Electricity within the body(PHR 177)Course

Prof. Dr. Moustafa. M. MohamedVice Dean

Faculty of Allied Medical SciencePharos University

Alexandria

Dr. Mervat MostafaDepartment 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

• The equilibrium potential difference for an ion can be found from the Nernst equation:

Nernst Equation

𝑉=±2.3𝐾𝑇𝑒

𝑙𝑜𝑔𝐶1𝐶 2

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;

0.0267 V = 26.7 mV

V*26.7 mV(

So the Nernst Potential is:

V

• 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,

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 nerve fiber • 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 themembrane 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 the potential difference created across the cell membrane by the metabolic processes of the fiber 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:

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.