ME01 - Membrane Electrophysiology Flashcards by Kate Alyssa Caton

Characteristics of Ion Channels

Ion Channel Selectivity
Ion Channel Gating
Conductance of Ion Channel

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Ion channels permit the passage of some ions, but not other

Ion Channel Selectivity

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Ion Channel Selectivity is based on:

Channel size
Distribution of charges that line it
Size and charge of ions
How much water the ion attracts and holds around it

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Ion channels are either GATED or NON-GATED

Ion Channel Gating

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What are Gated ion channels and it’s types

Gated ion channels have gates that can open or close the channel
Voltage-gated - gates are controlled by voltage (difference in membrane potential)
Ligand-gated - open or closed by hormones, second messengers or neurotransmitters

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Ion channels that are always open

Also called as leakage channels

Non-Gated Ion Channels

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Conductance of Ion Channels depend on ______

If the channel is OPEN

The higher the probability that a channel is open, the higher the conductance or permeability

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Location of ION CHANNELS

Non-Gated: Cell membrane on dendrites, Cell Body and Axon
Ligand-Gated: dendrites, cell body
Voltage-Gated: Axon hillock, unmyelinated axons, along the nodes of Ranvier in myelinated axons

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Functions of ION CHANNELS

Non-Gated : RMP
Ligand-Gated: Synaptic potentials (incoming signals to neurons)
Voltage-Gated: Generation and Propagation of action potential (outgoing signals from neurons)

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Types of Membrane Potentials

Diffusion Potential

Equilibrium Potential

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Potential difference generated across a membrane because of a concentration difference of an ion

DIFFUSION POTENTIAL

Can be generated only if the membrane is permeable to the ion

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Factors related to the Diffusion Potential
Size
Sign

Size depends on the size of the concentration gradient

Sign depends on whether the diffusing ion is positively or negatively charged

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Diffusion Potential do not result in changes in concentration of the diffusing ions

TRUE.

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Diffusion potential that exactly balances (opposes) the tendency for diffusion caused by a concentration difference

Equilibrium Potential

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Chemical and Electrical Driving forces that act on an ion are equal and opposite
No net diffusion of the ion occurs

Electrochemical Equilibrium

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In the presence of a nondiffusable ion, diffusable ions distribute themselves so that at equilibrium their concentration ratios are equal
For any pair of cation/anion

Gibbs-Donnan Equilibrium

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Formula for Gibbs-Donnan Equilibrium

[K+]in x [Cl-]in = [K+]out x [Cl-]out

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Implications and Consequence of Gibbs-Donnan Equilibrium

(1) Presence of charged proteins in cells, there are more osmotically active particles in cells than in IF
C: Animal cells have flexible walls; Osmosis make cells swell and rupture; Prevented by Na/K ATPase

(2) At equilibrium, distribution of ions across membrane is assymetric
C: Electrical potential difference exists across membrane (MP)
Magnitude can be determined by Nernst equation

(3) There are more proteins in the plasma than in the IF
C: Gibbs-Donnan equilibrium will affect ion movement across capillary walls

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Calculates the equilibrium potential at a given concentration difference of a permeable ion across cell membrane

Nernst equation

EMF = 61+log concentration inside/conc. outside

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Signs to remember in Nernst equation

sign of potential is (+) if diffusing ion is a negative ion

sign of potential is (-) if diffusing ion is a positive ion

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Calculates the membrane potential in the inside of a membrane when a membrane is permeable to several different ions

Goldman-Hodgkin-Katz Equation

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Measured potential difference across the cell membrane in millivolts (mV)

RMP

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Intracellular potential relative to the extracellular potential

RMP

RMP of -70 means 70mV cell negative

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Characteristics of EMP

Established by diffusion potentials that result from concentration differences of permeable ions
Each permeable ion attempts to drive the membrane potential toward its equilibrium potential

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ORIGIN OF RMP

Contribution of K+ Diffusion Potential Contribution of Na+ Diffusion Contribution of Na/K ATPase

If potassium ions were the only factor causing the resting potential, the resting potential INSIDE THE FIBER would be equal to ________

-94 millivolts

Potassium vs Sodium in Diffusion and Permeability

Diffusion of K contributes far more to membrane potential than diffusion of Na Permeability to K is about 100 times as great as its permeability to Na

Contribution of Na/K ATPase to the Original RMP

Direct electrogenic contribution of the pump (3Na2K) is small This creates an additional degree of -4mV

Short range change in voltage Incoming Na ions diffuse for short distances inside the plasma membrane Produce a current that travels from the point of stimulation toward the cell's trigger zone

LOCAL POTENTIALS

Any change in which membrane voltage shifts to a less negative value

Depolarization

Two possible effects of Depolarization

Local potential - generation of a nonpropagated response | Action potential - generation of a propagated response

Characteristics of LOCAL POTENTIAL

Graded. (Stronger the stimulus, stronger the response) Decremental. (Gets weaker as they spread from point of stimulation) Reversible. (If stimulation ceases, K diffusion out of cell to return membrane voltage to its resting potential) Either Excitatory or Inhibitory. Excitatory - produce an action potential Inhibitory - less likely to produce an action potential

Types of Local Potential

Synaptic Local Potentials Generator/Receptor Potentials Electrotonic Potentials

Alteration in the Membrane Potential of cell resulting from activation at the synapse

Synaptic Potential

Types of Synaptic Potential

Excitatory Post-Synaptic Potential - Intracellular voltage increases Inhibitory Post-Synaptic Potential - Intracellular voltage decreases

Transmembrane potential difference produced in sensory receptor Occurs as depolarization resulting from inward current flow

Generator/Receptor Potential

Influx of current will often bring the membrane potential of the sensory receptor towards the threshold for triggering an action potential

Generator/Receptor Potential

Passive spread of charge inside a neuron due to local changes in ionic conductance Ionic charge enters in one location and dissipates to others, losing intensity as it spreads (graded response)

Electrotonic Potential

Rapid changes in the membrane potential that spread along the nerve fiber

Action Potential

How do you conduct a nerve signal

The action potential moves along the nerve fiber until it comes to the fiber's end

Characteristics of Action Potential

Follow All-or-None Law. If stimulus depolarizes neuron to threshold, neuron fires at its maximum voltage. Above threshold, stronger stimuli do not produce stronger action potential. Nondecremental. AP do not get weaker with distance Irreversible. AP goes to completion. AP cannot be stopped once it begins.

``` Fill the table. Parameter LOCAL POTENTIAL ACTION POTENTIAL Mediator Voltage Change Intensity Reversibility Distance of Effect Progression ```

Parameter LOCAL POTENTIAL ACTION POTENTIAL Mediator Ligand-gated ion channels Voltage-gated ion channel Voltage Change Depolarizing/Hyperpolarizing Depolarizing Intensity GRADED ALL-OR-NONE Reversibility Reversible Irreversible Distance of Effect Short (Local) Long (distant) Progression Decremental Non-decremental

Steps in the Generation of Action Potential

Resting Stage - RMP, before AP begins, -90mV Depolarization - Threshold potential is reached (-65mV) Voltage-gated Na overwhelm K+, permeable to Na ions Overshoot - great excess of positive Na ions causes MP to overshoot beyond the zero level

Ionic Basis of Depolarization | Necessary factor in both depolarization and repolarization of the nerve membrane

Voltage-Gated Sodium Channel

Feedback Control of Depolarization

Positive Feedback

Sodium channels begin to close and the K channels open more than normal Rapid diffusion of K ions to the exterior reestablishes resting membrane potential

Repolarization

Ionic Basis of Repolarization | Also plays an important role in increasing the rapidity of repolarization of the membrane

Voltage-Gated Potassium Channel

Feedback Control of Hyperpolarization

Negative Feedback

Afterhypolarization K+ conductance remains higher than at rest after closure of the Na+ channels Membrane potential is driven very close to the K+ equilibrium potential

Undershoot

Effects of Ion Fluxes on Action Potential Structure Contribution K+ leak channel Na+/K+ ATPase pump Voltage-gated Na+channels Voltage-gated K+ channels

Structure Contribution K+ leak channel Resting Membrane Potential Na+/K+ ATPase pump Maintenance of the Concentration gradient Voltage-gated Na+channels Depolarization and Hyperpolarization Voltage-gated K+ channels Hyperpolarization

Effect on | DECREASE extracellular Na+ concentration

Reduce the size of AP | No effect on RMP

Effect on: | INCREASE extracellular K+ concentration

Decrease RMP

Effect on: | DECREASE extracellular Ca2+ concentration

INCREASE excitability | DECREASE threshold

Effect on: | INCREASE extracellular Ca2+ concentration

DECREASE excitability | INCREASE threshold

Effect on: INCREASE intracellular Cl- concentration INCREASE intracellular Protein concentration

DECREASE RMP

How are AP propagated?

By the spread of local currents to ADJACENT areas of membrane, which are then depolarized to threshold and generate action potentials

Direction of AP Propagation

No single direction of propagation | An AP travels in both directions away from the stimulus until the entire membrane has become depolarized

Depolarization process travels over the entire membrane if conditions are right, or it does not travel at all if conditions are not right

All-Or-Nothing Principle

Factors Affecting Conduction Velocity

Fiber Size - Increase diameter, faster | Myelination - myelin acts insulator; saltatory conduction (5-50 fold)

Action potentials can be generated only at gaps in the myelin sheath called as

Nodes of Ranvier

Importance of Saltatory conduction

It conserves energy for the axon | Only the nodes depolarize, allowing 100 times less loss of ions than would otherwise be necessary

Time periods after an action potential during which new stimulus cannot be readily elicited

Refractory Period

Another AP cannot elicited, no matter how large the stimulus is Coincides with almost the entire duration of the action potential

Absolute Refractory Period

Ionic basis of Absolute Refractory Period

Inactivated gates of Na+ channel are closed when membrane potential is depolarized and remain closed until repolarization occurs No AP can occur until the inactivations gates open

Begins at the end of the Absolute refractory period and continues until the membrane potential returns to the resting level AP ca be elicited only if a larger than usual inward current is provided

Relative Refractory period

Ionic Basis of Relative Refractory Period

K+ conductance is higher than the rest MP is closer to the K+ equilibrium potential and farther from the threshold More inward current is required to bring the membrane to threshold

Integral proteins that span the cell membrane | When open, permit the passage of certain ions

Ion Channels