electricity

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 outside.
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 sensory nerves was measured its typical value are 40-60 m/sec







……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.



Potential R R

p T P

Q S

0 0.5 1 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 ? waves more alert person has above 13 Hz or ? 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 encephalogram
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.