Types of neuronesTypes of neuronesHistological ClassificationHistological Classification

1- Unipolar.1- Unipolar.

2- Bipolar2- Bipolar

3- Multipolar3- Multipolar

Functional classificationFunctional classification : :

1- Sensory ( afferent )1- Sensory ( afferent ) : : conduct conduct impulses from receptors to C.N.S .impulses from receptors to C.N.S .

2- Motor (efferent):2- Motor (efferent): conduct conduct impulses from the C.N.S to the impulses from the C.N.S to the effector organ.effector organ.

3-Interneurons3-Interneurons:: are located are located inside the C.N.S connect each inside the C.N.S connect each other & connect sensory & motor other & connect sensory & motor neurons.neurons.

Cross section in spinal cord

Structure of neuroneStructure of neurone Cell body ( soma )Cell body ( soma ) : : The soma also contains :The soma also contains : Nissil bodiesNissil bodies : : responsiple for protein responsiple for protein

synthesis.synthesis. NeurofibrilsNeurofibrils : : extend into cell processes extend into cell processes

& form the main support of the neuron.& form the main support of the neuron. Microtubules & MicrofilamentsMicrotubules & Microfilaments : : they they

extend into the axon & have supportive extend into the axon & have supportive and transport function ( axoplasmic and transport function ( axoplasmic transporttransport ). ).

NO centrosomeNO centrosome

Neuron structure

2- Cell processes:2- Cell processes:

a-Dendrites :a-Dendrites :

- Multiple short processes - Multiple short processes extend from the cell body extend from the cell body ..

- Collect impulses & - Collect impulses & transmit them towards transmit them towards the cell body.the cell body.

b- Axonb- Axon : :

- - Single long process that conduct Single long process that conduct impulses away from the cell body. impulses away from the cell body.

- Vary in length from few millimeter - Vary in length from few millimeter to about one meter .to about one meter .

- Its origin from the cell body is called - Its origin from the cell body is called the axon hillock .the axon hillock .

- It branches freely near its terminal .- It branches freely near its terminal .- The terminal branches end by - The terminal branches end by

swollen part (synaptic knobs) which swollen part (synaptic knobs) which contain chemical transmitter contain chemical transmitter vesicles .vesicles .

Types of axonsTypes of axonsAxons are one of two types : Axons are one of two types :

1-1- Myelinated Myelinated::

covered by myelin sheath& covered by myelin sheath& neurilemmal sheath.neurilemmal sheath.

2-2-UnmyelinatedUnmyelinated::

covered only by neurilemmal covered only by neurilemmal sheath but no underlined sheath but no underlined myelin sheath .myelin sheath .

Myelin sheathMyelin sheathlipoprotein in nature & lipoprotein in nature & insulatorinsulator

formed in peripheral nervous formed in peripheral nervous system by wrapping of cell system by wrapping of cell membrane of membrane of Schwan cellsSchwan cells around the axon while in the around the axon while in the C.N.S. it is formed by C.N.S. it is formed by oligodendroglial cellsoligodendroglial cells . .

Myelination of neurones

Myelination in PNS & CNS

absent from axon hillock & absent from axon hillock & terminal branches .terminal branches .

interrupted at intervals of interrupted at intervals of about 0.1 – 1.0 mm by about 0.1 – 1.0 mm by nodes of Ranviernodes of Ranvier

responsiple for the white responsiple for the white colour of myelinated colour of myelinated nerve& the white matter of nerve& the white matter of C.N.S .C.N.S .

Resting membrane Resting membrane potentialpotential( R.M.P.)( R.M.P.)

DefinitionDefinition:: It is the It is the potential difference potential difference between inside & between inside & outside the membrane outside the membrane at rest at rest

ValueValue :: it is – 70 mv. it is – 70 mv.

MeasurementMeasurement : : it is measured by special it is measured by special apparatus with apparatus with microelectrodes , microelectrodes , amplifier ,and sensitive amplifier ,and sensitive voltmeter . voltmeter .

One microelectrode is put in One microelectrode is put in the inner side of the the inner side of the membrane & the other membrane & the other microelectrode is put on the microelectrode is put on the outside surface .outside surface .

Cathode ray oscilloscope

Causes of RMPCauses of RMP

1- Selective 1- Selective permeability of the permeability of the membrane.membrane.

2- Sodium – potassium 2- Sodium – potassium pump.pump.

1-Selective permeability of the 1-Selective permeability of the membranemembrane

POTASSIUM ION DIFFUSIONPOTASSIUM ION DIFFUSION : :

The cell membrane is about The cell membrane is about 100 times more permeable for 100 times more permeable for potassium ions than for potassium ions than for sodium ions. sodium ions.

Potassium ions are Potassium ions are concentrated inside 30-40 concentrated inside 30-40 times more than outside.times more than outside.

Potassium ions tend to move Potassium ions tend to move from inside the cell to from inside the cell to outside. outside.

Potassium ion diffusion Potassium ion diffusion continue untill the positive continue untill the positive charge outside the membrane charge outside the membrane reach a level that repells the reach a level that repells the outflow of more potassium outflow of more potassium ions ( ions ( equilibrium ofequilibrium of potassium ion diffusionpotassium ion diffusion ) . ) .

SODIUM ION DIFFUSIONSODIUM ION DIFFUSION : : Sodium ion concentration Sodium ion concentration

outside the membrane is 10-outside the membrane is 10-15 times the concentration 15 times the concentration inside. inside.

Sodium tends to diffuse to Sodium tends to diffuse to inside the cell. inside the cell.

Sodium ion diffusion is limited Sodium ion diffusion is limited by the low permeability of the by the low permeability of the resting membrane to sodium.resting membrane to sodium.

CHLORIDE ION DIFFUSION :CHLORIDE ION DIFFUSION :Chloride ions tend to diffuse from Chloride ions tend to diffuse from

outside to inside the cell outside to inside the cell (concentrated 25 times outside (concentrated 25 times outside more than inside ) . more than inside ) .

This diffusion is prevented by This diffusion is prevented by repulsionrepulsion force caused by the force caused by the negativity negativity insideinside & also by the & also by the attractionattraction force between chloride & force between chloride & sodium ions sodium ions outsideoutside the membrane the membrane ..

PROTEIN DIFFUSIONPROTEIN DIFFUSION : :

The cell membrane offers The cell membrane offers absolute barrier to the absolute barrier to the passage of organic anions passage of organic anions (proteins), because the (proteins), because the protein molecules are of protein molecules are of large size . large size .

They cannot pass through They cannot pass through the cell membrane. the cell membrane.

movement of ions at rest movement of ions at rest occur through the occur through the leak leak protein channelsprotein channels which are which are always opened. always opened.

the most important ion at the most important ion at rest is the potassiumrest is the potassium ion ion because of highest because of highest concentration gradient & concentration gradient & highest membrane highest membrane permeabilitypermeability

Potassium

equilibrium

Nernest equation

Ek = - 61. log Conc inside / Conc outside.

= - 61 .log 140/4.5

= - 91 mv.

Electrochemical potential & ionic current

2-Sodium – potassium 2-Sodium – potassium pumppump

Function of sodium-potassium Function of sodium-potassium pumppump

1- It 1- It keepskeeps high high Na + concentrationNa + concentration outside the cell & high outside the cell & high K+ concentrationK+ concentration inside the cell .inside the cell .

2- It is an 2- It is an electrogenic pump.Itelectrogenic pump.It pumps pumps three positive charges to outside & only three positive charges to outside & only two positive charges to inside with the two positive charges to inside with the result of one negative charge inside , result of one negative charge inside , help negativity of R.M.P. help negativity of R.M.P.

3- 3- keeps normal cell volumekeeps normal cell volume by by preventing accumulation of electrolytes preventing accumulation of electrolytes inside the cell .inside the cell .

Action potentialAction potential DefinitionDefinition : : it is the it is the rapid rapid

propagatedpropagated change in membrane change in membrane potential as a result of potential as a result of stimulation by an effective stimulation by an effective stimulus stimulus

MONOPHASIC ACTION POTENTIALMONOPHASIC ACTION POTENTIAL:: ItIt is recorded by putting one is recorded by putting one

microelectrode in the inside microelectrode in the inside surface of the membrane & the surface of the membrane & the other on the outer surface. other on the outer surface.

Monophasic action potentialMonophasic action potential

- 70

Zero

+ 35

L.P

Dep

ola

riza

tion R

ep

ola

rizatio

n After depolarization

After hyperpolarization

Duration (m.Sec.)

Volt

ag

e (

m.v

.)

Latent periodLatent periodit is the it is the timetime passed since passed since

application of stimulus until the application of stimulus until the start of recording action start of recording action potential.potential.

It depends on the It depends on the velocity of velocity of conductionconduction in the nerve & the in the nerve & the distance between the stimulating distance between the stimulating and the recording electrode .and the recording electrode .

It is inversely proportional to It is inversely proportional to the velocity of conduction but the velocity of conduction but directly proportional to the directly proportional to the distance between the distance between the stimulating & recording stimulating & recording electrode .It can be used to electrode .It can be used to calculate the velocity of calculate the velocity of conduction as follows :conduction as follows :

Velocity = Velocity =

Distance / Latent period. Distance / Latent period.

Na + & K+ conductance

during action potential

Na + & K + conductance during Action potential

Spike potential :Spike potential :

It is composed of two limbs :It is composed of two limbs :

a-a-ascending limbascending limb

( Depolarization wave ) . ( Depolarization wave ) .

b-b-descending limbdescending limb

( Repolarization wave). ( Repolarization wave). Duration of the spike Duration of the spike

potential is 0.5 – 1.0 m.secpotential is 0.5 – 1.0 m.sec

Sodium channels during action potential occurence

The ascending limb The ascending limb ( Depolarization ):( Depolarization ):

--It is due to Na+ influx & it is of two phases:It is due to Na+ influx & it is of two phases:

a-Slow phase :a-Slow phase : from R.M.P. till firing level from R.M.P. till firing level

( from –70 mv to –55 mv ) it is due to ( from –70 mv to –55 mv ) it is due to increase in Na+ permeability by the effect increase in Na+ permeability by the effect of the stimulus of the stimulus

b-Rapid phase :b-Rapid phase : from –55 mv to + 35 mv from –55 mv to + 35 mv

It is due to rapid Na+ influx caused by It is due to rapid Na+ influx caused by opening of Na+ voltage gated channels . opening of Na+ voltage gated channels .

The descending limb The descending limb ( repolarization ) :( repolarization ) :

It represents the major part of It represents the major part of the process of repolarization the process of repolarization (about 70 %). (about 70 %).

It is due to closure of Na+ It is due to closure of Na+ voltage gated channels & voltage gated channels & opening of K+ voltage gated opening of K+ voltage gated channels resulting in rapid k+ channels resulting in rapid k+ efflux while Na+ influx stops efflux while Na+ influx stops

Voltage gated Na+ & K+ Voltage gated Na+ & K+ channels open by the same channels open by the same signal ( change of signal ( change of membrane potential from membrane potential from resting state to – 55 mv ).resting state to – 55 mv ).

Na+ voltage gated Na+ voltage gated channels are rapid to open channels are rapid to open & rapid to close while K+ & rapid to close while K+ voltage gated channels are voltage gated channels are slow . slow .

Depolarazation triggers

Phases

Of

A.P

Ascend-ing

limb

Peak

Desce-

nding

Limb

Na channel activation

Gate open rapidlyPositive

Feedback

loop

Na enter

cells

More

depolarization

To stop cycle

Na inactivation

gate close

Slow K

Channels open

Potassium

Leaves

cells

aa-After depolarization :-After depolarization :

( Negative after potential )( Negative after potential )duration duration 4 m.sec.4 m.sec.it is the it is the slow repolarizationslow repolarization

following the descending limb following the descending limb of the spike till R.M.P. is of the spike till R.M.P. is reached reached

it is due to it is due to slow K+ outfluxslow K+ outflux due due to closure of some voltage to closure of some voltage gated K+ channels. .gated K+ channels. .

b-b-After hyperpolarizationAfter hyperpolarization : :( Positive after potenial )( Positive after potenial )

duration duration 40 m.sec.40 m.sec. it is it is small hyperpolarizationsmall hyperpolarization

following the after depolarization following the after depolarization till the R.M.P. is reached . till the R.M.P. is reached .

it is caused by : it is caused by :

a- some K+ voltage gated channels a- some K+ voltage gated channels still opened still opened

b- increased activity of Na+ - K+ b- increased activity of Na+ - K+ pump. pump.

Conduction of action potential

Saltatory conductionPoint to point conduction

This is an This is an active processactive process that needs that needs energy .energy .

The The depolarized point make a potential depolarized point make a potential difference with the adjacent point.difference with the adjacent point.

A current develop between that point A current develop between that point & the adjacent point& the adjacent point which become which become depolarized to the firing level depolarized to the firing level producing a new action potential in the producing a new action potential in the new point.new point.

The new action potential produces The new action potential produces another action potential at the another action potential at the adjacentadjacent point & so on. point & so on.

Differences between propagation in Differences between propagation in myelinated & unmyelinated nerve myelinated & unmyelinated nerve

fibers :fibers :Myelinated Myelinated nerve fibersnerve fibers

Unmyelinated Unmyelinated nerve fibersnerve fibers

Type of Type of conductionconduction

SaltatorySaltatory

( jumping )( jumping )Point to pointPoint to point

Velocity of Velocity of conductionconduction

RapidRapidSlowSlow

Energy Energy consumptionconsumption

Consume less Consume less energyenergy

Consume more Consume more energyenergy

Biphasic action potentialBiphasic action potential It is measured by putting the It is measured by putting the two two

microelectrodesmicroelectrodes on the on the outer surface.outer surface. As the depolarization wave reaches the As the depolarization wave reaches the

electrode near the stimulator , the electrode near the stimulator , the electrode becomes negative relative to electrode becomes negative relative to the other electrode & a downward the other electrode & a downward deflection is recorded .deflection is recorded .

When the wave passes to the part of When the wave passes to the part of the nerve fiber between the two the nerve fiber between the two electrodes , the potential difference electrodes , the potential difference returns to zero .returns to zero .

When the wave reaches the second When the wave reaches the second electrode , it becomes negative electrode , it becomes negative relative to the first electrode & an relative to the first electrode & an upward deflection is recorded .upward deflection is recorded .

When the depolarization wave When the depolarization wave leaves the second electrode , the leaves the second electrode , the potential difference returns again to potential difference returns again to zero.zero.

Therefore , the record shows a Therefore , the record shows a downward deflection followed by an downward deflection followed by an isoelectric interval & then an upward isoelectric interval & then an upward deflection .deflection .

Biphasic action potential

Depolarization

In the nearby

electrode

Depolarization

In the distant

electrode

Volt

ag

e

Duration

EXCITABILITYEXCITABILITYDefinitionDefinition ::

It is the ability of It is the ability of nerve fiber or muscle nerve fiber or muscle fiber to respond to fiber to respond to stimuli .stimuli .

Stimulus :Stimulus : It is the change in the It is the change in the

surrounding environment . surrounding environment . Types of stimuli :Types of stimuli :

1- 1- ElectricalElectrical : : it is of two types : it is of two types :

a-Galvanica-Galvanic : : direct current direct current obtained from a battery of low obtained from a battery of low intensity & of long duration.intensity & of long duration.

b-Farradicb-Farradic : : high intensity & of high intensity & of short duration .short duration .

Electrical stimuli are Electrical stimuli are commonly used in commonly used in

experimental work because experimental work because they are : they are :

1- easily applied .1- easily applied .

2- accurately adjusted as strength 2- accurately adjusted as strength & duration .& duration .

3- similar to physiological process 3- similar to physiological process of excitation of excitation

4- cause no or minimal damage to 4- cause no or minimal damage to the tissuethe tissue

2 - 2 - ChemicalChemical : : Chemical tranasmittersChemical tranasmitters: :

acetyl choline , adrenaline & acetyl choline , adrenaline & noradrenaline.noradrenaline.

Hormones.Hormones.Drugs.Drugs.IonsIons : Na+,K+& Ca++ . : Na+,K+& Ca++ .GasesGases : O2 & CO2. : O2 & CO2.

3- 3- PhysicalPhysical : :

a-a-ThermalThermal : : cooling & cooling & warming warming

b-b-MechanicalMechanical : stretch , : stretch , touch & pressure .touch & pressure .

c-c-ElectromagneticElectromagnetic : : light light stimuli to the retina of the stimuli to the retina of the eye. eye.

Factors affecting Factors affecting effectiveness of stimuli :effectiveness of stimuli :

1-Strength (intensity) of 1-Strength (intensity) of stimulistimuli..

2- Duration (time of 2- Duration (time of application) of stimuli.application) of stimuli.

3- Rate of rise of 3- Rate of rise of intensity. intensity.

1-Strength (intensity) of 1-Strength (intensity) of stimulistimuli : :

According to the stimulus strength According to the stimulus strength the following types of stimuli are the following types of stimuli are known :known :

Sub minimal stimulusSub minimal stimulus (sub (sub threshold): threshold): it produces no action it produces no action potential ,but only produces localized potential ,but only produces localized change (local excitatory state) change (local excitatory state)

Minimal (threshold ) stimulusMinimal (threshold ) stimulus : : it is it is the weakest stimulus which can the weakest stimulus which can produce an action potential.produce an action potential.

Supra threshold stimulusSupra threshold stimulus..Maximal stimulus.Maximal stimulus.Supramaximal stimulus.Supramaximal stimulus. Threshold ,supra threshold Threshold ,supra threshold , maximal & supra maximal , maximal & supra maximal stimuli produce the same stimuli produce the same action potential in single action potential in single nerve fiber because it nerve fiber because it obeys the all or non law.obeys the all or non law.

Action potential

Local excitatory

state

Threshold stimulus

Subthreshold stimuli

Suprathreshold stimuli

Resting potential

Threshold

2-Duration (the time of 2-Duration (the time of application) of stimulus :application) of stimulus :

The duration of stimulus is The duration of stimulus is closely related to the strength.closely related to the strength.

The strength-duration curve can The strength-duration curve can be obtained by stimulating the be obtained by stimulating the nerve with electrical stimuli of nerve with electrical stimuli of different intensities and recording different intensities and recording the time needed by each the time needed by each stimulus to start the response.stimulus to start the response.

Strength-Duration curve

R

2R

Duration

Str

en

gth

C

R: Rheobase

C: Chronaxie

From the curve we observe From the curve we observe that :that :

1-Within limits1-Within limits , the stronger the , the stronger the stimulus , the shorter will be the stimulus , the shorter will be the duration .duration .

22-There is a -There is a minimal timeminimal time , less than , less than this time , no stimulus whatever its this time , no stimulus whatever its strength can produce response .strength can produce response .

33-There is a -There is a minimal strength minimal strength ((Rheobase)Rheobase) , less than this strength , , less than this strength , no stimulus whatever its duration can no stimulus whatever its duration can produce response .produce response .

4-The 4-The ChronaxieChronaxie is the time is the time needed by stimulus strength needed by stimulus strength double rheobase to produce double rheobase to produce response.response.

N.B. :N.B. :

It is a It is a measuremeasure of excitability . of excitability .

It is It is inversily proportionalinversily proportional to to excitability .excitability .

3- Rate of rise of intensity3- Rate of rise of intensity ::

The stimulus becomes effective The stimulus becomes effective only if the rate of rise only if the rate of rise exceeds a exceeds a certain limit.certain limit.

When the current rises too When the current rises too slowly,slowly, the the nerve nerve accommodatesaccommodates itself to the rise itself to the rise of the current (the threshold of of the current (the threshold of stimulation is increased).stimulation is increased).

Excitability changes Excitability changes during an action potentialduring an action potential

During conduction of a nerve During conduction of a nerve impulse , the excitability of the impulse , the excitability of the nerve fibers varies & it passes in the nerve fibers varies & it passes in the following phases: following phases:

1-Temporal rise of excitability1-Temporal rise of excitability : :

it is associated with the local it is associated with the local response (local depolarization ) in response (local depolarization ) in the nerve fiber before the firing the nerve fiber before the firing level .level .

Excitability changes during Excitability changes during A.P.A.P.

- 70

Zero

+ 35

Exc.100%

Zero

1

2

3

45

1- Initial increase.

2- Absolute refractory period (ARP).

3- Relative refractory period (RRP).

4- Super-normal phase.

5- Subnormal phase

Absolute & relative refractory period

Absolute refractory period Absolute refractory period (A.R.P):(A.R.P):

it corresponds to the ascending limb it corresponds to the ascending limb & the first third of the descending & the first third of the descending limb of the spike potential .The limb of the spike potential .The excitability is completely lost = zero excitability is completely lost = zero ( no stimulus whatever its strength ( no stimulus whatever its strength can excite ) .can excite ) .

Significance :Significance : A.R.P. limits the A.R.P. limits the number of impulses that can be number of impulses that can be produced & conducted by the nerve produced & conducted by the nerve fibers fibers

Mechanism :Mechanism : During the ascending limb During the ascending limb the Na+ voltage gated the Na+ voltage gated channels are already opened channels are already opened ( no effect of other stimuli).( no effect of other stimuli).

During the first third of During the first third of descending limb Na+ descending limb Na+ channels are closed by the channels are closed by the inactivation gate and can inactivation gate and can not be opened.not be opened.

Relative refractory period Relative refractory period ( R.R.P. ):( R.R.P. ):

It corresponds to the second two It corresponds to the second two thirds of the descending limb of thirds of the descending limb of spike potential .spike potential .

The excitability is less than normal The excitability is less than normal (supra threshold stimulus strength (supra threshold stimulus strength can excite).can excite).

MechanismMechanism : : it is due to difficulty to it is due to difficulty to open the closed voltage gated Na+ open the closed voltage gated Na+ channels .channels .

4- Supernormal phase:4- Supernormal phase:

it corresponds to the after it corresponds to the after depolarization ( the depolarization ( the membrane is partially membrane is partially depolarized) .depolarized) .

The excitability is higher The excitability is higher than normal than normal

(sub threshold stimulus can (sub threshold stimulus can excite)excite)

5- 5- Subnormal phase : Subnormal phase :

It corresponds to the after It corresponds to the after hyper polarization . hyper polarization .

The excitability is lower than The excitability is lower than normal normal

Supra threshold stimuli are Supra threshold stimuli are needed to excite .needed to excite .

Factors affecting Factors affecting excitability of nerve excitability of nerve

fibersfibers1- Physical factors1- Physical factors

2- Chemical factors2- Chemical factors

3- Electrical factors3- Electrical factors

1- Physical factors1- Physical factors

ThermalThermal : :

Warming increase excitability due to Warming increase excitability due to increase of the metabolic increase of the metabolic reactions .reactions .

Cooling decrease excitability due to Cooling decrease excitability due to decrease of the metabolic decrease of the metabolic reactions .reactions .

MechanicalMechanical :: deep pressure deep pressure decrease excitability decrease excitability

2- Chemical factors2- Chemical factors : :

A- Local anaestheticsA- Local anaesthetics : :

Local anaesthetics as Local anaesthetics as cocaine , novocaine & cocaine , novocaine & xylocaine decrease xylocaine decrease excitability by decreasing excitability by decreasing Na+ permeability .Na+ permeability .

B- Ions :B- Ions :

- - Calcium :Calcium :

Increase E.C.F calcium decreases Increase E.C.F calcium decreases excitability by decreasing Na+ excitability by decreasing Na+ permeability & decrease E.C.F. permeability & decrease E.C.F. calcium do the reverse.calcium do the reverse.

- Sodium:- Sodium:

Increase E.C.F. sodium increase Increase E.C.F. sodium increase excitability while its decrease excitability while its decrease decreases excitability.decreases excitability.

Potassium :Potassium :

increase of E.C.F. potassium increase of E.C.F. potassium increase excitability while its increase excitability while its decrease decreases excitability .decrease decreases excitability .

Oxygen lack & carbon dioxide Oxygen lack & carbon dioxide excess:excess: decreases excitability . decreases excitability .

Acidosis:Acidosis: decreases excitability decreases excitability while while alkalosisalkalosis increases it. increases it.

3-Electrical factors 3-Electrical factors ( Electrotonus ) :( Electrotonus ) :

These are the changes that These are the changes that occur at the cathode occur at the cathode (catelectrotonus) & at the (catelectrotonus) & at the anode (anelectrotonus) when anode (anelectrotonus) when a subthreshold galvanic a subthreshold galvanic current is applied to a nerve current is applied to a nerve fiber .fiber .

CatelectrotonusCatelectrotonus : : slight slight depolarizationdepolarization occurs near occurs near the the cathodecathode which is which is accompanied by accompanied by increased increased excitabilityexcitability . .

AnelectrotonusAnelectrotonus : : slight slight hyper hyper polarizationpolarization occurs near the occurs near the anodeanode which is accompanied which is accompanied by by decreased excitabilitydecreased excitability..

Types of nerve fibersTypes of nerve fibersTypeTypeAABBCC

DiameterDiameter3-20 um3-20 um1-3 um1-3 um0.5-1.0 um0.5-1.0 um

MyelinationMyelinationMyelinatedMyelinatedMyelinatedMyelinatedunmyelinatedunmyelinated

Velocity of Velocity of conductionconduction

10-120 m/sec10-120 m/sec5-10 m/sec5-10 m/sec0.5-5.0 m/sec0.5-5.0 m/sec

Sensitivity toSensitivity to

PressurePressure

Local Local anestheticanesthetic

+ + + ++ + + +

+++ ++ +

+ ++ +++

+ + + ++ + + +

ExampleExampleSensory & Sensory & motor somaticmotor somatic

Preganglionic Preganglionic autonomicautonomic

PostganglioniPostganglionic autonomicc autonomic

All or non lawAll or non law

Action potential either Action potential either occurs maximally or it does occurs maximally or it does not occur at all , provided not occur at all , provided that all other conditions that all other conditions remain constant .remain constant .

It is obeyed by a single It is obeyed by a single nerve or single muscle fiber . nerve or single muscle fiber .

Action potential

Local excitatory

state

Threshold stimulus

Subthreshold stimuli

Suprathreshold stimuli

Resting potential

Threshold

Sub minimal ( sub threshold ) Sub minimal ( sub threshold ) stimulus does not produce an stimulus does not produce an action potential .action potential .

Threshold stimulus produces Threshold stimulus produces the maximal action potential .the maximal action potential .

Supra threshold , maximal & Supra threshold , maximal & supra maximal stimuli produce supra maximal stimuli produce the same action potential .the same action potential .

Differences between action Differences between action potential & local excitatory statepotential & local excitatory state

Local Local excitatory excitatory

state (L.E.S.)state (L.E.S.)

Action Action potentialpotential

(A.P.)(A.P.)

CauseCauseSub threshold Sub threshold stimulus.stimulus.

Threshold & more.Threshold & more.

All or non lawAll or non lawDoesn’t obey.Doesn’t obey.Obeys.Obeys.

GradationGradationCan be graded.Can be graded.No gradation.No gradation.

SummationSummationCan be summated.Can be summated.No summation.No summation.

ConductionConductionDecreased Decreased gradually till gradually till disappear.disappear.

Conducted with the Conducted with the same intensity.same intensity.

Action potential in mixed Action potential in mixed nervenerve

(compound action potential)(compound action potential)Mixed nerve contains different types of Mixed nerve contains different types of

nerve fibers with nerve fibers with different different excitabilityexcitability & & different different velocity of conductionvelocity of conduction . .

Action potential in mixed nerve:Action potential in mixed nerve:

1- Has multiple peaks1- Has multiple peaks : : the first peak the first peak is caused by the most rapid fibers is caused by the most rapid fibers followed by the peak of less rapid followed by the peak of less rapid fibers & lastly the peak of the slowest fibers & lastly the peak of the slowest fibers.fibers.

2- Doesn't obey all or non law2- Doesn't obey all or non law : : Sub threshold stimulusSub threshold stimulus produces no produces no

action potential .action potential . Threshold stimulusThreshold stimulus produces an action produces an action

potential potential Supra thresholdSupra threshold stimulus produces more stimulus produces more

intense action potential intense action potential Maximal stimulusMaximal stimulus produces the maximal produces the maximal

action potential due to stimulation of all action potential due to stimulation of all nerve fibers .nerve fibers .

Supra maximal stimulusSupra maximal stimulus produces the produces the same maximal action potentiasame maximal action potentia

Action potential in a mixed Action potential in a mixed nervenerve

(Compound action potential)(Compound action potential)

DurationDuration

Volt

ag

e