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PHAR20300 > EXCITABLE CELLS > Flashcards

EXCITABLE CELLS Flashcards

1
Q

What is potential difference?

A

Difference in voltage between two points

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2
Q

What is membrane potential (Vm)?

A

Difference in voltage across the cell membrane (inside and outside of the cell)

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3
Q

What is resting membrane?

A

Difference in voltage across the cell membrane when a cell is a rest

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4
Q

What is graded potential?

A

Change in membrane potential that may or may not result in an action potential (leads to hyperpolarization)

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5
Q

What is an action potential?

A

A large, rapid change in membrane potential produced by depolarisation of an excitable cell’s plasma membrane to threshold

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6
Q

What is an equilibrium potential?

A

The membrane potential that results in no NET diffusion of an ion

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7
Q

What makes cells excitable?

A

Ability of a cell to be stimulated to create an electrical current that generates an action potential

(ion concentrations are extremely different inside the cell compared to the extracellular fluid)

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8
Q

Give examples of excitable cells (5)

A
  • All neurones
  • Skeletal muscles
  • Cardiac muscles
  • Some smooth muscle
  • Some endocrine cells (e.g. pancreatic β cells -
  • basically cells that need to transfer info quickly
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9
Q

Give examples of non-excitable cells

A

All other body cells e.g:

  • fibroblasts
  • epithelial cells, endothelial cells
  • adipocytes
  • blood cells
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10
Q

How do you measure resting potential?

A

The resting membrane can be measured using a microelectrode (recording electrode), electrode and voltmeter

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11
Q

What is the resting potential membrane dependent on?

A
  • Factor 1 – conc gradient of ions
  • Factor 2 - presence of ion channels in the plasma membrane
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12
Q

What pumps control the resting potential’s conc. gradient?

A

Na+/K+, H+/K+, H+, Ca2+ ATPases

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13
Q

What ion channels control the resting potential?

A

K+, Na+, Ca2

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14
Q

What are the three states an ion channel can be in?

A
  • Closed (resting) - equilibrium of the equation to left
  • Open (activated) - when a depolarising stimulus arrives, the equilibrium shifts to the right = opens
  • Inactivated (refractory/ desensitised) - occurs during sustained depolarisation (requires repolarization)
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15
Q

Define Nernst (equilibrium) potential

A

The electrical potential that balances the chemical potential (so no net movement of ions) - for membranes with only one ion

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16
Q

Give the Nernst (equilibrium) potential equation

A
  • VN = The Nernst Equilibrium Potential (mV)
  • R= Ideal Gas Constant
    • 8.314 J.K-1.mol-1 (Joules per Kelvin per mole).
  • T = Temperature (in Kelvin)
    • K = °C + 273
    • RT at room temp is 61
  • z = the charge of the ion (valence)
  • F = Faraday’s number
  • 96485 C.mol-1 (Coulombs per mole).
17
Q

How to calculate the Vm (voltage inside relative to outside), when there are multiple penetrating solutes? which equation?

A
  • Goldman-Hodgkin-Katz (GHK) equation
  • Contribution of membrane potential is weighted according to permeability
  • pK is the membrane permeability for K+.
  • pNa is the relative membrane permeability for Na+.
  • pCl is the relative membrane permeability for Cl-
18
Q

What does Van Hoff equation state?

A

Chemical energy is proportional to solute concentration

19
Q

What is the implication of the 3 equations below combined?

  • Van Hoff equation
  • Nernst equation
  • Goldman/GHK equation
A

Gated ion channels can rapidly change the membrane voltage, without any change in bulk intracelluar concentrations, on a time scale as fast as ~1ms (nerve action potential)

20
Q

What is the implication of the 3 equations below combined? Van Hoff equation Nernst equation Goldman/GHK equation

A

Gated ion channels can rapidly change the membrane voltage, without any change in bulk intracellular concentrations, on a time scale as fast as ~1ms (nerve action potential)

21
Q

What does automaticity mean?

A

Ability of a tissue or organ to function without external control

22
Q

Describe how the heart is able to be automatic?

A
  • Cells store energy as a concentration gradient of ions across the plasma membrane (inside differs from outside)
  • Membrane proteins control passage of ions across the membrane (creates electrical activity and excitability
  • Electrical signal can travel rapidly to neighbouring cells - Electrical activity is coupled to mechanical action (contraction)
23
Q

Describe the appearance of cardiac muscle on a cellular level

A
  • Short (100-200um)
  • Strong, branching cells held together at intercalated discs
  • Electrical (gap) junctions allow cell-to-cell spread of action potentials (in 3D) without chemical synpases
24
Q

Describe an action potential in contractile cells? (5)

A
  1. Resting membrane potential (maintained by open K+ channels and closed Na+/K+
  2. Depolarization - threshold reached = Fast Na+ channels open = rapid depolarization
  3. Partial repolarization -due to influx on Na+ channels, so Na+ channels close
  4. Plateau - Ca2+ channels move out of cell = maintains depolarization
  5. Repolarization - Ca2+ channels close and K+ channels reopen (back to beginning)
25
Q

What four things do contractile myocytes contain?

A
  • Actin
  • Myosin
  • Troponin
  • Sarcoplasmic reticulum
26
Q

What is the sequence of events in electrical activity of cardiac contractile cells?

A
  • AP causes Ca2+ to enter the cell via L-type voltage-gated Ca2+ channels
  • This triggers the release of more Ca2+ from the sarcoplasmic reticulum
  • Contraction occurs when cytosolic Ca2+ binds to troponin (allows troponin to interact with myosin)
  • Relaxation occurs when cytosolic [Ca2+] falls
    • caused by the active transport of Ca2+ into the SR and extracellular fluid
  • Fall in cytoplasmic [Ca 2+] renders troponin inactive = fibres dissociate
27
Q

What is the action potential upstroke in a nodal cell graph due to?

A

Due to voltage-gated Ca2+ channels

28
Q

What is the action potential downstroke due to?

(in nodal cells)

A

Increase in concentration of K+

29
Q

What are the currents in nodal cells activated by?

A 
sympathetic stimulation (AdRs, e.g. noradrenaline)
which open Na+ and Ca2+ channels and increase their concentration
30
Q

In pacemaker cells, describe how spontaneous slow depolarisation occurs

A

Na+ (and K+)
Transient Ca2+ channels

31
Q

In pacemakers cells, describe how rapid depolarisation occurs

A

due to slow Ca2+ channels

32
Q

How does repolarisation in pacemaker cells occur?

A

K+ channels

33
Q
A

PHAR20300 (25 decks)

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