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CH. 7/ Electricity within the body
Across membrane of neuron an electrical potential (voltage) due to
presence of more negative ions on the inside of the membrane than
Neuron is to be polarized, the inside of the cell is 60-90 mV more
negative than out side and this represent the resting potential . if
stimulation heat, cold, light, sound cause change in action potential
Fig a : resting potential of axon= - 80mV
- - - - ++ - - - ----
- ---------
++ -- - -
- - - - -- - - - - -
+ + + +- - - - - + + + +
b . depolarized
a. polarized
inside go positive to 50 mV. The action potential last few m sec for most
neuron and muscles, and last 150-300 m sec for cardiac muscle. The
membrane of some axons is covered with fatty insulating layer called
myelin has small uninsulated gaps called nodes of ranvier.
The action potential decreases in amplitude as it travels through
myelinated segment just on electrical signal is attenuated when it passes
through a cable . the reduced signal then acts like a stimulates at the next
node of ranvier (gap) to restore the action so its original size and shape,
this process repeats a long the axon. The action potential seems to jumb
from one anode to the next , it travels by salutatory conduction.
Electrical signals from muscles …Electromyogram.
The record of the potentials from muscles during movement is called
Electromyogram . the conduction velocity of
measured its typical value are 40-60 m/sec
sensory nerves was
……Stimulus1 …
0.25 m
…stimulus 2
Last figure is a method of measuring the motor nerve conduction
velocity. The latency period for the response to stimulus 1 is 4 sec longer
than that for response to stimulus 2.
... t = 4*10-3
X= 0.25 m difference in distance
V=x/t = 62.5 m/sec = the nerve conduction velocity.
Electrical signals from the heart electrocardiogram :
The electrical signals from SA node (sinoaterial) or pacemaker initiate the
depolarization of the nerve and muscles of both atria, causing atria to
contract and pump blood into ventricles.
The electrical signals then passes through aterioventricle AV anode,
which initiates depolarization of right and left ventricles causing them to
contract and force blood to pulmonary and general circulations. The
ventricle nerves and muscles then repolarized and the sequence begins
again. The major electrical events of the normal heart cycle as:
1. the atrial depolarization, which produce (p) wave
2. the atrial repolarization, which is rarely seen and is unlabeled .
3. the ventricular depolarization which produces (QRS).
4. the ventricular repolarization which produces (T) wave.
time sec
Electrical signal from the brain : electroencephalogram
the recording of brain signals is called (EEG). The frequencies of EEG
signals depend on mental activity.
For example: relaxed person has EEG signals pf 8-13 Hz or α waves
more alert person has above 13 Hz or β waves.
Application of EEG :
1. EEG useful in diagnosis of epilepsy
2. in confirming brain tumors, since electrical activity is reduced in
the region of tumor.
3. used as monitor in surgery when ECG can not use also useful for
indicating the anesthesia level of the patient.
Electrical signals from the eye electroretinogram and electrooculogram
The recording of potential changes produced by the eyes when the retina
is exposed to flash of light called ERG . The recording of potential
changes due to eye movement called EOG.
Magnetic signals from heart and brain magneto cardiogram and magneto
Since a flow of electrical charge produces magnetic field. A magnetic
field produced by the current in the heart during depolarization and
repolarization. The recording of heart magnetic field is MCG it provide
information in diagnosis if injury current exists in the heart prior to heart
attack. The recording of magnetic field surrounding the brain called MEG
Current research involving electricity in the body
Bone contains of : collagen which is piezoelectric material when force is
applied to collagen, small electrical potential is generated. Collagen
behaves like N-type semiconductor its current like negative charge.
Mineral crystals of bone apatite close to collagen behave like P-type
semiconductor its current by positive charge.
At junction current flows from P to N type. The force on bones produces
potential by piezoelectric and PN junction of collagen –apatite produce
currents that induce and control bone growth.
Another small direct current arises in injured zone called injury current
which is higher than that in surrounding areas. This high potential
associated with limb regeneration in animals like salamander. Stimulation
of fracture sites with direct current of 1-3 micro ampere has been found to
promote healing of bone fractures.