Electrical Current and the Body Reflects the flow of ions rather than electrons There is a...
-
Upload
myles-fisher -
Category
Documents
-
view
213 -
download
0
Transcript of Electrical Current and the Body Reflects the flow of ions rather than electrons There is a...
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