Lecture 1- Introduction Flashcards
What is a passive response or passive potential?
-associated with molecules coming in (information from other neurons) -change in the membrane potential, but not yet the action potential (that happens when the +40mV threshold is reached= when the membrane is depolarised and an action potential is initiated)
What is the resting membrane potential?
-65mV
What is the threshold for generating an action potential?
+40mV
What is an action potential like?
-non graded, either occurs or it doesn’t - it is the patterning of APs (number and frequency) that determines what sort of information is received/sent
What is a neuron’s cell membrane like normally?
-sits in a position of equilibrium, balance of charges -it is negatively charged inside compared to outside
What set up the resting membrane potential and any subsequent changes in membrane potential?
-ion channels -balance of charges is due to charged ions
Where around the membrane is there more K+ (Potassium)?
-much higher inside than outside the cell -intracellular= 140 (mM) -extracellular= 5 (mM)
Where around the membrane is there more Na+ (Sodium)?
-much more outside than inside cell -intracellular= 5-15 (mM) -extracellular= 145 (mM)
Where around the membrane is there more Cl- (Chloride)?
-much more outside than inside the cell (smaller gradient than Na+) -intracellular= 4-30 (mM) -extracellular= 110 (mM)
Where around the membrane is there more Ca2+ (Calcium)?
-more outside than inside the cell -intracellular= 0.0001 (mM) -extracellular= 1-2 (mM) -large gradient thanks to the small concentration inside the cell
How are the gradients set up along the cell membrane?
- via ion transporters
- actively move ions against concentration gradients
- create ion concentration gradients
What is the Na/K ATPase?
- pumps sodium out of cells, while pumping potassium into cells. It has antiporter-like activity but is not actually an antiporter since both molecules are moving against their concentration gradient
- these are the crucial to set up the concentration imbalance across the membrane of the cell
- move ions against conc gradients
- 70% of energy use of the brain is involved in this movement of ions
What is another ATPase in the brain?
- Ca2+ pump
- large gradient across the membrane so need the pump to remove Ca2+ from inside the cell
How is the Na/K pump important for resting membrane potential?
-In order to maintain the cell membrane potential, cells keep a low concentration of sodium ions and high levels of potassium ions within the cell (intracellular). -The sodium-potassium pump moves 3 sodium ions out and moves 2 potassium ions in, thus, in total, removing one positive charge carrier from the intracellular space. (for each ATP that is broken down)
What is the structure of the Na/K pump?
- composed primarily of one alpha subunit (approx. 1000 amino acids) and one beta subunit
- The alpha subunit is responsible for most of the enzyme’s pumping function. It contains the specialized sequences of amino acids which bind to sodium, potassium, and ATP.
- The beta subunit is involved in the routing of the alpha-beta complex to the cell membrane, and it also functions to occlude, or to make inaccessible to either side of the membrane, potassium ions during conformation change
- The alpha subunit dominates both the cytoplasmic and transmembrane regions of the enzyme, while the beta subunit is primarily on the extracellular side of the membrane. This arrangement makes sense given their specific functions.
What is the mechanism by which the Na/K ATPase operates?
- The pump, after binding ATP, binds 3 intracellular Na+ ions.
- ATP is hydrolyzed, leading to phosphorylation of the pump at a highly conserved aspartate residue and subsequent release of ADP.
- A conformational change in the pump exposes the Na+ ions to the outside. The phosphorylated form of the pump has a low affinity for Na+ ions, so they are released.
- The pump binds 2 extracellular K+ ions.
- This causes the dephosphorylation of the pump, reverting it to its previous conformational state, transporting the K+ ions into the cell.
- The unphosphorylated form of the pump has a higher affinity for Na+ ions than K+ ions, so the two bound K+ ions are released. ATP binds, and the process starts again.
What are ion exchangers?
- Ion exchangers are either cation exchangers that exchange positively charged ions (cations) or anion exchangers that exchange negatively charged ions (anions). There are also amphoteric exchangers that are able to exchange both cations and anions simultaneously.
- set up and control the membrane potential as well
- these are energy independent -drive for Na to go in, down the gradient, this is used to make the conformational changes, energy from sodium drives Ca out (works same in the other types)
- this is for ion homeostasis as well as for neurotransmitters in some cases
- many types: c) Na+/Ca2+ exchanger, d) Na+/K+/Cl- cotransporter (can do more than 1:1), e) K+/Cl- cotransporter, f) Na+/H+ exchanger, g) Na+/neurotransmitter transporter (GABA, Dopamine)
How does ion selectivity work?
- channels that can differentiate between Na and K, not much difference in size of the ions
- how how do they differentiate:
- potassium channel= made up of four subunits= together they come together to form the channel
- potassium ions that exist in the hydrated form in the membrane, intra and extracellular space= go through ion selectivity filter= make the ions line up (4)
- 4 lined up -like the ball toy (metal one bumps and the next bumped)
- ion selectivity filter= what makes the difference between potassium and sodium?
- water= polar potassium ion is surrounded by water
- most energy efficient form, the position is exact for that ideal form -amino acids in the selectivity filter= have oxygen molecules that form the structure of teh channel= they mimick exactly the position of K and water position
- so the ion (K) can sit in the channel in their idealised form and when there is a concetration gradient
- sodium, cl etc don’t have the same size as ideal with water so they don’t fit
What is gating and how does it work?
- gating refers to the opening (activation) or closing (by deactivation or inactivation) of ion channels
- 3 types: voltage gated, ligand gates and mechanically gated -some stimulus comes along, opens it and allows for the ions to go through
- name ‘gating’ derives from the idea that an ion channel protein includes a pore that is guarded by a gate or several gates, and the gate(s) must be in the open position for any ions to pass through the pore. A variety of cellular changes can trigger gating, depending on the ion channel, including changes in voltage across the cell membrane (voltage-gated ion channels), drugs or hormones interacting with the ion channel (ligand-gated ion channels), changes in temperature,[2] stretching or deformation of the cell membrane, addition of a phosphate group to the ion channel (phosphorylation), and interaction with other molecules in the cell (e.g., G proteins).
- multiple types of gate -ligand= binding of a ligand allows for conformational change that allows ions through -mechanical= on skin= allows you to feel sth as it mechanically changes the shape and allows ions through
What is the selectivity filter?
eg. in the KcsA potassium channel:
- this model describes the importance for passage of the dehydration of the potassium ion.
- The main chain carbonyl oxygen atoms that make up the selectivity filter are held at a precise position that allows them to substitute for water molecules in the hydrated shell of the potassium ion, but they are too far from a sodium ion.
- Potassium ion channels remove the hydration shell from the ion when it enters the selectivity filter.
- The selectivity filter is formed by a five residue sequence, TVGYG, termed the signature sequence, within the P loop of each subunit.
- This signature sequence is highly conserved, with the exception that an isoleucine residue in eukaryotic potassium ion channels often is substituted with a valine residue in prokaryotic channels.
- This sequence in the P-loop adopts a unique structure, having their electro-negative carbonyl oxygen atoms aligned toward the centre of the filter pore and form a square anti-prism similar to a water-solvating shell around each potassium binding site.
- The distance between the carbonyl oxygens and potassium ions in the binding sites of the selectivity filter is the same as between water oxygens in the first hydration shell and a potassium ion in water solution, providing an energetically favorable route for de-solvation of the ions.
- The selectivity filter opens towards the extracellular solution, exposing four carbonyl oxygens in a glycine residue (Gly79 in KcsA).
- The next residue toward the extracellular side of the protein is the negatively charged Asp80 (KcsA). This residue together with the five filter residues form the pore that connects the water-filled cavity in the centre of the protein with the extracellular solution
How many types of channels are there in the brain?
- vast array
- eg.: voltage gated: Na+ channel, Ca2+ channel, K+ channel, Cl- channel
- ligand-gated channels: Neurotransmitter receptor, Ca2+ activated K+ channel, Cyclic nucleotide gated channel
- there are 100s of K+ channels, multiple different types - do different things -this gives us the different responses that neurons give
- Kv1.1= the main subunit but can also join in with other types and then give different types of channels, different rates of hyperpolarisation etc.
- it gives us subtle differences in their ability to respond
What are the inward rectifiers and what do they do?
- A channel that is “inwardly-rectifying” is one that passes current (positive charge) more easily in the inward direction (into the cell) than in the outward direction (out of the cell). It is thought that this current may play an important role in regulating neuronal activity, by helping to stabilise the resting membrane potential of the cell.
- rectifier= in process of change in voltage, the ability to transmit an ion through changes
- tend to move the ion inside rather than outside (usually work when hyperpolarised cell)
- over the course of voltage change the passage of the ion will also change (as the conc gradient is less)
What channel from these can be the one responsible for the resting membrane potential?
- permeability to K+= it is about the channels
- voltage gated= active only in response to thershold so no
- open and at rest= that one will be the crucial one for the setting up of the membrane potential
- K+ activated by Ca+= closed at rest -HERG= channel only opens when cell coming back to hyperpolarised cell
- inward rectifier= opened by hyperpolarisation, not at depolarisation, open at resting potential= this is the one responsible and ones that determine the resting membrane potential
- RMB= if changed= affects balance of the neuron, different response
What is this?
- knockout= no TREK2 (red) with it loses the responsivness to chloroform, takes longer to be anesthesised
- have to have higher conc. to react
- when the channels are altered then play a role in anesthesia -they are specific! the repsonse to pentobarbital is not affected in the knockout
- resting potential is incredibly crucial in how the cell responds
- a general anesthetics= change the responsiveness of cells (the resting membrane potential)
- loss of writing reflex (the rolling over) when exposed to chloroform
What are the things that K2P channels respond to?
-K2P channels are involved in many things, respond to lipids, anesthetics, connected to cytoskleton, heat, voltage, acidity, secondary messengers, all of these affect their activity
