Membrane Potentials (Sept 12)

Question 1

Potential difference (membrane potential)

Answer 1

-separation of electrical charge -they are attracted to each other (+ve on outside of membrane, -ve on inside) -can do work if there is a channel that allows the charges to pass through the membrane

Question 2

Ions across the membrane

Answer 2

-Na+ and K+ are main ions (Ca2+ and Cl- are also there) -negatively charged proteins also contribute to potential difference (and stay inside cell) -more -ve ions on inside than outside -difference is concentrated at plasma membrane (goes away as you go deeper into cell)

Question 3

Resting membrane potential of a cell

Answer 3

-if it’s alive, should have resting membrane potential of -40mV to -70mV

Question 4

Excitable cells

Answer 4

-have resting membrane potential of -70mV -neurons, skeletal cells, cardiomyocytes -capable of generating electrical signals like action potentials

Question 5

K+ key concepts

Answer 5

-always trying to leave the cell (efflux) -concentration of K+ inside the cell is much greater than it is inside the cell so it wants to go down its concentration gradient -when K+ leaves cell, potential difference is greater because the voltage becomes more negative with positive charge leaving the cell

Question 6

Na+ key concepts

Answer 6

-Na+ always trying to enter cell (influx) -Na+ more abundant outside the cell than inside so it will flood in going down its concentration gradient -when Na+ enters the cell, the inside of the cell becomes less negative (maybe even +ve) -if interior of cell is usually -ve, there is attractive force between Na+ and inside of cell (charge gradient) -electrochemical gradient -potential difference is lessened (voltage moves towards 0) because positive ions decrease -ve charge in cell

Question 7

Why is K+ not affected by charge gradient?

Answer 7

-inside of cell is -ve and K+ is +ve so you would think it would stay in there -charge gradient however isn’t as strong as the concentration gradient

Question 8

Extracellular fluid is rich in _______ and cytosol is rich in _________

Answer 8

1. Na+ and Cl- 2. K+ and -ve proteins

Question 9

How is RMP maintained?

Answer 9

-membrane is differentially permeable (less permeable to Na+ than K+) -sodium potassium ATPase pump

Question 10

Differentially permeable membrane

Answer 10

-leak channels that randomly open and close -K+ movement out of cell is greater than Na+ coming into cell -few Na+ leaking in so net result is negative (losing more +ve things than you’re gaining)

Question 11

Sodium potassium pump

Answer 11

-shuttles K+ that leaked out back in and Na+ out that got in -pumps 3 Na+ ions out for every 2 K+ ions that it brings in -net negative charge is left -electrogenic pump (creates electrical separation across membrane)

Question 12

Dead cell

Answer 12

-if leak channels were allowed to flow forever then eventually amount of K+ on the outside versus inside would be the same and the ions would stop flowing -RMP would be 0

Question 13

Uses of membrane potentials

Answer 13

-movement of sensory messages to CNS -processing messages in CNS -activation of motor outputs from CNS -these are electrically encoded to move quickly

Question 14

What are electrically encoded messages?

Answer 14

-in the format of changes in membrane potential -two types of changes: graded and action

Question 15

Voltage gated ion channels

Answer 15

-change in RMP causes this channel to open -depending on what the channel conducts, it might let sodium in or potassium out -contribute to action potentials

Question 16

Ligand gated ion channels

Answer 16

-contribute to graded potentials -has a receptor site to allow chemical (usually an NT) to bind to it which opens the ion channel -ligand dissociates and channel closes -would find this in a synapse

Question 17

Depolarization

Answer 17

-when binding of ligand leads to opening of Na+ channels -+ve ions rush in and potential difference decreases (less -ve, closer to 0)

Question 18

Hyperpolarization

Answer 18

-when binding of ligand leads to opening of K+ channel -K+ escapes, inside becomes more -ve so separation of charges becomes greater so inside of cell becomes more -ve -increases potential difference

Question 19

Graded potentials

Answer 19

• short lived -localized channels (don’t spread far in plasma membrane)
• can be summed in space and time
• if you stimulate ligand channels in one area you can get bigger changes
• can depolarize or hyperpolarize (these can cancel out)

–ve feedback mechanisms work to restore RMP (leak channels and ATPase pump)

-usually sit on the neuron within dendrite and body

Question 20

What kind of graded potential does this graph indicate?

Answer 20

Hyperpolarizing graded potential

Question 21

What kind of graded potential does this graph show?

Answer 21

Depolarazing graded potential

Question 22

What is this graph showing?

Answer 22

• graded potential doesn’t reach far spatially
• change in RMP where the channel is actually opening
• change in RMP a few microns away from channel is miniscule

Question 23

Where can Action Potential occur? What does it do? How does it work?

Answer 23

• can occur in any excitable membrane (axons in neurons, muscle membrane)
• graded potentials get added up in base of axon at axon hillock where changes in membrane potential can evoke an action potential because voltage gated ion channels are along axon and are important for creating action potential
• causes a brief reversal of voltage
• never summed
• all or none phenomenon
• works by positive feedback mechanism

Question 24

How does graded potential become converted to action potential?

Answer 24

• voltage threshold (-55mV) for opening voltage sensitive Na+ channels is reached
• Na+ floods into cell and RMP is decreased which causes even more Na+ to come into the cell because there is even more change in RMP (+ve feedback)
• goes until all Na+ has rushed in which is action potential

Question 25

What is happening in #1

Answer 25

• resting phase
• voltage gated Na+ and K+ closed
• leak channels and ATPase pump working in background

Question 26

What is happening in #2

Answer 26

• graded potentials come
• stimulus causing voltage to rise, eg. ligand gated channels open
• brought RMP to threshold
• getting neuron close to threshold so that the voltage gated channels can open

Question 27

What is happening in #3

Answer 27

• voltage gated Na+ channels open
• flood of Na+ and inside becomes positive

Question 28

What is happening in #4

Answer 28

• repolarizing phase
• K+ channels start to open
• K+ voltage gated ion channels need huge stimulus (depolarization) for them to open
• K+ comes from inside to cell outside which makes RMP more negative
• Na+ channels closing as well at same time
• K+ stay open longer than they need to so we get slightly more -ve than RMP (hyperpolarizing phase)
• leak channels and ATPase shuttle Na+ and K+ back to where they should be to get back to RMP

Question 29

Refractory period

Answer 29

• absolute refractory period for AP: if you’re in middle of creating an AP you can’t make a new one (this one must develop and finish because you can’t add them one on top of the other)
• relative refractory period: in hyperpolarizing phase- because Na+ channels have closed, and now we are back to RMP you could get it to fire another AP if you have a huge stimulus (you are further way from threshold so need a big stimulus)

Question 30

Muscle fibre contraction

Answer 30

• message for muscle fibres to contract has to travel from cell body of neuron along axon to the end of the neuron
• muscle fibre and end of neuron have gap (synapse)
• turn electrical signal into chemical signal to float across synapse to generate another electrical signal in muscle by opening ligand gated ion channels and eventually voltage gated ion channels
• this process is neurotransmission