: consist of brain, spinal cord & the peripheral nerves.
Nerve fibers (neurons) that transmit sensory information to brain or spinal cord are
"afferent nerves" and those that transmit information from brain or spinal cord to the
muscles & glands are "efferent nerves".
2 – Autonomic Nervous System: controls various internal organs such as heart "intestines &
glands. The control of this system is involuntary.
Neuron: is the basic structure unit of nervous system. This is specialized for reception,
interpretation & transmission of electrical signals.
Contents of neuron (nerve cell):1 – Cell body: receives electrical messages from another nervous through contacts called
"synapses", which are located on the "dendrites".
Dendrites: are the part of the neuron specialized for receiving information from other cells,
(or stimuli)
2 – Axon: (or nerve fibers) this carries the electrical signal to muscles, glands, or other
neurons. As long as 1 m.
It's usually covered by myelin sheath, except some parts called "Nodes of Ranvier"
Synapse: permit the transparent of a signal on one direction & prevent it from going back.
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Medical physics/Al-Kindy college of medicine
Dr. Ekhlass Jawad
Electrical Potentials of Nerves:
*Across the surface or membrane of every neuron is an
electrical potential (voltage) difference due to the presence of more "negative ions" on the
inside of the membrane than the outside, 60 to 90 mv it's polarized.
* This potential difference is called Resting Potential of the neuron. When the neuron is
stimulated, a momentary charge will happen from positive to negative; this potential change is
called Action Potential. Propagates along the axon.
The stimulation may be caused by heat, cold, light, sound and odors.
By convention, we measure the transmembrane potential difference by taking the
potential inside minus the potential outside. In virtually all cells, this gives a
negative voltage. Nerve cells, for example, have a membrane potential of about
-70 mV, with the negative sign indicating that the inside is negative, i.e. has an
excess of negative charges:
The first question is why this potential difference exists--what produces the
separation of change across the cell membrane?
There are two basic reasons:
1. The chemical compositions of the fluids inside and outside the cell are different.
2. The membrane allows some ions to enter and leave the cell more easily than
others.
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Medical physics/Al-Kindy college of medicine
Dr. Ekhlass Jawad
 When membrane is stimulated:
- becomes permeable to Na+ ions which diffuse due
to the negative charge
 Potential rises initially to 0 mV (depolarisation)
& then to +30 mV (reverse polarisation)
 Membrane becomes impermeable to Na+ ions &
they are trapped within nerve cell
 K+ ions diffuse out of the membrane which restores
the potential (repolarisation)
 Process takes about 2 ms
Then the K+ ions are pumped out, process takes about
50 m
* Signals are stopped at nodes of Ranvier, & by having another amplification the signal
will transmit to another node till it reaches the muscles or the other cell, this is done by the
action potential.
Two primary factors affect the velocity of propagation of the action potential:1- The Resistance within the core of the membrane.
2- The Capacitance (or the charge stored) across the membrane.
*A decrease in either will increase the propagation velocity.
* Internal resistance of axon decrease when its diameter increased, that will cause
increasing in the velocity of propagation of action potential than that of a small diameter.
* The greater the stored charge on a membrane, the longer it takes to depolarize it i.e. the
slower propagation speed.
* Hence the charge stored in a myelinated section of axon is very small i.e. very faster the
unmyelinated section which high charge stored.
Electrical Signals from Muscles {electromyogram} ( EMG):
*The record of potentials from muscles during movement is called EMG.
* A muscles made up of many motor units, cache one consist of a single branching neuron
from spinal cord.
* Resting potential across the membrane of muscle fibers is similar to that of a nerve fiber.
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Medical physics/Al-Kindy college of medicine
Dr. Ekhlass Jawad
Muscle Action: This is initiated by an action potential that travels along an axon & is
transmitted across the motor and plates into the muscle fibers, causing them the contraction.
* Single muscle cells are usually not monitored in EMG examination because it's difficult
to isolate a single fiber so there are 2 methods for obtaining EMG:1 - Surface electrode (plate electrode).
2 – Concentric needle electrode.
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Medical physics/Al-Kindy college of medicine
Dr. Ekhlass Jawad
Surface electrode that attached to the skin
measures the electrical signals from many
motor units. While needle electrode inserted
under the skin measures single motor unit
activity by means of insulated wires
connected to its point.
*Action of EMG:
1 – Action potential appears in EMG after a
latency period
(Time between stimulation and the beginning
of the response).
2 – EMGs of symmetrical muscles of the body
are compared to each other, or to those of
normal individuals, to determine the action
potential & latency periods.
3 – Determining the velocity of the action
potential in motor nerves many times. Nerve
damage may result decrease in velocity.
Electrical Signals From the Heart –{Electrocardiogram} ECG:
1- The action of the heart is controlled by an electrical signal initiated by "Sinoatrial (SA)
node" which is spontaneous stimulation of special muscle cells located in the right atrium and
called the natural pacemaker of the heart.
2-* SA node controls the pulse rate (70 pulses per min); it decreases or increases the
pulse rate according to the demands of body to the blood.
3-* The electrical signal from SA node initiate depolarization of nerves & muscles of
both Atria causing them contraction & pumping blood into ventricles, Repolarization of atria
follows.
4-* Electrical signal passes into artrioventricual (AV) node, this initiates depolarization
of right & left ventricles, causing them contraction & force blood into the two systems, then
ventricles are repolized & so forth.
Major Electrical Events of Normal Hear t:
1 – Atrial depolarization (contraction)
produce P wave.
2 – Atrial repolarization (relaxation)
rarely seen & is unlabeled.
3 – Ventricles depolarization
QRS complex.
4 – Ventricles repolarization
T wave.
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Medical physics/Al-Kindy college of medicine
Dr. Ekhlass Jawad
Measuring ECG:
The ECG is the recording of the electrical potential of the heart that extend to the body
surface. By placing the electrodes of an ECG instrument on the skin surface, you can record
the waves of depolarization and repolarization that are generated by the cardiac muscle. The
apparatus used is called the electrocardiograph;
A standard ECG consists of 12 leads:
3 Bipolar standard limbs lead (I, II, III).
3 unipolar limb leads (aVR, aVL, aVF).
6 unipolar chest leads.
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Medical physics/Al-Kindy college of medicine
Dr. Ekhlass Jawad
Notice:1- The potentials measured on the surface of the body depend on the location of the
electrodes.
2- Surface electrodes for obtaining ECG are most commonly located in left arm (LA),
right arm (RA), & left leg (LL).
Surface Electrodes in the Frontal Plane:These leads record the differences between the potentials in
2 limbs, by applying electrodes usually at the wrist and
ankle. The 3 standard bipolar limb leads include:
Lead I: This records the difference between the potential in
the left arm (LA) and that in the right arm (RA).
Lead 11: This records the difference between the potential
in the right arm (RA) and that in the left leg (LL).
Lead III: This records the difference between the potential
in the left leg (LL) and that in the left arm (LA).
* The potential between any two electrodes gives the relative amplitude & direction of
the electric dipole vector in the frontal.
The Rest Augmented lead configurations
Unipolar limb leads (aVR, aVL, aVF):
These measure the actual potential at a certain point. This is
carried out by applying one electrode from the electrocardiograph to
the desired point (it is active, +ve or exploring electrode) while the
other electrode represents a common reference point inside the
instrument; it is the -ve electrode (0 potential) i.e. the unipolar leads measure the potential
differences between active electrodes and zero potential.
aVR: one side of the electrodes connect to RA the other connect to the center of the
resistance between LL & LA
aVL : one side of the electrodes connect to LA& the other connect to the center of the
resistance between LL & RA
aVf : one side of the electrodes connect to
LL and the other connect to the center of the
resistance between RA & LA
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Medical physics/Al-Kindy college of medicine
Dr. Ekhlass Jawad
Unipolar chest leads:
Unipolar leads (chest leads) record the absolute potential at 6 standard points on the anterior
chest wall designated as V1 to V6, the locations of which are as follows:
V1: At the right margin of the sternum in the 4th right intercostal space.
V2: At the left margin of the sternum in the 4th left intercostal space.
V3: Midway between V2 and V4.
V4: At the left midclavicular line in the 5th intercostal space.
V5: At the left anterior axillary line in the 5th intercostal space.
V6: At the left midaxillary line in the 5th intercostal space.
Leads V1 & V2 look at the right ventricle and reflect its activity, V3 & V4 look at the
interventricular septum and reflect its activity, while leads V5 & V6 look at the left ventricle
and reflect its activity
The ECG can be used to diagnose problems with the heart which include:
 Arythmia, an irregular pumping pattern which is quite common in young people,
increased by exercise.
 Blockage of part of the blood supply for the heart. This can lead to the heart muscle
getting tired, or in extreme cases death of the heart muscle, myocardial infarction
(heart attack).
 Fibrillation in which there is no co-ordinate pumping activity, which will lead to death
if not treated very quickly.
Defibrillation is a dramatic intervention
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Medical physics/Al-Kindy college of medicine
Dr. Ekhlass Jawad
Electrical Signals from the Brain – {Electroencephalogram} EEG –
The recording of the signals from the brain is called electroencephalogram or (EEG),
which are due to primarily to the electrical activity of the neurons in the cortex of the brain.
* Electrode are small discs made of chloride silver, these are attached to the head at
locations that depend upon the part of the brain to be studied.
* These electrodes out on the head at some areas, we record only the potential difference
response to that area.
* The reference electrode is attached to the ear (A1 & A2). In routine exams 10 to 20
channels are recorded.
* Asymmetrical activity is often an indication of brain disease, the right side signals
compared to the left one.
* The amplitude of EEG signals is low (~ 50 v( .
* The external noise is controlled, the potentials of muscle activity, such as eye
movement, can cause artifacts in the record.
EEG is generally described in terms of its frequency band. The amplitude of the EEG
shows a great deal of variability depending on external stimulation as well as internal mental
states. Delta, theta, alpha, beta and gamma are the names of the different EEG frequency
bands which relate to various brain states, as described in the following pages.
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Medical physics/Al-Kindy college of medicine
Dr. Ekhlass Jawad
Evoked Response:
Signal that result when the brain receives external stimuli, such as flashing lights or
pulses of sound, if there is no response then this is lack of response which is called
habituation.
Electrical Signals from the Eye:
I – Electroretinogram ERG:
The recording of potential changes produced by the eye when retina is exposed to a
flash of light.
One electrode is located in a contact lens that fits over the cornea & the other one
attached to the ear or forehead to approximate the potential at the back of eye.
* ERG is more complicated than a nerve axon signal because it’s the sum of many effects
taking place within the eye.
B wave is the most interesting clinically since it
arise in the retina .Its absent in the ERG of patients with
inflammation of the retina.
II – Electroculogram EOG:
It’s the recording of potential changes due to eye
movement.
* A pair of electrodes is attached near the eye.
* EOG provides.
1 – Information on orientation of eye.
2 – Its angular velocity.& its angular acceleration.
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