potentials lectures 10_12
TRANSCRIPT
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Resting Potentials and Action Potentials
Lecture 10PSY391S
John Yeomans
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Special Properties of Neurons
• Excitability--Action Potential in Axons.• Conduction--Action Potential in Axons.• Transmission--Synapses, Electrical &
Chemical.• Integration--Postsynaptic Cell.• Plasticity--Presynaptic Terminal and
Postsynaptic Membrane.
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Resting and Action Potentials
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Pumps Exchange Ions
• All cells have pumps and resting potentials (-40 to -90 mV).
• Pumps use ATP to exchange ions.• Na+/K+ pump: 3 Na+ exchanged for 2 K+.• Ca++ pump: Keeps powerful Ca++ ions
out.
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Concentration of Ions and State of Channels at Rest
-65 mV
Concentrations maintained by Na+/K+ and Ca++ pumps.
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Potentials• All potentials result from ions moving across
membranes.• Two forces on ions: Diffusion (from high to low
concentration); Electrical (toward opposite charge and away from like charge).
• Each ion that can flow through channels reaches equilibrium between two forces.
• Equilibrium potential for each ion determined by Nernst Equation.
• K+ make - potentials; Na+ make + potentials.
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Nernst Equation
• EK+ = +58 mV log10 ([K+] outside/[K+] inside).
(+58 mV for room temperature, squid axon).
• EK+ = 58 mV log10 1/20 = -75 mV.
• ENa+ = 58 mV log10 10/1 = + 58 mV.
• ECl- = -58 mV log10 15 = -68 mV.
• ECa++ = +58 mV log10 10,000 = +220 mV.
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Resting Potential Results from Passive K+ Channels and EK+
• At rest, membrane potential is -60 to -70 mV in most neurons. Why?
• K+ is most permeable, due to leak of K+ through passive K+ channels.
• Therefore, K+ ions leave, making the inside more negative.
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Action Potential Results from Voltage-gated Na+ Channels
ENa+ = +58 mV
EK+ = -75 mv
Closed!
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Action Potentials
• Only neurons and muscles have action potentials (not all neurons).
• Due to voltage-gated Na+ channels.• Most in axons, at initial segment (axon
hillock) and nodes of Ranvier. A few in big dendrites where depolarizations need a boost.
• Channel ionic currents are studied by voltage clamps and patch clamps.
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Voltage Clamp• Used to measure ion currents in squid giant
axons (Hodgkin & Huxley).• Study single ion by changing ions in axon.• Hold voltage constant by injecting current with
large electrode. Measured current I.• Measured Na+ or K+ current during action
potential: INa+ = V/R = K/R ~ Na+ conductance.• Measure “channel” permeability changes.• Predicted action potential changes from Nernst
Eq and channel permeabilities.
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Single Channels
Study electrical properties,Ionic properties, Pharmacology (toxins, agonists, antagonists)Molecular biology (mutant channels)
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Voltage-gated Na++Channel: MolecularStructure and Gating
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All Na+ channels open in APabsolute refractory period.
(No voltage-gated K+ channels in mammalian unmyelinated axons)
(>1 m/s in mammals)
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(1-120 m/s)
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Synapses and Postsynaptic Potentials
Lecture 11PSY391S
John Yeomans
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Release and Ca++
• Transmitter is synthesized and stored in vesicles.
• Action potential opens voltage-gated Ca++ channels near release sites.
• Ca++ activates proteins that move vesicles to release sites.
• Exocytosisrelease and diffusion of transmitter.• EPSPs, IPSPs (depending on ions) .• Reuptake or enzyme breakdown of transmitter.
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Chemical Receptors
Nicotinic, AMPA Na+ Muscarinic, Dopamine, GABAB
GABAA Cl- Gs, Gi
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Ionotropic Receptors
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Receptors are now defined by genes
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Second Messengers
• cAMP and cGMP, IP3, DAG (G-coupled)• Ca++, etc.• Kinases (dozens, e.g. A, CaMK)• Gene transcription (CREB)• Plasticity • Retrograde messengers NO and CO.
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Other Receptor Types
• Steroid receptors--Lipophilic molecules pass through membrane to act in neurons.
• Tyrosine kinase receptors--NGF activates enzymes and kinases.
• Slower growth effects.
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Summation
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PSPs
• Excitatory: Na+ or Ca++ entry.• Inhibitory: K+ efflux or Cl- entry.• Also blocking open channels (e.g. rods
and cones).• Slow potentials: seconds to hours.
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Integration of Potentials
Lecture 12John Yeomans
PSY391S
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Computation in Single Neurons
• Thinking requires complex computation. How?
• Neural computation occurs in postsynaptic cells, by integration of PSPs, and by changes in synapses.
• We still have no idea how thoughts are represented in neurons or circuits, only rough ideas of which brain regions are important.
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Integration in the Cell and Axon
PSPs decay with distance.
Integration occurs at axon hillock.
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Synapses on Soma, Dendrites and Spines
Thousands of synapses, of many types, on each output neuron.
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• Strongest near axon, usually inhibitory.• Next strongest on soma and proximal
dendrite shafts.• Weakest synapses on spines, usually
excitatory.• Larger neurons usually have more
synapses, more spines. Why?
Synapse Strength
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Spines
• Problem: Too many synapsestoo much ion leakage along dendrites.
• Solution: Place synapses on isolated spines.
• All spine synapse have equal access to dendrite shafts.
• Spine shapes change in minutes: mushrooms less, slivers moreplasticity.
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Plasticity
• Facilitation and depression of PSPs.• Presynaptic changes: transmitters,
vesicles, release, retrograde NO.• Postsynaptic changes: Receptors can be
added and subtracted. Channels can be phosphorylated;
• Second messengers and kinases can change postsynaptic response;
• Spines can grow or shrink; New proteins.
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Integration of Brain Potentials
• Most recordings are extracellular, or outside brain. Averages across many or millions of neurons.
• Electrode size and distance determines how many neurons are measured.
• Human studies are mainly from surface of brain. Brain-waves are correlated with thoughts (Dreams, meditation, stimuli).
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Human Potentials
• Strong potentials in muscles--EMG, ECG (electromyogram and electrocardiogram).
• Weaker potentials from brain--EEGs.• Evoked potentials allow study of changes.• Computer averaging allows study of deep
brain potentials: Event-related potentials in sensory systems and cognition.
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EEG and ERP
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Electroencephalogram
• Shows widespread activity of brain, mainly from PSPs.
• Sleep stages, waking, slow wave, REM.• Most intense in seizures of different types,
petit mal, grand mal etc.• Can find lobes that are most active (e.g.,
occipital for alpha waves, temporal or frontal lobe or for seizures).
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Event-Related Potentials
• Warning and CNV: Cortex mainly.• I-VI : Brain stem auditory paths.• No-P3 : Cortical processing of auditory
stimulus. Primary to association areas. • Temporal resolution better than spatial
resolution.• Brain imaging (fMRI) localizes thoughts
better, but not to neurons.