Membrane Potential/Action Potential

Question 1

What is membrane potential?

Answer 1

The difference in electrical charges across the plasma membrane - potential inside relative to extracellular solution

Question 2

How do you measure membrane potential?

Answer 2

Measured using a microelectrode - fine glass pipette with tip diameter less than 1um penetrates cell membrane without bursting cell

Question 3

What two factors must be present for establishment of a membrane potential?

Answer 3

Asymmetric distribution of ions across membrane (ion gradients)

Selective ion channels - primarily K, Na, Cl (also H, Ca), may be gated

Question 4

In most cells, what ion channels dominate membrane ion permeability?

Answer 4

Open K+ channels

Question 5

What is the effect of a K+ chemical gradient and electrical gradient that are opposite and equal?

Answer 5

No net movement of K+

Negative charge across membrane = resting membrane potential

Question 6

If a membrane is selectively permeable for K+ then what will membrane potential be?

Answer 6

Membrane potential = Ek (K equilibrium potential)

Question 7

If there are more K+ channels than Na/Ca channels then membrane potential is___________

Answer 7

Closer to Ek ~ -100mV

Question 8

If there are more Na/Ca channels than K+ channels then membrane potential is___________

Answer 8

Closer to Ena or Eca ~ 0mV

Question 9

What is depolarisation?

Answer 9

Decrease in size of membrane potential from its resting value (becomes less negative)

Question 10

What is hyperpolarisation?

Answer 10

Increase in size of membrane potential from its resting value (becomes more negative)

Question 11

What is the effect of increasing membrane permeability to a particular ion?

Answer 11

Membrane potential moves towards the Ei (equilibrium potential) for that ion

Question 12

Opening what channels causes hyperpolarisation?

Answer 12

K+

Cl-

Question 13

Opening what channels causes depolarisation?

Answer 13

Na+

Ca2+

Question 14

What is conductance of a membrane?

Answer 14

How permeable a membrane is to a particular ion, dependent on number of channels for that particular ion are open

Question 15

What is the Goldman-Hodgkin-Katz (GHK) equation used to work out?

Answer 15

The membrane potential of “real membranes” (take into account all channels in the membrane)

Question 16

Give an example of mechanical gating

Answer 16

Hair cells in the inner ear, K+ channels close in response to membrane deformation, causes membrane depolarisation and ca2+ channels open. Results in vesicles containing neurotransmitter to fuse with the basement membrane (to afferent nerve). Neurotransmitter binds receptor on post synaptic plate - generates action potential to the CNS for interpretation

Question 17

Where are synapses found?

Answer 17

Between: nerve cell - nerve cell
Nerve cell - muscle cell
Nerve cell - gland cell
Sensory cell - nerve cell

Question 18

What dictates fast transmission at a synapse?

Answer 18

The receptor on the post synaptic membrane is an ion channel (neurotransmitter caused opening)

Question 19

What do excitatory neurotransmitters cause at the synapse?

Answer 19

Opening of ligand gated channels that cause membrane depolarisation (Na, Ca) - “excitatory post-synaptic potential”

E.g acetyl choline, glutamate, dopamine

Question 20

What do inhibitory neurotransmitters cause at the synapse?

Answer 20

Opening of ligand gated channels that cause hyperpolarisation (K+, Cl-) “inhibitory post-synaptic potential”

E.g glycine, GABA

Question 21

What dictates slow transmission at a synapse? And give two examples?

Answer 21

The receptor and ion channel are separate proteins.

1. Direct G protein gating - localised, rapid
2. Gating bus intracellular messenger - throughout cell, amplification by cascade, slower

Question 22

What things can also influence membrane potential?

Answer 22

Changes in ion concentration - most importantly extracellular K+ concentration - alters membrane excitability

Electrogenic pumps - e.g. Na/K ATPase adds one + charge out each time, contributes a few mV to membrane potential (more negative)

Question 23

What does GABA do?

Answer 23

GABA works to open Cl- channels (in nerve cells) causing hyperpolarisation - has an inhibitory effect

Question 24

How is the action potential “all or nothing”?

Answer 24

As the stimulus must cause depolarisation to reach threshold in order to cause action potential propagation. Also the action potential is the same size no matter how large the stimulus is (larger stimulus, more frequent action potentials)

Question 25

What is the axon hillock?

Answer 25

Last part of neuron cell body that connect to the axon. The last site where membrane potentials propagated from synaptic inputs at summated before transmission to axon

Question 26

What is the cause of the upstroke of the AP?

Answer 26

Due to large increase in permeability to Na, Na enters cell, depolarisation

Question 27

What is the effect of reduced extracellular Na on upstroke of the AP?

Answer 27

Lower extracellular Na, less depolarisation when Na channels open, therefore smaller overshoot

Question 28

How can you measure voltage currents?

Answer 28

Voltage clamps - used to measure the ion currents through membranes of excitable cells (at set membrane potential)

Question 29

What are the differences in conductance of Na and K channels?

Answer 29

Na channels - show inactivation - inactivated very quickly after opening K channels - don't show inactivation, close much more slowly (return to MP slower) and remain Koenig after repolarisation causing hyperpolarisation

Question 30

What allows inactivated Na channels to recover?

Answer 30

Hyperpolarisation of the membrane (inside of cell more negative) allows Na channels to become closed where they are excitable and open in response to a stimulus

Question 31

What is the absolute refractory period (ARP)?

Answer 31

When nearly all of the Na channels are in the inactivated state therefore no matter how strong a stimulus is, an action potential cannot be initiated

Question 32

What is the relative refractory period (RRP)?

Answer 32

Na channels are beginning to recover from inactivation and excitability returns towards normal as number of channels in inactivated state decreases, can initiate action potential if stimulus is strong enough

Question 33

What are the main structural features of voltage gated Na channels? (5)

Answer 33

Single peptide subunit (containing 4 repeating domains) Each domain has 6 TMS alpha helices Between TMS helices 5 and 6 there is a pore region TMS region 4 is positively charge Between TMS helices 3 and 4 there is an inactivation particle

Question 34

What does the pore region dictate?

Answer 34

Pore region dictates what ions can pass through the channel

Question 35

What is the positively charged region of each domain for?

Answer 35

The positively charged region of each domain, causes a voltage field across the membrane (contributes to voltage sensitivity). When ion binds, causes conformational change and pore opens.

Question 36

What is the function of the inactivation particle in VG Na channels?

Answer 36

The inactivation acts to block the channel pore, Na cannot enter pore

Question 37

What are the main structural features of voltage gated K channels? (4)

Answer 37

Made up of 4 identical peptides (4 alpha subunits) Each peptide subunit has 6 TMS alpha helices Between TMS helices 5 and 6 there is a pore region TMS region 4 is positively charge

Question 38

What channels do local anaesthetics often act on?

Answer 38

Local anaesthetics act mainly by blocking VG Na channels therefore blocking depolarisation, therefore no action potential (When channel open, more likely to become occupied by anaesthetic - block channel, pain decreases.

Question 39

When axon diameter decreases, what's the effect on conduction velocity?

Answer 39

Smaller axon diameter, slower conductance velocity

Question 40

What is local current circuit theory?

Answer 40

The theory that depolarisation of a small region of an axon which spreads to neighbouring regions, depolarising them. Causes immediate local change. When Na enters cell, repels other + charges, attracts - charges, sets up local current Depolarisation causes further (Na) channels to open thus propagating AP

Question 41

What is the length constant (λ)?

Answer 41

Distance it takes ice the potential to fall to 37% of its original value

Question 42

What is capacitance (Cm)?

Answer 42

The ability to store electrical charge - property of the lipid bilayer

Question 43

What two things do spread of local current depend on?

Answer 43

Capacitance | Membrane resistance

Question 44

What is membrane resistance?

Answer 44

Depends on the number of ion channels open Lower resistance = more ion channels open = change in voltage spreads less far down axon

Question 45

High resistance means what?

Answer 45

Less ion channels open | Local surround spreads further down axon

Question 46

What's the effect of axon diameter on travel of the action potential?

Answer 46

Wider the axon diameter causes local depolarisation to spread further

Question 47

What is myelination?

Answer 47

Fatty, tightly packed membrane substance that surrounds the axon of some nerve cells. Formed of a Schwann cell. Forms an electricity insulating layer.

Question 48

What are nodes of ranvier?

Answer 48

Gaps between myelin sheath, contain high density of Na channels

Question 49

What is saltatory conduction?

Answer 49

the propagation of action potentials (depolarisation) along myelinated axons from one node of Ranvier to the next node ("jumps"). Much faster, AP only at nodes (local currents depolarise next node to threshold, initiate AP)

Question 50

How is conduction velocity proportional to diameter of myelinated and unmyelinated axons?

Answer 50

Conduction velocity is directly proportional to diameter of myelinated axon Conduction velocity is proportional to the √diameter of unmyelinated axon

Question 51

What is the maximum conduction velocity in myelinated and unmyelinated axons?

Answer 51

myelinated axons - max velocity ~120 m/s | unmyelinated axons - max velocity ~20 m/s

Question 52

How does the myelin sheath improve conduction? (4)

Answer 52

Large increase in membrane resistance Large decrease in membrane capacitance Increase in the length constant Slight decrease in time constant

Question 53

Name two diseases that effect conduction of the action potential in the CNS

Answer 53

Multiple sclerosis (all CNS nerves, most common) Devic's disease (optic and spinal cord nerves only)

Question 54

Name a disease that effects conduction of the action potential in the PNS

Answer 54

landry guillain barre syndrome

Question 55

What do diseases that effect conduction of the action potential result from?

Answer 55

Breakdown/damage of the myelin sheath, show plaques of demyelination (autoimmune damage) In regions of demyelination, density of action current is reduced because of resistive and capacitive shunting therefore depolarisation doesn't reach next node of ranvier, threshold not met.