s 1 28 september topics graded potentials action potentials properties of v-gated ion channels ap...

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28 September TopicsGraded potentialsAction PotentialsProperties of V-gated ion channelsAP conduction

Concerning Test # 1Types of questions

Recall informationApply to situationsChapter questionsRefugee physiology

Check Moodle site for 1QQ grades

Family WeekendDr. Davis in Office or Lab10-11:30 am

1QQ # 10 for 8:30 class1. At resting membrane potential

a) The concentration gradient favors the entry of Na+

b) The electrostatic driving force favors the entry of Na+

c) The concentration gradient favors the exit of K+

d) The electrostatic driving force favors the exit of K+

e) The membrane is more permeable to Na+ than to K+.

2. Suppose you have a cell with the normal concentrations of Na+ and K+ on either side of the membrane, and you were able to make the membrane absolutely and completely impermeable to Na+. A) What would the membrane potential be? (include the units.)B) What equation would you use to calculate the membrane potential?

1QQ # 10 for 9:30 class1. At the equilibrium potential for Na+

a) The concentration gradient favors the entry of Na+

b) The electrostatic driving force favors the entry of Na+

c) The concentration gradient favors the exit of K+

d) The electrostatic driving force favors the exit of K+

e) The membrane is more permeable to Na+ than to K+.

2. Suppose you have a cell with the normal concentrations of Na+ and K+ on either side of the membrane, and you were able to make the membrane absolutely and completely impermeable to K+. A) What would the membrane potential be. (include the units.)B) What equation would you use to calculate the membrane potential?

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Excitable membranes & special structures make Neurons good Electrical Communicators

}receiving

}sending

Ligand-gated ion channels in membranes of dendrites and soma…. Graded potential

conducted decrementally

Voltage-gated ion channels in membrane of axon hillock and axon…..Action potentials = “all or nothing!”

Axon hillock “integrates” and produces an “all-or-nothing” Action Potential conducted non-decrementally and unidirectionally.

Synapse onother neurons, skeletal muscle, smooth muscle, cardiac muscle,glands

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Leak ChannelsGated Channels….. Ligand-gated….. Mechanically-gated….. Voltage-gated

Interneurons & Motoneurons

Sensory neuronTypes and locations of Ion Channels

Intracellular RecordingElectrode orStimulating Electrode

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w/ LGCs and MGCs

w/ VGCs

w/ LGCs

How is the intensity of a stimulus encoded by action potential if all action potentials have the same size (amplitude)?

What happens when the membrane is depolarized by more than about 15 mV?

Action potentials are all or nothing.Analogy of shutter release pressure on a camera, either trips shutter or not.

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Relevance of the GHK equation

To reset from inactivated state to closed state, membrane must repolarize.

Compare and contrast voltage-gated Na and K channels based on time to open and duration of open time.

Open at -55 mV

Membrane must repolarize to “reset” Na+ Channels to be capable of opening again.

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Voltage-gated Na+ channel

scienceblogs.com/.../upload/2006/03/channel.jpg

Tetrodotoxin from ovary ofPuffer fish, used in Japanese sushi (fugu)

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What types of ion-channels are labeled in this neuron in red?

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TTX with red fluorescent marker

Relative permeabilitiesDuration of APRefractory periods

absolute RPrelative RP

Properties of V-gated Na+ and K+ channels account for the shape of the action potential and the refractory periods.

Why does the peak of the action potential not reach ENa?

RisingPhase

FallingPhase

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Natural ways to Initate an Action Potential

Graded depolarization in cell body reach threshold at axon hillock

Graded depolarization in in receptive membranes of sensory neurons reach threshold for AP. i.e. nociceptors and stretch receptors.

Unstable membrane potential cycles: pacemaker potentials in pacemaker cells of heart, smooth muscles of gut, and medullary neurons for respiratory rhythm.

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Who Cares?

Novacaine, lydocaine, xylocaine, All block voltage-gated Na+ channelsPrevent action potentials, so stimulus does not result in an action potential in sensory neurons which would convey that information to the brain where person would be conscious of the stimulus!

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Axon Hillock

Axon

Questions About Action Potential Conduction:How does an action potential move along the axon? Why doesn’t the amplitude get smaller with distance?Why is the conduction of an action potential unidirectional? What is the absolute refractory period and what is going on with voltage gated sodium channels that accounts for the absolute refractory period?What is the relative refractory period and what is going on with voltage gated sodium channels that accounts for the relative refractory period?

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In unmyelinated axons, action potential must be generated at each point along the membrane, a relatively slow process that involves influx of Na+ which sets up positive feedback cycle.

In myelinated axons, action potential must be generated only at the nodes of Ranvier, which allows AP to be conducted much faster and with fewer ions moving, and thus less energetically expensive.

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