PROPERTIES OF NERVE FIBRES

DR NILESH KATE

MBBS,MD

ASSOCIATE PROF

DEPT. OF PHYSIOLOGY

ELECTRICAL PROPERTIES OF NERVE FIBRES

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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, May 3, 2023

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

BASIC CONCEPTSBASIC CONCEPTS

Ions – charged particlesIons – charged particles

Anions – Negatively charged particlesAnions – Negatively charged particles

Cations – Positively charged particlesCations – Positively charged particles

BASIC CONCEPTSBASIC CONCEPTSFORCES THAT DETERMINE IONIC FORCES THAT DETERMINE IONIC

MOVEMENTMOVEMENT

Electrostatic forcesElectrostatic forcesOpposite charges attractOpposite charges attractIdentical charges repelIdentical charges repel

Concentration forcesConcentration forcesDiffusion – movement of ions through semi Diffusion – movement of ions through semi

permeable membranepermeable membraneOsmosis – movement of water from region Osmosis – 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/Transmembrane 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.



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.



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 POTENTIAL1. 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. REPOLARISATIONA 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 – TYPESABSOLUTE 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.



Sub threshold stimulus – no response.

Threshold Stimulus – initially stimulate axons with low threshold.

Further increase in intensity of stimulus – more & more axons stimulated.



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.


Thank You

PROPERTIES OF NERVE FIBRES

Health & Medicine

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