EXCITABLE CELLS

Question 1

What is potential difference?

Answer 1

Difference in voltage between two points

Question 2

What is membrane potential (Vm)?

Answer 2

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

Question 3

What is resting membrane?

Answer 3

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

Question 4

What is graded potential?

Answer 4

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

Question 5

What is an action potential?

Answer 5

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

Question 6

What is an equilibrium potential?

Answer 6

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

Question 7

What makes cells excitable?

Answer 7

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)

Question 8

Give examples of excitable cells (5)

Answer 8

• 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

Question 9

Give examples of non-excitable cells

Answer 9

All other body cells e.g:

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

Question 10

How do you measure resting potential?

Answer 10

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

Question 11

What is the resting potential membrane dependent on?

Answer 11

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

Question 12

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

Answer 12

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

Question 13

What ion channels control the resting potential?

Answer 13

K+, Na+, Ca2

Question 14

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

Answer 14

• 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)

Question 15

Define Nernst (equilibrium) potential

Answer 15

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

Question 16

Give the Nernst (equilibrium) potential equation

Answer 16

• 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).

Question 17

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

Answer 17

• 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-

Question 18

What does Van Hoff equation state?

Answer 18

Chemical energy is proportional to solute concentration

Question 19

What is the implication of the 3 equations below combined?

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

Answer 19

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)

Question 20

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

Answer 20

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)

Question 21

What does automaticity mean?

Answer 21

Ability of a tissue or organ to function without external control

Question 22

Describe how the heart is able to be automatic?

Answer 22

• 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)

Question 23

Describe the appearance of cardiac muscle on a cellular level

Answer 23

• 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

Question 24

Describe an action potential in contractile cells? (5)

Answer 24

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)

Question 25

What four things do contractile myocytes contain?

Answer 25

* Actin * Myosin * Troponin * Sarcoplasmic reticulum

Question 26

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

Answer 26

* 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

Question 27

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

Answer 27

Due to voltage-gated Ca2+ channels

Question 28

What is the action potential downstroke due to? | (in nodal cells)

Answer 28

Increase in concentration of K+

Question 29

What are the currents in nodal cells activated by?

Answer 29

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

Question 30

In pacemaker cells, describe how spontaneous slow depolarisation occurs

Answer 30

Na+ (and K+) Transient Ca2+ channels

Question 31

In pacemakers cells, describe how rapid depolarisation occurs

Answer 31

due to slow Ca2+ channels

Question 32

How does repolarisation in pacemaker cells occur?

Answer 32

K+ channels