Physiology Flashcards
What is the meaning of a gated ion channel?
Part of the membrane can block or undergo an conformational change to block the pore
Describe the Kv structural ion channel family
-6 transmembrane spanning domains
-The 4th transmembrane domain is the voltage sensor. Voltage dependent ion channels are sensitive to the membrane potential which is mediated by amino acids in the 4th transmembrane domain.
-Pore region is where amino acids which construct the pore are located.
-4 subunits must come together to create a functioning potassium channel.
Describe the structure of Kir ion channels
-2 transmembrane spanning domains
-4 subunits must come together to create a functioning potassium channel.
Describe the structure of Nav or Cav ion channels
-24 transmembrane spanning domains
-4 pore regions
-Come together to make one channel
-Beta subunit
Describe the structure of the CFTR Cl- ion channels
-12 transmembrane spanning domains
-Regulatory domains
Describe the structure of Ach receptors
-Ligand gated ion channel
-Binding of Ach opens the channel and depolarises the membrane potential.
What is the function of the patch clamp technique and how is it performed?
-Allows us to evaluate ion channel physiology
-When there are mutations we can look at ion channel pathophysiology
-Glass pipette is filled with salt solution
-A reference electrode is used
-Glass pipette touches cell membrane
-Suck on the end of the pipette. Membrane of cell seals onto pipette
-You can look at single ion channels moving across membrane, current can be measured
-Ripping away part of the membrane allows you to record currents from the whole membrane of the cell
What is the function of ion channels?
When open, they drive the membrane potential towards the Nernst potential for the channel
What is FHEIG?
Disease which cases Bi-temporal narrowing, hypertrichosis (hair), thin upper lip, bushy long eyebrows.
Also causes delayed development of intellectual ability and motor skills.
Seizures and Egg anomalies
Mutants have larger currents in potassium channels
This impacts the interstitial space, more potassium is lost which goes into interstitial space and potassium accumulates. Depolarises Nernst potential for potassium. Membrane potential in other neurons set by Nernst potential for potassium. They then have more positive resting membrane potential and more likely to fire action potentials.
What is Vm measurement and how is it collected?
-The voltage across the plasma membrane
-Reference electrode in extracellular solution
-Sharp glass electrode inserted through cell membrane into extracellular fluid
-Majority of cells measure -70mV
Describe sodium potassium ATPase’s role in setting the membrane potential
-Hydrolyses ATP to move 3 sodium ions out of the cell in exchange for 2 potassium ions.
-Electrogenic (net charge movement)
-Loss of positive charge
Describe the Nernst potential for potassium
-Potassium channels in cell membrane
-High intracellular potassium, low extracellular concentration
-Large anionic proteins which are not membrane permeable
-Concentration gradient for potassium to leave the cell
-Potassium carries positive charge leaving behind a negative charge
-Generates a potential gradient which works in opposite gradient, moving potassium ions in
-Number of ions leaving balances ions coming in so there is no net movement of potassium.
the potential at which there is no net movement is the Nernst potential for potassium
Describe the nernst potential for cells with sodium channels
-High extracellular sodium and low intracellular sodium
-Concentration gradient moves sodium down conc gradient into cell
-Movement of sodium carries charge
-Positive sodium moves in, positive potential.
-Sodium then passed out due to potential gradient
-At the point where potential and concentration channels are equal and opposite, this is the Nernst potential
Describe the movement at the membrane potential
-Sodium potassium ATPase (3 Na+ out and 2 K+ in) sets low intracellular sodium concentration
-K+ channels drive membrane potential in negative direction
-At rest cells around the body have negative resting membrane potentials
-If sodium channels are opened, sodium will move in down the electrochemical gradient. This moves the balance of selectivity
Describe the action at the sodium amino acid co-transporter
-Sodium shifts the membrane potential towards the Nernst potential for sodium
-Activation of potassium leads to repolarisation
-Sodium is still being transported, the balance just shifts
What are normal conditions of intra and extracellular Na+?
-Extracellular; high 145mM
-Intracellular; low 15mM
What is the function of the thick ascending limb of the loop of Henle?
-Reabsorption of NaCl in preference to H2O
-Apical membrane is impermeable to water, so NaCl is absorbed
-Creates transepithelial osmotic gradient responsible for counter current multiplication
Describe action at the thick ascending limb of the loop of Henle
On the basolateral membrane:
-Sodium potassium ATPase pumps sodium out and potassium in. This keeps intracellular sodium low
On the apical membrane:
-NKCC2, sodium at this transporter uses sodium gradient to drive uptake of one sodium, one potassium and 2 chloride ions into cell.
-Sodium leaves via pump, chloride leaves down electrochemical gradient via basolateral chloride conductance (CICKB)
-Potassium recycled over membrane
-Activity of NKCC depends on inward Na+ gradient
-If sodium inside cell increases too high, this reduces NKCC driving force so its function impaired. Leads to diuresis.
What are the normal conditions of intra and extra cellular Ca2+?
Extracellular; high 1,000,000nM
Intracellular; low 100nM
What is the role of Ca2+ in acinar cells?
Under control of different second messengers or hormones leads to stimulation of pancreatic acinar cells leading to increase in calcium, aiding binding of secretory vesicles to apical membrane and no enzymes are released.
What keeps in intracellular calcium low?
There are 2 mechanisms involves; Na+/Ca2+ exchanger and Ca2+ ATPase
If inwards Na+ gradient is 10 fold and Ca2+ gradient is 10,000 fold, how can the Na+/Ca2+ exchanger keep Ca so low?
-Normally exchanges extracellular Na+ for ca2+
-Na+/Ca2+ exchanger is electrogenic
-Stoichiometry is 3Na+:1Ca2+
-This means the effect of Na+ gradient is magnified
-The effect of the 10 fold gradient is cubed.
Describe the role of PMCA
-Virtually inactive at physiological Ca2+
-Increases Ca2+ which activates calmodulin
-Removal of auto inhibition and activation of PMCA
-At resting levels is the major mechanism for controlling Ca2+
Describe role of the Na+/Ca2+ exchanger
-Major role when Ca2+ rises above resting levels
-Important when there are large influxes of Ca2+
Describe the relationship between Ca2+ release from stores and store operated calcium channels
-Store operated calcium channels are involved in the filling of stores
-Agonist binds to GPCR stimulating PLC, breaking PIP2 into DAG and IP3
-IP3 will activate IP3 receptors in store membrane
-Ca2+ rushes out of store into cytoplasm
-Ca2+ must be replenished inside store
-Mechanisms linked to SOCC, SOCC are activated, calcium moved in and selectively refills stores.
-SOCC located in plasma membrane are inactive at rest.
-In membrane there is STIM1 protein.
-Starts with high levels in store, when store is depleted, calcium binding to STIM is removed, leading to activation of STIM.
-Complexes are formed. conformational change allowing STIM to interact with plasma membrane.
-Can activate SOCC, Ca2+ can move from extracellular domain to store.
-PMCA pumps are slightly inhibited, SERCA pumps are stimulated. Entry of Ca2+ is favoured into cytoplasmic space. Exit pathway is inhibited, movement of calcium across plasma membrane. Stocks are replenished.
What happens if the calcium balance goes wrong?
-Chronically de myelinated axons on MS
-Decrease in expression of Na+/K+ ATPase
-The impact is; inefficient transmission of action potentials. Impacts on function of NCX, rise in Ca2+ leading to Ca2+ mediated axon degeneration
Why is it important to control intracellular pH?
-pH is a logarithmic scale
-A small change in pH reflects a large change in protein concentration
-Proteins act to buffer changes in pH
-Change in pH changes protein charge which changes protein conformation and changes protein function.
What are the 3 factors involved in control of intracellular pH?
-Buffering
-Acid extrusion
-Acid loading
How does buffering control intracellular pH?
-pH buffer is any system that moderates the effects of an acid or alkali load by reversibly consuming or releasing protons.
-Buffering systems act to minimise pH changes and help protect the cell from damage.
-A buffer cannot reverse the changes in pH caused by an acid or alkali load, any recovery is due to the presence of acid loading/ extrusion mechanisms.
BUFFERS CANNOT PREVENT CHANGE IN PH, THEY MERELY MINIMISE THE MAGNITUDE OF THE CHANGE.
What is the buffering power?
The amount of strong base (or acid) that must be added to a solution in order to raise (or lower) the pH by a given amount.
Describe acid extrusion
-Na/H exchanger:
-Under normal physiological conditions, the sodium proton exchanger moves sodium into cell and protons out.
-The action of the Na/H exchanger relies upon sodium potassium ATPase.
What is the role of NHE1?
Housekeeping function; primary roles in regulation of pH and in control of cell volume.
Inhibited by ‘low’ concentrations of amiloride and its analogue EIPA. Found in basolateral membrane of epithelial cells.
Describe the acid loading system
-Cl/HCO3 exchanger- anion exchanger family
-Under normal conditions, direction of transport is inward movement of Cl for exchange of HCO3
-Removal of bicarbonate, leaves proton behind leading to acidification
-Like the Na/H exchanger, the activity of the Cl-/HCO3- is modulated by pH
-low activity at acid pH which increases as pH becomes alkaline
what are the three types of airflow?
Laminar
Unstable
Turbulent
How is the flow type determined?
It is governed by the Reynolds number which is influenced by the viscosity and density of the gas
Re < 2000 is laminar flow
Re = 2000-3000 is unstable flow
Re > 3000 is turbulent flow
Describe laminar flow
Steady flow down a tube in a uniform direction and speed. Flow rate is maximal in the centre of the tube and reduces towards the edges
Describe turbulent flow
If flow rate is beyond a critical value, irregular currents lead to vortices developing. The rate of gas movement is proportional to square root of the pressure difference. A greater pressure gradient is needed to obtain the same flow rate seen under laminar conditions.
What is COPD?
A group of progressive obstructive lung diseases characterised by increase in airway resistance and decreased airflow
It includes; chronic bronchitis and emphysema
How is COPD treated?
A progressive disease and so there is no cure, only the symptoms can be controlled
-Bronchodilators; anticholinergics or B2 adrenoreceptor agonists
-Glucocorticosteroids
What is poiseuille’s law?
-Airway resistance is proportional to gas viscosity and the length of the tube is inversely proportional to the fourth power of the radius
-This means that small changes in the airway diameter have big impacts on the resistance and hence the flow rate
What is the difference in airway resistance in someone with COPD?
In a normal individual, airway resistance is 1.5cm H2O.s.litres-1
In an individual with COPD, airway resistance is 5.0cm H2O.s.litres-1
Describe the effect of inspiration on resistance
The lungs and chest wall expand and intrapleural pressure becomes more negative. Large pressure gradient in alveoli allowing air to rush into lungs. The airways expand during inspiration which decreases resistance