Memrane Physiology Flashcards
Function of cell membrane
Defines boundaries of cell
Enclose cell organelles
Enables cell to create an internal environment
Describe the compartment within confines of the cell membrane
Intracellular environment
Largely aqueous
Describe the compartment outside the cell membrane
Extracellular compartment:
Interstitial space
Largely aqueous
Examples of charge carrying particles in the intracellular environment
Organelles
Proteins
Anions or cations
Chemical gradient?
Concentration gradient of ions between the Intracellular and extracellular compartments of the cell
Electrical gradient?
Known as membrane potential
Electrical polarity between inside and outside of cells due to charged particles
Electrochemical gradient?
Combined electrical and chemical gradients
Standing electrochemical gradient because continuously present
What is Resting membrane potential?
Cells of different tissue have different resting potentials
Electrical gradient between inside and outside of the cell
Maintained via ion pump
Leak channels open
RMP of nerve cell?
-70 mv
Characteristic of electrically excitable tissues?
They able to momentarily discharge the standing electrical potential between the Intracellular and extracellular compartments
What is death in terms of electrically excitable tissue?
The permanent discharge of resting membrane potentials
Factors that decide what can cross cell membrane
Size
Polarity
Solubility in water
Ion channel/ protein
Types of ion channels
Selective
Leak
Transmembrane spanning proteins?
Protein assemblies that span the full thickness of the cell membrane
Common on ion channels
Voltage gated - generate AP
Ligand gated - generate Em at synapse with NT
Leak - set RMP
Mechanically gated
What is the function Ion pumps in nerve cells?
Maintain resting potential, dont set it up
Require ATP
they provide the starting point for the ability of the cell to generate electrical signals.
What is a Graded depolarisation?
Depolarisation is proportional to the strength of stimulation applied
Once threshold reached same AP always generated
What is level of membrane potential at which graded depolarisations become an action potential known as?
‘Threshold level’
Different neuronal cell types have different threshold levels
~ -55mV in neurones
Value of Resting membrane potential
-70mV
Nerst equation used for
Calculates membrane potential at eqm for each of the ions in question
Need to know relative conc of ion intracellularly and extracellularly
Epm potential of K+
-80mV
Eqm potential of Na+
+60mV
What is conductance of ion channels
Represents the activity of ion channels
Directly proportional to how many channels are opening in membrane
Each ion has its own conductance
When are voltage gates channels open
RMP closed
Depolarisation Na+ open
Repolarisation K+ open
Goldman Hodgkin Katz equation
Modification of the nerst equation
Takes into account relative permeabilities of ions in question
What happens during depolarisation
Na+ enters the cell
Membrane depolarises
Depolarisation reaches threshold
More Voltage gated Na+ channels open
Repolarisation events
K+ channels open - K+ efflux
Na+ channels closed - Na+ influx stopped
Extracellular voltage?
Zero mV
Channels that determine resting membrane potential?
Leak channels control permeability at rest
Electrogenic?
Many active transport mechanisms are electrogenic
Produce change in voltage because there is net movement of anions/ cations to one side of membrane
Chemical force
Differences in ion concentration
Electrical force?
Due to electrical charge intracellularly/ extracellularly
Membrane potential of the cell
What is eqm potential?
When chemical force = electrical force
Equal and opposite
Resting membrane potential will be equal to eqm potential if membrane permeable to one ion only. True/ False?
True
Skeletal muscle cells/ glial cells only permeable to k+
So membrane potential = K+ eqm potential
Ion composition of Na and K
• K+ concentration is high inside the cell, low outside
• Na+ concentration is low inside the cell, high outside
Driving an ion across the membrane electrically requires:
• the membrane possesses channels permeable to that ion to provide conductance
• There is an electrical potential difference across the
membrane
How does Em remain constant
Na influx = K efflux
- Driving force of K small and cell has large permeability to K
- Driving force of Na large but small permeability to Na
Why is the Na driving force large at Em (resting membrane potential)
Na influx due to chemical and electrical force
Big electrical difference. Na eqm is +60mV and Em is -65mV.
More Na outside due to pump so steep Chemical gradient.
What does resting potential depend on
Permeability of the ions
The ions driving forces
What is a nerve cell more permeable to?
K
More K leak channels
Why is there a small K driving force at RMP
Eqm potential of K+ is -80mV and Em is -65m. So small difference.
Chemical gradient (causing efflux) is greater than electrical force (causing influx)
Which ion is resting membrane most permeable to
K+
What are the permeabilities of ions during hyperpolarisation
Increased k+ permeability
Decreased Na+ permeability
What is the conductance of ions during depolarisation
Sodium has higher conductance than potassium
So sodium influx
What is refractory period caused by
Inactivation of na+ channels
Delayed closing of K+ channels
Do AP spread backwards?
No
