Membrane Potentials Flashcards
Membrane potential
The potential difference between the inside and outside of a cell (measured in millivolts: mV).
Membrane potential of extracellular fluid
0 mV by definition
Microelectrode
- Fine glass pipette
- Tip diameter < 1 μm
- Can penetrate cell membrane
- Filled with conducting solution (KCl)
Animal cell resting membrane potential
-20 to -90 mV
Cardiac and skeletal muscle cell resting membrane potential
-80 to -90 mV
Nerve cell resting membrane potential
-50 to -75 mV
Effect of -70 mV membrane potential
Outward chemical gradient exceeds inward electrical gradient resulting in net efflux of K+ ions.
Effect of -90 mV membrane potential
Equilibrium potential- chemical and electrical gradients equal but opposite- no net flow of K+.
The Nernst equation
Ek = (61/z)log([K+]o/[K+]i
Ek: membrane potential at which K+ at eqm
z: valency (+1 for K+)
[K+]o,[K+]i: concentrations
Setting up the resting potential
- Open K+ channels dominate membrane ionic permeability at rest
- At eqm no net movement of K+ but -ve membrane potential
- Resting membrane potential arises as membrane more permeable to K+ at rest than other ions
Depolarisation
- Decrease in the size of membrane potential from normal value
- Cell interior less negative
Hyperpolarisation
- Increase in the size of membrane potential from normal value
- Cell interior more negative
Effect of increasing membrane permeability to a particular ion
Membrane potential moves towards the equilibrium potential for that ion
Equilibrium potential for K+ (Ek)
-90 mV
Equilibrium potential for Cl- (Ecl)
-70 mV
Equilibrium potential for Na+ (Ena)
+70 mV
Equilibrium potential for Ca2+ (Eca)
+120 mV
Movement of Na+ ions
Na+ moves into the cell to move Vm closer to Ena
Movement of K+ ions
K+ moves out of the cell
Movement of Cl- ions
Cl- moves into cell (has a -ve charge)
Ligand gating
Channel opens or closes in response to binding of a chemical ligand e.g. channels at synapses that respond to extracellular transmitters.
Voltage gating
Channel opens or closes in response to changes in membrane potential e.g. channels involved in action potentials.
Mechanical gating
Channel opens or closes in response to membrane deformation e.g. channels in mechanoreceptors.
Where can synaptic connections occur between?
nerve cell - nerve cell
nerve cell - muscle cell
nerve cell - gland cell
sensory cell - nerve cell
Basic synaptic mechanism
Chemical transmitter released from presynaptic cell binds to receptors on postsynaptic membrane.
Excitatory synapses
- Excitatory transmitters open ligand-gated channels that cause membrane depolarisation
- Permeable to Na+, Ca2+ or cations in general
- Resulting change called excitatory post-synaptic potential (EPSP)
- Excitatory transmitters include acetylcholine, glutamate
Inhibitory synapses
- Inhibitory transmitters open ligand-gated channels that cause hyperpolarisation
- Permeable to K+ or Cl-
- Resulting change called inhibitory post-synaptic potential
- Inhibitory transmitters include glycine, ɣ-aminobutyric acid
Factors that influence membrane potential
- Changes in ion concentration
- most important extracellular K+ conc (~5mM) - Electrogenic pumps - Na+/K- ATPase
- contributes very little to potential
- one +ve charge moved out per cycle
Significance of active transport of ions on membrane potential
Indirectly responsible for entire membrane potential, because it sets up and maintains ionic gadients.
