PHYSIOLOGY 1 LECTURE 11 PROPAGATION of ACTION POTENTIALS

PHYSIOLOGY 1PHYSIOLOGY 1

LECTURE 11LECTURE 11

PROPAGATION of ACTION PROPAGATION of ACTION POTENTIALSPOTENTIALS

PROPAGATION of the PROPAGATION of the ACTION POTENTIALACTION POTENTIAL

The movement of the action potential The movement of the action potential along a biological membrane is along a biological membrane is dependent on the properties of the dependent on the properties of the voltage gated ion channels and the voltage gated ion channels and the electrical cable properties of the cell.electrical cable properties of the cell.

Basically, the action potential is Basically, the action potential is regenerated at each new site along regenerated at each new site along the membrane.the membrane.

PROPAGATION of the PROPAGATION of the ACTION POTENTIALACTION POTENTIAL

Properties of the Voltage Gated Properties of the Voltage Gated ChannelsChannels - -– 1. 1. ThresholdThreshold - Threshold is determined - Threshold is determined

by the protein structure of the voltage by the protein structure of the voltage gated channelsgated channels

– 2. 2. All or None EventAll or None Event - Once initiated - Once initiated the action potential goes to completion the action potential goes to completion - protein cycle- protein cycle

– 3. 3. Local EventLocal Event

PROPERTIES OF VOLTAGE PROPERTIES OF VOLTAGE GATED CHANNELSGATED CHANNELS

ThresholdThreshold - - Changes in the Changes in the

electrical electrical environment of the environment of the voltage gated ion voltage gated ion channels causes channels causes the protein the protein structure to change structure to change shape opening the shape opening the channel.channel.


All or none eventAll or none event - - Once the voltage Once the voltage

gated ion channels gated ion channels have been have been stimulated they stimulated they must cycle must cycle through the through the complete protein complete protein shape change in shape change in order to reset.order to reset.


Local EventLocal Event - - The cycling of a single The cycling of a single

voltage gated channel voltage gated channel only moves 5 or 6 ions. only moves 5 or 6 ions. This is insufficient to This is insufficient to activate the entire activate the entire membrane. However, membrane. However, if a second channel is if a second channel is close enough to be close enough to be activated the action activated the action potential is potential is propagated.propagated.

CABLE PROPERTIES OF CABLE PROPERTIES OF CELLSCELLS

ELECTRONIC CURRENT ELECTRONIC CURRENT FLOWFLOW

1. Ion movement across cell 1. Ion movement across cell membranesmembranes

2. Ions flow from positive to 2. Ions flow from positive to negativenegative

3. Resistance to current flow3. Resistance to current flow– A. Membrane resistanceA. Membrane resistance– B. Cytosolic resistanceB. Cytosolic resistance



4. Space and Time Constants - 4. Space and Time Constants - determines the velocity of propagationdetermines the velocity of propagation

5. Decrement in signal strength5. Decrement in signal strength 6. Depolarization of adjacent 6. Depolarization of adjacent

membranemembrane 7. Threshold7. Threshold 8. Generation of sequential “local” 8. Generation of sequential “local”

action potentials - Propagationaction potentials - Propagation



Ion movement Ion movement across cell across cell membranesmembranes - -

Movement of electrical Movement of electrical charge implies current charge implies current flow. Movement of a flow. Movement of a positive ion implies a positive ion implies a negative charge is left negative charge is left behind. Like charges behind. Like charges repel while unlike repel while unlike chargescharges attractattract..



Ions flow from Ions flow from positive to negativepositive to negative --

More properly More properly stated positive ions stated positive ions flow form positive flow form positive to negative while to negative while negative ions flow negative ions flow from negative to from negative to positivepositive



Resistance to Current FlowResistance to Current Flow Membrane ResistanceMembrane Resistance (Rm)(Rm) - The - The

hydrophobic nature of the cellular hydrophobic nature of the cellular membrane make ion diffusion difficult membrane make ion diffusion difficult so most ions pass the cellular so most ions pass the cellular membrane by use of protein membrane by use of protein transporters. Since the transporters transporters. Since the transporters have maximum rates there exists have maximum rates there exists resistance to ion movement.resistance to ion movement.



Resistance to Current FlowResistance to Current Flow Cytosolic Resistance - (Rc)Cytosolic Resistance - (Rc) Ions must move through the Ions must move through the

cellular cytosol along with cytosolic cellular cytosol along with cytosolic proteins other ions and cellular proteins other ions and cellular products. Therefore, resistance products. Therefore, resistance occurs as interference to free occurs as interference to free movement begins to take effect.movement begins to take effect.



Space and Time ConstantsSpace and Time Constants Both the space (Both the space () and time constants ) and time constants

(() are mathematical means of ) are mathematical means of normalizing data between different cell normalizing data between different cell types (1/e).types (1/e).

They are the determents of the velocity They are the determents of the velocity of action potential propagation.of action potential propagation.

Vel. = Vel. = / /



Space Constant - Space Constant - The space The space

constantconstantmeasures measures the distance it the distance it takes for the initial takes for the initial depolarization depolarization voltage of the voltage of the action potential to action potential to decline by 1 / e or decline by 1 / e or about 63%.about 63%.



Time Constant - Time Constant - The time constant The time constant

measures the measures the time it takes for time it takes for the initial action the initial action potential voltage potential voltage to decline by 1 / e to decline by 1 / e or about 63 %.or about 63 %.



VELOCITY OF PROPAGATIONVELOCITY OF PROPAGATION

= (= (dd x Rm /4 x Rc) x Rm /4 x Rc)1/21/2

= Rm x Cm= Rm x Cm andand

Vel = Vel = / /

= 1 / Cm x (= 1 / Cm x (dd / 4 x Rm x Rc) / 4 x Rm x Rc)1/21/2



Decrement of Decrement of Signal Strength -Signal Strength -

The opening of the The opening of the voltage gated ion voltage gated ion channels during channels during action potential action potential generation only allow generation only allow 5 to 6 ions to move, 5 to 6 ions to move, hence, as these hence, as these disperse away from disperse away from the area voltage falls.the area voltage falls.



Depolarization of Depolarization of Adjacent MembraneAdjacent Membrane - -

Dispersal of the Dispersal of the entering Na+ ions entering Na+ ions away from their away from their entry site rises the entry site rises the membrane potential membrane potential of adjacent of adjacent membrane segmentsmembrane segments



ThresholdThreshold - - Threshold is an Threshold is an

intrinsic property of intrinsic property of the voltage gated the voltage gated transport proteins.transport proteins.

As the membrane As the membrane potential changes the potential changes the voltage gated voltage gated channels twist to channels twist to accommodate amino accommodate amino acid charges.acid charges.



Generation of Generation of Sequential “Local” Sequential “Local” Action PotentialsAction Potentials - -

If a second set of If a second set of voltage gated ion voltage gated ion channels exists close channels exists close enough to the first set enough to the first set for the membrane for the membrane potential to still be potential to still be above threshold than a above threshold than a new action potential is new action potential is generated - Propagationgenerated - Propagation

Saltatory ConductionSaltatory Conduction

1. Effects of the myelin sheaths 1. Effects of the myelin sheaths (Schwann cells) - Forces the action (Schwann cells) - Forces the action potential to jump from one node of potential to jump from one node of Ranvier to the next.Ranvier to the next.

2. Influence of the space and time 2. Influence of the space and time constants (Velocity of Conduction)constants (Velocity of Conduction)


Myelin SheathsMyelin Sheaths - - Schwann cells Schwann cells

wrap themselves wrap themselves around the axon around the axon several times several times forcing the forcing the voltage gated ion voltage gated ion channels into the channels into the Nodes of Ranvier.Nodes of Ranvier.


Effects of space and time constants - Effects of space and time constants - Vel = 1 / Cm x (d / 4 x Rm x Rc)Vel = 1 / Cm x (d / 4 x Rm x Rc)1/21/2

The myelin sheaths increase Rm but The myelin sheaths increase Rm but to a much greater degree they to a much greater degree they decrease the cell membrane decrease the cell membrane capacitance (Cm), hence, capacitance (Cm), hence, significantly increasing velocity of significantly increasing velocity of conduction.conduction.

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PHYSIOLOGY 1 LECTURE 11 PROPAGATION of ACTION POTENTIALS