Electrical Current and the BodyElectrical Current and the Body

Reflects the flow of ions rather than Reflects the flow of ions rather than electronselectrons

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 The membrane provides a resistance to ion

flowflow

PLAYPLAY

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 Chemically gated channels – open with

binding of a specific neurotransmitterbinding of a specific 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 Mechanically gated channels – open and close

in response to physical deformation of in response to physical deformation of receptorsreceptors

Operation of a Gated ChannelOperation of a Gated Channel

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

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

the cellthe cell Open when a neurotransmitter is attached Open when a neurotransmitter is attached

to the receptorto the 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 Closed when the intracellular environment

is negative is 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 Movement is along their electrochemical

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

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

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

PLAYPLAY

Resting Membrane Potential Resting Membrane Potential (V(Vrr))

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

It is generated by different concentrations It is generated by different concentrations of Naof Na++, 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 cell to NaDifferential permeability of the cell to Na++ and and

KK++

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

Resting Membrane Potential Resting Membrane Potential

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 Membrane potential changes are produced by: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

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 Hyperpolarization – the inside of the

membrane becomes more negative than the membrane becomes more negative than the resting potentialresting potential

Changes in Membrane PotentialChanges in Membrane Potential

Figure 11.9

Action Potentials (APs)Action Potentials (APs)

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

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

They do not decrease in strength over They do not decrease in strength over distancedistance

They are the principal means of neural They are the principal means of neural communicationcommunication

An action potential in the axon of a neuron An action potential in the axon of a neuron is a nerve impulseis a nerve impulsePLAYPLAY

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 Kand K++

Each NaEach Na++ channel has two voltage-regulated channel has two voltage-regulated gates 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 As sodium gates close, voltage-sensitive

KK++ gates open gates 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 Restores the resting electrical conditions of the neuronthe neuron

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

conditions is restored by the sodium-conditions is restored by the sodium-potassium pumppotassium 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

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 Positive ions in the axoplasm move toward the polarized (negative) portion of the the polarized (negative) portion of the membranemembrane

Sodium gates are shown as closing, open, Sodium gates are shown as closing, open, or closedor closed

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

Ions of the extracellular fluid move toward Ions of the extracellular fluid move toward the area of greatest negative chargethe area 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 The impulse propagates away from its point of originpoint of origin

Propagation of an Action Propagation of an Action Potential Potential

Figure 11.13b

Propagation of an Action Propagation of an Action Potential Potential

The action potential moves away from the The action potential moves away from the stimulusstimulus

Where sodium gates are closing, Where sodium gates are closing, potassium gates are open and create a potassium gates are open and create a current flowcurrent flow

Propagation of an Action Propagation of an Action Potential Potential

Figure 11.13c

Threshold and Action PotentialsThreshold and Action Potentials

Threshold – membrane is depolarized by 15 to Threshold – membrane is depolarized by 15 to 20 mV20 mV

Established by the total amount of current Established by the total amount of current flowing through the membrane flowing 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 All-or-none phenomenon – action potentials either happen completely, or not at alleither happen completely, or not at all

EPSP and IPSPEPSP and IPSP Excitatory post synaptic potentialExcitatory post synaptic potential Inhibitory post synaptic potentialInhibitory post synaptic potential Graded potentialGraded potential The Challenge – Come up with an analogy The Challenge – Come up with an analogy

about APabout AP Post them on mycourses message boardPost them on mycourses message board Everyone go read themEveryone go read them Send me an email with your voteSend me an email with your vote Yes, you can vote for yourselfYes, you can vote for yourself

Your competitionYour competition

““When I want pizza in the dorm, I have to collect When I want pizza in the dorm, I have to collect enough money.  Each bill or coin that I find enough money.  Each bill or coin that I find throughout the room is an EPSP, coming closer throughout the room is an EPSP, coming closer to pizza threshold.  Each roommate or hall-mate to pizza threshold.  Each roommate or hall-mate who stops by and reminds me that I owe them who stops by and reminds me that I owe them money is an IPSP, taking me further away from money is an IPSP, taking me further away from pizza threshold.  When I reach threshold, I go pizza threshold.  When I reach threshold, I go online and order the pizza.  Once I’ve hit “send”, online and order the pizza.  Once I’ve hit “send”, the signal travels away.  It is all-or-none, in that it the signal travels away.  It is all-or-none, in that it doesn’t go faster or slower, regardless of how doesn’t go faster or slower, regardless of how hungry I am.”hungry I am.”

Conduction Velocities of AxonsConduction Velocities of Axons

Conduction velocities vary widely among Conduction velocities vary widely among neuronsneurons

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

faster the impulsefaster the impulse Presence of a myelin sheath – myelination Presence of a myelin sheath – myelination

dramatically increases impulse speeddramatically increases impulse speedPLAYPLAY

Saltatory ConductionSaltatory Conduction

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

Voltage-gated NaVoltage-gated Na++ channels are channels are concentrated at these nodesconcentrated at 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

Nerve Fiber ClassificationNerve Fiber Classification

Nerve fibers are classified according to:Nerve fibers are classified according to: DiameterDiameter Degree of myelinationDegree of myelination Speed of conductionSpeed of conduction

SynapsesSynapses

A junction that mediates information A junction that mediates information transfer from one neuron:transfer from one neuron: To another neuronTo another neuron To an effector cellTo an effector cell

Presynaptic neuron – conducts impulses Presynaptic neuron – conducts impulses toward the synapsetoward the synapse

Postsynaptic neuron – transmits impulses Postsynaptic neuron – transmits impulses away from the synapseaway from the synapse

SynapsesSynapses

Figure 11.17

PLAYPLAY InterActive Physiology ®: Nervous System II: Anatomy Review, page 6

Electrical SynapsesElectrical Synapses

Electrical synapses:Electrical synapses: Are less common than chemical synapsesAre less common than chemical synapses Correspond to gap junctions found in other cell typesCorrespond to gap junctions found in other cell types Are important in the CNS in:Are important in the CNS in:

• Arousal from sleepArousal from sleep• Mental attentionMental attention• Emotions and memoryEmotions and memory• Ion and water homeostasisIon and water homeostasis

Chemical SynapsesChemical Synapses

Specialized for the release and reception Specialized for the release and reception of neurotransmittersof neurotransmitters

Typically composed of two parts: Typically composed of two parts: Axonal terminal of the presynaptic neuron, Axonal terminal of the presynaptic neuron,

which contains synaptic vesicles which contains synaptic vesicles Receptor region on the dendrite(s) or soma of Receptor region on the dendrite(s) or soma of

the postsynaptic neuronthe postsynaptic neuron

PLAYPLAY InterActive Physiology ®: Nervous System II: Anatomy Review, page 7

Synaptic CleftSynaptic Cleft

Fluid-filled space separating the Fluid-filled space separating the presynaptic and postsynaptic neuronspresynaptic and postsynaptic neurons

Transmission across the synaptic cleft: Transmission across the synaptic cleft: Is a chemical event (as opposed to an Is a chemical event (as opposed to an

electrical one)electrical one) Ensures unidirectional communication Ensures unidirectional communication

between neuronsbetween neurons

PLAYPLAY InterActive Physiology ®: Nervous System II: Anatomy Review, page 8

Synaptic vesiclescontaining neurotransmitter molecules

Axon of presynapticneuron

Synapticcleft

Ion channel(closed)

Ion channel (open)

Axon terminal of presynaptic neuron

PostsynapticmembraneMitochondrion

Ion channel closed

Ion channel open

Neurotransmitter

Receptor

Postsynapticmembrane

Degradedneurotransmitter

Na+Ca2+

1

2

34

5

Action

potential

Figure 11.18

Synaptic Cleft: Information Synaptic Cleft: Information TransferTransfer

Termination of Neurotransmitter Termination of Neurotransmitter EffectsEffects

Neurotransmitter bound to a postsynaptic Neurotransmitter bound to a postsynaptic neuron: neuron: Produces a continuous postsynaptic effectProduces a continuous postsynaptic effect Blocks reception of additional “messages” Blocks reception of additional “messages” Must be removed from its receptorMust be removed from its receptor

Removal of neurotransmitters occurs when Removal of neurotransmitters occurs when they:they: Are degraded by enzymesAre degraded by enzymes Are reabsorbed by astrocytes or the Are reabsorbed by astrocytes or the

presynaptic terminals presynaptic terminals Diffuse from the synaptic cleftDiffuse from the synaptic cleft

NeurotransmittersNeurotransmitters

Chemicals used for neuronal Chemicals used for neuronal communication with the body and the communication with the body and the brainbrain

50 different neurotransmitters have been 50 different neurotransmitters have been identifiedidentified

Classified chemically and functionallyClassified chemically and functionally

Neurotransmitters: Neurotransmitters: AcetylcholineAcetylcholine

First neurotransmitter identified, and best First neurotransmitter identified, and best understoodunderstood

Released at the neuromuscular junctionReleased at the neuromuscular junction Synthesized and enclosed in synaptic Synthesized and enclosed in synaptic

vesiclesvesicles

Neurotransmitters: Neurotransmitters: AcetylcholineAcetylcholine

Degraded by the enzyme Degraded by the enzyme acetylcholinesterase (AChE)acetylcholinesterase (AChE)

Released by:Released by: All neurons that stimulate skeletal muscleAll neurons that stimulate skeletal muscle Some neurons in the autonomic nervous Some neurons in the autonomic nervous

systemsystem