Resting membrane potential Flashcards
divide body fluid into 2 compartments:
- intracellular fluid (ICF) = cytosol
- extracellular fluid (ECF)
extracellular fluid: general features
- constant chemical environment vital for survival of cells (homeostasis)
consists of:
- interstitial fluid (ISF)
- plasma
extracellular fluid: define ISF
solution that bathes the non blood cells
extracellular fluid: define plasma
extracellular compartment of the blood
extracellular fluid: level of capillaries
- interstitial fluid and plasma separated by single layer of endothelial cells intersperse w water filled pores
extracellular fluid: function of separation of interstitial fluid/ plasma
permits rapid change of all substances up to size of small protein btw plasma/ interstitial fluid
= plasma and ISF same conc. of solutes/ ions
difference in ICF and ECF: potassium
- higher in ICF
- much lower in ECF
difference in ICF and ECF: sodium
- lower in ICF
- much higher in ECF
difference in ICF and ECF: chloride
- lower in ICF
- much higher in ECF
define amphipathic and eg:
- having both -ve and +ve parts
- phospholipid molecules polar phosphate head (hydrophilic)
- non polar lipid tail (hydrophobic)
substances which can cross membrane via simple diffusion:
- small non polar lipophilic
- v small uncharged polar molecules
substances which can’t cross membrane via simple diffusion:
- larger uncharge polar molecules
- charged molecules and ions
substances which can’t cross membrane via simple diffusion: eg
- amino acids
- glucose
- lactate
- nucleotides
- H+, K+, Na+, calcium, magnesium, chloride, bicarbonate (HCO3-)
substances which can cross membrane via simple diffusion:
- O2, CO2, N2, fatty acids, steroid hormones
- H2O, urea, glycerol, ethanol
ion channels: features
- pore forming integral proteins that span lipid bilayer
- hydrophilic pore allows diffusion of charged ions across membrane DOWN electrochemical gradient
- passive transport
- no. and type of channels determines flow across membrane
list 5 types of ion channels:
- selective
- non gated
- gated
- fast
- bidirectional
ion channels: selective
either:
- allow only 1 ion species to pass (eg. Na+)
- allow polarity of ion species (cation/ anion)
ion channels: non gated
aka leak channels: open/ close randomly
ion channels: gated and types
open and close based on specific stimuli:
- ligand gated (extra/intracellular chemical)
- voltage (change in volt across membrane)
- mechanically (mechanical deformation)
- thermally (temp)
ion channels: fast
1x10(8) ions pass in 1 second
ion channels: bidirectional
- net ion flux depends on electrochemical gradient
leak channels: features
- selective but non gated channels
- open/ close randomly
- allow ions (K, Na, Cl) to diffuse passively across membrane DOWN electrochemical gradients
- crucial for establishing resting membrane potential of cell
ion pumps and coupled carriers: features
- integral proteins spanning lipid bilayer
- move solutes (ions, glucose) against/ UP electrochemical gradient
- active transport
ion pumps and coupled carriers: active transport requires metabolic energy by
directly:
- form of ATP
indirectly:
- form of chemical potential energy provided by another ion moving DOWN electrochemical gradient
ion pumps and coupled carriers: eg
sodium potassium pump
- primary active transport of K (inwards) and Na (outwards)
impermeant anions:
- many organic molecules are negatively charged (anions A-)
- which are too big to be diffused through the bilayer
- carry charge and also osmotically active
name two biophysical principals for correct functioning of the body:
- principal of equimolality
- principal of electrical neutrality
principal of equimolality:
conc. of osmotically active particles inside cell (ICF) should be approx = to outside (ECF)
imbalance: water going into
- ICF -> ECF: cell shrinkage
- ECF -> ICF: cell swelling
principal of electrical neutrality:
total cation (+) conc. outside cell MUST = total anion (-) outside cell - same for inside cell
principal of electrical neutrality: plasma acidosis
- high Cl/HCO3 or low Na = more H in plasma
- lowers pH
principal of electrical neutrality: plasma alkalosis
- low Cl/HCO3 or high Na = more OH in plasma
- raises pH
impermeant anions: ICF balance
A- balanced by K = electrical neutrality
impermeant anions: ECF balance
Cl- balanced by Na = electrical netrality
impermeant anions: ECF and ICF balance
equimolality
membrane potential=
Vm
membrane potential: features
- Vm measured relative to out of cell (ECF= 0)
- excitable cells capable of rapid change in response to stimulation
resting membrane potential:
RMP
- cell at rest
typical resting Vm: neurons
70 mV
exceptions: K+
- most common cation in ICF (balances A-)
- accumulated from sodium/potassium pump
- K will try to diffuse high to low (into ECF)
- down its chemical concentration gradient
chemical gradients:
- chemical driving forces
- ions move down chemical conc. gradient
- from high to low
charge separation:
- some intracellular K+ diffuses out (via leak channels) leaving unmatched -ve charges (impermeant A-)
- small difference in no. of charged ions btw inside/outside cell
- charges will cluster near membrane
charge separation: positive and negative charges found where?
- slight +ve charges (outside)
- slight -ve charges (inside)
electrical gradients: features
- opposite charges attract
- like charges repel
- charge separation creates electrical driving force
electrochemical gradient=
chemical gradient + electrical gradient
electrochemical force=
net driving force combo of chemical and electrical driving force
Na/K ATPase process:
- K inwards, Na outwards
- both move AGAINST electrochemical gradients
- ATP -> ADP
- 3 Na out of cell
- 2 K into cell
equilibrium occurs:
chemical and electrical driving forces are:
- opposite in direction, equal in magnitude
theoretical condition called ‘Equilibrium Potential E’
importance of Equilibrium potential E:
- tells which way ion will move across cell membrane at given membrane potential (Vm)
Nernst equation for K:
61.3 ÷ (+1) or z times log [K]out ÷ [K]in
forces on Na:
- high [Na] outside -> low [Na] inside
- chemical driving force pushes Na INTO cell
- diffuses through leak channels
- cell becomes less -ve, now exterior slight excess of -ve charges
- membrane potential Vm occurs -> electrical driving force attracts Na back out
E of K+
-95mV
E of Na+
+66mV
electrochemical force acting btw K and Na:
- this force will move ion across membrane in direction bring Vm closer to E of ion
- cells permeable to both ions
- RMP (resting Vm) will fall btw EK and ENa
typical neuron vs K:
neuron: -70mV vs EK= -95mV
EK < Vm
- chemical force drives K out > electrical force pulling K in
- net outward electrochemical force acting on K ions
- net outward flux/current of K ions
typical neuron vs Na:
- ENa = +66mV
- Vm «_space;ENa
- both chemical and electrical force direct inwards
- net inward electrochemical force acting on Na ions
- net inward flux/current of Na ions
define ion currents:
- movement of ions across membrane
- ions carry electrical charges and mass across membrane
magnitude of electrochemical force acting on particular ion is proportional to:
Vm - E
membrane permeability:
- also affects movement of ions
- most cells K is 25x more permeable than Na
Goldman-Hodgkin-Katz (GHK) equation
used to see relationship btw RMP and both ion conc. and ion permeabilities (P) for cell
maintaining RMP Vm:
- without other mechanisms, chemical gradients would gradually dissipate due to diffusion = electrically neutral both sides (Vm=0)
- cell actively transports K out and Na in using Na/K ATPase
Na/K ATPase: features
- uses ATP
- in electrically active neurons (60-70%) of cell energy budget used to drive pump
- removes 3 Na for every K brought in
- electrogenic (net outward K current)
Na/K ATPase: function
- maintains steep Na gradient across cell for regulation of:
- cytoplasmic pH
- interacellular [Ca]
- Na
- cellular volume (osmotic balance)
