Electricity Definitions Voltage (V) – measure of potential energy generated by separated charge Voltage (V) – measure of potential energy generated by.

Electricity DefinitionsElectricity Definitions

Voltage (V) – measure of potential energy generated Voltage (V) – measure of potential energy generated by separated chargeby separated charge

Potential difference – voltage measured between two Potential difference – voltage measured between two pointspoints

Current (I) – the flow of electrical charge between Current (I) – the flow of electrical charge between two pointstwo points

Resistance (R) – hindrance to charge flowResistance (R) – hindrance to charge flow Insulator – substance with high electrical resistanceInsulator – substance with high electrical resistance Conductor – substance with low electrical resistanceConductor – substance with low electrical resistance

Electrical Current and the BodyElectrical Current and the Body

Reflects the flow of ions rather than electronsReflects the flow of ions rather than electrons There is a potential on either side of There is a potential on either side of

membranes when:membranes when: The number of ions is different across the The number of ions is different across the

membranemembrane The membrane provides a resistance to ion flowThe membrane provides a resistance to ion flow

Role of Ion ChannelsRole of Ion Channels

Types of plasma membrane ion channels:Types of plasma membrane ion channels: Passive, or leakage, channels – always openPassive, or leakage, channels – always open Chemically gated channels – open with binding of a Chemically gated channels – open with binding of a

specific neurotransmitterspecific neurotransmitter Voltage-gated channels – open and close in Voltage-gated channels – open and close in

response to membrane potentialresponse to membrane potential Mechanically gated channels – open and close in Mechanically gated channels – open and close in

response to physical deformation of receptorsresponse to physical deformation of receptors

Operation of a Gated ChannelOperation of a Gated Channel

Example: NaExample: Na++-K-K++ gated channel gated channel Closed when a neurotransmitter is not bound Closed when a neurotransmitter is not bound

to the extracellular receptorto the extracellular receptor NaNa++ cannot enter the cell and K cannot enter the cell and K++ cannot exit the cannot exit the

cellcell Open when a neurotransmitter is attached to Open when a neurotransmitter is attached to

the receptorthe receptor NaNa++ enters the cell and K enters the cell and K++ exits the cell exits the cell

Operation of a Gated ChannelOperation of a Gated Channel

Figure 11.6a

Operation of a Voltage-Gated Operation of a Voltage-Gated ChannelChannel

Example: NaExample: Na++ channel channel Closed when the intracellular environment is Closed when the intracellular environment is

negative negative NaNa++ cannot enter the cell cannot enter the cell

Open when the intracellular environment is Open when the intracellular environment is positive positive NaNa++ can enter the cell can enter the cell

Operation of a Voltage-Gated Operation of a Voltage-Gated ChannelChannel

Figure 11.6b

Gated ChannelsGated Channels

When gated channels are open: When gated channels are open: Ions move quickly across the membrane Ions move quickly across the membrane Movement is along their electrochemical gradientsMovement is along their electrochemical gradients An electrical current is createdAn electrical current is created Voltage changes across the membraneVoltage changes across the membrane

Electrochemical GradientElectrochemical Gradient

Ions flow along their chemical gradient when Ions flow along their chemical gradient when they move from an area of high concentration they move from an area of high concentration to an area of low concentrationto an area of low concentration

Ions flow along their electrical gradient when Ions flow along their electrical gradient when they move toward an area of opposite chargethey move toward an area of opposite charge

Electrochemical gradient – the electrical and Electrochemical gradient – the electrical and chemical gradients taken togetherchemical gradients taken together

Resting Membrane Potential (VResting Membrane Potential (Vrr))

The potential difference (–70 mV) across the The potential difference (–70 mV) across the membrane of a resting neuronmembrane of a resting neuron

It is generated by different concentrations of It is generated by different concentrations of NaNa++, K, K++, Cl, Cl, and protein anions (A, and protein anions (A))

Ionic differences are the consequence of:Ionic differences are the consequence of: Differential permeability of the neurilemma to NaDifferential permeability of the neurilemma to Na++

and Kand K++

Operation of the sodium-potassium pumpOperation of the sodium-potassium pump

Measuring Mebrane PotentialMeasuring Mebrane Potential

Figure 11.7

Resting Membrane Potential (VResting Membrane Potential (Vrr))

Figure 11.8

Membrane Potentials: SignalsMembrane Potentials: Signals

Used to integrate, send, and receive Used to integrate, send, and receive informationinformation

Membrane potential changes are produced by:Membrane potential changes are produced by: Changes in membrane permeability to ionsChanges in membrane permeability to ions Alterations of ion concentrations across the Alterations of ion concentrations across the

membranemembrane Types of signals – graded potentials and action Types of signals – graded potentials and action

potentialspotentials

Changes in Membrane PotentialChanges in Membrane Potential

Changes are caused by three eventsChanges are caused by three events Depolarization – the inside of the membrane Depolarization – the inside of the membrane

becomes less negative becomes less negative Repolarization – the membrane returns to its Repolarization – the membrane returns to its

resting membrane potentialresting membrane potential Hyperpolarization – the inside of the membrane Hyperpolarization – the inside of the membrane

becomes more negative than the resting potentialbecomes more negative than the resting potential

Graded PotentialsGraded Potentials

Short-lived, local changes in membrane Short-lived, local changes in membrane potentialpotential

Decrease in intensity with distanceDecrease in intensity with distance Magnitude varies directly with the strength of Magnitude varies directly with the strength of

the stimulusthe stimulus Sufficiently strong graded potentials can Sufficiently strong graded potentials can

initiate action potentialsinitiate action potentials

Graded PotentialsGraded Potentials

Figure 11.10

Graded PotentialsGraded Potentials

Voltage changes are decrementalVoltage changes are decremental Current is quickly dissipated due to the leaky Current is quickly dissipated due to the leaky

plasma membraneplasma membrane Only travel over short distancesOnly travel over short distances

Action Potentials (APs)Action Potentials (APs)

A brief reversal of membrane potential with a A brief reversal of membrane potential with a total amplitude of 100 mVtotal amplitude of 100 mV

Action potentials are only generated by muscle Action potentials are only generated by muscle cells and neuronscells and neurons

They do not decrease in strength over distanceThey do not decrease in strength over distance They are the principal means of neural They are the principal means of neural

communicationcommunication An action potential in the axon of a neuron is a An action potential in the axon of a neuron is a

nerve impulsenerve impulse

Action Potential: Resting StateAction Potential: Resting State

NaNa++ and K and K++ channels are closed channels are closed Leakage accounts for small movements of NaLeakage accounts for small movements of Na++ and K and K++

Each NaEach Na++ channel has two voltage-regulated gates channel has two voltage-regulated gates Activation gates – Activation gates –

closed in the resting closed in the resting state state

Inactivation gates – Inactivation gates – open in the resting open in the resting statestate

Figure 11.12.1

Action Potential: Depolarization Action Potential: Depolarization PhasePhase

NaNa++ permeability increases; membrane permeability increases; membrane potential reversespotential reverses

NaNa++ gates are opened; K gates are opened; K++ gates are closed gates are closed Threshold – a critical level of depolarization Threshold – a critical level of depolarization

(-55 to -50 mV)(-55 to -50 mV) At threshold, At threshold,

depolarization depolarization becomes becomes self-generatingself-generating

Figure 11.12.2

Action Potential: Repolarization Action Potential: Repolarization PhasePhase

Sodium inactivation gates closeSodium inactivation gates close Membrane permeability to NaMembrane permeability to Na++ declines to declines to

resting levelsresting levels As sodium gates close, voltage-sensitive KAs sodium gates close, voltage-sensitive K++

gates opengates open KK++ exits the cell and exits the cell and

internal negativity internal negativity of the resting neuron of the resting neuron is restoredis restored

Figure 11.12.3

Action Potential: Action Potential: HyperpolarizationHyperpolarization

Potassium gates remain open, causing an Potassium gates remain open, causing an excessive efflux of Kexcessive efflux of K++

This efflux causes hyperpolarization of the This efflux causes hyperpolarization of the membrane (undershoot)membrane (undershoot)

The neuron is The neuron is insensitive to insensitive to stimulus and stimulus and depolarization depolarization during this timeduring this time

Figure 11.12.4

Action Potential: Action Potential: Role of the Sodium-Potassium Role of the Sodium-Potassium

PumpPump Repolarization Repolarization

Restores the resting electrical conditions of the Restores the resting electrical conditions of the neuronneuron

Does not restore the resting ionic conditionsDoes not restore the resting ionic conditions Ionic redistribution back to resting conditions Ionic redistribution back to resting conditions

is restored by the sodium-potassium pumpis restored by the sodium-potassium pump

Phases of the Action PotentialPhases of the Action Potential

1 – resting state1 – resting state 2 – depolarization 2 – depolarization

phasephase 3 – repolarization 3 – repolarization

phasephase 4 – 4 –

hyperpolarizationhyperpolarization

Figure 11.12

Propagation of an Action Propagation of an Action Potential Potential

(Time = 0ms)(Time = 0ms) NaNa++ influx causes a patch of the axonal influx causes a patch of the axonal

membrane to depolarizemembrane to depolarize Positive ions in the axoplasm move toward the Positive ions in the axoplasm move toward the

polarized (negative) portion of the membranepolarized (negative) portion of the membrane Sodium gates are shown as closing, open, or Sodium gates are shown as closing, open, or

closedclosed

Propagation of an Action Propagation of an Action Potential Potential

(Time = 0ms)(Time = 0ms)

Figure 11.13a

Propagation of an Action Propagation of an Action Potential Potential

(Time = 2ms)(Time = 2ms) Ions of the extracellular fluid move toward the Ions of the extracellular fluid move toward the

area of greatest negative chargearea of greatest negative charge A current is created that depolarizes the A current is created that depolarizes the

adjacent membrane in a forward directionadjacent membrane in a forward direction The impulse propagates away from its point of The impulse propagates away from its point of

originorigin

Propagation of an Action Propagation of an Action Potential Potential

(Time = 2ms)(Time = 2ms)

Figure 11.13b

Propagation of an Action Propagation of an Action Potential Potential

(Time = 4ms)(Time = 4ms) The action potential moves away from the The action potential moves away from the

stimulusstimulus Where sodium gates are closing, potassium Where sodium gates are closing, potassium

gates are open and create a current flowgates are open and create a current flow

Propagation of an Action Propagation of an Action Potential Potential

(Time = 4ms)(Time = 4ms)

Figure 11.13c

Threshold and Action PotentialsThreshold and Action Potentials

Threshold – membrane is depolarized by 15 to 20 mVThreshold – membrane is depolarized by 15 to 20 mV Established by the total amount of current flowing Established by the total amount of current flowing

through the membrane through the membrane Weak (subthreshold) stimuli are not relayed into Weak (subthreshold) stimuli are not relayed into

action potentialsaction potentials Strong (threshold) stimuli are relayed into action Strong (threshold) stimuli are relayed into action

potentialspotentials All-or-none phenomenon – action potentials either All-or-none phenomenon – action potentials either

happen completely, or not at allhappen completely, or not at all

Coding for Stimulus IntensityCoding for Stimulus Intensity

All action potentials are alike and are All action potentials are alike and are independent of stimulus intensityindependent of stimulus intensity

Strong stimuli can generate an action potential Strong stimuli can generate an action potential more often than weaker stimulimore often than weaker stimuli

The CNS determines stimulus intensity by the The CNS determines stimulus intensity by the frequency of impulse transmissionfrequency of impulse transmission

Stimulus Strength and AP Stimulus Strength and AP FrequencyFrequency

Figure 11.14

Absolute Refractory PeriodAbsolute Refractory Period

Time from the opening of the NaTime from the opening of the Na++ activation activation gates until the closing of inactivation gates gates until the closing of inactivation gates

The absolute refractory period:The absolute refractory period: Prevents the neuron from generating an action Prevents the neuron from generating an action

potentialpotential Ensures that each action potential is separateEnsures that each action potential is separate Enforces one-way transmission of nerve impulsesEnforces one-way transmission of nerve impulses

Absolute and Relative Refractory Absolute and Relative Refractory PeriodsPeriods

Figure 11.15

Relative Refractory PeriodRelative Refractory Period

The interval following the absolute refractory The interval following the absolute refractory period when:period when: Sodium gates are closedSodium gates are closed Potassium gates are openPotassium gates are open Repolarization is occurringRepolarization is occurring

The threshold level is elevated, allowing The threshold level is elevated, allowing strong stimuli to increase the frequency of strong stimuli to increase the frequency of action potential eventsaction potential events

Conduction Velocities of AxonsConduction Velocities of Axons

Conduction velocities vary widely among Conduction velocities vary widely among neuronsneurons

Rate of impulse propagation is determined by:Rate of impulse propagation is determined by: Axon diameter – the larger the diameter, the faster Axon diameter – the larger the diameter, the faster

the impulsethe impulse Presence of a myelin sheath – myelination Presence of a myelin sheath – myelination

dramatically increases impulse speeddramatically increases impulse speed

Saltatory ConductionSaltatory Conduction

Current passes through a myelinated axon only Current passes through a myelinated axon only at the nodes of Ranvierat the nodes of Ranvier

Voltage-gated NaVoltage-gated Na++ channels are concentrated channels are concentrated at these nodesat these nodes

Action potentials are triggered only at the Action potentials are triggered only at the nodes and jump from one node to the nextnodes and jump from one node to the next

Much faster than conduction along Much faster than conduction along unmyelinated axonsunmyelinated axons

Saltatory ConductionSaltatory Conduction

Figure 11.16

Multiple Sclerosis (MS)Multiple Sclerosis (MS)

An autoimmune disease that mainly affects An autoimmune disease that mainly affects young adultsyoung adults

Symptoms: visual disturbances, weakness, loss Symptoms: visual disturbances, weakness, loss of muscular control, and urinary incontinenceof muscular control, and urinary incontinence

Nerve fibers are severed and myelin sheaths in Nerve fibers are severed and myelin sheaths in the CNS become nonfunctional sclerosesthe CNS become nonfunctional scleroses

Shunting and short-circuiting of nerve Shunting and short-circuiting of nerve impulses occursimpulses occurs

Multiple Sclerosis: TreatmentMultiple Sclerosis: Treatment

The advent of disease-modifying drugs The advent of disease-modifying drugs including interferon beta-1a and -1b, Avonex, including interferon beta-1a and -1b, Avonex, Betaseran, and Copazone:Betaseran, and Copazone: Hold symptoms at bayHold symptoms at bay Reduce complicationsReduce complications Reduce disabilityReduce disability