Membranes & Receptors (S1-5)

Question 1

What are the five functions of a biological membranes? (S1)

Answer 1

Continuous highly selective permeability membrane
Control of the enclosed chemical environment
Communication
Recognition
Signal generate in response to stimuli

Question 2

What is the membrane composition of a dry membrane? How much water (%) is present in a normal, hydrated membrane? What is the composition of cholesterol (%) out of the total lipid? (S1)

Answer 2

Approximately 40% lipid, 60% protein and 1-10% carbohydrate.

20% water.

45% cholesterol.

Question 3

What is an amphipathic molecule? Are membrane lipids amphipathic? (S1)

Answer 3

It is a molecule that contains a hydrophobic and hydrophillic group. Yes

Question 4

In membrane lipids, what is n usually in the fatty acid group? What does this allow? (S1)

Answer 4

It is usually 16 or 18, although this can vary from 14 to 24. This allows the membrane to be the same width in general.

Question 5

What does a Cis double bond do? (S1)

Answer 5

It introduces a kink in the fatty acid chain. This reduces phospholipid packing.

Question 6

How does a phospholipid’s structure differ from triacylglycerol? (S1)

Answer 6

It is similar to triacylglycerol but one of the fatty acid groups is replaced by a phosphate-head group.

Question 7

In phosphatidylcholine what is the head group? Can you name a few other polar head groups? (S1)

Answer 7

Choline. Amines, amino acids and sugars.

Question 8

Why is sphingomyelin not a classical phospholipid? What is it structurally similar to? Is it a plasmalogen? (S1)

Answer 8

It does not have a glycerol backbone, having a fatty backbone instead
If the phosphocholine moiety was replaced with a sugar it would be a glycolipid.
Lipids not based on glycerol are plasmalogens, so therefore it is.

Question 9

If the head group is a single carbohydrate then we call this…?
If the head group has oligosaccharides (sugar multimers) then we call this…?
What purpose do these sugars serve? (S1)

Answer 9

Cerebrosides.

Gangliosides.

They perform signalling functions.

Question 10

What inherent tendency in the phospholipid allows us to have lipid bilayers? (S1)

Answer 10

The tendency to form lipid bilayers rather than micelles (which are a spherical distribution where hydrophillic head groups are on the outside and hydrophobic tail groups are on the inside. These hydrophobic tail groups will form van der Waal’s forces between each other). The formation of bilayers is spontaneous in water.

Question 11

Can phospholipds move? If so, how? (S1)

Answer 11

The phospholipid membrane is fluid in structure and constantly moving. It can move by...:
Flexion
Fast axial rotation
Lateral diffusion
Flip flop

Question 12

What is flexion? (S1)

Answer 12

It is a lot like vibration between the phospholipids.

Question 13

What is fast axial rotation? (S1)

Answer 13

The phospholipid spins around and does not move out of place

Question 14

What is lateral diffusion? (S1)

Answer 14

It is where a phospholipid moves by diffusion across the same side of the membrane.

Question 15

What is flip flop? Is this common? (S1)

Answer 15

A piece of footwear ideal for Summer. Alternatively when the hydrophilic heads move through the hydrophobic domain in order to flip around.
No it is rare as it requires a great deal of energy.

Question 16

What are the properties of cholesterol and what effect do they have on membrane stability? (S1)

Answer 16

Cholesterol has a polar head group, a rigid steroid ring and a non-polar hydrocarbon tail.
This increases stability. It stops phospholipids forming islands of lipids within the bilayer (this would cause fractures in the bilayer and lead to leakage of ions).

Question 17

How does cholesterol reduce the movement of the membrane? (S1)

Answer 17

The polar hydroxyl group of cholesterol binds to the carbonyl oxygen of the fatty acid group; this locks the cholesterol onto the phospholipid. It will reduce vibrational motion of the phospholipid, thus partially reducing the movement of the membrane.

Question 18

Why does cholesterol have a paradoxical effect? (S1)

Answer 18

Cholesterol packs between the phospholipids increasing distance between them. This makes the bilayer more fluid due to increased potential motion. This is a paradoxical effect because cholesterol also reduces flexion of the phospholipids through binding to them.

Question 19

What is the evidence for membrane proteins? (S1)

Answer 19

There is functional evidence: facilitated diffusion, ion gradients and specificity of cell responses - i.e. insulin is only recognised by receptor cells.
There is also biochemical evidence: we can freeze fracture the membrane or use membrane fractionation and gel electrophoresis; we can fracture the membrane and then analyse by SDS-PAGE

Question 20

How can membrane proteins move? (S1)

Answer 20

They can move by conformational change (vibrational), lateral diffusion and rotation. They cannot move by flip flop. This is because a huge amount of energy would be needed for their hydrophillic moieties to go through the hydrophobic domain.

Question 21

Can membrane protein movement be restricted? (S1)

Answer 21

Yes. There are several ways in which it can be restricted. Proteins can form aggregates: this will mean they will diffuse slower. Proteins can be tethered to the basement membrane (basolateral junctions) or the internal cytoskeleton of the cell - this is seen in nerve cells where proteins are tethered into the synapse. In cell adhesion, membrane proteins interact with each other. This tethering and adhesion will stop the protein moving. In addition to these protein-protein effects, there are lipid mediated effects - proteins tend to separate out into the fluid phase or cholesterol poor region.

Question 22

What are the two ways membrane proteins can associate with the lipid bilayer? (S1)

Answer 22

They can either be integral: spanning the entirety of the membrane at least once, or peripheral: associated with one side of the membrane.

Question 23

What interactions do the peripheral proteins have with the membrane? (S1)

Answer 23

Electrostatic and hydrogen bond interactions.

Question 24

Can integral proteins be removed by changes in pH or ionic strength? (S1)

Answer 24

No. Only peripheral proteins can be removed by changes in pH or ionic strength. Integral proteins can only be removed by agents that compete for non-polar interactions e.g. detergents and organic solvents.

Question 25

What can protein content in the membranes vary from? (S1)

Answer 25

18% in myelin to 75% in mitochondria.

Question 26

Normally what will the amino acids be in the hydrophobic domain of the cell membrane? (S1)

Answer 26

They will be small, hydrophobic and polar, uncharged.

Question 27

What can hydropathy plots be used to do? (S1)

Answer 27

They can be used to see if the amino acids making up the membrane protein are hydrophobic or hydrophillic. As the cell membrane is normally spanned by 18-22 amino acids, if there are c. 20 consecutive hydrophobic amino acids then we can assume this membrane protein is integral and interacts extensively with hydrophobic domains of the lipid bilayer.

Question 28

Can membranes be multiple trans-membrane spanning? (S1)

Answer 28

Yes, they can weave in and out of the cell membrane.

Question 29

What is membrane protein topology? (S1)

Answer 29

It refers to the fact proteins have a right way to face, (i.e. the N-terminus on the cytosolic side or the non-cytosolic side.)

Question 30

Outline the protein secretory pathway.

Answer 30

There is protein synthesis - this is due to a free ribosome initiating synthesis from an mRNA molecule… A hydrophobic N-terminal signal sequence is produced and recognised and bound to by the signal recognition particle (SRP). Protein synthesis stops…
GTP-bound SRP directs the ribosome synthesising the secretory protein to SRP receptors on the cytosolic face of the ER. The SRP dissociates.
Protein synthesis continues and the newly formed polypeptide is fed into the ER via a pore in the membrane (peptide translocation complex).
Signal sequence is removed by a signal peptidase once the entire protein has been synthesised.
The ribosome dissociates and is recycled.

Question 31

How does membrane protein synthesis differ from that of secretory proteins? (S1)

Answer 31

The difference is membrane proteins need to be able to span a vesicle, not just fit inside it. There is the addition of a stop transfer signal. When the membrane protein is being translated into the ER lumen it comes across the highly hydrophobic stop signal. This will normally be after 18-20 amino acids (i.e. the distance to span the membrane). The stop transfer signal remains in the ER membrane; the rest of the protein is translated into the cytoplasm. The protein spans the membrane therefore.

Question 32

Why is the plasma membrane described to be fluid? (S1)

Answer 32

This is because the hydrophobic integral components in the membrane such as lipids and membrane proteins move laterally.

Question 33

Why is the plasma membrane described to be mosaic? (S1)

Answer 33

This is because the membrane is made up of lots of different components - integral / peripheral proteins, cholesterol, glycoproteins, phospholipids.

Question 34

What is hereditary spherocytosis? What does it result in? What is it caused by? (S1)

Answer 34

The sufferer has spherical RBCs because they are missing 40-50% of their spectrin. The erythrocyte’s cytoskeleton is less intact and when they go through capillaries or the spleen they will burst. This means there are less mature RBCs and will lead to haemolytic anaemia. It is caused by a mutation in one of the two genes for spectrin and occurs in 1 in 20,000.

Question 35

What is hereditary eliptocytosis? (S1)

Answer 35

The RBCs are fragile eliptoid cells due to a defect in the gene coding for the spectrin molecule. This prevents the joining up of spectrin, NOT the formation. RBCs are unable to form stable heterotetramers.

Question 36

What does the cell cytoskeleton do? (S1)

Answer 36

It gives the membrane its shape.

Question 37

Which proteins are found extensively in the cytoskeleton? (S1)

Answer 37

Spectrin, Actin and Ankyrin.

Question 38

Describe the structure of spectrin. How is it attached to the membrane? (S1)

Answer 38

Spectrin is composed of alpha (a) and beta sections (B). The two sections come together to form a rod (a2B2). Two rods are then bound together by actin protofilaments.
Ankyrin, an ‘adaptor’ protein, attaches the spectrin to the membrane.

Question 39

How could you see spectrin in an erythrocyte? (S1)

Answer 39

If we broke a RBC and took the cytoplasmic side of it and subsequently low-angled shadowed it then we would see spectrin (and how it is glued together by ankyrin and actin.

Question 40

What types of molecules can permeate a lipid bilayer? (S2)

Answer 40

Hydrophobic molecules such as O2, CO2, N2 and benzene can diffuse through the phospholipid bilayer. Small, uncharged polar molecules such as H2O (surprisingly! water can cross the hydrophobic domains?), urea and glycerol are relatively permeable.
However large, uncharged polar molecules – glucose and sucrose – cannot permeate the membrane. The same goes for ions (H+, Na+, K+, Ca2+, Mg2+, Cl-, HCO3-.)

Question 41

Experimentally how would you go about seeing which molecules can permeate a lipid bilayer? (S2)

Answer 41

This can be seen in a beaker with a septum containing a lipid bilayer black film.

Question 42

What are the transport processes’ important roles? (S2)

Answer 42

They are the:
maintenance of ionic composition,
maintenance of intracellular pH,
regulation of cell volume,
concentration of metabolic fuels and building blocks,
the extrusion of waste products of metabolism and toxic substances,
the generation of ion gradients necessary for the electrical excitability of nerve and muscle.

Question 43

What are the three (most important) types of transport? (S2)

Answer 43

Ping-pong, facilitated diffusion and active transport.

Question 44

What are ping-pong transporters? (S2)

Answer 44

The channel is open to ions from one side. When the ion moves and binds to the middle of the membrane, the channel opens up to the other side and lets the ion move through to the other side.

Question 45

What is facilitated diffusion? What is the primary difference between this and active transport? (S2)

Answer 45

It is where there is an ion channel that allows a substance to pass through.

Active transport requires energy from the hydrolysis of ATP whereas facilitated diffusion does not.

Question 46

What is a ligand-gated channel? (S2)

Answer 46

A ligand-gated ion channel (e.g. a nAChR or ATP-sensitive K+ channel) will change its conformation when bound to by a ligand. The nACHr receptor is normally closed, but when ACH binds to it, it opens the intrinsic channel. Predominantly Na+ (although all cations will do the same) moves through. In an ATP-sensitive K+ channel, the channel is normally open, but on the binding of the ligand, ATP, the channel closes preventing the facilitated diffusion of ATP.

Question 47

What is a voltage-gated channel? (S2)

Answer 47

A voltage-gated ion channel has transmembrane segments which are charged. This means if the potential changes, there would be a driving force on that transmembrane segment of protein to move within the field. This movement of the segment drives a conformational change in the protein which carries ions through.

Question 48

Is transport using proteins saturable?

Answer 48

Yes.

Question 49

What are the extracellular and intracellular concentrations of Na+ respectively? Which way does the gradient go? (S2)

Answer 49

Inwards - 145mM –> 12mM

Question 50

What are the extracellular and intracellular concentrations of K+ respectively? Which way does the gradient go? (S2)

Answer 50

Outwards - 4mM <– 155mM

Question 51

What are the extracellular and intracellular concentrations of Ca2+ respectively? Which way does the gradient go? (S2)

Answer 51

Inwards - 1.5mM –> 10^-7mM

Question 52

What are the extracellular and intracellular concentrations of Cl- respectively? Which way does the gradient go? (S2)

Answer 52

Inwards - 123mM –> 4.2mM

Question 53

Is the Na+/K+ Pump electrogenic? Explain. (S2)

Answer 53

It is electrogenic because it pumps 3 Na+ out of the cell (against its concentration gradient) for every 2K+ in to the cell (again against its concentration gradient).

Question 54

Does the Na+-K+-ATPase contribute to the negative resting membrane potential? (S2)

Answer 54

Not really. It contributes c. -5mV with respect to the resting membrane potential (RMP). It does however create the K+ gradient that influences the RMP.

Question 55

Does the Na+ Pump require energy to function? (S2)

Answer 55

It requires energy from the hydrolysis of ATP. In fact around 25% of BMR is used for the pump.

Question 56

Describe the structure of the Na+-K+-ATPase. (S2)

Answer 56

There is an alpha subunit which allows the transport of K+, Na+ and ATP and has an ouabain binding site. The beta subunit is a glycoprotein that directs the pump to the surface.

Question 57

What will ouabain do to the Na+ pump? (S2)

Answer 57

It will bind, block and inhibit the Na+ pump. This will have secondary effects such as on the function of the NCX.

Question 58

Is the NCX a primary transporter? (S2)

Answer 58

No it is a secondary transporter. It relies on the Na gradient created by the Na+ pump in order to move Ca2+ out of the cell against its gradient. (3Na+ move out of the cell for every Ca2+)

Question 59

Does the NCX have a high affinity? (S2)

Answer 59

No. It has low affinity and high capacity.

Question 60

What is and does PMCA do? (S2)

Answer 60

It is the plasma membrane Ca2+ ATPase. It takes ATP and hydrolyses it. It is therefore a primary active transporter as it takes this energy, couples it to the protein, in order to transport Ca2+ against its concentration gradient out of the cell. H+ is transported into the cell - acidifying it.

Question 61

Is PMCA a high/low affinity and capacity transporter respectively? (S2)

Answer 61

It is a high affinity, low capacity transporter. High affinity means only a little Ca2+ is required and the transporter will still function, whereas low capacity refers to the fact it will only remove ‘residual calcium’.

Question 62

What is and does SERCA do? (S2)

Answer 62

SERCA is the sarco(endo)plasmic reticulum Ca2+ ATPase. It, like PMCA, is a primary active transporter. It drives Ca2+ into the intracellular store and extrudes H+ from the store.

Question 63

Is SERCA a high/low affinity and capacity transporter respectively? (S2)

Answer 63

It, like PMCA, is high affinity, low capacity.

Question 64

What is and does NHE do? (S2)

Answer 64

NHE is the Na+-H+-exchanger. It is a secondary active transporter and uses the gradient formed by Na+ from the Na+ pump (3Na+ out to 2K+ in).

Question 65

Does NHE play a role in pH control? (S2)

Answer 65

Yes. Na+ moves into the cell, H+ moves out. This means acid is extruded and leads to cell alkalinisation.

Question 66

What is AE? (S2)

Answer 66

It is the anion exchanger. Base, HCO3-, is extruded and Cl- intrudes - this leads to cell alkalinisation. It is electrochemically neutral i.e. not electrogenic.

Question 67

What does the Na+-Glucose co-transporter do? (S2)

Answer 67

Using the Na+ gradient created by the Na+-K+ pump, Na+ and glucose are co-transported into the cell (symport). Glucose can move against its concentration gradient. This will be found in the gut, to move glucose into the small intestine.

Question 68

What is the CFTR and what role does it play in cystic fibrosis sufferers? (S2)

Answer 68

It is the cystic fibrosis transmembrane conductance regulator. It is seen in many epithelial cells. If we take the lung epithelial cells, it transports chloride ions out of the cell into the lumen of the alveoli. Due to the presence of the Na+-2Cl- -K+ co-transporter into the cell, Cl- will build up in the cell (this is because Na+ moves out of the cell due to the Na+ pump and this transporter serves to move Na+ back into the cell… Cl- ‘cannot escape’). This will cause osmolytes to be dragged into the cell leading to clinical signs such as thicker mucous.

Question 69

In cases of diarrhoea what happens to the CFTR? (S2)

Answer 69

The CFTR is phosphorylated by protein kinase A. This results in lots of Cl- moving across the lumen. If this occurs in the epithelia of the small intestine, then lots of water will move into the lumen making faeces more watery.

Question 70

When can ‘reversal of mode’ of NCX be seen? (S2)

Answer 70

Where ATP is low, i.e. ischaemia, the Na+ pump will become inhibited. This means Na+ is not driven out of the cell. Instead it accumulates in the cell. If the Na+ accumulates to a certain level, NCX - instead of driving Na+ into the cell with the concentration gradient, may on occasion reverse mode - Ca2+ is driven into the cell - and its toxic.

Question 71

Outline bicarbonate reabsorption by the proximal tubule. (S2)

Answer 71

This process is vital in maintaining whole body pH.
Na is taken back into the proximal tubular cell by the NHE. The Na pump maintains Na+ gradients, pumping Na+ back into the capillary (3Na+ out, 2K+ in)
The NHE pumps H+ into the proximal tubule. H+ will join with the carbonate to form H2CO3, carbonic acid. This is broken down by carbonic anhydrase to form H2O and CO2. CO2 will diffuse across the membrane to the renal proximal tubular cell. In the proximal tubular cell H2O and CO2 will react to form carbonic acid. H+ will dissociate and bicarbonate will move into the capillary via the AE.

Question 72

Why may renal reabsorption of Na+ be controlled? (S2)

Answer 72

It is often a first line treatment for mild hypertension. This is because the more Na+ reabsorbed.. the more water reabsorbed.. this means more fluid and a higher blood pressure.

Question 73

Outline how reabsoprtion of Na+ takes place in the thick ascending limb? (S2)

Answer 73

In the thick ascending limb there is a Na-K+ ATPase that maintains a low Na+ concentration in the cell (Na+ out of the cells and into the capillaries, K+ into the cell). The NKCC takes Na+ up from the filtrate. To maintain the ionic balance in the cell ROMK (K+ from the filtrate into the cell) must be present as well as a K+-Cl- symporter into the capillary. There is also a Cl- channel uniporter moving said ions into the capillaries.

Question 74

Where do loop diuretics act and what do they do? (S2)

Answer 74

Loop diuretics act around the loop of Henlé at the thick ascending limb. They block the NKCC – the Na+-K+-2Cl- transporter preventing Na+ uptake from the thick ascending limb to the surrounding cells and ultimately to the capillaries.

Question 75

What do the thiazides family of drugs do? (S2)

Answer 75

At the distal convoluted tubule, they block the co-transport of Na+ and Cl- from the filtrate to the tubular cells.

Question 76

Where does amiloride act and what does it do? (S2)

Answer 76

Amiloride acts at the distal convoluted tubule and the cortical collecting duct. It blocks ENaC (the epithelial sodium channel) and stops the transport of sodium from the filtrate into the surrounding cells. It also blocks the action of the NHE in the proximal tubule. This reduces bicarbonate reabsorption.

Question 77

What effect does aldosterone have in the nephron? (S2)

Answer 77

Aldosterone up-regulates the transporters in the cortical collecting duct. In patients with hyperaldosteronism, they can have a high blood pressure because they are reabsorbing too much Na+ out of the filtrate.

Question 78

How could you treat hyperaldosteronism? (S2)

Answer 78

Amiloride could be used to block transport at the ENaC. Spironolactone could be used as well.

Question 79

What is a membrane potential? (S3)

Answer 79

All cells have an electrical potential (voltage) difference across their plasma membrane. This can vary from -50 to -75mV in nerve cells to -80 to -90mV in cardiac and skeletal muscle. They are always expressed as the potential inside the cell relative to the extracellular solution.

Question 80

How could we measure the resting potential of a cell? (S3)

Answer 80

We have a voltmeter attached to two electrodes. One electrode is attached across the extracellular medium, with the other electrode inserted into a glass pipette that can be inserted into the cell. The tip diameter is < 1um and is filled with conducting solution (KCl). The voltmeter for all cells will always be negative.

Question 81

Explain the concept of selective permeability and how the selective permeability of cell membrane arises. (S3)

Answer 81

The phospholipid bilayer has a hydrophobic interior and so is very impermeable to charged molecules or ions. It is permeable to small uncharged molecules, (respiratory gases, H2O and ethanol) however. In order for ions to cross the membrane, proteins that enable ions to cross cell membranes must be present. They have an aqueous pore through which ions flow by diffusion. These channels have a number of properties, they are selective for one or a few ion species. They show gating, the pore can open or close by a conformational change in the protein, and rapid ion flow which is always down the electrochemical gradient.

Question 82

Describe how the resting potential is set up given the distribution of ions across cell membranes. (S3)

Answer 82

The ion selectivity of channels and the types of channel that are open makes the whole cell membrane selectively permeable to ions. For most cells, open K+ channels dominate the membrane ionic permeability at rest. Thus the resting membrane potential arises because the membrane is selectively permeable to K+.

Question 83

Explain the term equilibrium potential for an ion. (S3)

Answer 83

At the equilibrium potential for an ion, the electrical and chemical (or diffusion) gradients for the ion balance so there is no net driving force for the ion across the membrane.

Question 84

How would you go about calculating the equilibrium potential for an ion? (S3)

Answer 84

An example calculation for K+ would be is 61.5/valency (or 1) log10 ([K+]out/[K+]inside). This would be 61 log10 [4.5/160] = -94.6mV. N.B outside / inside

Question 85

What are the equilibrium potentials of the 4 main ions? (S3)

Answer 85

Ek = -95mV; Ena = +70mV; Ecl = -96mV; Eca = +122mV

Question 86

What is depolarization, when may it occur? (S3)

Answer 86

Depolarization is where the membrane potential becomes less negative or more positive. It may occur when the membrane is more permeable to Na+ and Ca2+.

Question 87

What is hyperpolarization, when may it occur? (S3)

Answer 87

Hyperpolarization is where the membrane potential becomes more negative.
It may occur when membrane ion permeability to K+ and Cl- are increased i.e. K+ and Cl- channels open.

Question 88

What does increasing membrane permeability to a certain ion do? (S3)

Answer 88

It will move the membrane potential towards the equilibrium potential for that ion.

Question 89

Outline some of the roles of the membrane potential in signalling within and between cells. (S3)

Answer 89

Changing membrane potentials plays a role in:

1) Generating action potentials in nerve and muscle cells
2) Triggering and control of muscle contraction
3) Control of secretion of hormones and neurotransmitters.
4) Transduction of sensory information into electrical activity by receptors.
5) Postsynaptic actions of fast synaptic transmitters

Question 90

How can changes in ion concentration influence membrane potential? (S3)

Answer 90

Na/K-ATPase (and the NCX), electrogenic pumps that pump one positive charge out of the cell for each cycle). In some cells this contributes to the resting membrane potential.
Indirectly, Na-K-ATPase is responsible for the entire membrane potential, because it sets up and maintains the ionic gradients.

Question 91

Name the three types of gated channels. (S3)

Answer 91

Ligand-gated, voltage-gated and mechanical-gated.

Question 92

What are ligand-gated channels? (S3)

Answer 92

Ligand-gating: channel opens or closes in response to binding of a chemical ligand.
e.g. Channels at synapses that respond to extracellular transmitters; Channels that respond to intracellular messengers.

Question 93

What are voltage-gated channels? (S3)

Answer 93

Voltage-gating: channel opens or closes in response to changes in membrane potential. e.g. channels involved in action potentials

Question 94

What are mechanical-gated channels? (S3)

Answer 94

Mechanical-gating: channel opens or closes in response to membrane deformation. e.g. channels in mechanoreceptors: carotid sinus stretch receptors, hair cells.

Question 95

In ligand-gated channels, what is fast synaptic transmission? (S3)

Answer 95

In fast synaptic transmission, the receptor protein is also an ion channel. When the transmitter binds it causes the channel to open.

Question 96

What are excitatory transmitters? (S3)

Answer 96

Excitatory transmitters open ligand-gated channels that cause membrane depolarisation. They can be permeable to Na+, Ca2+ and sometimes cations in general (seen in nAChR) The resulting change in membrane potential is called an excitatory post-synaptic potential (EPSP). It has/is:

1. A longer time course than AP
2. Graded with amount of transmitter
3. Transmitters including: ACh and glutamate

Question 97

What are inhibitory transmitters? (S3)

Answer 97

Inhibitory transmitters open ligand-gated channels that cause hyperpolarisation. They are permeable to K+ or Cl-. The resulting change in membrane potential is known as an inhibitory post-synaptic potential (IPSP). Transmitters include: glycine, gamma-aminobutyric acid (or GABA)

Question 98

In ligand-gated channels, what is slow synaptic transmission? (S3)

Answer 98

It is where the receptor and channel are separate proteins. In slow transmission, the transmission of the information is between two proteins. There are two basic patterns: direct G-protein gating and gating via an intracellular messenger.

Question 99

What is direct G-protein gating? (S3)

Answer 99

In direct G-protein gating, the signal is coupled by binding their agonists to the receptor, releasing a G protein which goes on to bind to the channel (an effector protein). The G protein diffuses in the membrane taking the message from the activated protein to the channel. This occurs within the synapse and is slower because of the diffusion.

Question 100

What is gating via an intracellular messenger? (S3)

Answer 100

In gating via an intracellular messenger, the G-protein receptor is bound to by the agonist. The G protein then diffuses and reacts with the enzyme, causing a cascade ultimately leading to the production of an intracellular messenger or activated protein kinase which will activate the channel. This gives the possibility of amplification by cascade: one G-protein could lead to 1000s of produced intracellular messengers. It occurs throughout the cell.

Question 101

What are some of the properties of an action potential? (S4)

Answer 101

Action potentials are caused by changes in voltage across the membrane.
These changes are dependent on ionic gradients and relative permeability of the membrane.
They occur only if threshold is reached.
They are ‘All or Nothing’. This means there are no half or double action potentials.
They are propagated without loss of amplitude.

Question 102

What is the sodium hypothesis? (S4)

Answer 102

Once the membrane has been depolarised to the threshold voltage, voltage-gated Na+ channels open allowing Na+ influx. The membrane depolarizes further due to the influx of Na+ ions causing more voltage-gated Na+ channels to open and further depolarization. This positive feedback is the basis of the ‘All or Nothing characteristic of action potentials’.

Question 103

What actions contribute to repolarisation? (S4)

Answer 103

After a period of depolarisation, the Na+ channels become inactivated and close. Voltage-gated K+ channels open due to depolarisation moving the membrane potential towards its equilibrium potential of -95mV.

Question 104

How much Na+ moves into the cell during an action potential? (S4)

Answer 104

40uM. If resting [Na+] is 12mM then this represents an increase of ~ 0.3 or 0.4%

Question 105

What is conductance of an ion? (S4)

Answer 105

Conductance refers to the membrane’s permeability for an ion, if it is high there will be more channels for that ion open in the membrane bilayer.

Question 106

How can we investigate the mechanism of action potential generation? (S4)

Answer 106

• Voltage-clamping controls the membrane potential so that ionic currents can be measured.
• Using different ionic concentrations the contribution of various ions can be assessed
• Patch-clamping enables currents flowing through individual ion channels to be measured.

Question 107

What does voltage-clamping do? (S4)

Answer 107

It stops membrane potential moving freely:
[In an unclamped cell, membrane potential can change freely; a voltage clamp prevents the change in membrane voltage in response to membrane current].
The voltage-clamp enables membrane currents to be measured at a set membrane potential.

Question 108

What is the absolute refractory period? (S4)

Answer 108

During the ARP (absolute refractive period) nearly all Na+ channels are in the inactivated state. Excitability is at 0.

Question 109

What is the relative refractory period? (S4)

Answer 109

During the RRP (relative refractory period) Na+ channels are recovering from inactivation. Excitability returns towards normal as number of channels in the inactivated state decrease.

Question 110

What is accommodation? (S4)

Answer 110

The longer the stimulus the larger the depolarization necessary to initiate an action potential; the threshold effectively becomes more positive. This is because more Na+ channels become inactivated over time.

Question 111

Describe the structure of Na+ channels. (S4)

Answer 111

Na+ (and Ca2+) voltage-gated channels are similar in structure. Their main pore forming subunit is one peptide consisting of four homologous repeats. Each repeat consists of six transmembrane domains. One of these six domains (the 4th one) is unable to sense voltage across the membrane. A functional channel requires one α-subunit.

Question 112

Describe the structure of K+ channels. (S4)

Answer 112

Voltage-gated K+ channels are similar in structure to Na+ channels, except each repeat is a separate subunit. Each subunit still has six transmembrane domains, one of which is voltage sensitive. For the channel to be functional there must be 4 α-subunits.

Question 113

Give an example of a local anaesthetic. (S4)

Answer 113

Procaine.

Question 114

How do local anaesthetic act? (S4)

Answer 114

Local anaesthetics block small myelinated axons then un-myelinated axons and finally large myelinated axons. Due to this they tend to effect sensory before motor neurones.
Local anaesthetics are weak bases and cross the membrane in their unionized form. They do block Na+ channels when the channel is open and also have a higher affinity for Na+ channels in the inactivated state. This stops action potential generation.

Question 115

What is extracellular recording of action potentials? (S4)

Answer 115

Electrodes are used to raise the Vm to threshold generating an action potential. By recording changes in potential between the stimulating (cathode, negative) and recording (anode, positive) electrodes along an axon, conduction velocity can be calculated. The equation
v = d / t is used…

Question 116

What is local current theory? (S4)

Answer 116

Injection of current into an axon will cause the resulting charge to spread along the axon and cause an immediate local change in the Vm.

Question 117

What is the length constant, λ? (S4)

Answer 117

The length constant is the distance travelled for the potential to fall to 37% of its original value.

Question 118

What are local currents affected by? (S4)

Answer 118

Diameter: greater –> faster; Resistance: higher –> faster;
Capacitance: lower –> faster

“Velocity increases with diameter as there are more channels available in a larger surface area.
I = V/R states that the lower the resistance (there is a low cytoplasmic resistance because of a high diameter) the larger the current. The action potential will travel further because conduction velocity is increased. Resistance refers to how many available Na+ channels there are, with more available channels giving a higher resistance. This means a higher Vm, more voltage-gated Na+ channels open making it easier to reach threshold, conduction velocity is therefore increased. You want all of the energy you put into an AP to be translated into an AP. If there is high capacitance then some of that energy is being ‘stored’ and so not put into generating the AP. Therefore, a lower capacitance is preferable as more energy is available to go directly into spreading as a local current and conduction velocity will increase.”

Question 119

What is capacitance? (S4)

Answer 119

Capacitance, C, is the ability to store charge. It is a property of the lipid bilayer. The membrane resistance depends on the number of ion channels open. The lower the resistance the more ion channels are open.

Question 120

How does myelination affect local current theory? (S4)

Answer 120

Myelination acts to increase resistance and decrease capacitance (by approx. 100x in both). These increase the length constant (λ) and cause a slight decrease in the time constant. Conduction velocity is proportional to length constant, λ / time constant. Thus this results in a significantly increased conduction velocity.

Question 121

Why does myelination increase resistance and decrease capacitance? (S4)

Answer 121

The nodes of ranvier have a high concentration (~10,000) of voltage-gated Na+ channels increasing resistance and the myelin has low capacitance. In contrast unmelinated axons have an even distribution of Na+ channels.

Question 122

Which neurones are often myelinated: large or small? (S4)

Answer 122

Large diameter axons such as motorneurons are myelinated whereas smaller ones, sensory neurons say, are not.

Question 123

d / D = 0.7 for optimum conduction velocity. What are d and D here? (S4)

Answer 123

D is the diameter including the myelin sheath. d is the axon diameter.

Question 124

How does myelin help the propagation of action potentials? (S4)

Answer 124

The myelin sheath acts as a good insulator thereby causing the local circuit currents to depolarize the next node above threshold and initiate another action potential. The myelin sheath also enables saltatory conduction. Here the action potential ‘jumps’ from node to node allowing a much faster conduction velocity. An action potential occurs only at the nodes.

Question 125

What is conduction velocity proportional to in myelinated axons? And in unmyelinated axons? (S4)

Answer 125

In myelinated axons conduction velocity is proportional to diameter (D) of the nerve fibre. (It has a max. velocity of 120m/s – for motoneurones). In unmyelinated axons conduction velocity is proportional to the square root of fibre diameter (D). (Max. velocity of 20m/s).

Question 126

How is myelin formed? (S4)

Answer 126

Myelin is formed by special cells which envelop axons in their plasmalemma. Schwann cells myelinate peripheral axons and oligodendrocytes myelinate axons in the CNS.

Question 127

What are some examples of diseases that affect conduction of the action potential? (S4)

Answer 127

Multiple sclerosis and Devic’s disease are examples which affect the CNS. MS, an autoimmune disease, affects all CNS nerves whereas Devic’s only affects optic and spinal cord nerves.
Landry-Guillain-Barre syndrome and Charcot-Marie-Tooth disease are examples that affect the PNS. These diseases result from breakdown or damage to the myelin sheath. This can lead to decreased conduction velocity, complete block or cases where only some action potentials are transmitted.

Question 128

If fibre diameter is under 1um, is myelination effective? (S4)

Answer 128

Fibre diameter’s under 1um will result in a faster conduction velocity in unmyelinated axons.

Question 129

How do action potentials open Ca2+ channels? (S5)

Answer 129

The action potential arrives at the presynaptic membrane. This causes the opening of voltage-gated Ca2+ channels and the subsequent influx of calcium ions down their concentration gradient. [Ca2+ binds to synaptotagmin, which is membrane-bound. This action brings the vesicle close to the membrane. The snare complex, which is also membrane bound, makes a fusion pore with the vesicle and the neurotransmitter is secreted through this fusion pore]. In short the increase in cytosolic Ca2+ activates a group of proteins associated with the vesicle to promote exocytosis of ACh.

Question 130

Describe some aspects of diversity of Ca2+ channels. (S5)

Answer 130

Structurally voltage gated Ca2+ channels are very similar to voltage gated Na+ channels. However, Ca2+ channels have structural diversity – a blocker that blocks one calcium channel will not necessarily block another.
Different calcium channels have different primary locations, so selectively blocking one type of channel can have a localised effect.

Question 131

Where are voltage-gated L-type Ca2+ channels found? (S5)

Answer 131

In the muscle, neurones and the lungs. They can be blocked by dihyrdopyridines, e.g.Nifedipine.

Question 132

What happens in fast synaptic transmission? (S5)

Answer 132

In fast synaptic transmission, the receptor protein is also an ion channel. The binding of transmitter causes the channel to open.

Question 133

What does the ACh do after it is released? (S5)

Answer 133

It binds to the Nicotinic Ach Receptor on the post-junctional membrane to produce an end-plate potential. This depolarisation will raise the muscle above threshold so that an action potential is produced.

Question 134

What are the two types of blockers of nicotinic ACh receptors? (S5)

Answer 134

nAChRs, which are examples of ligand-gated ion channels, can be blocked by competitive blockers e.g. tubocuranine; and depolarizing blockers e.g. succinylcholine.

Question 135

What is the mechanism of action of competitive blockers on the nAChR? (S5)

Answer 135

Tubocurarine binds at the molecular recognition site for ACh.

Question 136

What is the mechanism of action of depolarizing blockers on the nAChR? (S5)

Answer 136

The presence of succinylcholine results in a maintained depolarisation at the post-junctional membrane. Adjacent Na+ channels will not be activated due to accommodation. Depolarizing blockers like succinylcholine are often used in operations to induce paralysis.

Question 137

What is myasthenia gravis? (S5)

Answer 137

Myasthenia Gravis is an autoimmune disease targeting nAChR.

Question 138

What are some of the clinical effects of myasthenia gravis? (S5)

Answer 138

Patients suffer drooping eyelids and profound weakness which increases with exercise.

Question 139

What causes myasthenia gravis? (S5)

Answer 139

Antibodies directed against nAChR’s on the postsynaptic membrane of skeletal muscle. Endplate potentials are reduced in amplitude, leading to muscle weakness and fatigue.

Question 140

What treatment is given to myasthenia gravis sufferers? (S5)

Answer 140

It is treated with ACh-esterase inhibitors, to increase the amount of time ACh is in the synaptic cleft.

Question 141

What are some examples of cellular processes sensitive to [Ca2+]? (S5)

Answer 141

Fertilisation, secretion, neurotransmission, metabolism, contraction, learning and memory, apoptosis and necrosis.

Question 142

How is the Ca2+ gradient set-up and maintained? (S5)

Answer 142

There is relative impermeability of the plasma membrane: Ca2+ channels are largely closed.
It is dependent upon the cells ability to expel Ca2+ across the plasma membrane.
Ca2+ buffers - Ca2+ diffuses more slowly than predicted from its ionic or hydrated radius. Ca2+ buffers limit diffusion – ATP and Ca2+-binding proteins such as parvalbumin, calbindin, calreticulin and calsequestrin, etc. The Ca2+ ion diffuses 0.1-0.5um before encountering a binding molecule. Ca2+ diffusion depends on concentration of binding molecules and their level of saturation.
Intracellular Ca2+ stores which can fall under rapidly releasable and non-rapidly releasable.

Question 143

How do cells expel Ca2+ across the plasma membrane? (S5)

Answer 143

Ca2+-ATPase
There is a feedback mechanism. If intracellular Ca2+ increases, then Ca2+ binds to calmodulin (a Ca2+-binding protein… a trigger protein) and the Ca2+-calmodulin binds to Ca2+-ATPase. The Ca2+-ATPase then removes Ca2+.
This exchanger is high affinity, low capacity.
Na+-Ca2+-exchanger (NCX)
The Na+ gradient is used as the driving force (this is dependent on the Na+-K+-ATPase). This transporter is electrogenic and so works best when the cell is at its resting membrane potential.

Question 144

How is intracellular Ca2+ elevated? (S5)

Answer 144

Through Ca2+ influx across the plasma membrane (i.e. altered membrane permeability to Ca2+). These two channels contribute to this effect:
Voltage-operated Ca2+ channels
Receptor-operated Ca2+ channels (i.e. through a ligand or agonist attaching to the receptor. They are essentially ligand-gated e.g. NMDA/AMPA receptors for glutamate)
Intracellular Ca2+ levels can rise tenfold.

Question 145

How is Ca2+ released from rapidly releasable intracellular stores? (S5)

Answer 145

Through the action of GPCRs and CICR.

Question 146

How are GPCRs involved in calcium release? (S5)

Answer 146

Stores of Ca2+ are set up inside the sarco-endoplasmic reticulum by the SERCA protein. Ca2+ is moved in using the energy from ATP hydrolysis and binds to proteins such as calsequestrin. This release of Ca2+ is mediated by G-protein coupled receptors (GPCRs) A ligand binds to the GPCR on the cell membrane activating its Gaq subunit. This subunit then binds to the membrane phospholipid PIP2, releasing IP3, which in turn binds to its receptor on the sarcoendoplasmic reticulum, triggering the release of calcium down its concentration gradient into the cell.

Question 147

What is Ca2+ induced Ca2+ release (CICR)? (S5)

Answer 147

Ca2+ binds to the Ryanodine receptor, which is on the side of the sarcoendoplasmic reticulum; this triggers the release of Ca2+ into the cell.

Question 148

Where does CICR have an important physiological function? (S5)

Answer 148

The cardiac myocyte. During depolarisation Na+ and 15% of Ca2+ (through voltage-operated Calcium channels) will move into the cell from the T-tubule. However 85% of the influx of Calcium goes through ryanodine receptors. The role of the NCX is reversed at some points in a cardiac action potential.

Question 149

How is Ca2+ released from non-rapidly releasable intracellular Ca2+ stores? (S5)

Answer 149

Ca2+ is taken up into mitochondria when intracellular [Ca2+] is high as a protective mechanism, but mitochondria also participate in normal Ca2+ signalling due to microdomains (areas of cytoplasm with a higher concentration of Ca2+ due to their proximity to a channel). Mitochondria take up Ca2+ to aid in buffering, regulating signalling, and stimulation of ATP production. They do this via a Ca2+ uniporter that is driven using respiration.

Question 150

How is intracellular Ca2+ returned to basal concentrations? (S5)

Answer 150

Termination of signal, Ca2+ removal and Ca2+ store refilling. Ca2+ is removed through the cell’s ability to expel Ca2+, the Ca2+-ATPase and the NCX.
Ca2+ store refilling takes place through the recycling of released cytosolic Ca2+ (e.g. cardiac myocyte) and voltage-operated Ca2+ channels and/or capacitative Ca2+ entry. Capacitative or store-operated channel (SOC) assists mitochondrial Ca2+ in replenishing sarcoplasmic reticulum stores.