what is RMP
- All cells have electrical potential (voltage) differences across their plasma membrane (membrane potential)
- > Membrane potential of cells under resting condition is defined as resting membrane potential (RMP)
why RMP ?
- Enables cells to function as batters – providing power to membrane proteins
- Exciteable cells (neurons, muscles…) large membrane potentials provide basis for signal transmission in form of depolarizing waves called action potentials
measuring RMP
- Microelectrode is a fine glass pipette and filled with conducting solution (KCL)
- Membrane potential expressed as voltage inside the cell relative to outside the cell
- Measured in millivolts (1mV= 0,001V)
RMP in different cell types
- Animal cells: negative potential at rest that ranges from -20 to -90 mV depending on cell type
- Cardiac and skeletal muscle cells have the largest RMP: -80 to -90 mV
- Nerve cells: -50 to -75 mV
what determines RMP
- Ionic concentrations on either side of the cell membrane
- Ionic permeability of the cell membrane
ion concentrations
- Ion selectivity and type of channels that are open make cell membrane selectively permeable to ions
ion concentration for typical mamalian cell
selective permeability of cell membrane
Phospholipid bilayer
- Hydrophobic interior
- Permeable to small uncharged molecules (O2. CO2, H2O, ethanol)
- Very impermeable to charged mole. (ions)
ion channels
- Proteins that enable ions to cross cell membranes
- Aqueous pore through which ions flow by diffusion (passive transport)
channel properties:
1. Non-gated leak channels are always open
2. Selectivity: for one or a few ion species – Na+, K+, Ca2+, Cl- ion channels
3. Rapid ion flow: always down the electro-chemical gradient
passive and active transport of ion across membrane
Passive:
- Ion channels
- Ions to diffuse down the conc. Gradient
- Selectively permeable to certain ions
Active:
- Actively (energy) move selected ions against conc. Gradient
- Create ion concentration gradients
setting up the RMP
- chemical and electrical gradient
-> - If the forces are equal there will be no net movement
K+ is main ion affecting RMP, Na+ is 2nd most
K+ chemical and electrical gradient
K+ inside (160 mM) —> K+ outside (4,5 mM)
electrical gradient is from extracellular to intracellular
-> when no net movement of K+ the RMP is negative
Na+ chemical and electrical gradient
Na+ outside (150 mM) -> Na+ inside (15mM)
electrical gradient is from intracellular to extracellular
-> when no Na+ net movement RMP is positive
Nernst equation
other ions and membrane permeabilities
- Got their own ionic gradients and membrane permeabilities -> contribute to resting membrane potential
Eg Na+ : Ena= +55 mV
why is the RMP not -70 mV and not -20 mV
Neuronal membranes have high relative permeability to K+ because they have more K+ leak channels than Na+ leak channels
calculating RMP for more ions: Goldman-Holding-Katz equation (GHK)
RMP and EK
For most cells open K+ channels dominate the resting membrane permeability:
- Cardiac muscle (-80mV) and nerve cells (-70mV): RMP quite close to Ek
- Smooth muscle (-50mV): resting membrane potential not so close to Ek as there is lower membrane selecticity for K+
- Skeletal muscle (-90mV): RMP close to ECl and Ek as there is high membrane selectivity for K+ and Cl-
Depolarizing and Hyperpolarization
a. Depolarization
- Decrease in size of membrane potential from normal value
- Cell interior becomes less negative
b. Hyperpolarization
- Increase in size of membrane potential from normal value
- Cell interior becomes more negative
maintaining concentration gradients
Primary Role: The Na⁺/K⁺ pump is responsible for maintaining the concentration gradients of Na⁺ and K⁺ by actively transporting these ions across the cell membrane against their concentration gradients.
Mechanism:
The pump moves 3 sodium ions (Na⁺) out of the cell and 2 potassium ions (K⁺) into the cell for every ATP molecule consumed.
This active transport is critical because it works against the natural diffusion of these ions (i.e., Na⁺ tends to move inward and K⁺ tends to move outward).
Net Effect:
Sodium (Na⁺): High concentration outside the cell (~145 mM) and low concentration inside the cell (~10-15 mM).
Potassium (K⁺): High concentration inside the cell (~140 mM) and low concentration outside the cell (~5 mM).
The energy for this process comes from the hydrolysis of ATP (adenosine triphosphate), which provides the necessary power to move ions against their gradients.
changing MP
Underlies many forms of signaling between and within cells:
1. Action potentials in nerve cells
2. Triggering and control of muscle contraction
3. Cell cycle re-entrance of post-mitotic cells
4. Transduction of sensory info into electrical activity by receptors
5. Postsynaptic actions of fast synaptic transmitters
controlling ion channel activity
- Channels can open and close: gated
1. Ligand gating - Channel opens or closes in response to binding of chemical ligand eg channels at synapses that respond to extracellular transmitters and channels that respond to intracellular messengers
2. Voltage gating - Channel opens or closes in response to changes in membrane potential eg channels involved in action potentials
3. Mechanical gating - Channel opens or closes in response to membrane deformation eg channels in mechanoreceptors: carotid sinus stretch receptors, hair cells
