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Flashcards in Electrophysiology Deck (40):
1

• integrated proteins that span the cell membrane
• When open, permit the passage of certain ions

Ion Channels

2

• Ion channels permit the passage of some ions, but not others
• Selectivity is based:
o channel size
o distribution of charges that line it
o size and charge of ions
o how much water the ion attracts and holds around it

Ion Channel Selectivity

3

- ion channels have gates controlled by voltage (differences in membrane potential)
-location: axon hillock, unmyelinated axons, along the nodes of Ranvier in myelinated axons
- responsible for generation and propagation of action potentials (outgoing signals from neurons)

Voltage-gated

4

- ion channels (chemically-gated channels) are opened or closed by hormones, second messengers, or neurotransmitters
- location: dendrites, cell body
- responsible for synaptic potentials (incoming signals to neurons)

Ligand-gated

5

- ion channels (leakage channels) are always open
-location: cell membrane on dendrites, cell body and axon
- responsible for resting membrane potential

Non-Gated

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• Potential difference generated across a membrane because of a concentration difference of an ion
• Can be generated only if the membrane is permeable to the ion
• SIZE depends on the size of the concentration gradient
• SIGN depends on whether the diffusing ion is positively or negatively charged
• Created by the diffusion of very few ions
• Do not result in changes in concentration of the diffusing ions

Diffusion Potential

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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

9

• In the presence of a nondiffusible ion, diffusible ions distribute themselves so that at equilibrium their concentration ratios are equal

Gibbs-Donnan Equilibrium

10

• Used to calculate the equilibrium potential at a given concentration difference of a permeable ion across a cell membrane

Nernst Equation

11

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

Goldman-Hodgkin-Katz Equation

12

• Measured potential difference across the cell membrane in millivolts (mV)
• Expressed as the intracellular potential relative to the extracellular potential
• A resting membrane potential of -70 mV means 70 mV, cell negative

Resting Membrane Potential (RMP)

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

Depolarization

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generation of a nonpropagated response

Local potential

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generation of a propagated response

Action potential

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• Vary in magnitude (voltage) according to the strength of the stimulus
• A more intense or prolonged stimulus opens more ion gates than a weaker stimulus

Local potentials are GRADED

17

• Get weaker as they spread from the point of stimulation
• As Na spreads out under the plasma membrane and depolarizes it, K flows out
• Prevents local potentials from having any long-distance effects

Local potentials are DECREMENTAL

18

• if stimulation ceases, K diffusion out of the cell quickly returns the membrane voltage to its resting potential

Local potentials are REVERSIBLE

19

• Alteration in the membrane potential of a cell resulting from activation at the synapse
• If the intracellular voltage increases, it is called an excitatory post-synaptic potential (EPSP)
• If the intracellular voltage decreases, it is called an inhibitory post-synaptic potential (EPSP)

SYNAPTIC POTENTIALS

20

• Transmembrane potential difference produced in sensory receptors
• Occurs generally as depolarization resulting from inward current flow
• Influx of current will often bring the membrane potential of the sensory receptor towards threshold for triggering an action potential

GENERATOR/RECEPTOR POTENTIALS

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• 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 POTENTIALS

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• Rapid changes in the membrane potential that spread along the nerve fiber
• To conduct a nerve signal, the action potential moves along the nerve fiber until it comes to the fiber's end

Action Potential

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• If a stimulus depolarizes the neuron to threshold, the neuron fires at its maximum voltage
• If threshold is not reached, the neuron does not fire at all
• Above threshold, stronger stimuli do not produce stronger action potentials (not graded)

Action potentials follow ALL-OR-NONE LAW.

24

• Action potentials do not get weaker with distance
• An action potential at the end of a nerve fiber will be just as strong as an action potential in the trigger zone up to a meter away

Action potentials are NON DECREMENTAL.

25

• If a neuron reaches threshold, the action potential goes to completion
• Action potentials cannot be stopped once it begins

Action potentials are IRREVERSIBLE.

26

• Resting membrane potential before the action potential begins
• Membrane is polarized because of the -90 mV membrane potential

Resting Stage

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• When the threshold potential (-65 mV) is reached, the voltage-gated Na+ channels overwhelm K+ and other channels
• Membrane suddenly becomes permeable to sodium ions

Depolarization

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• in large nerve fibers, the great excess of positive sodium ions causes the membrane potential to overshoot beyond the zero level

Overshoot

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o Necessary actor in both depolarization and repolarization of the nerve membrane

Voltage-Gated Sodium Channel

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• Sodium channels begin to close and the potassium channels open more than normal
• Rapid diffusion of potassium ions to the exterior re-establishes resting membrane potential

Repolarization

31

o also plays an important role in increasing the rapidity of repolarization of the membrane

Voltage-Gated Potassium Channel

32

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

After hyperpolarization (undershoot)

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• 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

34

Factors Affecting Conduction Velocity

1. Fiber Size
- Increasing the diameter of a nerve fiber results in decreased internal resistance and faster conduction velocity
2. Myelination
• Myelin acts as an insulator around nerve axons and increases conduction velocity
• Myelinated nerves exhibit saltatory conduction

35

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

Refractory Periods

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

Absolute Refractory Period

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• inactivation gates of the Na+ channel are closed when the membrane potential is depolarized and remain closed until repolarization occurs
• No action potential can occur until the inactivation gates open

Ionic Basis of Absolute Refractory Period

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

Relative Refractory Period

39

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

Ionic Basis of Relative Refractory Period

40

• Cell membrane is held at a depolarized level such that the threshold potential is passed without firing an action potential
• Occurs because depolarization closes inactivation gates on the Na+ channels

Accommodation