Changing Membrane Potential

Question 1

Give 3 examples of how changing membrane potentials leads to signalling between and within cells

Answer 1

1. Action potentials in nerve and muscle cells (e.g. knee-jerk reaction)
2. Triggering and control of muscle contraction
3. Control of hormone and neurotransmitter secretion

(4. Transduction of sensory information into electrical activity by receptors (ie photoreceptors in the eye and sensory receptors in the ear)
5. Postsynaptic actions of fast synaptic transmitters)

Question 2

Depolarisation

Answer 2

Decrease in size of membrane potential from it’s normal value
Cell interior = less negative (more positive)

• Action Potential upstroke in graph

Eg -70 to -50

Question 3

Hyperpolarisation

Answer 3

Increase in size of membrane potential from its normal value
Cell interior becomes more negative

Eg -70 to -90

Question 4

What is meant by repolarisation?
What shouldn’t it be confused with?

Answer 4

• Membrane potential returns to resting membrane potential
• Not to be confused with hyperpolarisation
  (Hyper, the graph goes below resting)

Question 5

Identify the points of repolarisation in the graph

Answer 5

• Turquoise arrow
• Blue dot
• As the membrane potential is returning to resting

Question 6

Why do membrane potentials arise?

Answer 6

• As a result of selective ionic permeability within the membrane ( SIP)

Question 7

How can we change membrane potential?

Answer 7

By changing ion selectivity

Changing permeability of an ion

Question 8

What happens if you increase the membrane permeability to a particular ion in regards to the equilibrium potential for that ion?

Answer 8

Moves the membrane potential towards the equilibrium potential for that ion

Question 9

State the equilibrium potentials for K+ Na+ Cl- and Ca2+

Answer 9

Question 10

What causes hyperpolarisation?

Answer 10

Opening K+

K+ leaks out

Question 11

What causes depolarisation?

Answer 11

Opening Na+ or Ca2+ channels

Question 12

What effect does opening Cl- channels have on the membrane potential?

Answer 12

• In some cells, opening chloride channels can be depolarising
• In other cells it can be hyper polarising
• It all depends on what Ecl (equilibrium potential of chloride) is in those cells

Question 13

What equation outlines the imperfect selectivity of cell membranes?

Answer 13

GHK (Goldman-Hodgkin-Katz) equation

(Real cell membranes have channels open for more than 1 type of ion)
(How permeable the membrane is to that ion depends on the number of open channels for each ion)

Question 14

Give an example of some ion channels that are less selective

Answer 14

• At the neuromuscular junction, motor neurone nerve endings release acetylcholine (ACh).
• Acetylcholine binds to receptors on the muscle membrane
• These are Nicotinic acetylcholine receptors

(These are ligand gated ion channels and they are pentameric (5 subunits))

Ligand = Acetylcholine

There are 2 alpha subunits and they have binding sites for Ach

Binding of 2 Ach to alpha subuntis = cause the channel open

-ve pore within channels so only allows cations through (Na+, Ca2+ and K+) down their electrochemical gradients

When these channels are open, they drive the membrane potential towards 0mV (between equilibriums for Na+, Ca2+and K+, all the ions that pass through it) —> This is called the reversal potential for this channel

When these channels open, they will cause a depolarisation because 0mV is a lot more positive than the resting membrane potential

Question 15

Summarise the properties of Nicotinic acetylcholine receptors

Answer 15

1. Nicotinic acetylcholine receptors Have an intrinsic ion channel
2. Opened by binding of acetylcholine causing a conformational change
3. Channel lets Na+, Ca2+ and K+ through, but not anions due to the negative charge of the pore
4. Moves the membrane potential towards 0 mV, intermediate between ENa, ECa and EK

Question 16

How is channel activity controlled?

Answer 16

Channels can open and close - they are gated

3 Types of gating:

Ligand gating
Voltage gating
Mechanical gating

Question 17

Describe Ligand gating channels (1)
Give 2 examples of ligand gating

Answer 17

• The channels Opens/closes in response to binding of a chemical ligand
• E.g. Channels at synapses that respond to extracellular transmitters (eg Ach)
• Channels that respond to intracellular messangers like calcium

Question 18

Describe Voltage gated channels
Give an example

Answer 18

• Channels open/close in response to changes in membrane potential
• E.g. Channels involved in action potentials

Question 19

Describe Mechanical gating
Give an example

Answer 19

• Channels that Open/close in response to membrane deformation e.g when cells swell
• E.g. channels in Mechanoreceptors (carotid sinus stretch receptor/hair cells)
• Tend to be involved in volume regulation

Question 20

Describe Synaptic trasnmission (5)

Answer 20

• At the synapse, a chemical transmitter is released from presynaptic cell/bouton
• The transmitter diffuses in the synaptic cleft
• It then binds to receptors on post synaptic membrane
• Those receptors are gated by the neurotransmitter
• Can be FAST or SLOW

Question 21

What do chemical synapses occur between? / Where can Synaptic connections occur between?

Answer 21

Nerve cell - nerve cell
Nerve cell - muscle cell (e.g. at neuromuscular junction)
Nerve cell - gland cell
Nerve cell and sensory cell (e.g. photoreceptor cell)

Question 22

Describe Fast synaptic transmission (5)
Give an example

Answer 22

• Transmission that occurs quickly with a very short delay, because
• The receptor protein is also an ion channel
• Transmitter binding causes channel to open
• And you get ions flowing across
• Can be excitatory or inhibitory

Nicotinic Ach receptor

Question 23

Name the 2 different classes of fast synaptic transmission

Answer 23

• Excitatory
• Inhibitory

Question 24

Describe Excitatory synapses (4)

Answer 24

• Excitatory synapses are where you have ligand-gated channels that open to cause membrane depolarisation
• They can be permeable to Na+, Ca2+ or general cations
• ‘Excitatory’ Because it is moving the membrane potential closer to the threshold for firing an action potential
• Whether it is excitatory or inhibitory depends on the receptor

Question 25

What makes synapses either inhibitory or excitatory?

Answer 25

The receptors

Question 26

Facts about excitory synapses (2)

Answer 26

Longer time course than action potential Graded with amount of transmitter- more transmitter = more depolarisation (more = more channels)

Question 27

Examples of Excitory transmitters/receptors found in excitatory synapses (2)

Answer 27

- Nicotinic Acetylcholine receptors - Some Glutamate receptors

Question 28

Describe Inhibitory synapses (3)

Answer 28

- Where you have ligand gated channels that open to cause hyperpolarisation - K+ or Cl- permeable (Inhibitory because membrane potential is being taken further away from action potential threshold potential/cell is less likely to fire an action potential)

Question 29

Name 2 receptors found in inhibitory synapses

Answer 29

- Glycine receptors - Gamma aminobuytyric acid A receptors (GABA-A receptors)

Question 30

What is change in membrane potential called in excitory synapse?/What do you call the resulting depolarisation of membrane potential in an excitatory transmitter? What is it called in inhibitory?

Answer 30

Excitory post synaptic potential (EPSP) Inhibitory post synaptic potential (IPSP)

Question 31

Describe Slow synaptic transmission

Answer 31

- Receptor and channel are seperate proteins

Question 32

Describe the 2 basic patterns that slow synaptic transmission follows

Answer 32

1) Direct G-protein gating - Localised - Quite rapid 2) Gating via an intracellular messanger or protein kinase - e.g slowing the heart rate by opening a potassium channel - Throughout cell - Amplification by cascade

Question 33

Describe Direct G protein gating (G coupled receptor) (2) What happens in it?

Answer 33

Localised, quite rapid | G protein activated and migrates to interact with channel

Question 34

Describe the steps involved in Gating via intracellular messanger

Answer 34

``` - Neurotransmitter binds to g protein coupled receptor - Activates and releases G protein - Activates enzyme - Signalling cascade - Intracellular messanger or protein kinase produced and acts as a ligand to open/close the channel ``` Throughout cell, amplification by cascade

Question 35

In synapses, what causes a change in membrane potential?

Answer 35

- Opening of an ion channel

Question 36

Name 2 Factors that influence membrane potential

Answer 36

1) Changes in ion concentration 2) Electrogenic pumps - e.g. Na+ K+ ATPase

Question 37

Describe how changes in ion concentration can influence membrane potential

Answer 37

- The most important change in ion concentration is a change in extracellular K+ conc. - It is usually about 4.5 mM - But it sometimes gets altered in clinical situations - Can alter membrane excitability in the heart and lead to arrythmias

Question 38

Describe how electrogenic pumps can influence membrane potential

Answer 38

Electrogenic pumps - Na+ K+ ATPase (sets up and maintains ionic gradients) For each cycle: - 3 Na+ pumped out of the cell - 2 K+ pumped into the cell - These ions are each being pumped against their electrochemical gradients - That requires energy - That energy comes from hydrolysing ATP - Active transport of ions maintains ionic gradients - One positive charge is moved out for each cycle - That generates and electrical current and In some cells, this contributes a few mV directly to the membrane potential, making it more negative (3, NA, get Out)