Membrane Potentials and Action potentials

Question 1

How do you measure membrane potential? What is it called?

Answer 1

• To measure membrane potential a reference electrode is placed outside the cell.
• This is the zero-volt level.
• Another electrode is placed inside the cell.
• It measures a voltage difference that is negative compared with the outside (i.e. reference).
• All cells have a membrane potential

Question 2

Describe the cell membrane

Answer 2

• Lipid (hydrophobic) cell membrane is a barrier to ion movement and separates ionic environments.
• The cell membrane can selectively change its permeability to specific ions
• Permeable pores in the membrane (ion channels) open and close depending on transmembrane voltage, presence of activating ligands or mechanical forces.\

Question 3

What are ion channels? What are they selective for?

Answer 3

Ion channels can be selective for different types of ion (K+, Na+, Cl-, Ca2+).

Movement across the membrane will occur when the concentration of the ion is different on one side of the membrane and cease upon equilibration

Question 4

What is the equilibrium potential?

Answer 4

the potential at which electrochemical equilibrium has been reached. It is the potential that prevents diffusion of the ion down its concentration gradient

Question 5

What is depolarisation?

Answer 5

Membrane potential increases from negative towards 0

Question 6

What is repolarisation?

Answer 6

Membrane potential decreases towards resting potential

Question 7

What is overshoot?

Answer 7

Membrane potential decreases towards resting potential

Question 8

What is hyper polarisation?

Answer 8

Membrane potential decreases beyond resting potential

Question 9

What are graded potentials?

Answer 9

• Change in membrane potential in response to stimulation
• Occur at synapses and in sensory receptors
• Contribute to initiating or preventing action potentials

Question 10

Where do action potentials occur?

Answer 10

• Occur in excitable cells (mainly neurons and muscle cells but also in some endocrine tissues)
• In neurons they are also known as nerve impulses and allow the transmission of information reliably and quickly over long distances

Question 11

What is the role of action potentials?

Answer 11

• Play a central role in cell-to-cell communication and can be used to activate intracellular processes
  e. g. in muscle cells, an AP is the first of a series of events leading to contraction
  e. g. in beta cells of the pancreas and AP stimulates insulin release

Question 12

How is the permeability of ion channels affected and how does it change in action potentials?

Answer 12

• Permeability depends on conformational state of ion channels
• Opened by membrane depolarisation
• Inactivated by sustained depolarisation
• Closed by membrane hyperpolarisation/repolarisation

Question 13

What happens when membrane permeability of an ion increases?

Answer 13

• When membrane permeability of an ion increases it crosses the membrane down its electrochemical gradient
• Movement changes the membrane potential toward the equilibrium potential for that ion
• Changes in membrane potential during the action potential are not due to ion pumps

Question 14

What are the five phases of action potentials?

Answer 14

1. Resting membrane potential
2. Depolarising stimulus
3. Upstroke
4. Repolarisation
5. After-hyperolarsiation

Question 15

What happens in resting membrane potential during an action potential?

Answer 15

1. Permeability for PK > PNa

2. Membrane potential nearer equilibrium potential for K+ (-90mV) than that for Na+ (+72mV)

Question 16

What happens in depolarising stimulus during an action potential?

Answer 16

1. The stimulus depolarises the membrane potential

2. Moves it in the positive direction towards threshold

Question 17

What happens at upstroke during an action potential?

Answer 17

1. Starts at threshold potential
2. ­increase PNa because voltage-gated Na+ channels open quickly [Na+ enters the cell down electrochemical gradient]
3. Increase ­PK as the voltage-gated K+ channels start to open slowly [K+ leaves the cell down electrochemical gradient]
4. Less than Na+ entering
5. Membrane potential moves toward the Na+ equilibrium potential

Question 18

What happens at repolarisation during an action potential?

Answer 18

1. Decrease PNa because the voltage-gated Na+ channels close - Na+ entry stops
   ­increase PK as more voltage-gated K+ channels open & remain open
2. K+ leaves the cell down its electrochemical gradient
   Membrane potential moves toward the K+ equilibrium potential

Question 19

Describe phase 4 at the start of repolarisation

Answer 19

• Absolute refractory period
• Activation gate is open
• Inactivation gate is closed
• New action potential cannot be triggered even with a very strong stimulus

Question 20

Describe phase 4 later in repolarisation

Answer 20

• Absolute refractory period continues

- Activation AND Inactivation gates closed

Question 21

What happens at after hyper polarisation during an action potential?

Answer 21

1. At rest voltage-gated K+ channels are still open
2. K+ continues to leave the cell down the electrochemical gradient
3. Membrane potential moves closer to the K+ equilibrium - some voltage-gated K+ channels then close
4. Membrane potential returns to the resting potential

Question 22

Describe phase 5 after hyperpolarisation

Answer 22

• Relative refractory period
• Inactivation gate is open
• Stronger than normal stimulus required to trigger an action potential