Signalling

Question 1

What is signalling

Answer 1

• The cascade of processes by which an extracellular stimulus (typically a neurotransmitter or hormone) effects a change in cell function
• Signalling cell produces a particular type of extracellular signal molecule that is detected by a target cell
• Target cells have receptors that recognise and respond specifically to the signal molecules
• examples - glutamate and adrenaline
• types of signalling - 1) signalling through ion channels 2) signalling through receptor
• Signals can act over a long or short range

Question 3

Signalling through ion channels

Answer 3

• ion channels are often selective for a particular ion
• Ca2+ is a ubiquitous ( present, appearing, or found everywhere) signalling ion - important because it regulates a large variety of cellular processes
• calcium is very versatile and works in processes such as: exocytosis contraction, metabolism, gene transcription, fertilisation and proliferation hypertrophy

Question 4

Distribution of ions in gradients:

Answer 4

• ions are charged entities so membranes are normally impermeable to ions - so we have a natural barrier to keep ions in or our
• we have evolved mechanisms to allow ions to cross membranes and be transported and overcome the barriers
• Carrier proteins and channel proteins
• these have allowed cells to have gradients which are key in signalling
• we have ionic gradients inside our cells for example ER, lysosomes and mitochondria
• Ion transport can be passive (ion channels , concentration gradient goes high to low - nor direct energy required) or active (movement of ions and other solutes against a concentration gradient - energy needs to be put into the system)
• passive ion transport is determined by the electrochemical gradient
• opposing forces - gradient wants them to come in electrochemical does not

Question 5

Summary 1

Answer 5

• Ions exist in gradients across both the PM and organelles
• Ion transport is affected by transporters and ion channels
• Transport can be passive or active

Question 6

How ions are generated

Answer 6

• ion gradients are generated by active transport
• active transport requires ATP - ADP + phosphate ion (uses water and energy as well)
• the Na+-K+ pump generates Na+ and K+ gradients across the plasma membrane
• the Na+-K+ pump is a P-type ATPase (rewatch lecture for this bit)
• Ca+ pumps generate Ca+ gradient across the plasma membrane
• pumps are ATPases because they break down ATP
• takes calcium from the cytoplasm and ejects it into the extracellular space (rewatch lecture for this bit as well)
• Ca+ pumps generate Ca+ gradients across intracellular stores
• (SERCA) - S- sarco, ER - endoplasmic, C= calcium, A = P type ATPase
• ion gradients can be used to drive secondary transport
• The Na gradient is used to drive Ca2+ effluent cia the Na+/Ca+ exchanger (antiporter)
  (rewatch lecture for this part)

Question 7

Summary 2

Answer 7

• ion gradients are generated by active transport (via P-type ATPases) and coupled secondary transport (exchangers)
• Na+ and K+ gradients are established by the Na+-K+ pump
• Ca2+ gradient across the PM is generated by PMCA and NCX
• Ca+ gradient across the ER is generated by SERCA

Question 8

Ion channels

Answer 8

• one way to classify them is by looking at how they are activated
• voltage gated ion channels, ligand-gated (extracellular ligand) and (intracellular gated), mechanically gated
• voltage gated ion channels are evolutionary related to each other
• voltage sensing domain is able to detect the
• voltage changes, pore domain is what allows ion s through
• voltage gated potassium channels have 4 subunits,
• calcium sodium channels are much bigger - building blocks are joined in one large chain
• ion selectivity in K+ channels is mediated by the selectivity filter
• pores - have specific structures that only allow potassium through
• the selectivity filter stabilises the larger dehydrated K+ ion. Oxygen atoms from the filter are perfectly spaced to stabilize potassium ion, but not Na. Potassium in much more favourable
• voltage sensing is mediated by positively charged residues in the S4 region
• regularly spaced arginine R and lysine K residues “sense” potential changes (look at lecture for this)

Question 9

Voltage sensing

Answer 9

Voltage sensing is mediate by positively charged residues in the S4 region

Question 10

Summary

Answer 10

• Voltage gated Na+, K+, and Ca2+ channels share a similar tetrameric structure
• these channels comprise voltage sensing and pore forming domains
• key residues in these domains allow selective ion transport in response to voltage changes

Question 11

Channels

Answer 11

Channels - discriminate based on the on size and electric charge (when channel is open any molecule that is small enough and has the right charge can pass through)

Question 12

Transporters

Answer 12

transfers only molecules that or ions that fit into specific binding sites on the proteins

For uncharged molecule the the direction of the passive transport is only determined by it concentration gradient but for charged ones membrane potential also influences - this net driving force is called the electrochemical gradient

Question 13

The membrane potential

Answer 13

• Difference in voltage between the interior and exterior of the cell
• typically -70mV at rest (but can vary) - units = thousandth of a volt
• passive ion transport is determined by the electrochemical gradient
• the membrane potential arises due to ion gradients
• excess of positive charge inside - there are fixed anions which absorb the difference in ionic charge

Look at notes to blurt this bit

• the first blue thing = fixed anions, there is not voltage difference but there is a conc gradient - so potassium moves down the gradients
• 2 negative charges are then uncovered (look at charges)
• electrical gradient generated to bring potassium ions back in so voltage difference is reduced
• at some point chemical and electrical gradient cancel out and we get equilibrium potential

Question 14

Nernst equation

Answer 14

• the Nernst equation allows calculation of the equilibrium potential for a given ion

Look at notes to blurt

Question 15

Nernst equation

Answer 15

• the Nernst equation allows calculation of the equilibrium potential for a given ion

Look at notes

Question 16

Membrane potential

Answer 16

• The resting membrane potential arises primarily due to K+ imbalance
• most cells are more permeable to K+ than other ions (‘leak channels’)
• Resting membrane potential is thus close to the equilibrium potential for K+ (-90mV)
• does not work even if the ion has a gradient but the channels are not permeable to it

permeability PK»PCl>PNa

Question 17

Summary membrane potential

Question 18

Neurotransmission

Answer 18

• when a neuron is stimulated membrane potential of plasma membrane decreases to below 0.
• action potentials rise due to the concerted action of voltage gated Na+and K+ channels
• voltage gated Na+ channels depolarize the membrane - sodium moves into cells as gate is open (because it is sensitive to membrane potential changes)
• Influx of positively charged ions depolarises the membrane further so more sodium ion channels will open = more depolarisation
• Process continues until the plasma membrane potential has shifted from resting = -60mV to +40mV
• If depolarisation is large enough voltage gated sodium ion channels will open
• Equilibrium is reached - Na+ has no further tendencies to leave or enter the cell
• If channels continued to respond to the altered membrane potential the cell would get stuck with mos of its voltage gated sodium ion channels open
• Due to depolarisation potassium ion channels open - K+ ions start to flow out of the cell down their electrical chemical gradient - them moving out brings the membrane potential back to resting (this happens much more quickly than if K+ had flowed out of their leak channels only.
• if sodium ion channels remain open = not good ?

Question 19

Voltage gated Ca+ ions and synapses

Answer 19

• When action potential reached the nerve terminal the synaptic vesicle fuses with the plasma membrane which causes the neurotransmitter to be released into the synaptic cleft
• Depolarisation of nerve terminal plasma membrane caused by arrival of action potential opens voltage gated Ca2+ channels - concentrated in the in the plasma membrane of presynaptic nerve terminal
• Concentration outside of terminal is greater which causes Ca2+ to rush into the terminal through the open channels
• The resulting increase of Ca2+ in the cytosol of terminal is what triggers the membrane fusion that releases the neurotransmitter successfully turning the electrical signal into a chemical signal to be released into the synaptic cleft
• this diffuses across the synaptic cleft and binds to its complementary neurotransmitter receptors in the post synaptic plasma membrane of the target cells
• Binding causes a change in the membrane potential of the target cell which if large enough triggers the cell to fire an action potential
• Neurotransmitter is then removed very quickly from the synaptic cleft Depolarisation of- either by enzymes destroying it or or by pumping it back into the nerve terminals that released it or by uptake into neighbouring non neuronal cells
• This rapid removal limits the duration and spread of the signal and ensures that when the presynaptic cells fall quiet the post synaptic cell will do the same

Question 20

Summary nmj

Answer 20

• Sequential (in)activation of voltage-gated Na+ and K+ channels underlie action potentials
• Opening of Na+ channels depolarizes the membrane toward the equilibrium potential for Na+.
• Opening the K+ channels repolarizes the membrane toward the equilibrium potential for K+
  Opening of voltage gated Ca2+ channels at nerve terminals results in neurotransmitter release

Question 21

Ligand-gated ion channels

Answer 21

ions can be used for signalling purposes through opening of ion channels

Question 22

Excitatory synapses

Answer 22

Electrical signalling is enhanced at excitatory synapses

Question 23

Inhibitory synapses

Answer 23

Electrical signalling is dampened at inhibitory synapses

Question 24

Inotropic glutamate receptors

Answer 24

Inotropic glutamate receptors - found at excitatory synapses - allow positive charge into postsynaptic membrane to start depolarisation

Question 25

Pore domain

Answer 25

= building block inverted pore topology - flipped poor domain inserted in a ligand gated channel

Question 26

GABAa

Answer 26

- GABAA - found at inhibitory synapses have disulfide bond - they are called cis loop receptors - pentamere - they have 5 subunits coming together - respond to neurotransmitter GABA - when they open that allow chloride through - these ions are negative so it makes it less likely for the neuron to be activated as it hyperpolarizes the membrane

Question 27

Nicotinic acetylcholine receptors

Answer 27

- Nicotinic acetylcholine receptors - found at excitatory synapses - allow sodium to come through (which are positive hence why they are excitatory) - nAChRs are also found at the NMJ - Activated nAChRs depolarize muscle cells - excitatory - depolarization initiates contraction - there is E-C coupling - Contraction involves Ca2+ release from the sarcoplasmic reticulum (SR)

Question 28

Summary GABA

Answer 28

- ligand-gated ion channels are activated by synaptically released neurotransmitters - iGluRs are tetramers that conduct Na+/Ca2+ and are found at excitatory synapses - GABAA receptors are pentamers that conduct Cl- and are found at inhibitory synapses - pentametric nAChRs conduct Na+ and are found at the excitatory synapses and the NMJ - Depolarization of muscle cells induces Ca2+ - dependent contraction