Excitable cells

Question 1

What is diffusion? What is created?

Answer 1

Movement of molecules across short distances down a concentration gradient. Reaches dynamic equilibrium. It is spontaneous and requires no energy.

Question 2

What is flux?

Answer 2

The number of molecules that cross a unit area per unit of time. So it is the rate of transfer of molecules. IT IS A MEASURE OF DIFFUSION. e.g. molecules; m-2s-1.

Question 3

What type of ions are mainly found in the body: positive or negative?

Answer 3

Positive.

Question 4

What is voltage? Unit of measure?

Answer 4

The potential difference. GENERATED by ions to produce a charge gradient. Measured in volts.

Question 5

What is current? Unit of measure?

Answer 5

Movement of ions due to the potential difference. Amps.

Question 6

What is resistance? Unit of measure?

Answer 6

Barrier that prevents the movement of ions. Ohms.

Question 7

How is electrical potential measured in the cell across a membrane?

Answer 7

Reference electrode placed outside the cell. This is the zero volt level. Another electrode is placed inside the cell. It measures a voltage difference compared with the reference electrode.

Question 8

What is the membrane potential in a neurone at rest?

Answer 8

Between -60mV and -70mV. So axon is relatively negative compared to the outside.

Question 9

Case 1: what happens when two compartments (one containing 0.15M NaCl; the other containing 0.15M KCl) are separated by a membrane with no channels?

Answer 9

No diffusion across the membrane despite concentration gradients. Membrane potential = 0mV. Cannot pass the hydrophobic membrane barrier.

Question 10

Case 2: what happens when two compartments (one containing 0.15M NaCl; the other containing 0.15M KCl) are separated by a membrane with K+ channels?

Answer 10

K+ crosses the membrane and the direction of flux is dictated by its concentration gradient. Process of diffusion. Positive charge builds up in the first compartment. This eventually prevents further influx on K+. Compartments are in a state of electrochemical equilibrium where the electrical gradient balances the force of the concentration gradient. There is still much more positive charge in the first compartment, but the high electrical positive force repels the diffusion of further positive K+ across the membrane.

Question 11

When is the electrochemical equilibrium?

Answer 11

When concentration and electrical gradients are exactly balanced. When electrical force prevents further diffusion across the membrane.

Question 12

What is the equilibrium potential?

Answer 12

The potential at which the electrochemical equilibrium has been reached.

Question 13

What does the Nernst equation tell us?

Answer 13

Calculates the equilibrium potential.

Question 14

What is the equation of the Nernst equation?

Answer 14

[I believe that the idea is to just perform calculations using the equation, so don’t need to remember formulae – I think]. E is given as mV.

Question 15

Assuming that T=37C, how can the Nernst equation be simplified and rewritten?

Answer 15

[I believe that the idea is to just perform calculations using the equation, so don’t need to remember formulae – I think]. Simplified by converting natural log to common log.

Question 16

Example of performing calculations with the Nernst equation: What is the equilibrium potential for K+ when intracellular mM = 150, and extracellular mM = 5? [PHOTO 3].

Answer 16

61 comes from -61/1 – the 1 comes from the +1 charge of the K+ ion.

Question 17

According Nernst’s equation, what is the equilibrium potential for K+ and Na+ across neuronal membrane at resting potential?

Answer 17

-90mV for Ek and +72mV for ENa.

Question 18

What is the resting potential of neurones? Why is it not anything like the equilibrium potentials derived by the Nernst’s equation?

Answer 18

In reality, neurones have resting potentials of -60mV to -70mV. This is because membranes have a different permeability to potassium as they have to sodium: at rest, permeability to sodium is very small; but there is some K+ permeability across the membrane. Potassium permeability is much larger, so the membrane tends to a more negative resting potential than a positive one which the Na+ would exert (look at the previous equilibrium potentials).

Question 19

What other factors contribute to the resting potential of the neuronal membrane?

Answer 19

Contribution by other ions like Cl-.

Question 20

What does an ion’s contribution to the membrane potential depend on?

Answer 20

Depends on the membrane permeability to the ion. For example, neuronal membrane much more permeable to K+ than any other ion. As a result, the membrane potential tends towards the equilibrium potential of the K+ more than any other ion.

Question 21

What are the typical concentrations of potassium and sodium inside and outside the neurone?

Answer 21

Na+ : 150mM extracellular; 10mM intracellular. K+: 5mM extracellular; 150mM intracellular.

Question 22

EXTRA INFORMATION ON CALCULATING RESTING POTENTIALS.

Answer 22

PLEASE READ. DON’T REMEMBER BUT COMPREHEND AND UNDERSTAND. Appreciate how this results in the membrane potential being between -60 and -70mV at rest. The last example reflects the actual scenario most well. SOME Na+ channels are open, and there’s some sodium leakage. Potassium channels are open also.

Question 23

What are the words that describe changes in membrane potential and what are their definitions? (x4)

Answer 23

Hyperpolarising means that the membrane polarises BELOW the RESTING POTENTIAL.

Question 24

What is a graded potential? What is the basis by which it is different from an action potential?

Answer 24

Action potential is an all-or-nothing response. Same size depolarisation regardless. In a graded potential, the size of the change in the membrane potential differs. Stimulus is not large enough to create an action potential, so the membrane depolarisation depends on the size of the stimulus. Graded potential only involve resting K+ channels because there’s no repolarisation as there’s no action potential. NOTE: graded potentials can involve or be, excitary (depolarise membrane) or inhibitory (hyperpolarise membrane).

Question 25

What factors affect a graded potential? (x3)

Answer 25

Stimulus: Small stimulus depolarises the membrane less than a stronger stimulus. Distance: Depolarisation becomes lesser the further away from the nucleus. Time: more time = less depolarisation. Potentials are graded on time, stimulus or distnace. Different from action potentials.

Question 26

Where do graded potentials typically occur? (x2)

Answer 26

Graded potentials tend to occur at synapses when there’s many inputs, or sensory receptors.

Question 27

What is the general function of graded potentials?

Answer 27

Contribute to initiating or preventing ACTION potentials. Remember, if graded potentials aren’t large enough, they stay as graded potentials.

Question 28

Why do graded potentials decay as they propogate down the axon?

Answer 28

Current moves down axon and charge starts to dissipate out of the axon all the way down. Decay = exponential.

Question 29

What makes graded potentials last longer i.e. what slows the rate of decay? (x2)

Answer 29

Axon myelination. Axon with a larger diameter.

Question 30

Generally, what causes voltage-gated ion channels to be i)opened, ii)inactivated, and iii)closed?

Answer 30

Channels are opened by membrane depolarisation. Inactivated by sustained depolarisation. Closed by membrane hyperpolarisation or repolarisation.

Question 31

What are the two ways the electrochemical equilibrium is restored after an action potential and repolarisation? (x2)

Answer 31

Mainly by movement of ions through non voltage-gated ion channels. Some via the sodium-potassium pump mechanism.

Question 32

What does the sodium-potassium pump do in a neurone?

Answer 32

Sodium-potassium pump does not cause an action potential or alter the membrane potential (although it does alter it by a TINY amount). The membrane potential changes during the action potential from ion channels instead. Sodium-potassium pumps restores the original concentration gradients that cause the action potential e.g. pump 3Na+ back out of axon so can move in again to depolarise the membrane.

Question 33

How is the membrane depolarised? (focus on both Na+ and K+ channels)

Answer 33

The stimulus depolarises the membrane potential; some Na+ channels open and some Na+ move into the axon. Moves it in the +ve direction towards the THRESHOLD. At the THRESHOLD POTENTIAL, there is sudden Na+ flux into the axon as Na+ channels open quickly. Na+ travels down its electrochemical (electrical and concentration) gradient. Na+ tends to 100% permeability and membrane potential tends towarsd the Na+ equilibrium potential. Voltage gated K+ channels start to ppen SLOWLY, and K+ leaves axon down its electrochemical gradient also – less than the number of Na+ entering though.

Question 34

What does PK and PNa mean?

Answer 34

Refers to the permeability of the membrane to potassium and sodium respectively.

Question 35

What happens in membrane repolarisation?

Answer 35

Na+ channels inactivate so the movement of Na+ into the axon drops significantly. Pk (permeability of membrane of potassium ions) increases as more and more voltage-gated K+ channels open and remain open. Membrane potential moves towarsd the K+ equilibrium potential.

Question 36

How does the sodium channel inactivate?

Answer 36

There’s a part of the Na+ voltage-gated channel (protein) that reacts to the change in voltage, and flips itself into the mouth of the channel, blocking the channel from Na+ movement even though the channel is still open. Ball-and-chain hypothesis. This happens very quickly hence why membrane never reaches Na+ equilibrium potential.

Question 37

What does the sodium channel inactivation do to the neurone?

Answer 37

Sodium channel inactivation is the underlying mechanism for the absolute refractory period. For a period of time after the action potential, the membrane cannot be stimulated – called the refractory period. No matter how strong the stimulus; remains inactive.

Question 38

What is the importance of the absolute refractory period? (x2)

Answer 38

Means that there’s not a barrage of impulses going down the neurone. Also prevents the propogation of an action potential back down the neurone. Ensures unidirectionality.

Question 39

How is the inactivation of sodium channels reversed?

Answer 39

Cell membrane must be repolarised. The sodium channel inactivation gate peptide falls away and becomes unblocked. Channel can now be activated again by next stimulus.

Question 40

Why does hyperpolarisation occur? (x2 points)

Answer 40

More channels open still: As membrane potential approaches resting, there are more potassium channels open than usual still. This gives rise to the small hyperpolarisation event. Channels close slowly: As it nears the resting potential, potassium channels begin to close, but because there’s so many potassium channels, they take longer to suitably close, hence hyperpolarisation.

Question 41

How does the refractory period change at hyperpolarisation? Why only large stimuli? What makes it relative and not absolute?

Answer 41

At hyperpolarisation, you are at a refractory period, but not an absolute refractory period. At hyperpolarisation, absolute refractory period ends, and theres a RELATIVE refractory period instead. Here, only a larger than normal stimuli will activate an action potential – stronger stimuli needed to overcome hyperpolarisation as well as depolarise the cell. Not absolute because the inactivation gate is open. What makes it refractory is the increased change in membrane potential required.

Question 42

How does PK and PNa difffer?

Answer 42

PK changes more slowly to PNa – we mentioned this as a reason for hyperpolarisation.

Question 43

What cells types are electronically excitable? (x3)

Answer 43

Neurons, muscle cells and SOME endocrine tissues.

Question 44

What is threshold?

Answer 44

Once this potential is reached in depolarisation, an action potential is triggered.

Question 45

What is meant by an action potential being ‘all or nothing’? Why is it ‘all or nothing’?

Answer 45

Regardless of the size of the stimulus a full-sized action potential will always occur if the membrane is depolarised beyond the threshold. i.e. a small stimulus will produce the same sized action potential as a strong stimulus, provided both reach the threshold. All or nothing occurs because beyond the threshold, positive feedback occurs which induces rapid Na+ channel opening. This positive feedback loop occurs anyway once the threshold is reached and is therefore independent of the size of the original stimulus.

Question 46

What is the regenerative relationship between sodium permeability (PNa) and membrane potential?

Answer 46

Postive feedback.

Question 47

What happens if the depolarisation is less than the graded potential?

Answer 47

Less than the threshold = graded potential instead.

Question 48

How does active propogation of the action potential occur? This is simple propogation down an axon; not saltatory conduction.

Answer 48

At the depolarised area, a local current will depolarise the adjacent area. If this is sufficient for the threshold, that area will generate an action potential. Unidirectional because of refractory period in previous regions of the neurone.

Question 49

What is the mechanism by which an impulse can move rapildy down a neurone?

Answer 49

Impulse jumps between nodes of Ranvier (where sodium channel concentration high) by saltatory conduction. If the extent of decay is not too much by the time the impulse is felt by the next node of Ranvier, then an action potential is stimulated. This stimulation travels by passive propogation (as seen on the diagram). Passive propogation is graded potential and lasts longer.

Question 50

What is the duration of an action potential – two examples combining two factors?

Answer 50

Axons with a large diameter and myelinated: 120m/s. Axons with a small diameter and non-myelinated: 1m/s.

Question 51

How does diameter and myelination affect speed of an action potential?

Answer 51

Large diameter = less internal resistance. Myelination allows for saltatory conduction.

Question 52

What pathological conditions affect conduction – action potential travelling down a neurone? (x1, x2, x4).

Answer 52

Re-growth after injury: reduced axon diameter. Multiple sclerosis and diptheria: reduced myelination. Cold, anoxia (absence of oxygen), compression, and drugs (some anaestheitcs) also slow conduction.