Molecules and Cells - Electrochemical Equilibria

• examples of neurotransmitters that generate IPSPs

  1)

  ROC= GABAAR

  neurotransmitter= γ-Aminobutyric acid

  ion flow= Cl- in

  
  

  2)

  ROC= GlycineR

  neurotransmitter= Glycine

  ion flow= Cl- in
• examples of neurotransmitters that generate EPSPs

  1)

  ROC= nAChR

  neurotransmitter= acetylcholine

  ion flow= Na+ (and Ca2+) in and K+ out

  
  

  2)

  ROC= 5-HT3R

  neurotransmitter= 5-HT

  ion flow= Na+ and Ca2+ in

  
  

  3)

  ROC= iGluR

  neurotransmitter= Glutamate

  ion flow= Na+ (and Ca2+) in

  
  

  4)

  ROC= P2XR

  neurotransmitter= ATP

  ion flow= Na+ (and Ca2+) in
• How are IPSPs generated?

  negative ions are let into cell ( it becomes more negative inside than outside cell)

  
  

  -1 to -40 mV

  lasts 1 to 40 ms
• How are EPSPs generated?

  positive ions are let into cell ( it becomes more positive inside than outside cell)

  
  

  1 to 40 mV

  lasts 1 to 40 ms
• what are post synaptic potentials?

  -Postsynaptic potentials are changes in the membrane potential of the postsynaptic terminal of a chemical synapse.

  
  

  - these are due to the action of neurotransmitters acting on excitable cells.

  - the neurotransmitters act on receptor operated ion channels (ROCs) or Ligand-gated ion channels (LGICs)

  
  

  - they can be depolarising ( excitatory post synaptic potentials, EPSP) or hyperpolarisng (inhibitory post synaptic potentials, IPSP)
• What is the Nernst equation?

  Ex= RT/zF x log ([X]out / [X]in)

  
  

  Ex= Equilibrium potential

  R= gas constant

  T= temperature

  z= valency of ion

  F= faradays constant

  [X]out= conc of X outside cell

  [X]in= conc of X inside cell
• what is Equilibrium potential, Ex?

  At the equilibrium potential, the chemical and electrical gradients are equal and opposite in direction.

  
  

  - the amount of ions travelling in and out due to electric gradient is the same as the amount of ions travelling in and out dur to concentration gradient and so there is no net flow.

  
  

  - recorded from outside cell

  - the equilibrium potential is unique for each ion

  - lasts 2 to 5 ms
• Ion concentration (mM) inside cells

  Na+

  K+

  Cl-

  Ca2+

  Na+ = 5 mM

  K+ = 130 mM

  Cl- = 25 mM

  Ca2+ = 0.0001 mM
• What is voltage a measure of?

  Measure of the work done in separating charges across the membrane
• How are electrochemical gradients established?

  1) The pump moves ions against their concentration gradients

  2)There is restricted ion movement through channels

  3)The membrane stores ionic charges on its inner and outer surfaces (CAPACITANCE)
• What are electrochemical gradients?

  a gradient of electrochemical potential, usually for an ion that can move across a membrane.

  The gradient consists of two parts, the chemical gradient, or difference in solute concentration across a membrane, and the electrical gradient, or difference in charge across a membrane.
• what does the electrical and ion concentration gradient do to ions?

  electrical gradient =

  ions are attracted to their opposite charge

  
  

  concentration gradient =

  ions move from high to low concentration
• Ion concentration (mM) outside cells ( in the blood)

  Na+

  K+

  Cl-

  Ca2+

  Na+ = 140mM

  K+ = 3mM

  Cl- = 150mM

  Ca2+ = 1mM
• why is the sodium potassium ATPase transporter pump electrogenic?

  because it makes the inside of the cell slightly more negative (more positive ions are taken out than put back in). This causes a difference in charge.
• why is the KCC transporter pump electroneutral?

  because there is no change in charge ( it remains neutral)

  -it takes a positive and negative ion out so it does not make a difference
• How is resting membrane potential generated?


  -sodium potassium ATPase pump actively transports 3 Na+ ions out and 2 K+ ions into the cell using ATP

  -voltage gated sodium ion channels are closed so Na+ ions do not re-enter

  - the increased conc of K+ which is established due to the Na+/K+ ATPase pump causes the KCC ( potassium and chloride co transporter to allow K+ and Cl- ions to leave the cell).

  -there are already negative permanent ions inside the cell

  
  

  ----> this means the KCC pump is a secondary transporter as it relies on the sodium potassium ATPase pump ( primary transporter) to work.
• why are there differences in Vm ( membrane potential) of different cells?

  depends on the density, number and size of the non volatge gated channels
• what is the resting membrane potential ?

  The resting potential is determined by the concentrations of the ions in the fluids on both sides of the cell membrane and the ion transport proteins that are in the cell membrane.

  
  

  it is approximately -70mV

  (from -65v to -85 mV)

  
  

  - the membrane is polarised (more negative inside cell that outside)

  -not active

  -not generated by neurotransmitter

  -continuous
• examples of excitable tissue

  -neurons (CNS and PNS)

  -muscle ( cardiac and skeletal)

  -a very small number of glial cells

  -pancreatic beta cells (calcium AP)
• what is an action potential?

  An action potential occurs when the membrane potential of the tissue cells rapidly rises and falls.

  
  

  becomes more positive inside than outside cell.

  at 100mV

  
  

  includes:

  resting potential->depolairation-> action potential reached->repolarisation-> hyperpolarisation-> resting potential
• what is an excitable tissue?


  a tissue that can generate an action potential