Excitable Tissues, Resting Membrane Potential & Action

PotentialProf. Vajira Weerasinghe

Professor of PhysiologyDepartment of Physiology

Faculty of Medicinewww.slideshare.net/vajira54

Objectives

1. Explain why some membranes are excitable

2. Describe the electrochemical basis of resting membrane potential

3. Describe the mechanism of generation and propagation of action potential

4. Explain the differences in action potentials of skeletal, smooth and cardiac muscles

Excitable Tissues

• Tissues which are capable of generation and transmission of electrochemical impulses along the membrane

Nerve

Excitable tissuesexcitable Non-excitable

Red cellGIT

neuron

muscle

•RBC•Intestinal cells•Fibroblasts•Adipocytes

•Nerve •Muscle

•Skeletal•Cardiac•Smooth

Membrane potential

• A potential difference exists across all cell membranes

• This is called – Resting Membrane Potential (RMP)

Membrane potential

– Inside is negative with respect to the outside– This is measured using microelectrodes and

oscilloscope – This is about -70 to -90 mV

Excitable tissues

• Excitable tissues have more negative RMP ( - 70 mV to - 90 mV)

excitable Non-excitable

Red cellGIT

neuron

muscle

• Non-excitable tissues have less negative RMP

-53 mV epithelial cells-8.4 mV RBC-20 to -30 mV fibroblasts-58 mV adipocytes

Resting Membrane Potential

• This depends on following factors– Ionic distribution across the membrane– Membrane permeability – Other factors

• Na+/K+ pump

Ionic distribution

• Major ions– Extracellular ions

• Sodium, Chloride

– Intracellular ions• Potassium, Proteinate

K+ Pr-

Na+ Cl-

Ionic distribution

Ion Intracellular Extracellular

Na+ 10 142

K+ 140 4

Cl- 4 103

Ca2+ 0 2.4

HCO3- 10 28

Gibbs Donnan Equilibrium

• When two solutions containing ions are separated by membrane that is permeable to some of the ions and not to others an electrochemical equilibrium is established

• Electrical and chemical energies on either side of the membrane are equal and opposite to each other

Flow of Potassium

• Potassium concentration intracellular is more• Membrane is freely permeable to K+

• There is an efflux of K+

K+ K+K+

K+K+ K+

K+

K+

K+K+

Flow of Potassium

• Entry of positive ions in to the extracellular fluid creates positivity outside and negativity inside

K+ K+K+

K+K+ K+

K+

K+

K+K+

Flow of Potassium

• Outside positivity resists efflux of K+

• (since K+ is a positive ion)

• At a certain voltage an equilibrium is reached and K+ efflux stops

K+ K+K+

K+K+ K+

K+

K+

K+K+

Nernst potential (Equilibrium potential)

• The potential level across the membrane that will exactly prevent net diffusion of an ion

• Nernst equation determines this potential

Nernst potential (Equilibrium potential)

• The potential level across the membrane that will exactly prevent net diffusion of an ion

Ion Intracellular Extracellular Nernst potential

Na+ 10 142 +60

K+ 140 4 -90

Cl- 4 103 -89

Ca2+ 0 2.4 +129

HCO3- 10 28 -23

(mmol/l)

Goldman Equation

• When the membrane is permeable to several ions the equilibrium potential that develops depends on– Polarity of each ion– Membrane permeability– Ionic conc

• This is calculated using Goldman Equation (or GHK Equation)

• In the resting state– K+ permeability is 50-100 times more than that of Na+

Ionic channels

• Leaky channels (leak channels)– Allow free flow of ions– K+ channels (large number) – Na+ channels (fewer in number)– Therefore membrane is more permeable to K+

Na/K pump

• Active transport system for Na+-K+ exchange using energy

• It is an electrogenic pump since 3 Na+ efflux coupled with 2 K+ influx

• Net effect of causing negative charge inside the membrane

3 Na+

2 K+

ATP ADP

Factors contributing to RMP

• One of the main factors is K+ efflux (Nernst Potential: -90mV)

• Contribution of Na+ influx is little (Nernst Potential: +60mV)

• Na+/K+ pump causes more negativity inside the membrane

• Negatively charged protein ions remaining inside the membrane contributes to the negativity

• Net result: -70 to -90 mV inside

Electrochemical gradient

• At this electrochemical equilibrium, there is an exact balance between two opposing forces:

• Chemical driving force = ratio of concentrations on 2 sides of membrane (concentration gradient)

• The concentration gradient that causes K+ to move from inside to outside taking along positive charge and

• Electrical driving force = potential difference across membrane

• opposing electrical gradient that increasingly tends to stop K+ from moving across the membrane

• Equilibrium: when chemical driving force is balanced by electrical driving force

Action potential

Action Potential (A.P.)

• When an impulse is generated– Inside becomes positive – Causes depolarisation– Nerve impulses are transmitted as AP

Dep

olar

isat

ion R

epolarisation

-70

+30

RMP

Hyperpolarisation

Inside of the membrane is• Negative

– During RMP

• Positive– When an AP is generated

-70

+30

• Initially membrane is slowly depolarised

• Until the threshold level is reached– (This may be caused by the stimulus)

-70

+30

Threshold level

• Then a sudden change in polarisation causes sharp upstroke (depolarisation) which goes beyond the zero level up to +35 mV

-70

+30

• Then a sudden decrease in polarisation causes initial sharp down stroke (repolarisation)

-70

+30

• Spike potential– Sharp upstroke and

downstroke

• Time duration of AP– 1 msec

-70

+30

1 msec

All or none law

• Until the threshold level the potential is graded

• Once the threshold level is reached – AP is set off and no one can stop it !– Like a gun

All or none law

• The principle that the strength by which a nerve or muscle fiber responds to a stimulus is not dependent on the strength of the stimulus

• If the stimulus strength is above threshold, the nerve or muscle fiber will give a complete response or otherwise no response at all

• Sub-threshold stimulus No action potential

• Threshold stimulus Action potential is triggered

• Supra-threshold stimulus Action potential is triggered

• Strength of the stimulus above the threshold is coded as the frequency of action potentials

Physiological basis of AP

• When the threshold level is reached– Voltage-gated Na+ channels open up– Since Na+ conc outside is more than the inside– Na+ influx will occur– Positive ion coming inside increases the positivity of the

membrane potential and causes depolarisation

-70

+30

outside

inside

Na+Voltage-gated Na+ channel

Physiological basis of AP

– When membrane potential reaches +30, Na+ channels are inactivated

– Then Voltage-gated K+ channels open up– K+ efflux occurs– Positive ion leaving the inside causes more negativity inside

the membrane– Repolarisation occurs

-70

+30

outside

inside

At +30

K+

• Depolarisation – Change of polarity of the membrane

• from -70 to + 30 mV

• Repolarisation – Reversal of polarity of the membrane

• from +30 to -70 mV

-70

+30

-70

+30

Hyperpolarisation

• When reaching the Resting level rate slows down

• Can go beyond the resting level– Hyperpolarisation

• (membrane becoming more negative)

-70

+30

Role of Na+/K+ pump

• Since Na+ has come in and K+ has gone out

• Membrane has become negative

• But ionic distribution has become unequal

• Na+/K+ pump restores Na+ and K+ conc slowly– By pumping 3 Na+ ions outward and 2+ K ions

inward

VOLTAGE-GATED ION CHANNELS

• Na+ channel– This has two gates

• Activation and inactivation gates

outside

inside

Activation gate

Inactivation gate

• At rest: the activation gate is closed• At threshold level: activation gate opens

– Na+ influx will occur– Na+ permeability increases to 500 fold

• when reaching +30, inactivation gate closes– Na influx stops

• Inactivation gate will not reopen until resting membrane potential is reached• Na+ channel opens fast

outside

inside

outside

inside

-70 Threshold level +30Na+ Na+

outside

inside

Na+m gate

h gate

Voltage-gated Na+ channel

• There is a voltage sensor

• Which opens up activation gate

• Na+ influx occurs

• Membrane becomes more positive

• When Na+ channel opens • Na+ influx will occur• Membrane depolarises• Rising level of voltage causes many channels to open• This will cause further Na+ influx• Thus there a positive feedback cycle • It does not reach Na+ equilibrium potential (+60mV)• Because Na+ channels inactivates after opening • Voltage-gated K+ channels open • This will bring membrane towards K+ equilibrium potential

VOLTAGE-GATED K+ Channel

• K+ channel– This has only one gate

outside

inside

– At rest: K+ channel is closed– At +30

• K+ channel open up slowly• This slow activation causes K+ efflux• This will cause membrane to become more negative • Repolarisation occurs

outside

inside

outside

inside

-70 At +30

K+ K+

n gate

Basis of hyperpolarisation

• After reaching the resting still slow K+ channels may remain open: causing further negativity of the membrane

• This is known as hyperpolarisation

-70

+30

outsideinside

K+

Summary

Animation

Refractory Period• Absolute refractory

period– During this period nerve

membrane cannot be excited again

– Because of the closure of inactivation gate

-70

+30

outside

inside

Refractory Period• Relative refractory

period– During this period nerve

membrane can be excited by supra threshold stimuli

– At the end of repolarisation phase inactivation gate opens and activation gate closes

– This can be opened by greater stimuli strength

-70

+30

outside

inside

Na+ and K+ concentrations do not change during an action potential

• Although during an action potential, large changes take place in the membrane potential as a result of Na+ entry into the cell and K+ exit from the cell

• Actual Na+ and K+ concentrations inside and outside of the cell generally do not change

• This is because compared to the total number of Na+ and K+ ions in the intracellular and extracellular solutions, only a small number moves across the membrane during the action potential

Propagation of action potential

(Basis of nerve conduction)

Propagation of AP

• When one area is depolarised• A potential difference exists between that site and

the adjacent membrane• A current flow is initiated• Current flow through this local circuit is completed

by extra cellular fluid

Propagation of AP

• This local current flow will cause opening of voltage-gated Na+ channel in the adjacent membrane

• Na+ influx will occur

• Membrane is depolarised

Propagation of AP

• Then the previous area become repolarised

• This process continue to work

• Resulting in propagation of AP

Propagation of AP

Propagation of AP

Propagation of AP

Propagation of AP

Propagation of AP

Propagation of AP

Propagation of AP

Propagation of AP

Animation

AP propagation along myelinated nerves

• Na+ channels are conc around nodes

• Therefore depolarisation mainly occurs at nodes

Distribution of Na+ channels

• Number of Na+ channels per square micrometer of membrane in mammalian neurons

50 to 75 in the cell body350 – 500 in the initial

segment < 25 on the surface

of myelin 2000 – 12,000 at the nodes of

Ranvier20 – 75 at the axon

terminal

AP propagation along myelinated nerves

• Local current will flow from one node to another• Thus propagation of action potential and therefore nerve

conduction through myelinated fibres is faster than unmyelinated fibre – Conduction velocity of thick myelinated A alpha fibres is

about 70-100 m/s whereas in unmyelinated fibres it is about 1-2 m/s

Saltatory conduction

• This fast conduction through myelinated fibres is called “saltatory conduction”

• Saltatory word means “jumping”• This serves many purposes

– By causing depolarisation process to jump at long intervals it increases the conduction velocity

– It conserves energy for the axon because less loss of ions due to action potential occurring only at the nodes

Animation

Na+/K+ pump

• Re-establishment of Na+ & K+ concentration after action potential – Na+/K+ Pump is responsible for this – Energy is consumed– Turnover rate of Na+/K+ is pump is much slower

than the Na+, K+ diffusion through channels

3 Na+

2 K+

ATP ADP

Clinical importance

• Local anaesthetics (eg. procaine) block voltage gated sodium channels in pain nerve fibres

• Thereby pain signal transmission is blocked

Clinical importance • Demyelinating diseases

– In certain diseases antibodies would form against myelin and demyelination occurs

– Nerve conduction slows down drastically • eg. Guillain-Barre Syndrome (a patient suddenly find difficult to walk,

weakness rapidly progress to upper limbs and respiratory difficulty will also occur)

Muscle action potentials

• Skeletal muscle

• Cardiac muscle

• Smooth muscle

Skeletal muscle

• Skeletal muscle is supplied by somatic nerve

• When there is a signal to the muscle it contracts and relaxes

• Thus there are two events in the skeletal muscle – Electrical - action potential – Mechanical - contraction

Muscle contraction

• Excitation - contraction coupling

– Excitation : electrical event– Contraction : mechanical event

Mechanical event follows electrical event

Skeletal muscle

• Electrical event in a skeletal muscle membrane is exactly similar to nerve action potential

• Same duration = 1 msec

• Same voltage difference = from -70 to +30 mV

Cardiac muscle

• Innervated by autonomic nerves (sympathetic and parasympathetic)

• Similar to skeletal muscle – Electrical event - action potential

– Mechanical event – muscle contraction

• However cardiac muscle action potential is completely different to that of skeletal muscle

Cardiac muscle action potential

Phases• 0: depolarisation• 1: short

repolarisation• 2: plateau phase• 3: repolarisation • 4: resting

Duration is about 200 to 300 msec

Cardiac muscle action potentialPhases• 0: depolarisation

(Na+ influx through fast Na+ channels)

• 1: short repolarisation (K+ efflux through K+

channels, Cl- influx as well)• 2: plateau phase

(Ca2+ influx through slow Ca2+ channels)

• 3: repolarisation (K+ efflux through K+

channels)• 4: resting

Differences

• Prolonged plateau phase

• Due to opening of slow Ca2+ channels

• Which causes Ca2+ influx

• Membrane is not repolarised immediately

• Mechanical event occurs together with the electrical event

Excitation : electrical eventContraction : mechanical event

Refractory period

• Cardiac muscle refractory period is about 200 msec

• Equal to the period of depolarisation, plateau phase & part of repolarisation phases

• The reason for this is that the fast sodium channels are not fully reactivated and therefore cannot reopen to normal depolarizing stimuli

Smooth muscle

• eg. gut wall, bronchi, uterus

• Controlled by nerve supply (autonomic nerves) or hormonal control

Smooth muscle

• Resting membrane potential may be about -55mV

• There are different types of action potentials

• Spikes – Slow waves - duration 10-50 ms

voltage from -55 to 0 mV– Plateau waves - similar to cardiac muscles

• Ca2+ influx is more important that Na+ influx

Pacemaker cells

• Cardiac – SA node

• Spontaneous action potentials

• RMP = -40 mV

Depolarisation

• Activation of nerve membrane

• Membrane potential becomes positive

• Due to influx of Na+ or Ca++

Hyperpolarisation

• Inhibition of nerve membrane

• Membrane potential becomes more negative

• Due to efflux of K+ or influx of Cl-

Channel blockers

• Tetrodotoxin (TTX) is a naturally-found poison that inhibits the voltage-gated Na+channels

• Tetraethyl ammonium (TEA), a quaternary ammonium cation, is an agent that inhibits the voltage-gated K+ channels

Other substances

• Oubain– Plant poison which blocks Na+/K+ pump

• Digitalis– Drug used in cardiac conditions – blocks Na+/K+ pump

Effect of serum hypocalcaemia • Concentration of calcium in ECF has a profound effect

on voltage level at which Na+ channels activated

• Hypocalcaemia causes hyperexcitability of the membrane

• When there is a deficit of Ca2+ (50% below normal) sodium channels open (activated) by a small increase in the membrane potential from its normal level – Ca2+ ions binds to the Na+ channel and alters the voltage

sensor

Effect of serum hypocalcaemia

• Therefore membrane becomes hyperexcitable

• Sometimes discharging spontaneously repetitively

– tetany occurs

• This is the reason for hypocalcaemia causing tetany

Carpopedal spasms

Membrane stabilisers• Membrane stabilisers (these decrease excitability)

• Increased serum Ca++– Hypocalcaemia causes membrane instability and spontaneous

activation of nerve membrane– Reduced Ca level facilitates Na entry– Spontaneous activation

• Decreased serum K+• Local anaesthetics• Acidosis• Hypoxia

Membrane stabilisers

• Membrane destabilisers (these increase excitability)

•Decreased serum Ca++•Increased serum K+•Alkalosis