4 & 5. Resting and Action potentials Flashcards

1
Q

Flux

A

the rate of transfer of molecules

= number of molecules that cross a unit area per unit time e.g. m-2. s-2

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2
Q

Is there net flux in dynamic equilibrium?

A

No

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3
Q

Equilibrium potential

A

The potential at which electrochemical equilibrium has been reached.
Concentration gradient balanced by electrical gradient

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4
Q

When is electrochemical equilibrium achieved?

A

when electrical force prevents further diffusion across the membrane

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5
Q

What does the Nernst equation show?

A

It allows you to calculate the equilibrium potential of a cell (dependent on temperature and charge)

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6
Q

What are the intracellular and extracellular concentrations of the 4 main ions involved in action potentials?

A

Na+: inside: 10 outside: 150
K+: inside: 150 outside: 5
Cl-: inside: 5 outside: 120
Ca2+: inside: 0.1 outside: 2

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7
Q

Explain why the resting membrane potential of most cells is around -70 mV

A

Equilibrium potential of K+ is -90 mV and Na+ is +72
Membranes have mixed K+ and Na+ permeability
But more permeable to K+ so resting membrane potential is closer to equilibrium potential of K+.

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8
Q

What does the Goldman-Hodkin-Katz equation show?

A

Accounts for relative permeability of the membrane at any one time to different ions so you can figure out the resting membrane potential.

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9
Q

Resting potential

A

-70mV

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10
Q

Depolarising

A

-70 to 0 (becoming more +)

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11
Q

Hyperpolarising

A

-70 to -90 (becoming more -)

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12
Q

Overshoot

A

0 to +

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13
Q

Repolarising

A

+ to -

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14
Q

What is the difference between graded potentials and action potentials?

A

APs are an all-or-nothing event (have the same amplitude every time)
GPs can vary in amplitude and the amplitude is affected by the strength of the stimulus. GPs can be positive or negative and they decrease in altitude as they travel away from the point of origin.

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15
Q

How can nerve cells use graded depolarisations and hyperpolarisations for signalling?

A

Contribute to initiating or preventing action potentials

May summate or cancel each other out, create a more integrated signal

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16
Q

Where do graded potentials occur?

A

Synapses

Sensory receptors

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17
Q

Where do action potentials occur?

A

In excitable cells
Mainly neurones and muscle cells e.g. in muscle cells lead to contraction
Also in some endocrine tissues e.g. In B cells of pancreas provoke release of insulin

18
Q

What does permeability to ions depend on?

A

Conformational state of ion channel

Most channels in neurology are voltage operated

19
Q

What are the 5 phases of an action potential?

A
  1. Resting membrane potential
  2. Depolarising stimulus
  3. Upstroke
  4. Repolarisation
  5. After hyperpolarisation
20
Q

Describe phase 1 of an action potential

A

Caused by K+ moving out of cells
Finite permeability to Na+
Membrane potential nearer equilibrium potential for K+ than for Na+

21
Q

Describe phase 2 of an action potential

A

Stimulus depolarises membrane potential
(Graded response- small and slow depolarisation)
Moves it in +ve direction towards threshold

22
Q

Describe phase 3 of an action potential

A

Starts at threshold potential
At threshold potential lots of Na+ channels respond
PNa increases because voltage-gated Na+ channels open quickly
Na+ ions enter the cell down their electrochemical gradient
Depolarisation
Membrane potential moves toward Na+ equilibrium potential
PK increases as the voltage-gated K+ channels start to open slowly
K+ ions leave the cell down their electrochemical gradient
(Slowly- Less than Na+ entering)

23
Q

Describe phase 4 of an action potential

A

PNa decreases because voltage-gated Na+ channels inactivate (Channels blocked)
Na+ entry stops
PK increases as more voltage-gated K+ channels open and remain open
K+ leaves the cell down its electrochemical gradient
Membrane potential moves toward the K+ equilibrium potential

24
Q

Describe phase 5 of an action potential

A

(At rest) voltage-gated K+ channels are still open
K+ continues to leave the cell down the electrochemical gradient
Membrane potential moves closer to the K+ equilibrium
Additional K+ channels are still open, so slight hyperpolarisation (Close slowly)
Some voltage-gated K+ channels then close

Membrane potential returns to the resting potential

25
What occurs at the start of repolarisation?
Part of channel protein reacts to change in voltage and blocks the channel so Na+ cant get through Na+ channel gate is open, but blocked by inactivation protein Na+ entry to cell stops Depolarisation recognised by K+ channels and more open
26
What is the underlying mechanism for the absolute refractory period? What is this?
Na+ channel inactivation is the underlying mechanism for the Absolute refractory period Inactivation gate is closed New AP cannot be triggered even with very strong stimulus
27
What happens later in repolarisation?
Absolute refractory period continues Na+ Channel activation gate closes Inactivation protein still present
28
What occurs during post-hyperpolarisation?
Relative refractory period Inactivation gate is open Stronger than normal stimulus required to trigger an action potential
29
How long does a normal action potential last?
2 ms
30
Threshold potential
Once this potential is reached an AP is triggered
31
Refractory state
Unresponsive to threshold depolarization
32
What is the regenerative relationship between PNa+ and membrane potential?
Once threshold is reached the cycle continues Positive feedback behaviour Cycle continues until the voltage-gated Na+ channels inactivate- closed and voltage-insensitive Membrane remains in a refractory (unresponsive) state until the voltage-gated Na+ channels recover from inactivation
33
Ion channels vs Ion pumps in an action potential
Ion channels: Main pathway for ion movement, rapid | Ion pumps: NOT directly involved in ion movement during an AP, slow, need ATP
34
Why can the AP only move in 1 direction?
Originating area is refractory
35
Nodes of ranvier
Unmyelinated gaps along the axon Allow rapid impulse propagation as impulse can jump to areas of high concentration of Na+ channels Allows movement via saltatory conduct
36
What influences conduction velocity?
Axon diameter | Axon myelination
37
Large diameter myelinated axons speed
120 m/s | Low internal resistance
38
Small diameter non-myelinated axons speed
1 m/s Can't travel by saltatory conduct High internal resistance
39
How does conduction velocity change with increased axon diameter?
Increases | Less resistance to current flow inside large diameter axons
40
How does conduction velocity change with myelination?
Increases | AP's only occur at nodes of Ranvier
41
What slows conduction velocity?
Reduced axon diameter (i.e. re-growth after injury) Reduced myelination (i.e. MS and diphtheria) Cold, anoxia, compression and drugs (some anaesthetics)