Electricity within the body
(PHR 177)Course
Prof. Dr. Moustafa. M. Mohamed
Vice Dean
Faculty of Allied Medical Science
Pharos University
Alexandria
Dr. Mervat Mostafa
Department of Medical Biophysics
Pharos University
Study Of Electricity Is Important For Two
Reasons
• All living organisms are controlled by electrical
signals derived from sensors which respond to
changes in the environment
• Electrical and electronic circuits are widely used
in biological measurements and nowadays highly
advanced electronic systems are used in medical
fields for diagnostic and therapeutic purpose.
• In a biological system, the electric
conduction is
quite complicated
because of the presence of different
types of electric current carriers.
• The mobility of the electrons are much
higher than the ionic or molecular
mobility
The Biological Function Of The Living Cell
1. Keep the composition of the cell inside.
2. Allows the transport of certain ions to inside
and outside the cell.
3. Receives electrical information from other
surrounding cells and passes this information to
the nucleus of the cell.
4. Responsible for cell to cell communications.
• In biological cell membranes and nerves, a resting
potential is caused by differences in the concentration
of ions inside and outside the biological membrane
and by biological membrane and by differences in the
permeability of the cell membrane to different ions.
The Cells Membranes
• The cells membranes have a common
characteristic relating to the presence of a
potential difference between the interior
difference between the interior and the exterior
of the membrane.
Ion concentrations
The Cell Membrane is Semi-Permeable
Nernst Equation
• The equilibrium potential difference for an ion
can be found from the Nernst equation:
𝐾𝑇
𝐶1
𝑉 = ±2.3
𝑙𝑜𝑔
𝑒
𝐶2
Where
• K is Boltzmann's constant (1.38x 10-23 J/K),
• T is the absolute temperature of the medium
[(273+t) Kelvin)] and
• "e" is the charge of the electron (1.6x10-19
Coulomb).
• The migration of the K+ ions through the membrane
will cause a potential difference between the two
faces of the membrane leading to the formation of a
potential hill against the movement of the
positive K+ ions, till an equilibrium state takes
place.
• This equilibrium occurs when the thermal energy of
the K+ ions equals the height of the potential hill
By applying Nernst Potential, at normal body
temperature (37℃) the quantity KT/e is;
𝐾𝑇
𝑒
=
(1.38∗10−23 𝐽/𝐾)(273+37)
1.6∗10−19 𝐶𝑜𝑢𝑙𝑜𝑚𝑏
=0.0267 V = 26.7 mV
So the Nernst Potential is:
V= ±2.3*26.7
𝐶1
mV(𝑙𝑜𝑔 )
𝐶2
V= ± 61.4
=
𝐶1
± 61.4 mV(𝑙𝑜𝑔 )
𝐶2
𝐶1
mV(𝑙𝑜𝑔 )
𝐶2
• For a nerve cell, the intracellular has a K+
ions concentration of 0.141 mol/l
• whereas the extracellular fluid has K+ ions
concentration of only 0.005 mol/l,
0.141
= −(61.4 mV)(𝑙𝑜𝑔
) = −89.2 mV
0.005
The structure of the nerve cells (neuron)
Schwann Cells
•
•
•
•
•
These cells form a multilayered myelin sheath.
Reducing the membrane capacitance.
Increasing its electrical resistance.
Increasing its electrical resistance.
This sheath allows a nerve pulse to travel further
without amplification.
• Reducing the metabolic energy required by the
nerve cell.
Node of Ranvier
• At the nodes, the amplifications of the nerve
pulses occur.
• Thus a myelinated axon act as a cable, with the
periodic amplification used to prevent the signal
from becoming too weak. from becoming too
weak.
• By the contrast, signals in unmyelinated axon
become weak in a very short distance and
required virtually continuous amplification.
• The cell membrane acts as a capacitor with its
negative charge on the inside and the positive
charge on the outside, the cell membrane acts as the
dielectric material.
Resting Membrane Potential
• At rest the inside of the cell is at -70 Mv.
• With inputs to dendrites inside becomes
more positive more positive
• if resting potential rises above threshold,
an action potential starts to travel from
cell body down the axon
Depolarization and The Action
Potential (AP)
• Action potential opens cell membrane to allow
sodium (Na+) in.
• inside of cell rapidly becomes more positive than
outside
• this depolarization travels down the axon as
leading edge of the AP.
Repolarization follows
• After depolarization potassium (K+) moves out
restoring the inside to a negative voltage
• This is called repolarization.
• The rapid depolarization and repolarization
produce a pattern called a spike discharge.
Finally, Hyperpolarization
• Repolarization leads to a voltage below the
resting potential, called hyperpolarization.
• Now neuron cannot produce a new action
potential.
• This is the refractory period
The passive electrical properties of nervefiber
• Axons act as cables that transmit bioelectric
impulses from one nerve cell to other cells or the
central nervous system.
• The wall of the axon tube is semipermeable
membrane which although a dielectric, allows ions
to migrate into and out of the fiber.
• At the resting state the permeability of the
membrane for Na+ ions is less than that for K+ and
Cl- ions.
• The resting state of the membrane is described as
the polarized stage of the membrane.
• The resting potential can be defined as thepotential
difference created across the cell membrane by the
metabolic processes of thefiber during rest.
• When a nerve is stimulated, stimulus causes a fall in
the potential difference across the plasma membrane
which lets the membrane to become much more
permeable to Na+ ions.
• These Na+ ions rapidly migrate from the
extracellular fluid to the interior of the fiber.
• This renders the potential of the membrane to
rise and eventually becomes positive with
respect to the outside.
• This process is called depolarization.
• The depolarization of the membrane is called
the action potential.
• Myelinated axons conduct differentlythan the
unmyelinated axon. The myelinatedsegment of an
axon with itslarge thichness (d) has very low
electrical capacitance, since the value of the
capacitance, since the value of thecapacitance (C) is:
𝐴
𝐶 = 𝐾𝜀
𝑑
The Sodium-Potassium Pump
• It is the most active transport mechanism in
the body.
• It transport Na+ out of the cell and this
transport is coupled with pumping of K+ in the
opposite direction.
• It is an active transport mechanism, why?
• Since it occurs against both concentration and
electrochemical gradient.
The speed of propagation of the action potential
• Two primary factors affect the speed of
propagation of the action potential;
1. The resistance (R) within the core of the
membrane.
2. The capacitance (c) across the membrane.
• The polarization and depolarization process
across the axon's membrane will depend on its
time constant "t".
t = RC
• For myelinated axons both of the capacitance
"C" and the internal resistance "R" are small and
very short time is needed to polarize and
depolarize the axon.
• Accordingly, we expected that the velocity of
the action potential impulses through mylinated
axons is extremely high.
• The velocity of propagation of the action
potential impulses across non myelinated axon
will be small, why?
• The non myelinated axon's have very small
thickness and hence high capacitance.
Accordingly, the time necessary for
polarization and depolarization of the axons
membrane will be high (RC is high).
Recording of the membrane potential:
• Action potential of a nerve can be recorded by
using certain apparatus, which is formed of:
• 1. Two microelectrodes
• 2. The electronic amplifier.
• 3. The cathode ray oscilloscope.