Cellular Neurophysiology (NEUR0007) Notes - Bioelectricity, Action Potentials & Ion Channels

Introduction to Electricity
Introduction to Electricity
 Electricity
o The force that drives out brain
 Allows communication between neurons
o Concepts
 Electricity = movement of charges
 Transfer of charges from one place to another
 Electrons are the carriers of charge in electrical circuits
 In biological systems – ions carry charge across cell membranes
o Information process in the CNS – results from the operation of channels + transporter proteins =
enable + regulate the movement of ions across membranes
 Conductors and insulators
o Electrons in an atom are confined in orbits around the nucleus
o Conductors
 Electrons from outer cells can move easily from atom to atom
 Cannot store charges
 Whenever a potential difference is applied to a conductor  electrons move in one
direction = impossible for conductor to store charges
o Insulators
 Electrons tend to stay in their own orbital – not free to move
 Can store charge
 Usually molecules – not elements
 Charge and its movement
o Charge (Q) – measured in coulombs (C)
o Charge of one electron or univalent ion = 1.6 x 10 -19C
o One coulomb = 6.35 x 1018 electrons
o Faradays constant – charge of one mole of univalent ions
 Faradays constant = avogadro’s number x charge on each ion
 F = Faradays constant = 96500 coulombs per mole
 Avagadro’s number = NA = 6.022 x 10 23
 Charge on each ion = e0 = 1.602 x 10-19
 Current electricity
o Electric current (I) = flow of electrons or ions from one place to another
 Rate of change of charge per unit time





 I = current
 Q = charge
 t = time
 Measured in amperes (A)
 1 ampere = the flow of 1 coulomb of charge in 1 second
 Potential difference
o Charge only moves if there is a potential difference between
two points = driving force of charge
o Measured in volts (V)
o Voltmeter – measures the potential difference
o Water analogy

,Introduction to Electricity
 2 reservoirs – at different heights
 Big height difference  strong flow = high current
 Small height difference  weak flow = low current
 Current = transfer of water from one reservoir to another
 Potential difference = difference i n height between two containers
 Voltmeter = measures difference in height between two reservoirs
 Resistance – Ohms law
o For any given potential difference – the current that flows through an element of a circuit =
determined by its resistance
o Ease with which a charge moves through a conductor
 Impedes the movement of charge
o Unit of resistance = ohm (Ω)
o Equation
 V = IR
 V = voltage
 R = resistance
 I = current
o Conductance = reciprocal of resistance
 G=1/R
 I = GV
o Unit of conductance = siemen (S)
o Water analogy
 Constriction in pipe
 Smaller constriction = less flow of water through resistance
 Circuits topology
o Elements in an electrical circuit can be arranged in series / parallel / combination of both





o Water analogy
 Pump = battery – provides potential difference
 Water = electrons
 Flow of water = electric current
 Resistance = narrowing of pipes
 Series circuit





 Parallel circuit
 Flow of water splits – F1 + F2
o F1 goes down one pipe
 Encounters one resistance
o F2 goes down another pipe

,Introduction to Electricity
 Encounters another resistance
 2 flows of water re-join after resistances
o Resistances in series and parallel
 Series





 Current is same throughout the circuit
o V = IR1 + IR2 = I(R1 + R2)
 Rtotal = R1 + R2
 Resistance in series add
 Voltage divider
o I = V / (R1 + R2)
o Voltage difference across resistances
 V1 = IR1
 V2 = IR2
o Battery voltage = addition of voltage across resistances
 Parallel





 I = I1 + I2 = V/R1 + V/R2 = V(1/R1 + 1/R2)
o 1/Rtotal = 1/R1 + 1/R2
 Resistances in parallel add as their reciprocal





 Capacitance
o An insulator can store charges
o Capacitor
 Arrangement of an insulator between two conductors = allows storage of
charges in an electrical circuit
o Capacitor connected to a battery  electrons build up on one plate – repelling
electrons from the other plate  resulting in one negative and one positive plate

, Introduction to Electricity
 Once each plate is fully charged up  electron flow stops  charge is stored on plates
o Charge stored in a capacitor = proportional to the applied voltage
 Q = CV
 Q = charge stored in capacitor
 C = capacitance – how much charge can be stored for a given voltage
o Higher capacitance = more charge can be stored
 V = voltage across 2 plates
o Units of capacitance = farad (F)
 1 F = capacitance of an element that can store 1C of charge given a 1V potential difference
o Factors affecting capacitance
 Plate area
 Larger plates = more capacitance
 Plate spacing
 Closer plates = more capacitance
 Dielectric material – insulator between the plates
 Good insulator between plates = more capacitance




 C = capacitance in Farads
 ε = absolute permittivity of dielectric
 A =area of plate overlap in m2
 D = distance between plates in meters
o Can be connected in series of in parallel
 In parallel – total capacitances = sum of all capacitances
 Ctotal = C1 + C2 + C3 + … + Cn
 In series – 1/total capacitances = sum of reciprocal capacitances
 1/Ctotal = 1/C1 + 1/C2 + 1/C3 + … + 1/Cn
o Water analogy
 Capacitor = elastic membrane in pipe
 Blow  pushes water in one direction
 Elastic membrane expands until reaching equilibrium with strength of water 
stops  resulting in no flow of water
 Current in a circuit containing a capacitor – depends on the rate of change of voltage




o Faster change in voltage = more current flows
 Direct and alternating current
o Direct current (DC) sources
 Direct flow of ions in one direction – without changes in polarity
 Generated by a battery
o Alternating current (AC) sources
 Periodically reverse their polarity
 UK mains – sinusoidal – frequency of 50Hz + voltage of 240V
 Time dependent circuits – the RC circuit
o Battery = generats a potential difference
o Resistor
o Capacitor