Electrical Properties of Nerve Fibres

DR NILESH KATE
MBBS,MD
ASSOCIATE PROF
DEPT. OF PHYSIOLOGY
ELECTRICAL
PROPERTIES OF
NERVE FIBRES

OBJECTIVES
 Electrical Properties of Nerve Fibres.
 Excitability
 Resting membrane potential
 Action potential.
 Phases & Ionic basis.
 Characteristics of nerve excitability vs stimulus.
 Membrane excitability during AP.
 Compound AP
 Injury potential.
Wednesday, December 7, 2016

EXCITABILITY.
 Property of Nerve Fibre
due to which it respond
to stimulus by
Generating Nerve
Signal.
 Stimulus – Mechanical,
Electrical, Chemical or
Thermal

RESTING MEMBRANE
POTENTIAL
 Recording of electrical
potential.
 Microelectrode.
 Cathode ray
oscilloscope.

Bioelectrical
Phenomena of the Cell

BASIC CONCEPTSBASIC CONCEPTS
VoltVolt
A charge difference between two
points in space

Ions – charged particlesIons – charged particles
Anions – Negatively charged particlesAnions – Negatively charged particles
Cations – Positively charged particlesCations – Positively charged particles

FORCES THAT DETERMINE IONICFORCES THAT DETERMINE IONIC
MOVEMENTMOVEMENT
Electrostatic forcesElectrostatic forces
Opposite charges attractOpposite charges attract
Identical charges repelIdentical charges repel
Concentration forcesConcentration forces
Diffusion – movement of ions through semiDiffusion – movement of ions through semi
permeable membranepermeable membrane
Osmosis – movement of water from regionOsmosis – movement of water from region
of high concentration to lowof high concentration to low

INTRODUCTION.
 Potential difference
across membrane of all
living cell – Membrane
Potential/Resting
membrane
potential/Transmemb
rane potential.
 Negative inside &
positive outside.

RMP
 Resting Means – Cell
not metabolically
quiescent but no
electrical change.
 Change in membrane
potential during
excitation – Action
potential.

GENESIS OF RMP
 Selective permeability
of cell membrane.
 Gibb’s – Donnan
Membrane Equilibrium.
 Nernst Equation.
 Constant Field
Goldmann equation.
 Na-K+ ATPase Pump.

SELECTIVE PERMEABILITY OF
CELL MEMBRANE.
 Membrane – Selectively
Permeable.
 Ions like Na+, K+, Cl-,
HCO3- are diffusible
 Proteins & organic
phosphates – Non-
diffusible.
 Gated Channels –
responsible for different
permeability.

GIBB’S – DONNAN MEMBRANE
EQUILIBRIUM.
 According to this, when
ionized solutions are
separated by semi
permeable membrane
 1. Each solution is
electrically equal – Total
charges on cation equal to
total charges of anion.
 2.Product of Diffusible
ions on both sides should be
equal.

Experiment.
 2 solutions, A & B
separated by semi
permeable membrane
M.
 According to Gibb’s-
Donnan equilibrium
 (Na+)A=(Cl-)A &
(Na+)B=(Cl-)B
 [(Na+)A Χ (Cl-)A=
(Na+)B Χ (Cl-)B

Experiment.
 But if one or more Non-diffusible ions are
present on one side.
 (Na+)A=(Cl-)A + (X-)A & (Na+)B=(Cl-)B so
(Na+)A+(Cl-)A + (X-)A > (Na+)B+(Cl-)B……..(1)
 Product of Diffusible ions
(Na+)A Χ(Cl-)A=(Na+)B Χ (Cl-)B……….(2)
From Eq 1&2
(Na+)A > (Na+)B and (Cl-)A < (Cl-)B

Inference .
 So there is unequal
distribution of diffusible
ions at equilibrium.
 Na+ more on side
which contains non-
diffusible ions & Cl-
more on other side.

Nernst Equation.
 Due to unequal
distribution of ions across
cell membrane creates
Concentration Gradient.
 But movement of ions
across membrane is
prevented by Electrical
Gradient which is due to
Non-diffusible anions.

Nernst Equation.
 Thus equilibrium is
reached resulting in
Equilibrium Potential
(Diffusion Potential)
 Magnitude is given by
Nernst Equation
E(m)=
± 61log(conc)0/(conc)i

CONSTANT FIELD GOLDMANN
EQUATION.
 Nernst Equation
calculate equilibrium
potential for
individual ion.
 But at any given time
membrane potential
depend on
distribution of ions &
its permeability

GOLDMANN-HODGKIN-KATZ
EQUATION
 SO membrane potential due integrated role
of different ions is given by
 V = RT Pk[K+]i +Pna+[Na+]i + PCl-[Cl-]o
----- In -----------------------------------------------------
F Pk Pk[K+]o +Pna+[Na+]o + PCl-[Cl-]I
V= membrane potential, R=Gas constant,
T= absolute temp, F=Faraday constant

Inference……..
 Most imp ion for
development – Na+, K+ &
Cl-
 Degree of Importance –
depend on membrane
permeability to that ion
 Positive ion conc. gradient
from inside to outside
 Signal Transmission – is
primarily due to change in
Na & K permeability.

Na-K+ ATPase Pump.
 Main Role of Na-K
pump lies in building
concentration
gradient.
 Its Electrogenic pump.
 Creates negativity
inside cell.

ACTION POTENTIAL
 Def – Any change in the
Resting Membrane
Potential when
stimulated by
Threshold stimulus.
 Sub threshold & sub
minimal threshold does
not produce action
potential but causes
change in local potential

PHASES OF ACTION
POTENTIAL
 Resting membrane
potential – straight base
line -70mv
 Stimulus Artifact – mild
deflection due to leakage
of current from
stimulating electrode to
recording electrode.

PHASES OF ACTION
POTENTIAL
 Latent period – Short
Isoelectric period between
application of stimulus &
onset of AP.
 Firing level –
Depolarization start slowly
up to level after which it
occurs rapidly.
 Overshoot – Depolarization
continues beyond zero.

PHASES OF ACTION
POTENTIAL
 Spike potential – sudden
depolarization followed
by Repolarization
produces spike.
Repolarization
 After depolarization –
slow Repolarization upto
RMP level
 After Hyperpolarization
– below RMP.

IONIC BASIS OF ACTION
POTENTIAL
 Polarization phase.
 Depolarization Phase.
 Repolarization Phase.
 After Depolarization.
 After
Hyperpolariization.

POLARIZATION PHASE.
 State of resting
membrane potential
 -70mv
 Due to distribution of
ions across cell
membrane.

DEPOLARIZATION PHASE.
 Threshold stimulus.
 Na+ permeability
Increases – reaches
firing level –
depolarization – more
Na channels open –
more Depolarization
Hodgkin-Cycle

FACTORS WHICH LIMIT FURTHER
DEPOLARIZATION
 Inactivation of Na
channels due to
activation of h-gates.
 During overshoot
direction of electrical
gradient is reversed.
 Opening of Voltage
gated K channels – K
efflux.

REPOLARIZATION PHASE.
 Decrease in Na influx
 Increase in K efflux
 Remains activated for
long time

AFTER DEPOLARIZATION.
 Its misnomer
 It is further
Repolarization.
 Due to slow efflux of
K+
 As K channels
inactivates very
slowly.

AFTER HYPERPOLARIZATION.
 Slow efflux of K
continues even after
RMP is reached.
 Membrane potential
becomes more
negative -72mv.
 Then K channels also
closes & RMP achieved
again by Na-K pump.

ROLE OF Ca IONS
 Conc of Ca in ICF is very low as compared o ECF.
 So when Na channels open some Ca ions also
moves inside along with Na.

STAGES OF ACTION POTENTIAL
1. RESTING:
Stage before action
potential develops.
The membrane is
polarised at the resting
state of a cell
Sodium gates are
inactivated in this state

2. DEPOLARISATION:
A reduction in the polarity of the membrane
potential caused by the opening of voltage-gated
Na+
channels

3. REPOLARISATION
A return of the membrane potential towards the
resting value caused by the closing of voltage-gated
Na+
channels and the opening of voltage-gated K+
channels
As a result of sodium influx, potassium
channels get open and potassium ions exit out
of the cell.

HYPERPOLARISATIONHYPERPOLARISATION
 The membrane potential reaches values more negative
than the resting value due to the slowly closing of voltage-
gated K+
channels

Characteristics of an Action
Potential #1 Triggered by external stimulant
 #2 Threshold potential needed to trigger the
action potential
 #3 Obeys All or None law
 #4 No Change along conduction path
 #5 Reverses Polarity
 #6 Refractory Period

CHARACTERISTICS OF NERVE
EXCITABILITY VIS-À-VIS STIMULUS.
 Strength duration curve.
 All or none response.
 Accommodation.
 Infatiguability.

STRENGTH DURATION CURVE.
 Relationship between strength
& duration of stimulus.
 Rheobase ® Minimum
intensity of stimulus applied
for adequate time produces
response
 Chronaxie –© Minimum
duration for which intensity of
double the Rheobase should
applied to produce response.

ALL OR NONE RESPONSE.
 When a stimulus of sub
threshold intensity is applied
then no AP is produced.
 If threshold stimulus applied
response in the form of spike
AP.
 If we increase strength of
stimulus more than threshold
no increase in magnitude of AP
is observed.

MEMBRANE EXCITABILITY
DURING ACTION POTENTIAL
Depending on response to
stimulus, period of AP is
divided into
 Refractory period.
 Supernormal period.
 Subnormal period.

REFRACTORY PERIOD.
 Period following AP
during which nerve
fibre either dose not
respond or respond sub
normally to threshold
or supra threshold
stimulus.

REFRACTORY PERIOD – TYPES
ABSOLUTE REFRACTORY PERIOD.
 Period during which 2nd
stimulus no matter how
strong it may be it
cannot produce
response.
 From firing levels to
1/3rd
of Repolarization.

IONIC BASIS
 During upstroke, m gates of
Na channels are opened
rapidly & during
Repolarization channels are
closed by closure of
inactivation gates (h) gates
of Na channels.
 These channels will not
reopens until potential
comes back to resting levels

RELATIVE REFRACTORY
PERIOD.
 Period during which
nerve fibre shows
response if strength of
stimulus is more than
normal.
 From end of absolute
refractory period to
start of after
depolarization.

Ionic basis
 During this Na channels
are coming out of
inactivation stage & K
channels are still
opened.
 Stronger stimulus open
more Na channels
through m gates &
produce response.

SUPERNORMAL PERIOD.
 During this membrane
is hyperexcitable i.e
threshold is
decreased.
 Correspond with after
depolarization stage.

IONIC BASIS
 During after
depolarization phase Na
channels have come out
of inactivated state & K
channels are mostly
closed & membrane
potential is nearer to
firing level.
 Threshold level
decreases.

SUBNORMAL PERIOD.
 During this membrane
excitability is low
 Threshold stimulus is
increased.
 It corresponds with
hyper polarization
phase of AP.

ACCOMMODATION
 When threshold stimulus is
applied quickly then AP is
produced.
 When threshold stimulus is
applied slowly no AP is
produced.
 This phenomenon of
adaptation to stimuli is
Accommodation.

IONIC BASIS OF ACCOMMODATION.
 When depolarization is rapid, Na channel opening overtake
Repolarization forces
 When depolarization is slow, more & more Na channels open
to get inactivated after 1ms ,while K channels remains open
which restores membrane potential & Repolarization
overtake depolarization.

INFATIGUABILITY.
 Nerve fibre cannot fatigued due to its absolute
refractory period.

COMPOUND ACTION POTENTIAL
 Its Monophasic
recording of AP from
mixed nerve.
 Mixed Nerve – which
contains different types
of nerve fibres with
different diameters.
 So it’s the algebraic
summation of AP of
many axons.

RESPONSE OF MIXED NERVE TO
STIMULI.
 It depends on
 Threshold of individual
axon
 Distance from
stimulating electrode.

RESPONSE OF MIXED NERVE TO
STIMULI.
 Sub threshold stimulus
– no response.
 Threshold Stimulus –
initially stimulate axons
with low threshold.
 Further increase in
intensity of stimulus –
more & more axons
stimulated.

RESPONSE OF MIXED NERVE
TO STIMULI.
 Maximal stimulus –
all axons are
stimulated
 Supramaximal
Stimulus – no further
axons are stimulated.

FEATURES OF COMPOUND
ACTION POTENTIAL
 Unique shape with
multiple peaks.
 Number & size of peaks
vary with type of fibre.
 When stimulus is less
than maximal – shape of
AP depend on number
& type of fibre
stimulated.

INJURY POTENTIAL
 Leakage Current –
due to disruption of
channels.

Electrical Properties of Nerve Fibres

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