Electricity in Medicine Flashcards
- Coulomb´s law and permittivity
Coulomb’s Law gives the magnitude of the electrostatic force F between two charges q1 and q2 that are seperated by a distance r. Ke= Coulomb´s constant = 8.987x10^9 Nm²C-² r= distance between charges q1,q2 are the magnitudes of the charges * The attractive or repulsive force F acting between 2 charges qo and q is directly proportional to their product and inversely proportional to the distance between them. -the electrostatic force can be attractive or repulsive in nature -it is dependent on -charge q → The greater the charge, the greater the electrostatic force -radius r → electrostatic force decreases as distance between the two charges increases -permittivity constant є → the greater the permittivity constant, the smaller the electrostatic force. permittivity constant є – a measure of a material’s ability to transmit an electrical field -a higher permittivity value is associated with a material’s ability to store the same amount of charge but with a smaller electrical field. This leads to increased capacitance. Value in a vacuum: -Permittivity values in insulators are higher than Eo. -This is the result of dielectric polarization which occurs in insulators and leads to a decreased force between the charges as compared to a vacuum. -Therefore, relative permittivity can be defined. This is the ratio of the permittivity of a given material to that of a vacuum. -The high permittivity of water enables good solubility of salts in water. Water molecules decrease the attractive forces between negatively and positively charged ions in solution. This hydration of ions is accompanied by a decrease in free enthalpy (thermodynamically favorable). -Various ions are hydrated to various extents: ~Positively charged ions have higher hydration numbers bc their positive charge induces a greater polarization effect in the electron shells of water molecules than does a negative charge. ~Smaller ions also possess higher hydration numbers than larger ions of the same size. For ex: K+ has a greater radius in crystalline structure than does Na+. However, in water solution, the effective radius of Na+ is greater than that of K+ due to its smaller size. This means that Na+ is hydrated to a greater extent and its effective radius increases. *This quality is important when considering the permeability of ions in cell membranes and the resulting membrane potential. Since Na+ is hydrated to a greater extent, it has more difficulty transversing the phospholipid bilayer of the cell membrane. This explains why cell membranes are more permeable to K+ than Na+.
Intensity of electric field, Electric current, voltage, resistance, impedance and their measurement, units
INTENSITY: The intensity of an electrical field is given by
E=F/q
The electrical field strength is defined as the electrostatic force felt by a positive test charge qo divided by its charge.
Units are Newtons/coulomb which equals volts/meter
Intensity can also refer to the amount of energy transmitted via acoustic or electromagnetic radiation. Intensity is the measure of average energy flux. For example in the case of sound, intensity = power/area
Intensity is a vector quantity and has units watts/m2
The Ampere (A) is the SI unit for electric current, it is equivalent to 1 Coulomb per second which is roughly equal to 6.241 x 1018 that of the elementary charge. Amperes are used to express flow rate of electric charge.
VOLTAGE (V)/ELCTRICAL POTENTIAL: Since work must be done to move a charge in an electrical field, voltage is the amount of work that must be performed to move a positive test charge q0 through an electric field from point a to point b. The voltage or potential difference is the difference in electric potential between two points.
-The work depends only on the electric potential at the two points and is independent of the path taken by the charge.
Unit is the Volt. 1V = 1J/1C
*voltage is a parameter of electricity which causes current to flow when circuit is compeleted.
* V is a scalar quantity whose sign is dependent on the sign of the charge
*0 electric potential is defined at an infinite distance away from the source.
RESISTANCE (R): opposition to the flow of charge in a direct current (electrical current).
*Resistance does not effect the flow of charge, it just results in a potenial drop.
R=U/I
The SI unit of resistance is the ohm.
R= resistance [Ohm]
U=Voltage [Volt]
I= Power? [Ampère]
Resistance of a conductor is dependent on its:
Length (L)- directly proportional –the longer the resistor, the greater the resistance (e- have to travel longer through the resistor-greater potential drop)
Crosssectional area (A**) –inversely proportional –the greater the crossectional area, the greater the resistance (an increase in the amount of conduction paths for the e-)
Resistivity (ρ)**- directly proportional – the greater the materials resistivity, the greater the resistance.
Resistivity is value that characterizes a material‘s intrinsic resistance to current flow
SI unit is ohm x meter
Temperature – most conductors have higher resistance at higher temperatures (due to more oscillations [Schwankungen] in the atoms of the conductor causing a resistance to e- flow)
*conductors have low values of resistance while resistors have high values of resistance
IMPEDANCE (Z): opposition to the the flow of charge in an alternating current
Z=U/I
-it is based on three components
Resistance (R)
Inductive reactance (L)
Reactive capacitance (Rc)
In tissues, inductive reactance can be neglected
Rest membrane potential
- Every neuron has a seperation of positive and negative electrical charge across its cell membrane..
- The electrical potential difference is about 60 – 70mV at rest and is known as the resting membrane potential.
- The negative resting membrane potential is a result of the membrane being significantly more permeable to K+ ions than Na+ ions. Thus K+ diffuses down its electrochemical gradient into the cell to join anionic proteins. Na+ remains outside of the cell.
Since , by convention, the potential outside the cell is arbitrarily defined as zero, and given the relative excess of negative charges inside the membrane; the potential difference across the membrane is expressed as a negative value.
*The potential difference across the membrane can be measured with two glass microelectrodes
Donnan´s equilibrium in a cell membrane
-Donnan’s equilibrium is the behavior of diffusable ions near a semipermeable membrane failing to distribute themselves evenly across the membrane due to the presence of another charged nondiffusable particle. This results in an uneven charge distribution across the membrane and the formation of an electrical potential between the two solutions (Donnan’s potential).
In the absence of a nondiffusable ion:
If a solution of KCl is added to one side (side 1) of a semipermeable membrane and water is added to the other side (side 2), KCl will dissociate and the two ions will move down their concent to side 2. The ions will continue to diffuse back and forth until equilibrium is acheived and there are equal concentraions of K+ and Cl- on either side of the membrane. At equilibrium, equal numbers of K+ and Cl- ions will diffuse back and forth across the membrane to maintain equal concentrations on either side and electrical neutrality.
In the presence of a nondiffusable ion:
If an impermeable anionic protein is added to side 1, there are equal concentrations of K+ on either side and no Cl- ions in side 1. In the beginning, some Cl- ions will diffuse down their concencentration gradient to side 1. This gives side 1 a net negative charge and establishes an electrical gradient for Cl- ions which restricts more Cl- ions from entering side 1. This electrical gradient also attracts K+ ions from side 2 to side 1. K+ ions will then move back to side 1 as a result of a K+ concentration gradient. At equilibrium, the product of the concentrations of the diffusable ions on either side is equal. However, there is a slight excess of cations on side 2 along with a sight excess of anions on side 1 producing an electrical potential between the two sides.
Sodium-Potassium Pump
- Enzyme located in the plasma membrane of all animal cells
- Pumps sodium out of cells while pumping potassium into cells
- It has an antiporter like activity
- Both molecules are going against their concentraction gradient
- Fkt: helps maintaining the resting potential and helps to regain it after an action potential
- Pumps out 3 sodium ions (Na+) and moves 2 potassium into the cell (K+)
Electrochemical potential
- Is the mechanical work done in bringing 1 mol of an ion from a standard state to a specified concentration and electrical potential.
- Electrochemical potential can be expressed as:
μ̅i = μi + ziFΦ
- -where:
- μ̅i is the electrochemical potential of species i, J/mol
- μi is the chemical potential of the species i, J/mol
- zi is the valency (charge) of the ion i, a dimensionless integer
- F is Faraday’s Constant, C/mol
- Φ is the local electrostatic potential, V.
- Book definition: The work required for transport of 1 mole of the i-th component (ion or electron) inside the given phase, defined as the sum of chemical and electrostatic components (F=96484 C/mol- Faraday´s constant, z- number of elementary charges of the i-th ion).
Measurement of electric conductivity in solutions
Both intracellular and extracellular fluids contain a sufficient amount of ions which makes them good conductors of electric charge.
The property of a solution to conduct electrical current is described by the
- Specific Conductivity K* which is inversely proportional to ρ (resistivity)
- Molar Conductivity:* used to relate the dependence of specific conductivity on ion concentration
*Molar conductivity is dependent on temperature. It increases slightly.
In dilute solutions, where ions move around independently, the molar conductivity is additive
Conductometry
*If only one type of electrolyte is present in solution, then electrical conductivity is directly proportional to the concentration of the electrolyte.
Therefore, concentration can be estimated by measuring specific conductivity (k)
Measurement of specific conductivity is carried out by using a conducting vessel with two platinum electrodes. The vessel contains the solution and is located in the Wheatstone bridge circuit whose resistance is unknown. (Wheatstone Bridge = A circuit with 2 legs of resistance one of the legs includes the Rx)
Before, specific conductivity can be calculated, l and q must be determined. The ratio of l/q can be determined experimentally by first filling the vessel with a standard solution of known specific conductivity. The ratio l/q is called the capacity C of the reacting vessel. With this information, the vessel is then filled with the solution and the resistance is measured. Specific conductivity is calculated and the concentration of the solution is estimated since specific conductivity and concentration are proportional
Action potential and its detection
- Action potentials are generated by neurons after being excited by a stimulus of sufficient magnitude.
- A neuron at rest has a membrane potential of -70 mV due to the unequal distribution of ions across the neuronal membrane. Thus the inside of the cell is negative (anionic proteins & higher K+) and the outside is positive (higher Na+).
- If a neuron is sufficiently stimulated, some Na+ channels open and sodium enters the cell down its electrochemical gradient. This causes the membrane potential to rise slowly. Once it reaches a threshold potential of -60 - -50 mV, an action potential is generated and all voltage gated Na+ channels open. Na+ rushes into the cell and depolarizes the membrane to a positive value.
- An action potential is an all or none response. This means that whenever a threshold potential is triggered, an action potential of consitent size and duration is produced irrespective of the strength of the stimulus.
- K+ channels then open and K+ rushes out of the cell down its electrochemical gradient causing repolarization of the membrane to a negative value. → hyperpolarization
- During an action potential it is impossible to invoke another → absolute refractory period
- Immediately following an action potential it is difficult to initiate a response from the neuron (need stronger stimuli) → relative refractory period.
*The refractory period is important in the one way conduction of the action potential along the length of the neuronal axon.
-Mylenated axons propagate action potentials much faster than unmyelinated axons through saltatory conduction. In this method, regions of the axon are surrounded by an insulating myelin sheath and the neuronal membrane is only permeable to ions at the Nodes of Ranvier. Thus the action potential jumps from one node to the next.
Detection of Action Potentials:
An axon can be stimulated using two electrodes and some external source of voltage. One electrode is placed inside the axon and the other on its surface.
- If the external electrode is positive, hyperpolarization of the membrane occurs (membrane potential decreases)
- If the internal electrode is positive, depolarization occurs (membrane potential increases)
- Rheobase –magnitude of current just sufficient to excite a given nerve or muscle.
- Chronaxie –duration of reponse time interval if the current of twice the rheobase is applied.
*resting membrane potential and the shape of the action potential varies for different types of excitable cells (cardiac, neuronal, muscle)
Action potentials of heart muscle and their detection
-The heart contracts due to electrical stimulation controlled by the SA node (pacemaker of the heart) in the right atrium which causes a path of depolarization to the AV nodes and the rest of the conducting cells of the heart.
SA node causes atrial contraction
AV node causes ventricular contraction
Repolarization occurs and the cycle repeats.
-The shape of the action potential in cardiac conducting cells is a wide plateau and the duration of the action potential is much longer relative to that of neurons.
An electrocardiogram is used to detect the action potential of the heart.
- P wave corresponds to atrial depolarization
- QRS corresponds to ventricular depolarization
- T wave- ventricular repolarization
Electric current and organism
ermmm?
Use of electricity in diagnostics
The major application of electricity in diagnostic medicine is the electrocardiogram.
- The electrocardiogram records heart action potentials produced by changes in polarity of cardiac cells.
- To record an ECG, electrodes are placed on different parts of the body. Each of these leads monitors distinct areas of the heart. Using combinations of these electrodes, different tracings of the heart’s electrical activity can be made and recorded on paper or in a computer.
Diagnoses- abnormal heart rates: bradycardia = slower rate, bachycardia= faster rate, amythmia= irregular rate; myocardium damage as a result of myocardial infarction
Other applications include the electroencephalogram which records brain activity by detecting electrical potential across regions of the brain.
Diagnoses: epilepsy, sleep patterns, brain death.
Use of electricity in therapy
Electrotherapy is based on the three different effects of electric current. The effects are dependent on the type of electrical current used.
- Electrolytic*:
- A direct current has strong electrolytic effects (bc the organisms fluid has many electrolytes, alkaline compounds deposit on cathode, acidic compounds deposit on anode). It results in the changed stimulation of nerves. Higher direct current densities can result in tissue damage.
- A weak electrolytic effect can be produced by low frequency alternating current
- Stimulatory*:
- Low frequency alternating current has a strong stimulatory effect (if this current passes through the heart it can disrupt the heart’s electrical activity and have lethal effects).
- Direct current can only produce stimulatory results at sudden changes of intensity.
- Thermal*
- A high frequency alternating current has a strong thermal effect.
- It is applied safely for heating tissues during diathermy.
*Electrotherapy is applied in physciatry, balneology, rehabilitation.
