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Flashcards in Membrane Potential & Action Potential Deck (29)
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1
Q

How can you measure membrane potential of a cell?

A
  • A reference electrode placed outside of the cell (zero-volt level)
  • Another electrode placed inside the cell : measures a voltage negative compared with outside
2
Q

What prevents free movement of ions across cell membrane?

A

Lipid (hydrophobic) cell membrane is a barrier to ion movement and separates ionic environments

3
Q

How can ions go through cell membrane?

A

Permeabe pores (ion channels) open and close, depends on transmembrane voltage, presence of activating ligands or mechanical forces

Selectivity change permeability for ions

4
Q

What is electrochemical equilibrium?

A

Electrical forces balance diffusional forces

Stable transmembrane potential is achieved

5
Q

What is equilibirum potential?

A

Potential at which electrochemical equilibrium is reacged

Potential that prevents diffusion of ion down its conc gradient

6
Q

What equation is used for calculating equlibrium potential?

A

-The Nernst Equation

E= RT/zF In X2/X1

R= gas constant T= temp in kelvin

z = charge on ion (-1 Cl- +2 Ca2+)

F = Faradays number- charge per mol of ion

In= natural logorithm (log to base e)

X2= intacellular ion conc X1= extracellular ion conc

7
Q

In mM what is concentrations of various ions?

A

Na+ : 150mM extracellular , 10mM intracellular

K+ : 5mM extracellular , 150mM intracellular

Ca2+ : 2mM extracellular , 10^-4 mM intracellular

Cl- : 110mM extracelullar , 5mM intracellular

8
Q

Why do we use the Goldman-Hodgkin-Katz (GHK) equation?

A

Ek & Ena are theoretical values

In reality biological membranes are not uniquely selective for an ion

Membranes have mixed and variable permeability to all ions

Typical RMP (Em) is -70mV not =90mV which is Ek

GHK describes membrane potential (Em) more accurately

9
Q

What is depolarisation?

A

Membrane potential becomes more positive towards 0 mV

10
Q

What is repolarisation?

A

Membrane potential decreases towards resting potential

11
Q

What is overshoot?

A

Membrane potential becomes positive (above 0 mV)

12
Q

What is hyperpolarisation?

A

Membrane potetial decreases beyond resting potential

13
Q

What is a graded potential?

A

They produce the initial change in membrane potential that determines what happens next- initiate or prevent action potentials

Response to external stimulation or neurotransmitters

Decay along axon

14
Q

When does an action potential occur?

A
  • When a graded potential reaches a threshold for activation of many Na+ channels - leads to ‘all or nothing’ event
  • Occur in excitable cells (neurons, muscle cells) and in some endocrine tissue
15
Q

Are membrane potential changes during action potential due to ion pumps?

A

No, they are not. They are due to movement of ions through ion channels

16
Q

What are the 5 phases of the action potential?

A
  • Resting membrane potential
  • Depolarising stimulus
  • Upstroke
  • Repolarisation
  • After-hyperpolarisation

(all over within millisecond and half ish)

17
Q

What happens in phase 1 of Action potential (RMP)?

A
  • Permeability for Pk > Pna
  • Membrane potential nearer equilibrium potential for K+ (-90 mV) than for Na+ (+72mV)
18
Q

What happens in phase 2 of action potential (depolarising stimulus)?

A
  • The stimulus depolarises the membrane potential
  • Moves it in posititve direction towards threshold
19
Q

What happens in phase 3 of action potential (upstroke)?

A

Starts at threshold potential

Pna increases becauses VGSC open quickly so Na+ enters cell - produces overshoot

Pk increases as VGKCs start to open slowly , K leaves cell, less than Na entering though

Membrane potential moves toward Na+ equilibrium potential

20
Q

What happens in phase 4 of action potential (repolarisation)?

A

When peak of action potential achieved :

  • Pna decreases VGSC channels close so Na+ entry stops
  • Pk increases as more VGKC channels open and remain open
  • K+ leaves cell down its electrochemical gradient
  • Membrane potential moves toward the K+ equilibrium
21
Q

What is the absolute refractory period?

A

At start of repolarisation:

  • All sodium channels are open but then undergo CONFORMATIONAL CHANGE called INACTIVATION
  • Stops ion flow through sodium channel
  • New action potential can’t be triggered despite strong stimulus
  • Inactivation removed when cell repolarised completely
22
Q

What happens in phase 5 of action potential (after-hyperpolarisation)?

A
  • At rest VGKCs still open, K+ leaves cell down electrochemical gradient
  • Membrane potential moves closer to K+ equilibrium - some VGKCs then close
  • Membrane potenital returns to resting potential
  • Enter Relative Refractory Period : some Na+ channels recover from inactivation & open, stronger than normal stimulus needed to trigger action potential
23
Q

What is meant by ‘all or nothing’ events?

A
  • Once threshold potential reached, AP triggered
  • Once triggered full sized action potential occurs- positive feedback
  • If graded potential doesn’t reach threshold, it decays along axon
24
Q

What restores the ion concentration gradients after an AP?

A
  • K+ and Na+ carried across membrane against concentration gradients by different types of ion transporter Na+/K+ ATPase
  • 3 Na + out // 2 K+ in
25
Q

What determines the decay of a site of depolarisation?

A

1) Diameter of axon : small neurones larger resistance
2) Insulation of neuron : myelinated or not , insulation allows graded potential to decay further along axon (internal& membrane resistance)

26
Q

How is AP propagated actively down axon?

A

Stimulatory pulse has depolarised membrane so sodium flows in cell

Depolarisation will decay down adjacent area that is at RMP

If that area reaches threshold potential, sodium channels activated and sodium influx

There is depolarisation and then graded potential decays down new adjacent area etc etc AP propagated

27
Q

What happens at the Nodes of Ranvier?

A
  • These are gaps in myelin sheath
  • Saltatory conduction occurs
  • Voltage-gated channels mostly located at nodes
  • Makes transmission of AP rapid over long distances
28
Q

Small diameter non-myelinated axon Vs large diameter myelinated axon :

A

1m/s Vs 120 m/s

29
Q

What is passive propagation?

A

When there is a small voltage change that does not reach threshold and voltage decays down